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Review of DC-DC Converter Topologies Based on Impedance Network with Wide Input Voltage Range and High Gain for Fuel Cell Vehicles

Review of DC-DC Converter Topologies Based on Impedance Network with Wide Input Voltage Range and... The development of fuel cell vehicles (FCVs) has a major impact on improving air quality and reducing other fossil-fuel- related problems. DC-DC boost converters with wide input voltage ranges and high gains are essential to fuel cells and DC buses in the powertrains of FCVs, helping to improve the low voltage of fuel cells and “soft” output characteristics. To build DC-DC converters with the desired performance, their topologies have been widely investigated and optimized. Aiming to obtain the optimal design of wide input range and high-gain DC-DC boost converter topologies for FCVs, a review of the research status of DC-DC boost converters based on an impedance network is presented. Additionally, an evaluation system for DC-DC topologies for FCVs is constructed, providing a reference for designing wide input range and high-gain boost converters. The evaluation system uses eight indexes to comprehensively evaluate the performance of DC-DC boost converters for FCVs. On this basis, issues about DC-DC converters for FCVs are discussed, and future research directions are proposed. The main future research directions of DC-DC converter for FCVs include utilizing a DC-DC converter to realize online monitoring of the water content in FCs and designing buck-boost DC-DC converters suitable for high-power commercial FCVs. Keywords Fuel cell vehicles · DC-DC boost converter · Evaluation systems and indexes · High gain topology AbbreviationsTL Three level CG Common groundVG Voltage gain CIC Continuous input currentVL Voltage lifting EMI Electromagnetic interferenceVM Voltage multiplier FC Fuel cellVS Voltage stress FCV Fuel cell vehicle ICR Input current ripple IVR Input voltage range 1 Introduction PEMFC Proton exchange membrane fuel cell PI Proportional-integral Fuel cell vehicles’ (FCVs) only by-product is water, truly PWM Pulse-width modulated achieving zero emissions. In recent years, FCVs have devel- SC Switched-capacitor oped rapidly in the field of new energy vehicles [1 , 2]. How- SI Switched-inductor ever the voltages of fuel cell (FC) stacks cannot match the bus voltages of vehicles. Moreover, due to the shortcomings of FCs, such as slow dynamic responses and wide output * Xiaogang Wu voltage ranges, auxiliary power sources (such as batteries) xgwu@hrbust.edu.cn are required to act as external energy storage systems [3, 4]. Institute of Electrical and Electronic Engineering, Harbin Therefore, DC-DC boost converters are utilized to achieve University of Science and Technology, Harbin 150080, voltage decoupling and power distribution [5, 6]. China The electrochemical reactions in FCs occur on the cata- Institute of Electrical and Information Engineering, Tianjin lytic layers. At present, Pt is the catalyst typically used in University, Tianjin 300072, China FCVs [7]. Current ripples can induce the failure of Pt in the State Key Laboratory of Automotive Safety and Energy, catalyst layer and even accelerate the degradation rate of Tsinghua University, Beijing 100084, China Vol.:(0123456789) 1 3 352 X. Wu et al. the proton exchange membrane, reducing the lives of proton averaging method, pulse-width modulated (PWM) switch exchange membrane fuel cells (PEMFCs). In Ref. [8], an modeling, and an equivalent transformer method. Owing to amplitude of low-frequency current ripple higher than 4% the "soft" output voltage characteristics of FCs and the vary- significantly reduced the durability and the life of the FC. In ing road conditions, DC-DC converters need to maintain a Ref. [9], the frequency of the current ripple in the FC system constant bus voltage and achieve dynamic regulation of out- was required to be higher than 1.25 kHz. FCs can operate put power. Realizing the control objective primarily includes reliably for low-frequency current ripple contents lower than following the given signal and suppressing the disturbance 5%. Therefore, to extend the lives of FCs, DC-DC boost signal. Therefore, the rapidity, robustness, accuracy, and low converters require low input current ripples (ICRs). complexity of the controller for the converter are impor- The output characteristic of FCs are “soft”, namely, the tant aspects in optimizing the performance of a DC-DC terminal voltage of an FC decreases with increasing output converter. Typical control methods for DC-DC converters current [10, 11]. The output characteristic curve of an FC is include proportional-integral (PI) control, sliding mode divided into polarization, ohmic, and concentration differ - control, adaptive control, robust control, fuzzy control, and ence sections [12]. The polarization section corresponds to novel integrated control strategies [24–27]. the initial stage of the reaction where the voltage drops rap- As part of FCVs, developing vehicle technology can pro- idly. This reduction is caused by the movement of electrons mote research on DC-DC converters. For instance, the pow- between the electrodes and the destruction and reformation ertrain of FCVs can be divided based on the different power - of chemical bonds during chemical reactions at the elec- train structures into energy-type and power-type, requiring trodes. The ohmic section corresponds to the deceleration of different types of DC-DC converters [28]. Permanent mag- the voltage drop rate, and the decline curve becomes linear. net synchronous motors, which have wide ranges of speed This is caused by the resistance of the FC modules and the regulation, are commonly used in FCVs; therefore, DC-DC ionic resistance of electrolytes. The concentration difference converters are required to regulate the output power quickly section corresponds to an increased voltage dip under a large [29]. Vehicle velocities and driving conditions affect the out- current, wherein the reactants reach the electrode surface by put powers of FCs. Research on vehicle velocity estimation, diffusion and convection. When the diffusion rate is lower motor torque, and stability control can provide guidelines than the rate of the electrode reactions, the reactant concen- for designing DC-DC converters [30, 31]. trates on the electrode surface and the electrolyte body differ Most of the previous reviews focused on the classifica- significantly [13]. In conclusion, considering the V-A curve tion of various DC-DC topologies, while ignoring the deri- of FC, DC-DC boost converters require wide input voltage vation of DC-DC topologies based on impedance network ranges (IVRs) to meet the current variations in FCs caused and the applicability of FCVs. The main contributions of by different road conditions. this study are as follows. (1) Based on the different types The voltage of a single FC is less than 1 V, and multiple of impedance networks, the non-isolated DC-DC converter FCs in series are used as an FC stack[14]. However, limited topology was divided into four types: inductor + inductor, space in the vehicle, in turn, limits the number of single capacitor + capacitor, inductor + capacitor, and hybrid / cas- FCs, and the output voltage of the FC stack remains low. At caded. On the basis of the proposed principles of impedance present, the predominant DC bus voltage of an FCV is 400 V transformation, 12 types of impedance network structures [15]. To achieve voltage matching, the voltage gain (VG) of were derived and classified. (2) Based on this classification, the DC-DC boost converter should be as high as possible the DC-DC topologies proposed in existing literature are and no less than 10 [16]. reviewed, and the applicability of each DC-DC converter Thus, to extend the lives of FCs, DC-DC boost convert- in the powertrain of FCVs is discussed. This provides the ers should maintain continuous input currents (CICs) and direction for designing DC-DC converters for FCVs. (3) An ICRs [17–20]. Considering that the input voltage of an FC evaluation system of a DC-DC converter for FCV is estab- decreases with increasing input current, the step-up DC-DC lished. The DC-DC converters proposed in mainstream lit- topology should have a wide IVR [21, 22]. Owing to the erature are then compared and evaluated, and their applica- limited space in FCVs, the converter should be miniaturized bilities are discussed. (4) The issues faced while researching and also have high frequency, efficiency, and reliability [23]. DC-DC converters for FCVs are summarized, and the future A DC-DC converter constitutes a typical nonlinear sys- research directions for DC-DC converters are presented. tem, and analyzing the dynamic operation characteristics The main contents of this study are as follows: Sect. 2 of the circuit is complex. Based on accurate modeling, the introduces the derivation process and the research status of static and dynamic characteristics of a DC-DC converter sys- existing step-up DC-DC topologies based on the impedance tem can be easily analyzed and evaluated, and the controller network. In Sect. 3, the evaluation system of DC-DC boost parameters can be matched. Currently, the modeling meth- converters for FCV is constructed, the comparison and eval- ods of DC-DC converters primarily include a state-space uation of typical DC-DC boost converters are performed, 1 3 Novel DC-DC boost topology Coupled inductor topology Abandoning electrical isolation of transformer Novel isolated topology Review of DC‑DC Converter Topologies Based on Impedance Network with Wide Input Voltage Range… 353 and suggestions for selecting the topology for FCVs are pro- and size [39–41]. Therefore, the converter based on coupled vided. Section 4 summarizes the key problems of DC-DC inductor is not suitable for the FCV powertrain. converters and prospects for future research directions. Sec- The non-isolated DC-DC boost converter is simple with tion 5 constitutes conclusions. no leakage inductance. It is, therefore, more suitable for FCVs. To ensure the conversion efficiency of the DC-DC converter, most of the existing converters for FCVs are 2 Research Status of DC‑DC Boost traditional boost converters, which have few devices [42]. Converters for FCVs However, owing to the influence of parasitic parameters, the boost converter cannot work under a limited duty cycle. There exist two types of DC-DC boost converters: isolated Therefore, it cannot achieve the high step-up ratio required and non-isolated DC-DC boost converters. Depending on the by FCVs in practice [43, 44]. In addition, an interleaved presence of a coupling structure, the non-isolated DC-DC boost converter topology can effectively reduce the require- boost converter is further divided into DC-DC boost con- ment for power devices and ICR and improve the lives of verters based on the coupled inductor and DC-DC boost FCs. Gao Dawei’s team from Tsinghua University success- converters based on impedance network [32]. fully applied an interleaved boost converter to an FC bus and A transformer is added between DC and DC in the non- obtained favorable results [45, 46]. However, the low VG isolated DC-DC boost converter to realize DC-AC-DC renders it unsuitable for a low power FC powertrain. step-up conversion. A high VG can be achieved by adjust- The step-up converter based on an impedance network ing the turn ratio of the transformer; however, a high turn (consisting of an inductor, capacitor, diode, and a switch) ratio would lead to a large leakage inductance, increasing does not face issues of voltage spikes and low power densi- the voltage stress (VS), thereby decreasing the efficiency ties caused by leakage inductance. A topology with high VG, [33]. In addition, the magnetic components would reduce the low ICR, and wide IVR can be designed from the perspec- power density, which proves disadvantageous to a vehicle tive of FCV applications. This study classie fi d and discussed with less available space [34, 35]. Therefore, the isolated this type of converter. The main classification of DC-DC DC-DC boost converter is inapplicable to FCVs. boost converters is shown in Fig. 1. A high VG topology based on coupled inductor can be obtained by terminating the electrical isolation of the trans- 2.1 Derivation and Classification of Boost Topology former and adding a coupled inductor to the non-isolated Based on Impedance Network converter [36–38]. The VG can also be effectively improved by selecting a high turn ratio. However, the risk of leak- By combining the energy storage elements (inductor and age inductance remains; thus, an additional snubber circuit capacitor) with a diode and switch according to the fol- should be added to the converter based on the coupled induc- lowing four principles, boost converter structures based on tor. This addition, however, increases the cost, complexity, impedance network can be obtained. Fig.1 Classification of DC-DC DC-DC boost topology topology Isolated DC-DC boost topology Non-isolated DC-DC boost topology Topology based on Basic isolated DC-DC boost impedance network topology Basic boost topology Half-bridge circuit Forward circuit Buck-Boost Full-bridge circuit Boost Cuk Flyback Push-pull circuit Sepic Zeta circuit 1 3 Principle 2 354 X. Wu et al. Principle 1: To increase the VG, inductors and capacitors Z Impedance Quasi-Z are charged from the power source in parallel and discharged SC Principle 3 source transformation source to the load in series. Principle 2: To increase the VG, when the switch is closed, energy is exchanged between the capacitors, and when it is open, both the capacitors and the power source supply energy to the load together. Capacitor Principle 3: To increase the VG, the diode is attempted to + Principle 4 TL Capacitor be replaced by an inductor without violating the circuit laws. Principle 4: To reduce the VS of the components, the capacitor should be added into the impedance network, forming a circuit loop with a switch, diode, and load. Different types of impedance networks mainly include: VM Modified VM Principle 3 (1) Inductor + Inductor Fig.2 Relationships among Capacitor + Capacitor topologies Two inductors are combined according to Principle 1, and their operating states depend on the states of the diodes (or switches). Thus, the switched-inductor (SI) structure can (4) Hybrid / Cascaded be obtained. If the input current of SI is pulsating, in order to reduce In addition, integrating and cascading the three types of the ICR, each inductor switches on in turn with the same trigger phase difference. Therefore, only one inductor is impedance networks above can optimize the performance of DC-DC boost converters and aid in obtaining certain novel charged at a time, and an interleaved topology is obtained. The diode is switched off while the inductors are charging. topologies. On the basis of the classification of topologies based on If this diode is replaced by a capacitor in such a way that the two inductors and the capacitor work according to Principle impedance network, as shown in Fig. 3, this study reviews existing literature from recent years, summarizes the advan- 1, the structure of the SI combined with a charge pump is obtained. tages and disadvantages of each topology to elucidate its applicability in the FCV powertrain, and provides the basis (2) Capacitor + Capacitor for designing DC-DC boost converters for FCVs. Two capacitors are combined according to Principle 1, 2.2 Impedance Network Composed of Inductor + Inductor and their operating states depend on the states of the diodes (or switches). The switched-capacitor (SC) structure can, 2.2.1 SI Structure thus, be obtained. According to Principle 2, a voltage mul- tiplier (VM), including Cockcroft–Walton and Dickson According to Principle 1, an SI structure can be obtained VMs, can be obtained by combining the two capacitors. The Z-source structure can be obtained by modifying the by utilizing two inductors to store and release more energy, as shown in Fig. 4. The components L1-D2 and L2-D1 in SC structure according to Principle 3. The quasi-Z-source structure can be obtained by the impedance transformation Fig. 4a L1-S2 and L2-S1 in Fig. 4b constitute the SI. The operating principle of SI was extensively analyzed of the Z-source structure. The modified structure of the VM can be obtained by modifying the VM according to Principle in Refs. [16], [47], and a family of SI topologies with high VG was proposed. In addition, a hybrid SI topology was 3. A