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A New Circuit Design of AC/DC Converter for T8 LED Tube

A New Circuit Design of AC/DC Converter for T8 LED Tube applied sciences Article 1 2 , Sunghwan Kim and Haiyoung Jung * Electrical Engineering Department, Inha University, 100 Inharo, Nam-gu Incheon 22212, Korea; saint1119@naver.com Fire and Disaster Prevention Department, Semyung University, 65 Semyung-ro, Jecheon-si, Chungcheongbuk-do 27136, Korea * Correspondence: hyjung@semyung.ac.kr; Tel.: +82-43-649-1695 Abstract: This study is about an improved high-quality light-emitting diode (LED) converter for a T8 LED tube. The converter is separated into the AC driving circuit and DC driving circuit. Also, the LED tube was applied with an output ripple eliminator for the optical performance. The AC driving circuit and DC driving circuit are assembled at the end of the LED tube in a G13 base and a G13 base dummy, respectively, and the output ripple eliminator is located on an LED PCB. The proposed LED converter is founded on a SSBB (single-stage buck-boost) converter topology and was designed for 10 W operation for a 600 mm T8 LED tube. The light waveform of the LED tube was measured by a photosensor. The waveform had almost no ripple and was the same as a straight line. The average calculated percent flicker of the proposed LED converter was an average of 1.9% at 100 and 240 VAC, 50 and 60 Hz. The proposed converter has lower power efficiency than a conventional converter by 2.7% at 100–240 VAC, but it still has high power efficiency (>87%). The measurement results represent that the LED output current regulation is below 0.92% at 100–240 VAC and the converter obtains the power factor more than 0.84 and the total harmonic distortion is less than 14.3%. All of the current harmonics reach the IEC 61000-3-2 Class D standards for high-quality LED converters. Keywords: high circuit efficiency; photosensor; power factor correction; SSBB converter topology; ripple Citation: Kim, S.; Jung, H. A New Circuit Design of AC/DC Converter 1. Introduction for T8 LED Tube. Appl. Sci. 2021, 11, LEDs have a lot of characteristics such as high luminous efficacy, energy saving 421. https://doi.org/10.3390/app properties and long lifetimes. These advantages have allowed LED lighting to replace other types of lighting quickly in the marketplace. Furthermore, as regulations for the use of traditional lighting such as incandescent lamps and florescent lamps become increasingly Received: 25 November 2020 stronger, the market share of LED lighting is expected to increase rapidly [1–3]. Accepted: 18 December 2020 An LED lighting product generally consists of various mechanical parts, an LED Published: 4 January 2021 module and an LED converter. An LED converter should provide stable and accurate current to the LED module to achieve good optical performance, since the luminance Publisher’s Note: MDPI stays neu- variation of LEDs depends on the variation of the current supplied to LEDs. In addition, the tral with regard to jurisdictional clai- LED converter requires high power efficiency, high power factor (PF), low total harmonic ms in published maps and institutio- distortion (THD), low total cost, and low light flicker [4,5]. nal affiliations. The light quality of LED lighting is mainly determined by the driving method and the key components of the LED converter. Due to the low circuit complexity and low cost, a single-stage power factor correction (PFC) driving method is usually used in many Copyright: © 2021 by the authors. Li- types of low-power LED lightings, including LED tubes. However, the output current censee MDPI, Basel, Switzerland. inevitably has a double line frequency ripple and light flicker is generated by variation This article is an open access article of the luminance. Various studies show that low-frequency light flicker caused by large distributed under the terms and con- output current ripple can adversely affect human health and cause headaches, visual ditions of the Creative Commons At- fatigue and epileptic attacks [6–9]. tribution (CC BY) license (https:// Figure 1 shows a block diagram of a typical single-stage buck-boost LED converter, creativecommons.org/licenses/by/ which is composed of many function blocks. An AC voltage with 50/60 Hz is supplied to a 4.0/). Appl. Sci. 2021, 11, 