three-level (TL) topology can be obtained by combin- ing two capacitors according to Principle 4. The relationship derived by combining passive and active SI units. In Ref. [48], a transformerless high VG DC-DC boost converter between the topologies of capacitor + capacitor is shown in Fig. 2. based on the SI structure was proposed for the FC system. Moreover, the capacitor and inductor were connected in par- (3) Inductor + Capacitor allel in an attempt to enhance the VG. As proposed in Ref. [49], a step-up DC-DC converter combined the traditional According to Principle 1, topologies based on impedance switched-boost network with an SI unit, exhibiting the char- acteristics of high VG, fewer components, low VS, and good network can be obtained by combining an inductor with a capacitor. scalability. In Ref. [50], a step-up topology was presented by 1 3 Principle 1 Review of DC‑DC Converter Topologies Based on Impedance Network with Wide Input Voltage Range… 355 Fig.3 Novel topologies based on impedance network Inductor Capacitor Four principles Inductor+Inductor Capacitor+Capacitor Inductor+Capacitor ͓ SC ͓ SI ͓ VM ͓ TL ͓ Interleaved ͓ SI combined with ͓ VL circuit ͓ Z source charge pump ͓ Quadratic boost ͓ Quasi-Z source Combination ͓ Hybrid ͓ Cascaded combining the active and passive SIs, exhibiting the charac- L1 teristics of high VG (> 10), low component stress, and high L1 D3 efficiency (95.5%). In Ref. [51], two inductors of quadratic boost were replaced with passive SIs, obtaining four derived structures that were extensively compared. D2 S1 S2 The SI can replace the inductor in existing DC-DC boost L2 D1 L2 converters, significantly increasing the VG of the topology. It also has the merits of simple and easy control. The DC-DC converter based on SI can effectively match the low output (a) SI composed of diodes (b) SI composed of switches voltage of an FC with the high bus voltage in the powertrain Fig.4 Two types of SI structures of an FCV. However, on connecting the SI to the FC stack, the output current of the latter would fluctuate, reducing the life of the FC. Therefore, this structure is inapplicable for the FCV powertrain with strict requirements for ICR. L1 D1 2.2.2 Interleaved Structure L2 D2 The interleaved structure was also composed of induc- S1 S2 tor + inductor. Its switches are switched on in turn with the same trigger phase difference; therefore, only one induc- tor is charged at a time. The energy was evenly distributed to the branches where the switches are located, effectively Fig.5 Two-phase interleaved step-up structure eliminating the ICR. As shown in Fig. 5, the drive signals of S1 and S2 differ by 180°. When L1 (L2) is charged, L2 (L1) supplies energy to the load. The two-phase interleaved boost structure was improved in Refs. [53], [45], respectively. These structures effectively in Ref. [52] in the context of renewable energy sources. improved the VG of converters; however, they faced the When different inductors supplied energy to the load, the issue of having no common ground (CG). An interleaved later-end capacitors worked in different states, effectively converter for FCV was proposed in Ref. [46]. An FC can improving the step-up ratio with a small ICR. The two- and operate in the boost or buck modes when its voltage was four-phase floating interleaved structures were investigated lower or higher than the DC bus voltage, respectively, 1 3 356 X. Wu et al. reducing the ICR. In Ref. [22], the interleaved structure was L3 D1 implemented at the front-end of the converter to reduce the ICR. In the back-end, two capacitors were combined with D2 a diode-capacitor network to increase the VG and reduce L1 D3 the VS. The interleaved structure has the advantage of a small L3 D1 ICR. For an n-phase interleaved converter, when the duty cycle is an integral multiple of 1/n, and the trigger phase difference is 360° /n , the ICR is eliminated [54]. Therefore, C1 L1 D2 in the field of FCVs, the interleaved (not floating interleaved) structure is suitable for the front-end of the boost converter to reduce the ICR. Fig.6 Structure of an SI combined with a charge pump Using the interleaved structure in the powertrain of FCVs can effectively reduce the current ripple of the FC stack, reducing the impact on the lives of FCs. Therefore, this structure is suitable as the front-end of the DC-DC converter. D2 However, there are obvious disadvantages that accompany this structure. First, this structure does not significantly L2 L1 D1 improve the VG and is not suitable for low power FC stacks that require high VG converters. Second, the switches switch on alternately, which requires highly precise trigger signals C1 S and a control circuit. Third, its input and output do not have a CG, producing additional electromagnetic interference (EMI) in the powertrain. Fourth, there were many switches, which increases the cost of the powertrain. Fig.7 Quadratic boost structure 2.2.3 SI Combined with Charge Pump Structure can be concluded that the structure combining an SI with a To improve the VG, the diode D2 in the SI structure was charge pump is not applicable for FCV systems. replaced by a capacitor C1, namely the structure of an SI combined with a charge pump is obtained, as shown in 2.2.4 Quadratic Boost Fig. 6. In Ref. [55], a structure combining an SI with a charge According to Principle 1, the capacitor is used as another pump was proposed, which improved the step-up ratio of power source to realize charging in parallel and discharging the converter and provided the circuit with resilience to in series of two inductors, significantly improving the step- component tolerance. As demonstrated in Ref. [56], an SI up ratio. The quadratic boost structure is shown in Fig. 7. combined with a charge pump replaced one inductor (the In Ref. [58], the operating principles of the quadratic inductor that was not directly connected to the power source) boost structure were analyzed, the energy transmission in the quasi-Z-source. The new step-up structure exhibited modes were investigated, and the ripple characteristics of the reduced ICR and VS and also a high VG. The novel non-iso- quadratic boost converter in the complete inductor supply lated DC-DC topology proposed in Ref. [57] was cascaded mode (CISM) and incomplete inductor supply mode (IISM) with charge pump and SC structures, reducing the VS of the were analyzed. In Ref. [59], a modified high gain quadratic components as well as the switching and conduction losses. boost-sepic converter was proposed, which produced a high The structure combining an SI with a charge pump can VG with a low duty cycle and reduced the VSs of the switch further improve the VG by modifying the SI structure, which and the diodes while exhibiting the advantages of a con- renders the structure suitable for the FCV powertrain that tinuous input current (CIC). In Ref. [60], a modified high requires a high VG converter. However, on connecting this VG quadratic boost-sepic converter, having both CIC and a structure with an FC stack, the output current of the FCs CG structure, was proposed. In Ref. [61], a modified high would become pulsating current. Moreover, when the switch VG step-up DC-DC topology based on the quadratic boost is switched off, the power source is directly connected to structure was presented. This converter improved the VG capacitor C1, and the initial inrush current would appear on while decreasing the VS across the switches as well as the the side of the power source. From the two points above, it overall converter losses. 1 3 Review of DC‑DC Converter Topologies Based on Impedance Network with Wide Input Voltage Range… 357 The step-up ratio of the quadratic boost converter, which 2.3.2 VM Structure has both CIC and a CG structure, is the square of that of the boost converter. When the quadratic boost converter is used According to Principle 2, a VM can be obtained by transfer- in the FCV powertrain, it provides a high VG to match the ring energy between two capacitors to multiply the output FC and DC bus voltages of the vehicle, and it also main- voltage [66, 67]. The VM includes both Cockcroft–Walton tains the continuity of the FC output current. Moreover, and Dickson VMs, as shown in Fig. 9a and b. The single- the CG characteristics of the quadratic boost converter can stage VM is shown in Fig. 9c. effectively reduce additional EMI in the vehicle powertrain. In Ref. [68], a high VG DC-DC topology with CIC and Therefore, such a converter is suitable as the front-end of the high efficiency was designed based on a Cockcroft– Wal- DC-DC boost converter for an FCV. ton VM for the FC. The novel topologies designed in Refs. [69], [70] implemented interleaved structures in the front- 2.3 Impedance Network Composed stage to reduce the ICR, and a Dickson VM lifted the volt- of Capacitor + Capacitor age in the post-stage to match the voltage between the DC bus and renewable energy sources. A Dickson VM was first 2.3.1 SC Structure attempted to be applied to a classic non-isolated DC-DC boost converter in Ref. [71], and zero-current-switching According to Principle 1, two capacitors can be utilized to was realized. The Dickson VM used in the post-stage of the store and release more energy, obtaining the SC structure. converter proposed in [69] was improved upon in Ref. [72]. This structure optimizes the topology with regards to the The high VSs on the post-stage capacitors of a multi-stage voltage conversion ratio, as shown in Fig. 8. Dickson VM were reduced. In Ref. [62], an SC structure was added to boost, buck- This VM exhibited the characteristics of a high VG and a boost, and Cuk converters, effectively improving the VGs simple structure, making it applicable for the post-stage of of the topologies. In Ref. [63], SCs (Fig. 8b) replaced the the DC-DC boost converter for an FCV. However, VSs of capacitors in Cuk, sepic, and Zeta converters, improving the components increase with the number of stages in the VM, VGs of the topologies and also achieving higher conversion which, in turn, increases the possibility of component defects efficiencies. After modifying the SC (Fig.  8a) in Ref. [64], a and decreases the reliability of the entire FCV powertrain. novel topology with a high step-up ratio and a good transient In addition, the high VS results in the converter requir- response was obtained. In Ref. [65], a novel DC-DC struc- ing components with high rated powers, which increases ture based on the SC network was proposed. This topology the cost and power loss in the powertrain. Therefore, when could significantly improve the output voltage and possessed smooth input current and output voltage. C'1C'2 C'n The SI structure could effectively improve the VG and is ... accompanied by the advantages of simple control and easy implementation. However, the input and output of the SC ... structure do not have CGs. Prior to application in the FCV D2 D'3 D1 D'1 D'2 D3 ... powertrain, the SC structure should be optimized from the C1 C2 Cn perspective of the CG. (a) Cockcroft-Walton VM ... C'1 C'2 C'n ... D3 D'3 D1 D'1D D2 '2 C1 D1 C1 C2 Cn ... (b) Dickson VM D1 D2 C1 C2 C'1 C2 D1 D2 D2 C1 (a) (b) (c) Single stage VM Fig.8 Two types of SC structures Fig.9 Three types of VM structures 1 3 358 X. Wu et al. Fig.10 Modified VM L1 C'1 C1 C2 D1 L1 C1 L2 designing DC-DC boost converters based on VMs, the topol- Fig.11 Z-source structure ogy should be optimized with respect to reducing VSs of the components. components into the traditional Z-source structure. In Ref. 2.3.3 Modified VM Structure [80], the Z-source structure was combined with a VM to obtain a novel CG high-gain topology, which was suitable By replacing the diode D2 in the single-stage VM with an for renewable energy sources. The Z-source structure was inductor L1 according to Principle 3, a modified VM struc- integrated with the quasi-Z-source one in Ref. [81], and a ture, as shown in Fig. 10, can be obtained. combined Z-source DC-DC structure was proposed for the New energy systems were selected as the research back- FC and photovoltaic systems. This converter exhibited good ground, and the boost topology was combined with a modi- step-up capability. fied VM in Ref. [73]. This effectively improved the VG and The Z-source structure can provide a high gain under the reduced the ICR. The novel step-up DC-DC topology, pro- non-limit duty cycle, which can effectively match the low posed in Ref. [74], combined sepic topology, a VM, and a output voltage of FCs with the high bus voltage. However, modified VM to effectively rectify the voltage mismatch in using the Z-source structure in the FCV powertrain would new energy systems. In Ref. [75], the VM was integrated generate a pulsating current in the FC, affecting its life. In with a modified VM to obtain a second-order hybrid boost addition, the input and output of the Z-source structure do topology, which was suitable for renewable energy sources. not have a CG. The two points above restrict the application The modified VM can effectively improve the VG of the of Z-source structures in the FCV powertrain. topology, making it suitable for the FCV powertrain that requires high VG converters. However, when the modi-2.3.5 Quasi‑Z‑Source Structure fied VM is cascaded, VSs of the post-stage components increases. Prior to applying the modified VM structure to To overcome the disadvantage of the Z-source structure FCVs, a new topology should be designed from the perspec- not having a CG, a quasi-Z-source can be obtained from tive of reducing VSs to reduce the cost and power loss in the the impedance transformation of the Z-source structure, as powertrain. shown in Fig. 12. The operating principles of the quasi-Z- and Z-source structures are similar. 2.3.4 Z‑Source Structure In Ref. [3], the quasi-Z-source structure was used as the front-end of the proposed topology, obtaining a high The Z-source structure can be obtained by transferring the VG converter for FCVs. In addition, the efficiency of this SC structure according to Principle 3, as shown in Fig. 11. converter reached 95.13%. Based on the investigations When S is closed, C1 and C2 charge L1 and L2, respec- conducted in Ref. [3], a PI controller of the quasi-Z-source tively; however, when S is open, the power source charges structure that combined composite feedforward and feed- C1 and C2. The power source, L1, and L2 supply power to back controls was proposed in Ref. [82]. The quasi-Z-source the load [76]. converter was subsequently tested under the worldwide har- A steady-state analysis of a Z-source step-up topology monized light-duty test cycle. A high voltage boost DC-DC operating in the continuous conduction mode (CCM) was converter composed of a quasi-Z-source converter and a conducted in Ref. [77], and the voltage ripple and loss of quadratic boost converter was proposed in Ref.[83]. A high the converter were calculated. In Ref. [78], two high VG VG quasi-Z-source topology for new energy sources was Z-source DC-DC converters suitable for renewable energy proposed in Ref.[43], not only improving the VG of the sources were proposed, and an appropriate control method quasi-Z-source structure but also maintaining the advantages was presented. Moreover, the application prospect in FCV of CIC and low VSs of capacitors. was discussed. As demonstrated in Ref. [79], a CG Z-source The VGs of the quasi-Z-source and Z-source structures high VG converter with low stresses, high efficiency, and were equal. When the duty ratio is nearly 0.5, a high VG small size was designed without inserting additional can be obtained. Moreover, the CIC of the quasi-Z-source 1 3 Review of DC‑DC Converter Topologies Based on Impedance Network with Wide Input Voltage Range… 359 C2 2.4 Impedance Network Composed of Inductor + Capacitor L2 L1 D2 According to Principle 1, a voltage lifting (VL) circuit can be obtained by using an inductor and a capacitor to store and release more energy, as shown in Fig. 14. C1 S Based on the basic boost topology, positive and neg- ative VL circuits, first proposed in Refs [88] and [89], respectively, effectively improved the step-up ratio. Regarding the issue of low VG in the VL circuit, a family Fig.12 Quasi-Z-source structure of VL topologies with geometrically increasing VG were designed in Refs [90] and [91]. In Ref. [92], the VL circuit structure is beneficial to FCs, low VSs of the devices are was combined with the quadratic boost circuit to lift the conducive to reducing the cost of the powertrain, and the VG of the converter and reduce the VS of the switch. In CG characteristics can effectively reduce the additional EMI Ref. [93], a novel topology with high VG and CIC was in the powertrain. Therefore, this structure is suitable for obtained by integrating the conventional boost circuit with FCV systems. a VL circuit. A single switch step-up topology, based on the VL circuit and having high VG, was proposed in Ref. 2.3.6 TL Structure [94]. However, the input and output sides did not have a CG. In Ref. [95], the VL circuit was improved and a con- A TL structure, which can reduce the VS to half of the out- verter with a high step-up ratio was proposed, which had put voltage as shown in Fig. 13, can be obtained by combin- fewer switches. ing capacitors according to Principle 4. The VL circuit can significantly improve the step-up In Ref. [84], the TL forms of six basic chopper circuits ratio, and its input and output have a CG. The flexible were derived and then optimized. On this basis, a feedfor- structure can be easily designed. However, this circuit has ward control strategy for these topologies was proposed. A a pulsating input current, which is unfavorable for the FC TL topology was implemented in Refs [85] and [86] as the stack. While designing the converter topology for FCVs, post stage of the step-up converter for an FCV, effectively optimization with regard to CIC should be performed to reducing VSs of the components. For the FC composite improve the durability of the FCV powertrain. energy storage system, the VS on the switch of the TL topol- ogy presented in Ref [87] equaled to only half of the output 2.5 Hybrid and Cascaded Structure voltage, and the dynamic response of the converter was sig- nificantly improved. 2.5.1 Hybrid Structure The effective reduction of VSs of the components makes the TL structure suitable for application as the post-stage A hybrid structure can be obtained by replacing a functional of a DC-DC boost converter for an FCV. However, the unit of the basic topology with one or multiple topologies (or disadvantages of high control accuracy and a large number a part of the topology). The novel boost topology can improve of switches necessitates extensive research on developing the step-up ratio, IVR, component stress, or ICR. Hybrid topol- an optimization scheme to reduce the cost and power loss ogy can optimize the performance of the topology according to of the powertrain. specific requirements, which is a common method of designing D1 D2 D1 S1 C1 LC D2 S2 S C2 Fig.14 VL circuit Fig.13 TL structure 1 3 360 X. Wu et al. DC-DC converters for FCVs. The schematic diagram of hybrid with CIC can be used as the hybrid unit. Alternatively, high topology construction is shown in Fig. 15. Moreover, Boost, VG topology can be used as the hybrid unit to effectively Cuk, and sepic circuits have CIC, which is appropriate for the match the low output voltage of FCs and the high bus volt- basic topology of the hybrid structure. age. Furthermore, the CG structure should be maintained as The characteristics of a few DC-DC boost converters much as possible to reduce the additional EMI in the FCV constructed using the hybrid structure are listed in Table 1. powertrain. To reduce the power loss of components and Using a hybrid structure constitutes an important method improve the reliability of the powertrain, reducing the VSs of of generating novel boost converters for FCVs by introduc- components is necessary. However, there are many devices ing functional units to optimize the performance of the in the hybrid topology, which would increase the cost of the topology. For instance, when designing DC-DC converters powertrain and decrease the efficiency. for FCVs, blending can be conducted according to the actual needs. To reduce the impact on the lives of FCs, the topology Functional unit 1 Functional unit 2 Functional unit 2 The basic topology Topology 1 Topology 2 Topology 2 The hybrid topology Fig.15 Schematic of hybrid structure construction Table 1 Configuration and Reference Topology configuration Advantages Appli- advantages of a few hybrid cable to topologies FCV [96] Boost + Cuk ●Low VS ●CIC ★ [97] Quadratic Boost + Cuk ●High VG ●CIC ●Low VS ★ [98] Sepic + Modified VM ●High VG● CIC ★★ [99] Z-source + Quasi-Z-source ●High VG ★ [100] Cockcroft–Walton VM + Dickson VM ●Low output voltage drop ★ ●Low capacitor VS● High VG [101] Interleaved + Multi-level ●High VG● Low ICR ★ [102] Interleaved + VM ●Low ICR● High VG ●Simple structure ★★★ [103] [23] Boost + Modified VM ●CIC ●High VG● Low VS ★★ [104] Boost + Modified VM ●CIC● High VG ●Low switch VS ★ [105] VL circuit + Quadratic Boost ●High VG ★ ★ particularly inappropriate, ★★ relatively appropriate, ★★★ particularly appropriate 1 3 Topology 3 Functional unit 3 Topology 1 Functional unit 1 Topology 3 Functional unit 3 Review of DC‑DC Converter Topologies Based on Impedance Network with Wide Input Voltage Range… 361 2.5.2 Cascaded Structure through cascading. The cascaded structure exhibits the advantages of variety and simple implementation. Cascading more than two topologies (the duplicate part can Many studies have been conducted on utilizing cascaded be shared) can generate a cascaded structure, as shown in structures to construct new boost topologies for FCVs. A few Fig. 16. The 1st converter utilizes the FC as its power source, configurations and advantages of the cascaded structure are and the subsequent converter, in turn, uses the output volt- listed in Table 2. age of the former one as the power source. Thus, a cascade The cascaded structure can flexibly combine sub- relationship is formed. The nth converter provides energy topologies, according to the requirements of the particular to the load. Implementing the cascaded structure constitutes application, to obtain a new topology. This new topology, another important method of constructing new DC-DC boost thus, inherits the advantages of each sub-topology. To converters for FCVs. The performance of the topology in ensure the suitability of the cascaded topology for the terms of ICR, VG, IVR, and VS can be effectively improved FCV powertrain, a topology with low ICR can be used in the front-stage of the cascaded topology, ensuring the low current ripple characteristics of FCs. The cascaded topology can use a high gain structure as an intermedi- ate link to maintain the bus voltage under different FC DC-DC DC-DC DC-DC ... + ++ topology n topology 1 topology 2 output powers, and thus, adapt to different driving condi- tions. To reduce the cost of the powertrain and improve reliability, the TL structure can be used as the end-stage of the cascade topology. However, the efficiency of the Power DC-DC DC-DC DC-DC cascaded topology is the product of the efficiency at each ... Load source topology 1 topology 2 topology n stage, resulting in reductions in the entire efficiency of the vehicle powertrain [117]. Fig. 16 Schematic of cascaded structure construction Table 2 Configurations and advantages of a few cascaded topologies Reference Topology configuration Advantages Appli- cable to FCV [69] Interleaved + VM ●Low ICR ●High VG ★★ [70] [106] [107] Capacitor clamped circuit + VM ●CIC ●High VG● Low VS ●Low power loss ★★★ [108] Interleaved + VM + TL ●High VG● Low VS ●Zero ICR ★★ [109] SI + VM ●CIC ●High VG● Low power loss ★ [3] Quasi-Z-source + VM ●High VG●Low VS ●Wide IVR ★★★ [85] Quasi-Z-source + TL ●High VG ●High conversion efficiency ●Wide IVR ★★★ [86] Capacitor clamped circuit + TL ●High VG ●Simple control ●CIC ●Low VS ★★ [90] VL circuit + VL circuit ●High VG ★ [91] [65] SC + SC ●CIC ●High VG ★ [110] Quadratic Boost + VM ●High VG● CIC ●Wide IVR ★★★ [111] ●High conversion efficiency [112] [80] Z-source + VM ●High VG● Low VS ●High conversion efficiency ★ [113] Boost + VM + Modified VM ●High VG ●Low VS● CIC ★★★ [114] Boost + VM + VM ●High VG● Low CS● CIC ★★★ ●Working at non-limit step-up ratio [115] Boost + SC + VM ●CIC● High VG ●Low VS ★★ ●High conversion efficiency [116] Boost + VL circuit ●High VG ●Low power loss ★★★ ●CIC ●Few components ★ particularly inappropriate, ★★ relatively appropriate, ★★★ particularly appropriate 1 3 362 X. Wu et al. To synopsize, Table 3 systematically summarizes the high VG, low ICR, and wide IVR. The universal require- advantages and disadvantages of typical impedance net- ments for a DC-DC boost converter include voltage and works and provides suggestions for constructing DC-DC current stress, conversion efficiency, power density, number boost converters for FCVs. of components, the slope of step-up ratio, and presence of CG. 3 Evaluation of DC‑DC Boost Converter 3.1 Indexes of DC‑DC Boost Converter for FCVs for FCVs 3.1.1 VG Existing evaluation indexes for DC-DC boost converters mainly consider the performance of DC-DC converters, VG of the converter is defined as the ratio of the output volt- while the proposed evaluation indexes also take the require- age to the input voltage. The strength of the step-up capabil- ments of connection with FC into consideration. Therefore, ity of the topology increases with the step-up ratio. DC-DC the proposed evaluation system is more suitable for DC-DC boost converters for FCVs require high VGs no less than ten converters for FCVs. times. Figure 17 compares the VGs of 13 topologies. The evaluation indexes of the DC-DC boost converter constitute the standard based on which the merits and demerits of the topology are assessed. The performance requirements of the DC-DC boost converter for applica- tion in the field of FCVs are reflected by three indexes: Table 3 Advantages and disadvantages of topologies based on the impedance network Classification Impedance network Advantages Disadvantages Inductor + Inductor SI ●High VG ●Easy to control ●Pulsating input current ●Easy to generate new topologies Interleaved ●Low ICR ●No obvious increase in VG ●High ●Easy to generate new topologies control requirements ●Input and output do not have CG (float- ing interleaved) ●Many switches SI combined with charge pump ●High VG ●Easy to control ●Pulsating input current ●Easy to generate new topologies ●Initial inrush current existing in the power source side Quadratic boost ●High VG ●Low ICR ●Easy to control ●High VS Capacitor + Capacitor SC ●High VG ●High conversion efficiency ●Input and output do not have CG VM ●High VG ●High VS ●Output voltage dropping noticeably Modified VM ●High VG ●High VS Z-source ●High VG in non-limit duty cycle ●Pulsating input current ●Input and output do not have CG ●Narrow duty cycle range Quasi-Z-source ●High VG in non-limit duty cycle ●Narrow duty cycle range ●Compared with Z-source, input–output in CG ●Low ICR TL ●Low VS ●High control requirements ●Many switches ●No obvious increase in VG Inductor + Capacitor VL circuit ●Easy to generate new topologies ●No obvious increase in VG ●Pulsating input current Hybrid / Cascaded Hybrid ●High VG ●Easy to generate new topolo- ●Many components ●Low conversion gies efficiency ●Low power density Cascaded ●High VG ●Easy to generate new topolo- ●Many components ●Low conversion gies efficiency ●Low power density 1 3 Review of DC‑DC Converter Topologies Based on Impedance Network with Wide Input Voltage Range… 363 3.1.2 ICR low reliability of the entire converter. In addition, the volt- age and current stresses determine the rated parameters During the high-frequency switching of DC-DC boost con- of power electronic components. Generally, the higher the verters, the internal energy storage components are also voltage and current stress the component can withstand, the charged and discharged at high frequencies. This would, more expensive and the larger the size. To reduce the cost in turn, generate current ripples at the input side of the and size, the power electronic components in the DC-DC converter. The input side of the DC-DC boost converter is boost converter for FCV should have low voltage and cur- directly connected to the FC; therefore, an excessively large rent stresses. ICR of the converter reduces the life of the FC. Therefore, reducing the ICR is imperative [118]. Although the current 3.2.2 Power Density ripple can be filtered using filters, the input current of the DC-DC boost converter for FCV should not be pulsating The ratio of the rated power to the volume of the step-up considering the limited vehicle space. converter is defined as the power density. Owing to the limited space in FCVs, the converter is required to process high power at a small size. Utilizing appropriate circuit 3.1.3 IVR models and analysis methods to accurately calculate the parameters of inductors, capacitors, switches, and other The output power of the FC varies with the road conditions. components in the topology is a common method for The DC bus voltage is power battery voltage; therefore, its improving the power density. Miniaturization and the use variation range is small, and the output current of the FC of a high-frequency converter constitute the most effective would significantly change. The output voltage of the FC methods of increasing the power density. However, with would noticeably drop for increasing output current. In addi- increasing operating frequencies of the component, severe tion, a higher VG of the topology partly implies a wider EMI issues begin to arise. voltage range that can be inputted. To ensure FCV operation under various road conditions, the width of the IVR is an 3.2.3 Number of Components important index to estimate the performance of the DC-DC boost converter for an FCV. The complex DC-DC topology can generally improve the performance in one (or several) aspect(s). However, the dis- 3.2 Universal Indexes of DC‑DC Boost Converter advantages of having many components are evident, espe- cially in the cascaded and hybrid structures. A large number 3.2.1 Voltage and Current Stress of components increase the cost of the DC-DC boost con- verter and reduce the power density. The number of compo- VS constitutes the maximum reverse voltage born by the nents should be reduced as much as possible to meeting the component when it is switched off. Current stress consti- requirements of FCVs. tutes the maximum current flowing through the component. The significant increase in the component stress, in turn, increases the possibility of component defects, leading to Fig.17 VG curves of typical novel topologies 1 3 364 X. Wu et al. 3.2.4 Conversion Efficiency evaluated ten typical impedance network DC-DC boost topologies in terms of eight aspects: VG, ICR, IVR, VS, Owing to the parasitic parameters and the frequent switching current stress, number of components, the slope of the step- on and off of the switches, DC-DC converter losses include up ratio, and CG, as shown in Fig.  18. Two points are to conduction and switching losses. This not only results in be explained: first, the advantages and disadvantages of the an output power lower than the input power but also raises indicators of topology are relative. The topologies evalu- the temperature of the converter. The conversion efficiency ated in Fig.  18 only reflect the relationship between ten of the DC-DC converter is defined as the ratio of output topologies. Secondly, no topology is excellent in all aspects. power to input power. Moreover, high efficiency is the objec- Certain performances of the topology should be optimized tive behind developing DC-DC topologies. The conduction from the perspective of an application when designing the loss increases with the current. The switching loss of the topology. converter, however, increases with the operating frequency. In Fig. 18, a larger shaded region indicates better indica- Soft-switching technology can be used to reduce switching tors. From the inside to the outside, there are three grades: loss. C, B, and A, representing excellent, medium, and poor, respectively. There are only two grades of C (no) and A 3.2.5 CG (yes) in the presence of a CG, and the input current only has two grades: C (pulsating) and A (non-pulsating). The The du/dt and di/dt caused by switches are the primary VG function cannot be directly compared; therefore, its EMI sources in the DC-DC converter. Implementing soft- value is taken to the one corresponding to duty cycle switching technology, designing filters, parasitic parameter d = 0.8 (the topology with duty cycle 0 < d < 0.5 as the optimization, designing shield structure, and adding a CG value corresponding to d = 0.4, and the topology of duty are effective measures to suppress EMI [119]. When the cycle 0.5 < d < 0.75 as the value corresponding to d = 0.7). DC-DC converter does not have a CG, the high-frequency The IVR considers the variable range of the duty cycle an PWM voltage difference would generate additional EMI. intuitive reference. A better index is shown in Fig. 18 for a Therefore, CG is a key index in evaluating the topology of larger range of d. The number of components is the sum of the DC-DC boost converter for FCVs [120]. the number of switches, diodes, inductors, and capacitors. The stress function cannot be compared directly; therefore, 3.2.6 Slope of Step‑up Ratio the voltage and current stresses are the values correspond- ing to d = 0.8 (the topology for 0 < d < 0.5 as the value cor- The slope of the step-up ratio is an index that describes the responding to d = 0.4, and the topology for 0.5 < d < 0.75 as change in the step-up ratio with the duty cycle. A high step- the value corresponding to d = 0.7). The slope of the step- up ratio slope implies that the step-up ratio increases rapidly up ratio is the value corresponding to d = 0.8 (the topology with a slight increase in the duty cycle. The designers of the for 0 < d < 0.5 as the value corresponding to d = 0.4, and DC-DC boost converter tend to favor a gentle slope of the the topology for 0.5 < d < 0.75 as the value corresponding step-up ratio because it indicates that the control requirement to d = 0.7). of the topology is easy to achieve. 