421. https://doi.org/10.3390/app11010421 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 421 2 of 14 Appl. Sci. 2020, 10, x FOR PEER REVIEW 2 of 13 Figure 1 shows a block diagram of a typical single-stage buck-boost LED converter, which is composed of many function blocks. An AC voltage with 50/60 Hz is supplied to bridge rectifier through a line filter, and a full-wave rectified sinusoidal voltage is supplied a bridge rectifier through a line filter, and a full-wave rectified sinusoidal voltage is sup- to the DC link capacitor C . Switching is conducted for power factor correction to LINK plied to the DC link capacitor CLINK. Switching is conducted for power factor correction to obtain high power factor, and smoothing is performed to reduce the output ripple and obtain high power factor, and smoothing is performed to reduce the output ripple and control the output current [5]. control the output current [5]. AC Line Filter Rectifier Inductor Capacitor LINK Voltage Sensing Controller Switch Current Sensing Figure 1. The block diagram of typical single-stage buck-boost light-emitting diode (LED) con- Figure 1. The block diagram of typical single-stage buck-boost light-emitting diode (LED) converter. verter. The smoothing to remove the ripple is not perfect, so the single-stage PFC driving The smoothing to remove the ripple is not perfect, so the single-stage PFC driving method always makes a low-frequency output ripple, and output current varies between a method always makes a low-frequency output ripple, and output current varies between maximum peak and minimum peak [10]. If the light has cyclic variation in the amplitude, a maximum peak and minimum peak [10]. If the light has cyclic variation in the ampli- percent flicker is very useful to represent the level of light flicker, which is defined as [11]: tude, percent flicker is very useful to represent the level of light flicker, which is defined A B as [11]: Percent Flicker (%) =  100 (1) A + B 𝐴 − 𝐵 (1) where A and B are the maxi Pemum rcent and Flickminimum er (%) = luminance, × 100 respectively. The Equation (1) 𝐴 + 𝐵 shows that the low percent flicker represents good performance. Several methods can where A and B are the maximum and minimum luminance, respectively. The Equation be considered to improve the light flicker caused by the double line frequency output (1) shows that the low percent flicker represents good performance. Several methods can ripple [12–15]. One of the obvious ways is to increase the link capacitor C ; however, DC be considered to improve the light flicker caused by the double line frequency output regulatory requirements for PF and THD performance may not be met. Therefore, this ripple [12–15]. One of the obvious ways is to increase the link capacitor CDC; however, method is usually suitable for non-regulated low power LED lighting. Another method is to regulatory requirements for PF and THD performance may not be met. Therefore, this increase the output capacitance for the smoothing operation. However, a large electrolytic method is usually suitable for non-regulated low power LED lighting. Another method is capacitor is needed to remove the output ripple completely, which increases the system size to in and crea cost. se thTher e output c efore, a this paci is ta not ncesuitable for the sm foro small othinlighting g operati appli on. Ho cations wever with , a lsmall arge el cir ec cuit tro- space. lytic ca V pa arious citor converter is needed topologies to remove th have e outpu been t pr rippl oposed e com to plrete emove ly, wh curr ich ent incr ripple eases [th 16 e –sy 18s- ]. Item si n [16],za e fa li n cd k e co r-f st re . e Ther LED efc oo re n,v th erits er is no com t sui pose ta dbo le f P fo FrC sm fly ab lla li ck gh cti on nv g a erppl ter a ica nd tia on bs wi idire th ct i sm ona al ll b ci urc ck ui /b t o sp os atce co . n Va ve ri rt o eus r to co re nd ve uc rte e tr hto e c pol urro egi ntes rip hp alv ee w ba ee s n p rpr opo opo sed sed to t ro ed re uc m eo th ve e c cur urr re en ntt rri ip pp ple le . T [1 h6 e– o 1u 8t]p . u In t c [ u 1r6 r] e,n a t w fli acker vefo- rfm ree is L alE m D osco t fn la v ter , w ter hi le co hm igpo h p sed ow eo rfe PF fficC ien flcyb y ia sck ma co inn ta v ier neter d [ 1a 6n ,1 d 7 ]a . However, due to the high circuit complexity, it