3.3 Comparison and Evaluation of Typical DC‑DC 4 Limitations and Prospects of DC‑DC Boost Boost Converter Converter for FCVs In this section, DC-DC boost converters based on imped- Herein, the derivation and research status of DC-DC boost ance network are compared in terms of eight aspects: the converters are analyzed, and the future research direction of technology used, VG, ICR, duty cycle range, voltage and using DC-DC boost converter for FCVs is presented. This current stresses, number of components, CG, and maxi- is done to further promote the development of FCVs and mum efficiency. The power density and conversion effi- the applications of DC-DC boost converters. Topologies ciency of the topology are affected by many factors, such of DC-DC conversion systems for FCVs have become a as the operating frequency, rated power, and the number of research hotspot with the development of new energy vehi- components. The distribution of indexes for several exist- cles. On the basis of DC-DC topology optimization, design- ing topologies are summarized in Table 4. The maximum ing an advanced DC-DC conversion system while meeting efficiency in Table  4 is the value obtained in the corre- the requirements for FCVs is significantly challenging. The sponding paper. main issues about DC-DC conversion systems for FCVs are To compare the DC-DC boost converter more intuitively, as follows: and aid the topology design of FCVs, this study qualitatively 1 3 Review of DC‑DC Converter Topologies Based on Impedance Network with Wide Input Voltage Range… 365 1 3 Table 4 Indexes of typical DC-DC topologies Topologies Technology used VG Input current Duty cycle range Voltage/ current stress of Number of compo- CG Efficiency Appli- switches nents cable to FCV VS Current stress L/C Q D 1 1 Boost - Non (0, 1) U 1/1 1 1 Yes – ★ o I 1−d 1−d Pulsating 1+d 2d [96] Hybrid Non (0, 1) 2/3 1 2 No – ★ o o 1−d 1+d 1−d Pulsating 2 2 3 1+d−d ) 2d+d +d [97] Hybrid Non (0, 1) 3/4 1 4 No 94.07% ★ 2 o 2 2 o 1+d−d (1−d) (1−d) Pulsating 1+3d 1+d 2 [16] SI Pulsating (0, 1) 4/1 2 4 No 95.8% ★ o o 1−d 1+3d 1−d 2 2 [55] SI combined with charge pump Pulsating (0, 1) U 2/2 2 2 Yes 90% ★ o I 1−d 1−d 2 U 4 [85] Quasi-Z-source + Non (0.5, 0.75) 2/4 2 3 Yes 95.66% ★★★ 3−4d 2 3−4d TL Pulsating 2 1+2d [3] Quasi-Z-source + Non (0, 0.5) 2/5 1 5 Yes 95.13% ★★★ 2 o 1−2d 2 d−2d VM Pulsating 1 2 [121] Z-source Pulsating (0, 0.5) U 2/3 1 2 No – ★ 1−2d 1−2d 1+d 1 3+d [122] Z-source + Pulsating (0, 0.5) 3/5 1 3 No 82.4% ★ U I o o 1−2d 1+d (1−d) SC 2 U 1−d [107] Capacitor clamped + Non (0, 0.5) 1/4 2 5 Yes 94.72% ★★★ 2 o 1−2d 2 d−2d VM Pulsating 2 U 1 [86] Capacitor clamped + Non (0, 0.5) 1/3 2 4 No 95.28% ★★ 1−2d d−2d TL Pulsating 2 3 3+d 1+d 1+3d−d −d [110] Quadratic boost + Non (0, 1) 2/4 2 7 Yes 95.04% ★★★ 2 o 2 3 o (1−d) 3+d d−2d +d VM Pulsating 2 U 1.5 [123] Interleaved Non (0, 0.5); 2/3 2 4 Yes 92.6% ★ 1−d 1−d Pulsating (0.5, 1) 6 3 [70] Interleaved + VM Non (0, 1) 2/6 2 6 No 96% ★★ 1−d 6 1−d Pulsating 2 U 1+d [71] VM Non (0, 1) 2/3 1 3 Yes – ★ 1−d 2 d−d Pulsating 2−d 1 1 [90] VL circuit Pulsating (0, 1) 1/2 1 2 Yes – ★ U I o 2 o 1−d 2−d d−d 1 2−d [58] Quadratic boost Non (0, 1) U 2/2 1 3 Yes – ★★ o I 2 2 o (1−d) (1−d) Pulsating ★ particularly inappropriate, ★★ relatively appropriate, ★★★ particularly appropriate 366 X. Wu et al. 1 3 Fig.18 Distribution of indexes of typical novel topologies Review of DC‑DC Converter Topologies Based on Impedance Network with Wide Input Voltage Range… 367 (1) With continuously improved bus voltage levels in the storage components in a converter. This is an effective powertrain of FCVs, a step-up converter is required means to improve the power density. However, when to continuously improve the VG to match the FC and the operating frequency of a circuit is too high, the DC bus voltages. With continuously improved energy EMI would become severe. Balancing the relationship efficiency requirements of the vehicle, stricter require- between the operating frequency and the electromag- ments are presented for the power conversion efficiency netic compatibility of a system in the application of of DC-DC converter. Therefore, optimizing topologies semiconductor components with wide band gaps would of converters are complicated considering the system also be an important topic in the research on topology loss and stresses in the power electronic components. design and the modeling and control of DC-DC boost (2) The output characteristic of an FC is soft (the voltage converters for FCVs. is lower for a heavier load), resulting in a wide range (7) The reliability of a DC-DC converter for an FCV is of FC voltages and a large output power fluctuation. A an important factor that seriously affects the develop - large current ripple reduces the life of an FC. There- ment of the FCV industry. Completely combining the fore, it should be ensured that the DC-DC converter can research of system detection, modeling, control strat- maintain the output voltage in a wide IVR and reduce egy, and fault diagnosis methods to realize the safe the ICR. It is important to address issues pertaining to and reliable operation of DC-DC conversion systems topology design, accurate modeling, and optimal con- would also be an important research problem regarding trol of the DC-DC boost converters for FCVs that need DC-DC converters for FCVs. to be solved urgently. (8) Installing high-power FCs on commercial FCVs with (3) A DC-DC converter is a time-varying and strong non- relatively sufficient space have become increasingly linear system. Under certain specific conditions, there popular, and the "voltage window" wherein the voltage would exist various types of bifurcation, chaos, and of the FC stack equals the DC bus voltage also appears. other nonlinear phenomena. These are embodied in the Considering the function of step-up and step-down con- increased voltage and current ripple of the converter, verters, designing and controlling DC-DC converters the increased harmonics, the decreased working effi - with wide step-up/down voltage ranges and low ICRs ciency, noise, oscillations, and even system collapse. is a challenge in developing commercial FCVs. The procedure for performing nonlinear modeling of DC-DC converters, analyzing converter stability, Considering the abovementioned problems faced dur- and implementing chaos control, constitute important ing research on DC-DC boost converters for FCVs, related research topics related to DC-DC converters. studies on the following aspects can be conducted. (4) The operation state of the FC stack determines the per- formance of FCVs. Internal resistance represents the (1) With regards to the issue of high VG and efficiency difficulty in the proton and electron transport within operation of the DC-DC boost converter for an FCV, the electrode. It also determines the efficiency of FC the advantages of components with wide band gaps, power generation, which directly reflects the health such as high frequency and low power loss, can be uti- of the FC stack. The humidity inside FCs is a factor lized. The optimization calculation method of the sys- that affects this internal resistance. Although DC-DC tem can be used to improve and optimize DC-DC con- converters can achieve voltage conversion and power verters based on existing SI, SC, cascaded boost, and decoupling, utilizing them to realize online monitor- other topologies. This can be done in terms of decreas- ing of the water content in FCs constitutes the research ing the size of energy storage components, reducing direction of future DC-DC converters. the stress of semiconductor components, and reducing (5) When the FC stack is operated at low temperatures, switching loss. the fuel supply pressure needs to be increased, leading (2) To solve the issue of a wide-range operation and a to a low FC efficiency. When the temperature is below low ICR of the DC-DC boost converter for FCVs, the zero, the water in the FC stack would freeze, hindering dynamic response and anti-interference ability of this gas diffusion and causing FC shutdown. Using DC-DC converter can be synthetically improved based on exist- converters to directly control the warm-up operating ing topology optimization, such as implementing an voltage of FCs and realizing the regulation of cold interleaved parallel structure, combined with research starts both constitute issues to be considered in future on accurate modeling and the adaptive control method. design of DC-DC converters. (3) To solve the issue of nonlinear modeling and con- (6) Using components with wide band gaps (SiC, GaN) trol of the DC-DC converter for FCVs, based on the can result in the circuit operating with a high switch- study of nonlinear dynamics and combined syntheti- ing frequency, significantly reducing the size of energy cal modeling methods, such as state variable equation 1 3 368 X. Wu et al. and discrete mapping, the nonlinear behavior caused 5 Conclusions by changes in the DC-DC converter parameter can be revealed by qualitative and quantitative methods. This The requirements of the DC-DC boost converter for FCV is done to apply chaos control to the converter and real- powertrains are summarized according to the characteris- ize the expected periodicity. tics of the powertrain and output characteristics of the FC, (4) To solve the issue of online water content monitoring which mainly include high voltage gain, low input current in the FC stack, based on the analysis of the equivalent ripple, and wide input voltage range. model of FC, the basis of determining the water content According to the different types of impedance net- using the internal resistance of an FC can be mastered. works, DC-DC boost converter topologies can be divided Additionally, the detection principle of the internal into inductor + inductor, capacitor + capacitor, induc- resistance of an FC can be summarized to construct tor + capacitor, and hybrid / cascaded. Based on the deri- an online monitoring model of water content in FCs. vation and classification of 12 types of existing DC-DC Subsequently, the integration of DC-DC converters and impedance networks, the research status and characteris- health monitoring systems is realized. tics of various impedance networks are reviewed. Among (5) To control the warm-up operating voltage using the them, interleaved, quasi-Z-source, and quadratic boost DC-DC converter and achieve a cold start, based on structures have continuous input current, which render the mastering of FC warm-up operating characteris- them suitable for connecting with FCs. Voltage multiplier, tics combined with the characteristics of the DC-DC switched-inductor switched-capacitor, and voltage lifting converter, the working frequency, inherent param- circuits can effectively improve the voltage gain. The TL eters, and given parameters of the converter can be structure reduces the voltage stress of the device and is comprehensively optimized. Thus, this provides a suitable for the post-stage of high voltage gain DC-DC basis for designing DC-DC converters with warm-up converters. Hybrid and cascading are common methods functions. for constructing DC-DC converters, but the conversion (6) Considering the issues regarding the application of efficiency and power density need to be considered. semiconductor components with wide band gaps in On this basis, an evaluation system for the DC-DC boost DC-DC boost converters, the electromagnetic com- topology for FCV is constructed from eight evaluation patibility characteristic of the system in different fre- aspects: input current ripple, voltage gain, input voltage quency domains can be analyzed, and an accurate EMI range, voltage stress, current stress, number of components, model can be subsequently built on the basis of EMI common ground, and slope of the step-up ratio. Moreover, modeling. This provides a reference for the topology the typical topologies proposed in existing literature are optimization design of DC-DC converters utilizing compared and evaluated, providing a guideline for design- semiconductor components with wide band gaps. ing DC-DC converters for the FCV powertrain. (7) To improve the reliability of the DC-DC boost con- Lastly, the issues regarding DC-DC converters for FCVs verter for FCVs, the typical dynamic behavior of the are summarized. To facilitate the development of FCVs, converter under different fault conditions, its fault future research directions are proposed, i.e., utilizing a behavior laws under different control strategies, its fault DC-DC converter to realize online monitoring of the water characteristics and influencing factors, and the quanti- content in FCs and designing buck-boost DC-DC converters tative extraction of key fault feature information of fault suitable for high-power commercial FCVs. external characteristics in each stage are analyzed. This Acknowledgements This work was sponsored thought the International is to realize the construction of a fault generation model Science & Technology Cooperation of China under 2019YFE0100200 of the DC-DC converter, providing a model basis for and the Fundamental Research Foundation for Universities of Hei- the fault diagnosis of DC-DC converters. longjiang Province (2018-KYYWF-1672). (8) To design a buck-boost DC-DC converter suitable for high-power commercial FCVs, based on the topology Declarations optimization of existing buck-boost DC-DC convert- ers combined with the advanced modeling theory, Conflict of interest On behalf of all the authors, the corresponding au- thor states that there is no conflict of interest. research on the precise control of the output voltage and the dynamic regulation of the output power of the Open Access This article is licensed under a Creative Commons Attri- converter can be conducted. Therefore, the comprehen- bution 4.0 International License, which permits use, sharing, adapta- sive improvement in the performance of the DC-DC tion, distribution and reproduction in any medium or format, as long converter with wide ranges of step-up and step-down as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes can be realized. were made. The images or other third party material in this article are 1 3 Review of DC‑DC Converter Topologies Based on Impedance Network with Wide Input Voltage Range… 369 included in the article's Creative Commons licence, unless indicated 19. Profumo, F., Tenconi, A., Cerchio, M., Bojoi, R., Gianolio, G.: otherwise in a credit line to the material. If material is not included in Fuel cells for electric power generation: peculiarities and dedi- the article's Creative Commons licence and your intended use is not cated solutions for power electronic conditioning systems. EPE permitted by statutory regulation or exceeds the permitted use, you will J. 16(1), 44–51 (2015) need to obtain permission directly from the copyright holder. To view a 20. Wahdame, B., Girardot, L., Hissel, D., et al.: Impact of power copy of this licence, visit http://cr eativ ecommons. or g/licen ses/ b y/4.0/ . converter current ripple on the durability of a fuel cell stack. 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Review of DC-DC Converter Topologies Based on Impedance Network with Wide Input Voltage Range and High Gain for Fuel Cell Vehicles

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Springer Journals