cannot be applied to applications such bidirectional buck/boost converter to reduce the current ripple was proposed to reduce as T8 LED tubes. A multiplexing ripple cancellation was proposed by adding a ripple the current ripple. The output current waveform is almost flat, while high power effi- cancellation unit to achieve flicker-free operation in [18]. Although this solution is very ciency is maintained [16,17]. competitive, it is still too large to mount inside a G13 base with a diameter of 28 mm However, due to the high circuit complexity, it cannot be applied to applications such for low-power LED tubes [19]. In this paper, an LED converter with an output ripple as T8 LED tubes. A multiplexing ripple cancellation was proposed by adding a ripple eliminator is proposed to remove the double line frequency flicker of a T8 LED tube, while cancellation unit to achieve flicker-free operation in [18]. Although this solution is very providing high power efficiency, high PF, low THD and precise output current regulation. competitive, it is still too large to mount inside a G13 base with a diameter of 28 mm for Due to the addition of the output ripple eliminator, the power efficiency has been slightly low-power LED tubes [19]. In this paper, an LED converter with an output ripple elimi- reduced by an average of 2.7%. Nevertheless, the power efficiency is still more than 87% nator is proposed to remove the double line frequency flicker of a T8 LED tube, while and light flicker is almost removed. The proposed LED converter is separated into AC and providing high power efficiency, high PF, low THD and precise output current regulation. DC driving circuit and designed to mount in a G13 base and G13 base dummy. Because of Due to the addition of the output ripple eliminator, the power efficiency has been slightly the size limitations, we propose a practical method for product manufacturing by locating reduced by an average of 2.7%. Nevertheless, the power efficiency is still more than 87% the output ripple eliminator on the LED module. In Section 2, the theory of the output and light flicker is almost removed. The proposed LED converter is separated into AC and ripple eliminator operation is explained. In Section 3, design specifications of the proposed DC driving circuit and designed to mount in a G13 base and G13 base dummy. Because LED converter are represented in detail. In Section 4, a 10 W prototype is introduced and of the size limitations, we propose a practical method for product manufacturing by lo- experimental results are discussed. Finally, conclusions are given in Section 5. cating the output ripple eliminator on the LED module. In Section 2, the theory of the output ripple eliminator operation is explained. In Section 3, design specifications of the + Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 13 proposed LED converter are represented in detail. In Section 4, a 10 W prototype is intro- Appl. Sci. 2021, 11, 421 3 of 14 duced and experimental results are discussed. Finally, conclusions are given in Section 5. 2. The Proposed LED Converter for a T8 LED Tube 2. The Proposed LED Converter for a T8 LED Tube 2.1. Power Stage 2.1. Power Stage Figure 2a shows the overall stages of the proposed LED converter, which consists of a Figure 2a shows the overall stages of the proposed LED converter, which consists of power stage for constant current control and a ripple remove stage to reduce the output a power stage for constant current control and a ripple remove stage to reduce the output ripple. The power stage is physically divided into two driving circuit that are electrically ripple. The power stage is physically divided into two driving circuit that are electrically linked though the LED module [5]. Figure 2b shows a simplified circuit diagram of the linked though the LED module [5]. Figure 2b shows a simplified circuit diagram of the proposed LED converter with separated driving circuit for a T8 LED tube. proposed LED converter with separated driving circuit for a T8 LED tube. AC Driving Part LF CX VA VDC BD N C LINK1 GND (a) Power Stage Ripple free Stage AC Rectifier PFC & DC/DC Ripple Eliminator PFC Controller DC Driving Part LED– VDC L1 C CLINK2 LED+ CONTROLLER D GND (b) Figure 2. (a) Overall stages of the proposed LED converter; (b) simplified circuit diagram of AC and Figure 