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2096-4250
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10.1007/s42154-021-00163-z
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Abstract

The development of fuel cell vehicles (FCVs) has a major impact on improving air quality and reducing other fossil-fuel- related problems. DC-DC boost converters with wide input voltage ranges and high gains are essential to fuel cells and DC buses in the powertrains of FCVs, helping to improve the low voltage of fuel cells and “soft” output characteristics. To build DC-DC converters with the desired performance, their topologies have been widely investigated and optimized. Aiming to obtain the optimal design of wide input range and high-gain DC-DC boost converter topologies for FCVs, a review of the research status of DC-DC boost converters based on an impedance network is presented. Additionally, an evaluation system for DC-DC topologies for FCVs is constructed, providing a reference for designing wide input range and high-gain boost converters. The evaluation system uses eight indexes to comprehensively evaluate the performance of DC-DC boost converters for FCVs. On this basis, issues about DC-DC converters for FCVs are discussed, and future research directions are proposed. The main future research directions of DC-DC converter for FCVs include utilizing a DC-DC converter to realize online monitoring of the water content in FCs and designing buck-boost DC-DC converters suitable for high-power commercial FCVs. Keywords Fuel cell vehicles · DC-DC boost converter · Evaluation systems and indexes · High gain topology AbbreviationsTL Three level CG Common groundVG Voltage gain CIC Continuous input currentVL Voltage lifting EMI Electromagnetic interferenceVM Voltage multiplier FC Fuel cellVS Voltage stress FCV Fuel cell vehicle ICR Input current ripple IVR Input voltage range 1 Introduction PEMFC Proton exchange membrane fuel cell PI Proportional-integral Fuel cell vehicles’ (FCVs) only by-product is water, truly PWM Pulse-width modulated achieving zero emissions. In recent years, FCVs have devel- SC Switched-capacitor oped rapidly in the field of new energy vehicles [1 , 2]. How- SI Switched-inductor ever the voltages of fuel cell (FC) stacks cannot match the bus voltages of vehicles. Moreover, due to the shortcomings of FCs, such as slow dynamic responses and wide output * Xiaogang Wu voltage ranges, auxiliary power sources (such as batteries) xgwu@hrbust.edu.cn are required to act as external energy storage systems [3, 4]. Institute of Electrical and Electronic Engineering, Harbin Therefore, DC-DC boost converters are utilized to achieve University of Science and Technology, Harbin 150080, voltage decoupling and power distribution [5, 6]. China The electrochemical reactions in FCs occur on the cata- Institute of Electrical and Information Engineering, Tianjin lytic layers. At present, Pt is the catalyst typically used in University, Tianjin 300072, China FCVs [7]. Current ripples can induce the failure of Pt in the State Key Laboratory of Automotive Safety and Energy, catalyst layer and even accelerate the degradation rate of Tsinghua University, Beijing 100084, China Vol.:(0123456789) 1 3 352 X. Wu et al. the proton exchange membrane, reducing the lives of proton averaging method, pulse-width modulated (PWM) switch exchange membrane fuel cells (PEMFCs). In Ref. [8], an modeling, and an equivalent transformer method. Owing to amplitude of low-frequency current ripple higher than 4% the "soft" output voltage characteristics of FCs and the vary- significantly reduced the durability and the life of the FC. In ing road conditions, DC-DC converters need to maintain a Ref. [9], the frequency of the current ripple in the FC system constant bus voltage and achieve dynamic regulation of out- was required to be higher than 1.25 kHz. FCs can operate put power. Realizing the control objective primarily includes reliably for low-frequency current ripple contents lower than following the given signal and suppressing the disturbance 5%. Therefore, to extend the lives of FCs, DC-DC boost signal. Therefore, the rapidity, robustness, accuracy, and low converters require low input current ripples (ICRs). complexity of the controller for the converter are impor- The output characteristic of FCs are “soft”, namely, the tant aspects in optimizing the performance of a DC-DC terminal voltage of an FC decreases with increasing output converter. Typical control methods for DC-DC converters current [10, 11]. The output characteristic curve of an FC is include proportional-integral (PI) control, sliding mode divided into polarization, ohmic, and concentration differ - control, adaptive control, robust control, fuzzy control, and ence sections [12]. The polarization section corresponds to novel integrated control strategies [24–27]. the initial stage of the reaction where the voltage drops rap- As part of FCVs, developing vehicle technology can pro- idly. This reduction is caused by the movement of electrons mote research on DC-DC converters. For instance, the pow- between the electrodes and the destruction and reformation ertrain of FCVs can be divided based on the different power - of chemical bonds during chemical reactions at the elec- train structures into energy-type and power-type, requiring trodes. The ohmic section corresponds to the deceleration of different types of DC-DC converters [28]. Permanent mag- the voltage drop rate, and the decline curve becomes linear. net synchronous motors, which have wide ranges of speed This is caused by the resistance of the FC modules and the regulation, are commonly used in FCVs; therefore, DC-DC ionic resistance of electrolytes. The concentration difference converters are required to regulate the output power quickly section corresponds to an increased voltage dip under a large [29]. Vehicle velocities and driving conditions affect the out- current, wherein the reactants reach the electrode surface by put powers of FCs. Research on vehicle velocity estimation, diffusion and convection. When the diffusion rate is lower motor torque, and stability control can provide guidelines than the rate of the electrode reactions, the reactant concen- for designing DC-DC converters [30, 31]. trates on the electrode surface and the electrolyte body differ Most of the previous reviews focused on the classifica- significantly [13]. In conclusion, considering the V-A curve tion of various DC-DC topologies, while ignoring the deri- of FC, DC-DC boost converters require wide input voltage vation of DC-DC topologies based on impedance network ranges (IVRs) to meet the current variations in FCs caused and the applicability of FCVs. The main contributions of by different road conditions. this study are as follows. (1) Based on the different types The voltage of a single FC is less than 1 V, and multiple of impedance networks, the non-isolated DC-DC converter FCs in series are used as an FC stack[14]. However, limited topology was divided into four types: inductor + inductor, space in the vehicle, in turn, limits the number of single capacitor + capacitor, inductor + capacitor, and hybrid / cas- FCs, and the output voltage of the FC stack remains low. At caded. On the basis of the proposed principles of impedance present, the predominant DC bus voltage of an FCV is 400 V transformation, 12 types of impedance network structures [15]. To achieve voltage matching, the voltage gain (VG) of were derived and classified. (2) Based on this classification, the DC-DC boost converter should be as high as possible the DC-DC topologies proposed in existing literature are and no less than 10 [16]. reviewed, and the applicability of each DC-DC converter Thus, to extend the lives of FCs, DC-DC boost convert- in the powertrain of FCVs is discussed. This provides the ers should maintain continuous input currents (CICs) and direction for designing DC-DC converters for FCVs. (3) An ICRs [17–20]. Considering that the input voltage of an FC evaluation system of a DC-DC converter for FCV is estab- decreases with increasing input current, the step-up DC-DC lished. The DC-DC converters proposed in mainstream lit- topology should have a wide IVR [21, 22]. Owing to the erature are then compared and evaluated, and their applica- limited space in FCVs, the converter should be miniaturized bilities are discussed. (4) The issues faced while researching and also have high frequency, efficiency, and reliability [23]. DC-DC converters for FCVs are summarized, and the future A DC-DC converter constitutes a typical nonlinear sys- research directions for DC-DC converters are presented. tem, and analyzing the dynamic operation characteristics The main contents of this study are as follows: Sect. 2 of the circuit is complex. Based on accurate modeling, the introduces the derivation process and the research status of static and dynamic characteristics of a DC-DC converter sys- existing step-up DC-DC topologies based on the impedance tem can be easily analyzed and evaluated, and the controller network. In Sect. 3, the evaluation system of DC-DC boost parameters can be matched. Currently, the modeling meth- converters for FCV is constructed, the comparison and eval- ods of DC-DC converters primarily include a state-space uation of typical DC-DC boost converters are performed, 1 3 Novel DC-DC boost topology Coupled inductor topology Abandoning electrical isolation of transformer Novel isolated topology Review of DC‑DC Converter Topologies Based on Impedance Network with Wide Input Voltage Range… 353 and suggestions for selecting the topology for FCVs are pro- and size [39–41]. Therefore, the converter based on coupled vided. Section 4 summarizes the key problems of DC-DC inductor is not suitable for the FCV powertrain. converters and prospects for future research directions. Sec- The non-isolated DC-DC boost converter is simple with tion 5 constitutes conclusions. no leakage inductance. It is, therefore, more suitable for FCVs. To ensure the conversion efficiency of the DC-DC converter, most of the existing converters for FCVs are 2 Research Status of DC‑DC Boost traditional boost converters, which have few devices [42]. Converters for FCVs However, owing to the influence of parasitic parameters, the boost converter cannot work under a limited duty cycle. There exist two types of DC-DC boost converters: isolated Therefore, it cannot achieve the high step-up ratio required and non-isolated DC-DC boost converters. Depending on the by FCVs in practice [43, 44]. In addition, an interleaved presence of a coupling structure, the non-isolated DC-DC boost converter topology can effectively reduce the require- boost converter is further divided into DC-DC boost con- ment for power devices and ICR and improve the lives of verters based on the coupled inductor and DC-DC boost FCs. Gao Dawei’s team from Tsinghua University success- converters based on impedance network [32]. fully applied an interleaved boost converter to an FC bus and A transformer is added between DC and DC in the non- obtained favorable results [45, 46]. However, the low VG isolated DC-DC boost converter to realize DC-AC-DC renders it unsuitable for a low power FC powertrain. step-up conversion. A high VG can be achieved by adjust- The step-up converter based on an impedance network ing the turn ratio of the transformer; however, a high turn (consisting of an inductor, capacitor, diode, and a switch) ratio would lead to a large leakage inductance, increasing does not face issues of voltage spikes and low power densi- the voltage stress (VS), thereby decreasing the efficiency ties caused by leakage inductance. A topology with high VG, [33]. In addition, the magnetic components would reduce the low ICR, and wide IVR can be designed from the perspec- power density, which proves disadvantageous to a vehicle tive of FCV applications. This study classie fi d and discussed with less available space [34, 35]. Therefore, the isolated this type of converter. The main classification of DC-DC DC-DC boost converter is inapplicable to FCVs. boost converters is shown in Fig. 1. A high VG topology based on coupled inductor can be obtained by terminating the electrical isolation of the trans- 2.1 Derivation and Classification of Boost Topology former and adding a coupled inductor to the non-isolated Based on Impedance Network converter [36–38]. The VG can also be effectively improved by selecting a high turn ratio. However, the risk of leak- By combining the energy storage elements (inductor and age inductance remains; thus, an additional snubber circuit capacitor) with a diode and switch according to the fol- should be added to the converter based on the coupled induc- lowing four principles, boost converter structures based on tor. This addition, however, increases the cost, complexity, impedance network can be obtained. Fig.1 Classification of DC-DC DC-DC boost topology topology Isolated DC-DC boost topology Non-isolated DC-DC boost topology Topology based on Basic isolated DC-DC boost impedance network topology Basic boost topology Half-bridge circuit Forward circuit Buck-Boost Full-bridge circuit Boost Cuk Flyback Push-pull circuit Sepic Zeta circuit 1 3 Principle 2 354 X. Wu et al. Principle 1: To increase the VG, inductors and capacitors Z Impedance Quasi-Z are charged from the power source in parallel and discharged SC Principle 3 source transformation source to the load in series. Principle 2: To increase the VG, when the switch is closed, energy is exchanged between the capacitors, and when it is open, both the capacitors and the power source supply energy to the load together. Capacitor Principle 3: To increase the VG, the diode is attempted to + Principle 4 TL Capacitor be replaced by an inductor without violating the circuit laws. Principle 4: To reduce the VS of the components, the capacitor should be added into the impedance network, forming a circuit loop with a switch, diode, and load. Different types of impedance networks mainly include: VM Modified VM Principle 3 (1) Inductor + Inductor Fig.2 Relationships among Capacitor + Capacitor topologies Two inductors are combined according to Principle 1, and their operating states depend on the states of the diodes (or switches). Thus, the switched-inductor (SI) structure can (4) Hybrid / Cascaded be obtained. If the input current of SI is pulsating, in order to reduce In addition, integrating and cascading the three types of the ICR, each inductor switches on in turn with the same trigger phase difference. Therefore, only one inductor is impedance networks above can optimize the performance of DC-DC boost converters and aid in obtaining certain novel charged at a time, and an interleaved topology is obtained. The diode is switched off while the inductors are charging. topologies. On the basis of the classification of topologies based on If this diode is replaced by a capacitor in such a way that the two inductors and the capacitor work according to Principle impedance network, as shown in Fig. 3, this study reviews existing literature from recent years, summarizes the advan- 1, the structure of the SI combined with a charge pump is obtained. tages and disadvantages of each topology to elucidate its applicability in the FCV powertrain, and provides the basis (2) Capacitor + Capacitor for designing DC-DC boost converters for FCVs. Two capacitors are combined according to Principle 1, 2.2 Impedance Network Composed of Inductor + Inductor and their operating states depend on the states of the diodes (or switches). The switched-capacitor (SC) structure can, 2.2.1 SI Structure thus, be obtained. According to Principle 2, a voltage mul- tiplier (VM), including Cockcroft–Walton and Dickson According to Principle 1, an SI structure can be obtained VMs, can be obtained by combining the two capacitors. The Z-source structure can be obtained by modifying the by utilizing two inductors to store and release more energy, as shown in Fig. 4. The components L1-D2 and L2-D1 in SC structure according to Principle 3. The quasi-Z-source structure can be obtained by the impedance transformation Fig. 4a L1-S2 and L2-S1 in Fig. 4b constitute the SI. The operating principle of SI was extensively analyzed of the Z-source structure. The modified structure of the VM can be obtained by modifying the VM according to Principle in Refs. [16], [47], and a family of SI topologies with high VG was proposed. In addition, a hybrid SI topology was 3. A three-level (TL) topology can be obtained by combin- ing two capacitors according to Principle 4. The relationship derived by combining passive and active SI units. In Ref. [48], a transformerless high VG DC-DC boost converter between the topologies of capacitor + capacitor is shown in Fig. 2. based on the SI structure was proposed for the FC system. Moreover, the capacitor and inductor were connected in par- (3) Inductor + Capacitor allel in an attempt to enhance the VG. As proposed in Ref. [49], a step-up DC-DC converter combined the traditional According to Principle 1, topologies based on impedance switched-boost network with an SI unit, exhibiting the char- acteristics of high VG, fewer components, low VS, and good network can be obtained by combining an inductor with a capacitor. scalability. In Ref. [50], a step-up topology was presented by 1 3 Principle 1 Review of DC‑DC Converter Topologies Based on Impedance Network with Wide Input Voltage Range… 355 Fig.3 Novel topologies based on impedance network Inductor Capacitor Four principles Inductor+Inductor Capacitor+Capacitor Inductor+Capacitor ͓ SC ͓ SI ͓ VM ͓ TL ͓ Interleaved ͓ SI combined with ͓ VL circuit ͓ Z source charge pump ͓ Quadratic boost ͓ Quasi-Z source Combination ͓ Hybrid ͓ Cascaded combining the active and passive SIs, exhibiting the charac- L1 teristics of high VG (> 10), low component stress, and high L1 D3 efficiency (95.5%). In Ref. [51], two inductors of quadratic boost were replaced with passive SIs, obtaining four derived structures that were extensively compared. D2 S1 S2 The SI can replace the inductor in existing DC-DC boost L2 D1 L2 converters, significantly increasing the VG of the topology. It also has the merits of simple and easy control. The DC-DC converter based on SI can effectively match the low output (a) SI composed of diodes (b) SI composed of switches voltage of an FC with the high bus voltage in the powertrain Fig.4 Two types of SI structures of an FCV. However, on connecting the SI to the FC stack, the output current of the latter would fluctuate, reducing the life of the FC. Therefore, this structure is inapplicable for the FCV powertrain with strict requirements for ICR. L1 D1 2.2.2 Interleaved Structure L2 D2 The interleaved structure was also composed of induc- S1 S2 tor + inductor. Its switches are switched on in turn with the same trigger phase difference; therefore, only one induc- tor is charged at a time. The energy was evenly distributed to the branches where the switches are located, effectively Fig.5 Two-phase interleaved step-up structure eliminating the ICR. As shown in Fig. 5, the drive signals of S1 and S2 differ by 180°. When L1 (L2) is charged, L2 (L1) supplies energy to the load. The two-phase interleaved boost structure was improved in Refs. [53], [45], respectively. These structures effectively in Ref. [52] in the context of renewable energy sources. improved the VG of converters; however, they faced the When different inductors supplied energy to the load, the issue of having no common ground (CG). An interleaved later-end capacitors worked in different states, effectively converter for FCV was proposed in Ref. [46]. An FC can improving the step-up ratio with a small ICR. The two- and operate in the boost or buck modes when its voltage was four-phase floating interleaved structures were investigated lower or higher than the DC bus voltage, respectively, 1 3 356 X. Wu et al. reducing the ICR. In Ref. [22], the interleaved structure was L3 D1 implemented at the front-end of the converter to reduce the ICR. In the back-end, two capacitors were combined with D2 a diode-capacitor network to increase the VG and reduce L1 D3 the VS. The interleaved structure has the advantage of a small L3 D1 ICR. For an n-phase interleaved converter, when the duty cycle is an integral multiple of 1/n, and the trigger phase difference is 360° /n , the ICR is eliminated [54]. Therefore, C1 L1 D2 in the field of FCVs, the interleaved (not floating interleaved) structure is suitable for the front-end of the boost converter to reduce the ICR. Fig.6 Structure of an SI combined with a charge pump Using the interleaved structure in the powertrain of FCVs can effectively reduce the current ripple of the FC stack, reducing the impact on the lives of FCs. Therefore, this structure is suitable as the front-end of the DC-DC converter. D2 However, there are obvious disadvantages that accompany this structure. First, this structure does not significantly L2 L1 D1 improve the VG and is not suitable for low power FC stacks that require high VG converters. Second, the switches switch on alternately, which requires highly precise trigger signals C1 S and a control circuit. Third, its input and output do not have a CG, producing additional electromagnetic interference (EMI) in the powertrain. Fourth, there were many switches, which increases the cost of the powertrain. Fig.7 Quadratic boost structure 2.2.3 SI Combined with Charge Pump Structure can be concluded that the structure combining an SI with a To improve the VG, the diode D2 in the SI structure was charge pump is not applicable for FCV systems. replaced by a capacitor C1, namely the structure of an SI combined with a charge pump is obtained, as shown in 2.2.4 Quadratic Boost Fig. 6. In Ref. [55], a structure combining an SI with a charge According to Principle 1, the capacitor is used as another pump was proposed, which improved the step-up ratio of power source to realize charging in parallel and discharging the converter and provided the circuit with resilience to in series of two inductors, significantly improving the step- component tolerance. As demonstrated in Ref. [56], an SI up ratio. The quadratic boost structure is shown in Fig. 7. combined with a charge pump replaced one inductor (the In Ref. [58], the operating principles of the quadratic inductor that was not directly connected to the power source) boost structure were analyzed, the energy transmission in the quasi-Z-source. The new step-up structure exhibited modes were investigated, and the ripple characteristics of the reduced ICR and VS and also a high VG. The novel non-iso- quadratic boost converter in the complete inductor supply lated DC-DC topology proposed in Ref. [57] was cascaded mode (CISM) and incomplete inductor supply mode (IISM) with charge pump and SC structures, reducing the VS of the were analyzed. In Ref. [59], a modified high gain quadratic components as well as the switching and conduction losses. boost-sepic converter was proposed, which produced a high The structure combining an SI with a charge pump can VG with a low duty cycle and reduced the VSs of the switch further improve the VG by modifying the SI structure, which and the diodes while exhibiting the advantages of a con- renders the structure suitable for the FCV powertrain that tinuous input current (CIC). In Ref. [60], a modified high requires a high VG converter. However, on connecting this VG quadratic boost-sepic converter, having both CIC and a structure with an FC stack, the output current of the FCs CG structure, was proposed. In Ref. [61], a modified high would become pulsating current. Moreover, when the switch VG step-up DC-DC topology based on the quadratic boost is switched off, the power source is directly connected to structure was presented. This converter improved the VG capacitor C1, and the initial inrush current would appear on while decreasing the VS across the switches as well as the the side of the power source. From the two points above, it overall converter losses. 1 3 Review of DC‑DC Converter Topologies Based on Impedance Network with Wide Input Voltage Range… 357 The step-up ratio of the quadratic boost converter, which 2.3.2 VM Structure has both CIC and a CG structure, is the square of that of the boost converter. When the quadratic boost converter is used According to Principle 2, a VM can be obtained by transfer- in the FCV powertrain, it provides a high VG to match the ring energy between two capacitors to multiply the output FC and DC bus voltages of the vehicle, and it also main- voltage [66, 67]. The VM includes both Cockcroft–Walton tains the continuity of the FC output current. Moreover, and Dickson VMs, as shown in Fig. 9a and b. The single- the CG characteristics of the quadratic boost converter can stage VM is shown in Fig. 9c. effectively reduce additional EMI in the vehicle powertrain. In Ref. [68], a high VG DC-DC topology with CIC and Therefore, such a converter is suitable as the front-end of the high efficiency was designed based on a Cockcroft– Wal- DC-DC boost converter for an FCV. ton VM for the FC. The novel topologies designed in Refs. [69], [70] implemented interleaved structures in the front- 2.3 Impedance Network Composed stage to reduce the ICR, and a Dickson VM lifted the volt- of Capacitor + Capacitor age in the post-stage to match the voltage between the DC bus and renewable energy sources. A Dickson VM was first 2.3.1 SC Structure attempted to be applied to a classic non-isolated DC-DC boost converter in Ref. [71], and zero-current-switching According to Principle 1, two capacitors can be utilized to was realized. The Dickson VM used in the post-stage of the store and release more energy, obtaining the SC structure. converter proposed in [69] was improved upon in Ref. [72]. This structure optimizes the topology with regards to the The high VSs on the post-stage capacitors of a multi-stage voltage conversion ratio, as shown in Fig. 8. Dickson VM were reduced. In Ref. [62], an SC structure was added to boost, buck- This VM exhibited the characteristics of a high VG and a boost, and Cuk converters, effectively improving the VGs simple structure, making it applicable for the post-stage of of the topologies. In Ref. [63], SCs (Fig. 8b) replaced the the DC-DC boost converter for an FCV. However, VSs of capacitors in Cuk, sepic, and Zeta converters, improving the components increase with the number of stages in the VM, VGs of the topologies and also achieving higher conversion which, in turn, increases the possibility of component defects efficiencies. After modifying the SC (Fig.  8a) in Ref. [64], a and decreases the reliability of the entire FCV powertrain. novel topology with a high step-up ratio and a good transient In addition, the high VS results in the converter requir- response was obtained. In Ref. [65], a novel DC-DC struc- ing components with high rated powers, which increases ture based on the SC network was proposed. This topology the cost and power loss in the powertrain. Therefore, when could significantly improve the output voltage and possessed smooth input current and output voltage. C'1C'2 C'n The SI structure could effectively improve the VG and is ... accompanied by the advantages of simple control and easy implementation. However, the input and output of the SC ... structure do not have CGs. Prior to application in the FCV D2 D'3 D1 D'1 D'2 D3 ... powertrain, the SC structure should be optimized from the C1 C2 Cn perspective of the CG. (a) Cockcroft-Walton VM ... C'1 C'2 C'n ... D3 D'3 D1 D'1D D2 '2 C1 D1 C1 C2 Cn ... (b) Dickson VM D1 D2 C1 C2 C'1 C2 D1 D2 D2 C1 (a) (b) (c) Single stage VM Fig.8 Two types of SC structures Fig.9 Three types of VM structures 1 3 358 X. Wu et al. Fig.10 Modified VM L1 C'1 C1 C2 D1 L1 C1 L2 designing DC-DC boost converters based on VMs, the topol- Fig.11 Z-source structure ogy should be optimized with respect to reducing VSs of the components. components into the traditional Z-source structure. In Ref. 2.3.3 Modified VM Structure [80], the Z-source structure was combined with a VM to obtain a novel CG high-gain topology, which was suitable By replacing the diode D2 in the single-stage VM with an for renewable energy sources. The Z-source structure was inductor L1 according to Principle 3, a modified VM struc- integrated with the quasi-Z-source one in Ref. [81], and a ture, as shown in Fig. 10, can be obtained. combined Z-source DC-DC structure was proposed for the New energy systems were selected as the research back- FC and photovoltaic systems. This converter exhibited good ground, and the boost topology was combined with a modi- step-up capability. fied VM in Ref. [73]. This effectively improved the VG and The Z-source structure can provide a high gain under the reduced the ICR. The novel step-up DC-DC topology, pro- non-limit duty cycle, which can effectively match the low posed in Ref. [74], combined sepic topology, a VM, and a output voltage of FCs with the high bus voltage. However, modified VM to effectively rectify the voltage mismatch in using the Z-source structure in the FCV powertrain would new energy systems. In Ref. [75], the VM was integrated generate a pulsating current in the FC, affecting its life. In with a modified VM to obtain a second-order hybrid boost addition, the input and output of the Z-source structure do topology, which was suitable for renewable energy sources. not have a CG. The two points above restrict the application The modified VM can effectively improve the VG of the of Z-source structures in the FCV powertrain. topology, making it suitable for the FCV powertrain that requires high VG converters. However, when the modi-2.3.5 Quasi‑Z‑Source Structure fied VM is cascaded, VSs of the post-stage components increases. Prior to applying the modified VM structure to To overcome the disadvantage of the Z-source structure FCVs, a new topology should be designed from the perspec- not having a CG, a quasi-Z-source can be obtained from tive of reducing VSs to reduce the cost and power loss in the the impedance transformation of the Z-source structure, as powertrain. shown in Fig. 12. The operating principles of the quasi-Z- and Z-source structures are similar. 2.3.4 Z‑Source Structure In Ref. [3], the quasi-Z-source structure was used as the front-end of the proposed topology, obtaining a high The Z-source structure can be obtained by transferring the VG converter for FCVs. In addition, the efficiency of this SC structure according to Principle 3, as shown in Fig. 11. converter reached 95.13%. Based on the investigations When S is closed, C1 and C2 charge L1 and L2, respec- conducted in Ref. [3], a PI controller of the quasi-Z-source tively; however, when S is open, the power source charges structure that combined composite feedforward and feed- C1 and C2. The power source, L1, and L2 supply power to back controls was proposed in Ref. [82]. The quasi-Z-source the load [76]. converter was subsequently tested under the worldwide har- A steady-state analysis of a Z-source step-up topology monized light-duty test cycle. A high voltage boost DC-DC operating in the continuous conduction mode (CCM) was converter composed of a quasi-Z-source converter and a conducted in Ref. [77], and the voltage ripple and loss of quadratic boost converter was proposed in Ref.[83]. A high the converter were calculated. In Ref. [78], two high VG VG quasi-Z-source topology for new energy sources was Z-source DC-DC converters suitable for renewable energy proposed in Ref.[43], not only improving the VG of the sources were proposed, and an appropriate control method quasi-Z-source structure but also maintaining the advantages was presented. Moreover, the application prospect in FCV of CIC and low VSs of capacitors. was discussed. As demonstrated in Ref. [79], a CG Z-source The VGs of the quasi-Z-source and Z-source structures high VG converter with low stresses, high efficiency, and were equal. When the duty ratio is nearly 0.5, a high VG small size was designed without inserting additional can be obtained. Moreover, the CIC of the quasi-Z-source 1 3 Review of DC‑DC Converter Topologies Based on Impedance Network with Wide Input Voltage Range… 359 C2 2.4 Impedance Network Composed of Inductor + Capacitor L2 L1 D2 According to Principle 1, a voltage lifting (VL) circuit can be obtained by using an inductor and a capacitor to store and release more energy, as shown in Fig. 14. C1 S Based on the basic boost topology, positive and neg- ative VL circuits, first proposed in Refs [88] and [89], respectively, effectively improved the step-up ratio. Regarding the issue of low VG in the VL circuit, a family Fig.12 Quasi-Z-source structure of VL topologies with geometrically increasing VG were designed in Refs [90] and [91]. In Ref. [92], the VL circuit structure is beneficial to FCs, low VSs of the devices are was combined with the quadratic boost circuit to lift the conducive to reducing the cost of the powertrain, and the VG of the converter and reduce the VS of the switch. In CG characteristics can effectively reduce the additional EMI Ref. [93], a novel topology with high VG and CIC was in the powertrain. Therefore, this structure is suitable for obtained by integrating the conventional boost circuit with FCV systems. a VL circuit. A single switch step-up topology, based on the VL circuit and having high VG, was proposed in Ref. 2.3.6 TL Structure [94]. However, the input and output sides did not have a CG. In Ref. [95], the VL circuit was improved and a con- A TL structure, which can reduce the VS to half of the out- verter with a high step-up ratio was proposed, which had put voltage as shown in Fig. 13, can be obtained by combin- fewer switches. ing capacitors according to Principle 4. The VL circuit can significantly improve the step-up In Ref. [84], the TL forms of six basic chopper circuits ratio, and its input and output have a CG. The flexible were derived and then optimized. On this basis, a feedfor- structure can be easily designed. However, this circuit has ward control strategy for these topologies was proposed. A a pulsating input current, which is unfavorable for the FC TL topology was implemented in Refs [85] and [86] as the stack. While designing the converter topology for FCVs, post stage of the step-up converter for an FCV, effectively optimization with regard to CIC should be performed to reducing VSs of the components. For the FC composite improve the durability of the FCV powertrain. energy storage system, the VS on the switch of the TL topol- ogy presented in Ref [87] equaled to only half of the output 2.5 Hybrid and Cascaded Structure voltage, and the dynamic response of the converter was sig- nificantly improved. 2.5.1 Hybrid Structure The effective reduction of VSs of the components makes the TL structure suitable for application as the post-stage A hybrid structure can be obtained by replacing a functional of a DC-DC boost converter for an FCV. However, the unit of the basic topology with one or multiple topologies (or disadvantages of high control accuracy and a large number a part of the topology). The novel boost topology can improve of switches necessitates extensive research on developing the step-up ratio, IVR, component stress, or ICR. Hybrid topol- an optimization scheme to reduce the cost and power loss ogy can optimize the performance of the topology according to of the powertrain. specific requirements, which is a common method of designing D1 D2 D1 S1 C1 LC D2 S2 S C2 Fig.14 VL circuit Fig.13 TL structure 1 3 360 X. Wu et al. DC-DC converters for FCVs. The schematic diagram of hybrid with CIC can be used as the hybrid unit. Alternatively, high topology construction is shown in Fig. 15. Moreover, Boost, VG topology can be used as the hybrid unit to effectively Cuk, and sepic circuits have CIC, which is appropriate for the match the low output voltage of FCs and the high bus volt- basic topology of the hybrid structure. age. Furthermore, the CG structure should be maintained as The characteristics of a few DC-DC boost converters much as possible to reduce the additional EMI in the FCV constructed using the hybrid structure are listed in Table 1. powertrain. To reduce the power loss of components and Using a hybrid structure constitutes an important method improve the reliability of the powertrain, reducing the VSs of of generating novel boost converters for FCVs by introduc- components is necessary. However, there are many devices ing functional units to optimize the performance of the in the hybrid topology, which would increase the cost of the topology. For instance, when designing DC-DC converters powertrain and decrease the efficiency. for FCVs, blending can be conducted according to the actual needs. To reduce the impact on the lives of FCs, the topology Functional unit 1 Functional unit 2 Functional unit 2 The basic topology Topology 1 Topology 2 Topology 2 The hybrid topology Fig.15 Schematic of hybrid structure construction Table 1 Configuration and Reference Topology configuration Advantages Appli- advantages of a few hybrid cable to topologies FCV [96] Boost + Cuk ●Low VS ●CIC ★ [97] Quadratic Boost + Cuk ●High VG ●CIC ●Low VS ★ [98] Sepic + Modified VM ●High VG● CIC ★★ [99] Z-source + Quasi-Z-source ●High VG ★ [100] Cockcroft–Walton VM + Dickson VM ●Low output voltage drop ★ ●Low capacitor VS● High VG [101] Interleaved + Multi-level ●High VG● Low ICR ★ [102] Interleaved + VM ●Low ICR● High VG ●Simple structure ★★★ [103] [23] Boost + Modified VM ●CIC ●High VG● Low VS ★★ [104] Boost + Modified VM ●CIC● High VG ●Low switch VS ★ [105] VL circuit + Quadratic Boost ●High VG ★ ★ particularly inappropriate, ★★ relatively appropriate, ★★★ particularly appropriate 1 3 Topology 3 Functional unit 3 Topology 1 Functional unit 1 Topology 3 Functional unit 3 Review of DC‑DC Converter Topologies Based on Impedance Network with Wide Input Voltage Range… 361 2.5.2 Cascaded Structure through cascading. The cascaded structure exhibits the advantages of variety and simple implementation. Cascading more than two topologies (the duplicate part can Many studies have been conducted on utilizing cascaded be shared) can generate a cascaded structure, as shown in structures to construct new boost topologies for FCVs. A few Fig. 16. The 1st converter utilizes the FC as its power source, configurations and advantages of the cascaded structure are and the subsequent converter, in turn, uses the output volt- listed in Table 2. age of the former one as the power source. Thus, a cascade The cascaded structure can flexibly combine sub- relationship is formed. The nth converter provides energy topologies, according to the requirements of the particular to the load. Implementing the cascaded structure constitutes application, to obtain a new topology. This new topology, another important method of constructing new DC-DC boost thus, inherits the advantages of each sub-topology. To converters for FCVs. The performance of the topology in ensure the suitability of the cascaded topology for the terms of ICR, VG, IVR, and VS can be effectively improved FCV powertrain, a topology with low ICR can be used in the front-stage of the cascaded topology, ensuring the low current ripple characteristics of FCs. The cascaded topology can use a high gain structure as an intermedi- ate link to maintain the bus voltage under different FC DC-DC DC-DC DC-DC ... + ++ topology n topology 1 topology 2 output powers, and thus, adapt to different driving condi- tions. To reduce the cost of the powertrain and improve reliability, the TL structure can be used as the end-stage of the cascade topology. However, the efficiency of the Power DC-DC DC-DC DC-DC cascaded topology is the product of the efficiency at each ... Load source topology 1 topology 2 topology n stage, resulting in reductions in the entire efficiency of the vehicle powertrain [117]. Fig. 16 Schematic of cascaded structure construction Table 2 Configurations and advantages of a few cascaded topologies Reference Topology configuration Advantages Appli- cable to FCV [69] Interleaved + VM ●Low ICR ●High VG ★★ [70] [106] [107] Capacitor clamped circuit + VM ●CIC ●High VG● Low VS ●Low power loss ★★★ [108] Interleaved + VM + TL ●High VG● Low VS ●Zero ICR ★★ [109] SI + VM ●CIC ●High VG● Low power loss ★ [3] Quasi-Z-source + VM ●High VG●Low VS ●Wide IVR ★★★ [85] Quasi-Z-source + TL ●High VG ●High conversion efficiency ●Wide IVR ★★★ [86] Capacitor clamped circuit + TL ●High VG ●Simple control ●CIC ●Low VS ★★ [90] VL circuit + VL circuit ●High VG ★ [91] [65] SC + SC ●CIC ●High VG ★ [110] Quadratic Boost + VM ●High VG● CIC ●Wide IVR ★★★ [111] ●High conversion efficiency [112] [80] Z-source + VM ●High VG● Low VS ●High conversion efficiency ★ [113] Boost + VM + Modified VM ●High VG ●Low VS● CIC ★★★ [114] Boost + VM + VM ●High VG● Low CS● CIC ★★★ ●Working at non-limit step-up ratio [115] Boost + SC + VM ●CIC● High VG ●Low VS ★★ ●High conversion efficiency [116] Boost + VL circuit ●High VG ●Low power loss ★★★ ●CIC ●Few components ★ particularly inappropriate, ★★ relatively appropriate, ★★★ particularly appropriate 1 3 362 X. Wu et al. To synopsize, Table 3 systematically summarizes the high VG, low ICR, and wide IVR. The universal require- advantages and disadvantages of typical impedance net- ments for a DC-DC boost converter include voltage and works and provides suggestions for constructing DC-DC current stress, conversion efficiency, power density, number boost converters for FCVs. of components, the slope of step-up ratio, and presence of CG. 3 Evaluation of DC‑DC Boost Converter 3.1 Indexes of DC‑DC Boost Converter for FCVs for FCVs 3.1.1 VG Existing evaluation indexes for DC-DC boost converters mainly consider the performance of DC-DC converters, VG of the converter is defined as the ratio of the output volt- while the proposed evaluation indexes also take the require- age to the input voltage. The strength of the step-up capabil- ments of connection with FC into consideration. Therefore, ity of the topology increases with the step-up ratio. DC-DC the proposed evaluation system is more suitable for DC-DC boost converters for FCVs require high VGs no less than ten converters for FCVs. times. Figure 17 compares the VGs of 13 topologies. The evaluation indexes of the DC-DC boost converter constitute the standard based on which the merits and demerits of the topology are assessed. The performance requirements of the DC-DC boost converter for applica- tion in the field of FCVs are reflected by three indexes: Table 3 Advantages and disadvantages of topologies based on the impedance network Classification Impedance network Advantages Disadvantages Inductor + Inductor SI ●High VG ●Easy to control ●Pulsating input current ●Easy to generate new topologies Interleaved ●Low ICR ●No obvious increase in VG ●High ●Easy to generate new topologies control requirements ●Input and output do not have CG (float- ing interleaved) ●Many switches SI combined with charge pump ●High VG ●Easy to control ●Pulsating input current ●Easy to generate new topologies ●Initial inrush current existing in the power source side Quadratic boost ●High VG ●Low ICR ●Easy to control ●High VS Capacitor + Capacitor SC ●High VG ●High conversion efficiency ●Input and output do not have CG VM ●High VG ●High VS ●Output voltage dropping noticeably Modified VM ●High VG ●High VS Z-source ●High VG in non-limit duty cycle ●Pulsating input current ●Input and output do not have CG ●Narrow duty cycle range Quasi-Z-source ●High VG in non-limit duty cycle ●Narrow duty cycle range ●Compared with Z-source, input–output in CG ●Low ICR TL ●Low VS ●High control requirements ●Many switches ●No obvious increase in VG Inductor + Capacitor VL circuit ●Easy to generate new topologies ●No obvious increase in VG ●Pulsating input current Hybrid / Cascaded Hybrid ●High VG ●Easy to generate new topolo- ●Many components ●Low conversion gies efficiency ●Low power density Cascaded ●High VG ●Easy to generate new topolo- ●Many components ●Low conversion gies efficiency ●Low power density 1 3 Review of DC‑DC Converter Topologies Based on Impedance Network with Wide Input Voltage Range… 363 3.1.2 ICR low reliability of the entire converter. In addition, the volt- age and current stresses determine the rated parameters During the high-frequency switching of DC-DC boost con- of power electronic components. Generally, the higher the verters, the internal energy storage components are also voltage and current stress the component can withstand, the charged and discharged at high frequencies. This would, more expensive and the larger the size. To reduce the cost in turn, generate current ripples at the input side of the and size, the power electronic components in the DC-DC converter. The input side of the DC-DC boost converter is boost converter for FCV should have low voltage and cur- directly connected to the FC; therefore, an excessively large rent stresses. ICR of the converter reduces the life of the FC. Therefore, reducing the ICR is imperative [118]. Although the current 3.2.2 Power Density ripple can be filtered using filters, the input current of the DC-DC boost converter for FCV should not be pulsating The ratio of the rated power to the volume of the step-up considering the limited vehicle space. converter is defined as the power density. Owing to the limited space in FCVs, the converter is required to process high power at a small size. Utilizing appropriate circuit 3.1.3 IVR models and analysis methods to accurately calculate the parameters of inductors, capacitors, switches, and other The output power of the FC varies with the road conditions. components in the topology is a common method for The DC bus voltage is power battery voltage; therefore, its improving the power density. Miniaturization and the use variation range is small, and the output current of the FC of a high-frequency converter constitute the most effective would significantly change. The output voltage of the FC methods of increasing the power density. However, with would noticeably drop for increasing output current. In addi- increasing operating frequencies of the component, severe tion, a higher VG of the topology partly implies a wider EMI issues begin to arise. voltage range that can be inputted. To ensure FCV operation under various road conditions, the width of the IVR is an 3.2.3 Number of Components important index to estimate the performance of the DC-DC boost converter for an FCV. The complex DC-DC topology can generally improve the performance in one (or several) aspect(s). However, the dis- 3.2 Universal Indexes of DC‑DC Boost Converter advantages of having many components are evident, espe- cially in the cascaded and hybrid structures. A large number 3.2.1 Voltage and Current Stress of components increase the cost of the DC-DC boost con- verter and reduce the power density. The number of compo- VS constitutes the maximum reverse voltage born by the nents should be reduced as much as possible to meeting the component when it is switched off. Current stress consti- requirements of FCVs. tutes the maximum current flowing through the component. The significant increase in the component stress, in turn, increases the possibility of component defects, leading to Fig.17 VG curves of typical novel topologies 1 3 364 X. Wu et al. 3.2.4 Conversion Efficiency evaluated ten typical impedance network DC-DC boost topologies in terms of eight aspects: VG, ICR, IVR, VS, Owing to the parasitic parameters and the frequent switching current stress, number of components, the slope of the step- on and off of the switches, DC-DC converter losses include up ratio, and CG, as shown in Fig.  18. Two points are to conduction and switching losses. This not only results in be explained: first, the advantages and disadvantages of the an output power lower than the input power but also raises indicators of topology are relative. The topologies evalu- the temperature of the converter. The conversion efficiency ated in Fig.  18 only reflect the relationship between ten of the DC-DC converter is defined as the ratio of output topologies. Secondly, no topology is excellent in all aspects. power to input power. Moreover, high efficiency is the objec- Certain performances of the topology should be optimized tive behind developing DC-DC topologies. The conduction from the perspective of an application when designing the loss increases with the current. The switching loss of the topology. converter, however, increases with the operating frequency. In Fig. 18, a larger shaded region indicates better indica- Soft-switching technology can be used to reduce switching tors. From the inside to the outside, there are three grades: loss. C, B, and A, representing excellent, medium, and poor, respectively. There are only two grades of C (no) and A 3.2.5 CG (yes) in the presence of a CG, and the input current only has two grades: C (pulsating) and A (non-pulsating). The The du/dt and di/dt caused by switches are the primary VG function cannot be directly compared; therefore, its EMI sources in the DC-DC converter. Implementing soft- value is taken to the one corresponding to duty cycle switching technology, designing filters, parasitic parameter d = 0.8 (the topology with duty cycle 0 < d < 0.5 as the optimization, designing shield structure, and adding a CG value corresponding to d = 0.4, and the topology of duty are effective measures to suppress EMI [119]. When the cycle 0.5 < d < 0.75 as the value corresponding to d = 0.7). DC-DC converter does not have a CG, the high-frequency The IVR considers the variable range of the duty cycle an PWM voltage difference would generate additional EMI. intuitive reference. A better index is shown in Fig. 18 for a Therefore, CG is a key index in evaluating the topology of larger range of d. The number of components is the sum of the DC-DC boost converter for FCVs [120]. the number of switches, diodes, inductors, and capacitors. The stress function cannot be compared directly; therefore, 3.2.6 Slope of Step‑up Ratio the voltage and current stresses are the values correspond- ing to d = 0.8 (the topology for 0 < d < 0.5 as the value cor- The slope of the step-up ratio is an index that describes the responding to d = 0.4, and the topology for 0.5 < d < 0.75 as change in the step-up ratio with the duty cycle. A high step- the value corresponding to d = 0.7). The slope of the step- up ratio slope implies that the step-up ratio increases rapidly up ratio is the value corresponding to d = 0.8 (the topology with a slight increase in the duty cycle. The designers of the for 0 < d < 0.5 as the value corresponding to d = 0.4, and DC-DC boost converter tend to favor a gentle slope of the the topology for 0.5 < d < 0.75 as the value corresponding step-up ratio because it indicates that the control requirement to d = 0.7). of the topology is easy to achieve. 3.3 Comparison and Evaluation of Typical DC‑DC 4 Limitations and Prospects of DC‑DC Boost Boost Converter Converter for FCVs In this section, DC-DC boost converters based on imped- Herein, the derivation and research status of DC-DC boost ance network are compared in terms of eight aspects: the converters are analyzed, and the future research direction of technology used, VG, ICR, duty cycle range, voltage and using DC-DC boost converter for FCVs is presented. This current stresses, number of components, CG, and maxi- is done to further promote the development of FCVs and mum efficiency. The power density and conversion effi- the applications of DC-DC boost converters. Topologies ciency of the topology are affected by many factors, such of DC-DC conversion systems for FCVs have become a as the operating frequency, rated power, and the number of research hotspot with the development of new energy vehi- components. The distribution of indexes for several exist- cles. On the basis of DC-DC topology optimization, design- ing topologies are summarized in Table 4. The maximum ing an advanced DC-DC conversion system while meeting efficiency in Table  4 is the value obtained in the corre- the requirements for FCVs is significantly challenging. The sponding paper. main issues about DC-DC conversion systems for FCVs are To compare the DC-DC boost converter more intuitively, as follows: and aid the topology design of FCVs, this study qualitatively 1 3 Review of DC‑DC Converter Topologies Based on Impedance Network with Wide Input Voltage Range… 365 1 3 Table 4 Indexes of typical DC-DC topologies Topologies Technology used VG Input current Duty cycle range Voltage/ current stress of Number of compo- CG Efficiency Appli- switches nents cable to FCV VS Current stress L/C Q D 1 1 Boost - Non (0, 1) U 1/1 1 1 Yes – ★ o I 1−d 1−d Pulsating 1+d 2d [96] Hybrid Non (0, 1) 2/3 1 2 No – ★ o o 1−d 1+d 1−d Pulsating 2 2 3 1+d−d ) 2d+d +d [97] Hybrid Non (0, 1) 3/4 1 4 No 94.07% ★ 2 o 2 2 o 1+d−d (1−d) (1−d) Pulsating 1+3d 1+d 2 [16] SI Pulsating (0, 1) 4/1 2 4 No 95.8% ★ o o 1−d 1+3d 1−d 2 2 [55] SI combined with charge pump Pulsating (0, 1) U 2/2 2 2 Yes 90% ★ o I 1−d 1−d 2 U 4 [85] Quasi-Z-source + Non (0.5, 0.75) 2/4 2 3 Yes 95.66% ★★★ 3−4d 2 3−4d TL Pulsating 2 1+2d [3] Quasi-Z-source + Non (0, 0.5) 2/5 1 5 Yes 95.13% ★★★ 2 o 1−2d 2 d−2d VM Pulsating 1 2 [121] Z-source Pulsating (0, 0.5) U 2/3 1 2 No – ★ 1−2d 1−2d 1+d 1 3+d [122] Z-source + Pulsating (0, 0.5) 3/5 1 3 No 82.4% ★ U I o o 1−2d 1+d (1−d) SC 2 U 1−d [107] Capacitor clamped + Non (0, 0.5) 1/4 2 5 Yes 94.72% ★★★ 2 o 1−2d 2 d−2d VM Pulsating 2 U 1 [86] Capacitor clamped + Non (0, 0.5) 1/3 2 4 No 95.28% ★★ 1−2d d−2d TL Pulsating 2 3 3+d 1+d 1+3d−d −d [110] Quadratic boost + Non (0, 1) 2/4 2 7 Yes 95.04% ★★★ 2 o 2 3 o (1−d) 3+d d−2d +d VM Pulsating 2 U 1.5 [123] Interleaved Non (0, 0.5); 2/3 2 4 Yes 92.6% ★ 1−d 1−d Pulsating (0.5, 1) 6 3 [70] Interleaved + VM Non (0, 1) 2/6 2 6 No 96% ★★ 1−d 6 1−d Pulsating 2 U 1+d [71] VM Non (0, 1) 2/3 1 3 Yes – ★ 1−d 2 d−d Pulsating 2−d 1 1 [90] VL circuit Pulsating (0, 1) 1/2 1 2 Yes – ★ U I o 2 o 1−d 2−d d−d 1 2−d [58] Quadratic boost Non (0, 1) U 2/2 1 3 Yes – ★★ o I 2 2 o (1−d) (1−d) Pulsating ★ particularly inappropriate, ★★ relatively appropriate, ★★★ particularly appropriate 366 X. Wu et al. 1 3 Fig.18 Distribution of indexes of typical novel topologies Review of DC‑DC Converter Topologies Based on Impedance Network with Wide Input Voltage Range… 367 (1) With continuously improved bus voltage levels in the storage components in a converter. This is an effective powertrain of FCVs, a step-up converter is required means to improve the power density. However, when to continuously improve the VG to match the FC and the operating frequency of a circuit is too high, the DC bus voltages. With continuously improved energy EMI would become severe. Balancing the relationship efficiency requirements of the vehicle, stricter require- between the operating frequency and the electromag- ments are presented for the power conversion efficiency netic compatibility of a system in the application of of DC-DC converter. Therefore, optimizing topologies semiconductor components with wide band gaps would of converters are complicated considering the system also be an important topic in the research on topology loss and stresses in the power electronic components. design and the modeling and control of DC-DC boost (2) The output characteristic of an FC is soft (the voltage converters for FCVs. is lower for a heavier load), resulting in a wide range (7) The reliability of a DC-DC converter for an FCV is of FC voltages and a large output power fluctuation. A an important factor that seriously affects the develop - large current ripple reduces the life of an FC. There- ment of the FCV industry. Completely combining the fore, it should be ensured that the DC-DC converter can research of system detection, modeling, control strat- maintain the output voltage in a wide IVR and reduce egy, and fault diagnosis methods to realize the safe the ICR. It is important to address issues pertaining to and reliable operation of DC-DC conversion systems topology design, accurate modeling, and optimal con- would also be an important research problem regarding trol of the DC-DC boost converters for FCVs that need DC-DC converters for FCVs. to be solved urgently. (8) Installing high-power FCs on commercial FCVs with (3) A DC-DC converter is a time-varying and strong non- relatively sufficient space have become increasingly linear system. Under certain specific conditions, there popular, and the "voltage window" wherein the voltage would exist various types of bifurcation, chaos, and of the FC stack equals the DC bus voltage also appears. other nonlinear phenomena. These are embodied in the Considering the function of step-up and step-down con- increased voltage and current ripple of the converter, verters, designing and controlling DC-DC converters the increased harmonics, the decreased working effi - with wide step-up/down voltage ranges and low ICRs ciency, noise, oscillations, and even system collapse. is a challenge in developing commercial FCVs. The procedure for performing nonlinear modeling of DC-DC converters, analyzing converter stability, Considering the abovementioned problems faced dur- and implementing chaos control, constitute important ing research on DC-DC boost converters for FCVs, related research topics related to DC-DC converters. studies on the following aspects can be conducted. (4) The operation state of the FC stack determines the per- formance of FCVs. Internal resistance represents the (1) With regards to the issue of high VG and efficiency difficulty in the proton and electron transport within operation of the DC-DC boost converter for an FCV, the electrode. It also determines the efficiency of FC the advantages of components with wide band gaps, power generation, which directly reflects the health such as high frequency and low power loss, can be uti- of the FC stack. The humidity inside FCs is a factor lized. The optimization calculation method of the sys- that affects this internal resistance. Although DC-DC tem can be used to improve and optimize DC-DC con- converters can achieve voltage conversion and power verters based on existing SI, SC, cascaded boost, and decoupling, utilizing them to realize online monitor- other topologies. This can be done in terms of decreas- ing of the water content in FCs constitutes the research ing the size of energy storage components, reducing direction of future DC-DC converters. the stress of semiconductor components, and reducing (5) When the FC stack is operated at low temperatures, switching loss. the fuel supply pressure needs to be increased, leading (2) To solve the issue of a wide-range operation and a to a low FC efficiency. When the temperature is below low ICR of the DC-DC boost converter for FCVs, the zero, the water in the FC stack would freeze, hindering dynamic response and anti-interference ability of this gas diffusion and causing FC shutdown. Using DC-DC converter can be synthetically improved based on exist- converters to directly control the warm-up operating ing topology optimization, such as implementing an voltage of FCs and realizing the regulation of cold interleaved parallel structure, combined with research starts both constitute issues to be considered in future on accurate modeling and the adaptive control method. design of DC-DC converters. (3) To solve the issue of nonlinear modeling and con- (6) Using components with wide band gaps (SiC, GaN) trol of the DC-DC converter for FCVs, based on the can result in the circuit operating with a high switch- study of nonlinear dynamics and combined syntheti- ing frequency, significantly reducing the size of energy cal modeling methods, such as state variable equation 1 3 368 X. Wu et al. and discrete mapping, the nonlinear behavior caused 5 Conclusions by changes in the DC-DC converter parameter can be revealed by qualitative and quantitative methods. This The requirements of the DC-DC boost converter for FCV is done to apply chaos control to the converter and real- powertrains are summarized according to the characteris- ize the expected periodicity. tics of the powertrain and output characteristics of the FC, (4) To solve the issue of online water content monitoring which mainly include high voltage gain, low input current in the FC stack, based on the analysis of the equivalent ripple, and wide input voltage range. model of FC, the basis of determining the water content According to the different types of impedance net- using the internal resistance of an FC can be mastered. works, DC-DC boost converter topologies can be divided Additionally, the detection principle of the internal into inductor + inductor, capacitor + capacitor, induc- resistance of an FC can be summarized to construct tor + capacitor, and hybrid / cascaded. Based on the deri- an online monitoring model of water content in FCs. vation and classification of 12 types of existing DC-DC Subsequently, the integration of DC-DC converters and impedance networks, the research status and characteris- health monitoring systems is realized. tics of various impedance networks are reviewed. Among (5) To control the warm-up operating voltage using the them, interleaved, quasi-Z-source, and quadratic boost DC-DC converter and achieve a cold start, based on structures have continuous input current, which render the mastering of FC warm-up operating characteris- them suitable for connecting with FCs. Voltage multiplier, tics combined with the characteristics of the DC-DC switched-inductor switched-capacitor, and voltage lifting converter, the working frequency, inherent param- circuits can effectively improve the voltage gain. The TL eters, and given parameters of the converter can be structure reduces the voltage stress of the device and is comprehensively optimized. Thus, this provides a suitable for the post-stage of high voltage gain DC-DC basis for designing DC-DC converters with warm-up converters. Hybrid and cascading are common methods functions. for constructing DC-DC converters, but the conversion (6) Considering the issues regarding the application of efficiency and power density need to be considered. semiconductor components with wide band gaps in On this basis, an evaluation system for the DC-DC boost DC-DC boost converters, the electromagnetic com- topology for FCV is constructed from eight evaluation patibility characteristic of the system in different fre- aspects: input current ripple, voltage gain, input voltage quency domains can be analyzed, and an accurate EMI range, voltage stress, current stress, number of components, model can be subsequently built on the basis of EMI common ground, and slope of the step-up ratio. Moreover, modeling. This provides a reference for the topology the typical topologies proposed in existing literature are optimization design of DC-DC converters utilizing compared and evaluated, providing a guideline for design- semiconductor components with wide band gaps. ing DC-DC converters for the FCV powertrain. (7) To improve the reliability of the DC-DC boost con- Lastly, the issues regarding DC-DC converters for FCVs verter for FCVs, the typical dynamic behavior of the are summarized. To facilitate the development of FCVs, converter under different fault conditions, its fault future research directions are proposed, i.e., utilizing a behavior laws under different control strategies, its fault DC-DC converter to realize online monitoring of the water characteristics and influencing factors, and the quanti- content in FCs and designing buck-boost DC-DC converters tative extraction of key fault feature information of fault suitable for high-power commercial FCVs. external characteristics in each stage are analyzed. This Acknowledgements This work was sponsored thought the International is to realize the construction of a fault generation model Science & Technology Cooperation of China under 2019YFE0100200 of the DC-DC converter, providing a model basis for and the Fundamental Research Foundation for Universities of Hei- the fault diagnosis of DC-DC converters. longjiang Province (2018-KYYWF-1672). (8) To design a buck-boost DC-DC converter suitable for high-power commercial FCVs, based on the topology Declarations optimization of existing buck-boost DC-DC convert- ers combined with the advanced modeling theory, Conflict of interest On behalf of all the authors, the corresponding au- thor states that there is no conflict of interest. research on the precise control of the output voltage and the dynamic regulation of the output power of the Open Access This article is licensed under a Creative Commons Attri- converter can be conducted. Therefore, the comprehen- bution 4.0 International License, which permits use, sharing, adapta- sive improvement in the performance of the DC-DC tion, distribution and reproduction in any medium or format, as long converter with wide ranges of step-up and step-down as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes can be realized. were made. The images or other third party material in this article are 1 3 Review of DC‑DC Converter Topologies Based on Impedance Network with Wide Input Voltage Range… 369 included in the article's Creative Commons licence, unless indicated 19. Profumo, F., Tenconi, A., Cerchio, M., Bojoi, R., Gianolio, G.: otherwise in a credit line to the material. If material is not included in Fuel cells for electric power generation: peculiarities and dedi- the article's Creative Commons licence and your intended use is not cated solutions for power electronic conditioning systems. EPE permitted by statutory regulation or exceeds the permitted use, you will J. 16(1), 44–51 (2015) need to obtain permission directly from the copyright holder. To view a 20. Wahdame, B., Girardot, L., Hissel, D., et al.: Impact of power copy of this licence, visit http://cr eativ ecommons. or g/licen ses/ b y/4.0/ . converter current ripple on the durability of a fuel cell stack. 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Journal

Automotive Innovation – Springer Journals

Published: Nov 1, 2021

Keywords: Fuel cell vehicles; DC-DC boost converter; Evaluation systems and indexes; High gain topology

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