2. (a) Overall stages of the proposed LED converter; (b) simplified circuit diagram of AC DC driving circuit. and DC driving circuit. The proposed LED converter employs a single-stage buck-boost converter topology The proposed LED converter employs a single-stage buck-boost converter topology and consists of an AC driving circuit for rectification and a DC driving circuit for constant and consists of an AC driving circuit for rectification and a DC driving circuit for constant current control. The AC driving circuit consists of various filter components (CX, LF) for current control. The AC driving circuit consists of various filter components (CX, LF) for high frequency suppression, a full bridge rectifier (BD) and a DC link capacitor (C ). LINK1 high frequency suppression, a full bridge rectifier (BD) and a DC link capacitor (C LINK1). The DC driving circuit includes another DC link capacitor (C ), a power switch for LINK2 The DC driving circuit includes another DC link capacitor (CLINK2), a power switch for energy transfer (S), an inductor (L), a diode (D) and an output capacitor (C ) [5,20]. The full energy transfer (S), an inductor (L), a diode (D) and an output capacitor (C O) [5,20]. The bridge rectifier generates a positive full wave sinusoidal voltage from the AC line and full bridge rectifier generates a positive full wave sinusoidal voltage from the AC line and supplies it to the DC link capacitor in the AC driving circuit. The inductor is magnetized supplies it to the DC link capacitor in the AC driving circuit. The inductor is magnetized and stores rectified input energy from the AC driving circuit when the switch S turns on. and stores rectified input energy from the AC driving circuit when the switch S turns on. During the turn-off time, the inductor is demagnetized, and stored energy is trans- ferred to the output capacitor. In the single-stage driving method, the input voltage can be defined as: v (t) = V sin(2p f t) (2) I N in L + Appl. Sci. 2020, 10, x FOR PEER REVIEW 4 of 13 During the turn-off time, the inductor is demagnetized, and stored energy is trans- ferred to the output capacitor. In the single-stage driving method, the input voltage can Appl. Sci. 2021, 11, 421 4 of 14 be defined as: (2) ( ) 𝑣 (𝑡 ) = 𝑉 ∙ 2𝜋 𝑓 𝑡 𝐼𝑁 𝐿 where V is the amplitude of the input voltage and f is the line frequency of the input where 𝑉in is the amplitude of the input voltage and L 𝑓 is the line frequency of the input voltage. Since the controller is operated for power factor correction, the input current voltage. Since the controller is operated for power factor correction, the input current waveform follows the input voltage waveform. Therefore, the input current is defined as: waveform follows the input voltage waveform. Therefore, the input current is defined as: (3) i (t) = I sin(2p f t) (3) I N in L 𝑖 (𝑡 ) = 𝐼 ∙ (2𝜋 𝑓 𝑡 ) 𝐼𝑁 𝐿 where 𝐼 is the peak value of the input current. From Equations (1) and (2), the instanta- where I is the peak value of the input current. From Equations (1) and (2), the instanta- in neous output power can be expressed as [5,21]: neous output power can be expressed as [5,21]: (4) ( ) ( ) ( ) [ ( )] 𝑝 𝑡 = 𝜂 ∙ 𝑝 𝑡 = 𝜂 ∙ 𝑉 ∙ 𝐼 ∙ 2𝜋 𝑓 𝑡 = 𝑃 ∙ 1 − 4𝜋 𝑓 𝑡 𝑂𝑈𝑇 𝑓𝑓𝑒 𝐼𝑁 𝑓𝑓𝑒 𝑖𝑛 𝐿 𝑜 𝐿 p (t) = h  p (t) = h  V  I  sin (2p f t) = P [1 cos(4p f t)] (4) where 𝜂 is the expected power efficiency and 𝜂 ∙ 𝑉 ∙ 𝐼 o ⁄2is substituted with the OU T e f f I N e f f in in L L 𝑓𝑓𝑒 𝑓𝑓𝑒 amplitude of the output power 𝑃 . When 𝑉 is the output voltage, the instantaneous out- 𝑜 𝑜 where h is the expected power efficiency and h  V  I /2 is substituted with the e f f e f f in in put current can be expressed as: amplitude of the output power P . When V is the output voltage, the instantaneous output o o (5) current can be expressed as: 𝑖 (𝑡 ) = 𝐼 ∙ [1 − (4𝜋 𝑓 𝑡 )] 𝑈𝑇𝑂 𝑜 𝐿 where 𝐼 is the amplitude of the output current. The equation (5) shows that the single- i (t) = I [1 cos(4p f t)] (5) OU T L stage power factor correction driving method always contains double line frequency out- put ripple. If the output capacitance does not sufficiently smooth the output ripple, the where I is the amplitude of the output current. The equation (5) shows that the single-stage high and low amplitude current flowing through the LEDs generates light flicker [5,21– power factor correction driving method always contains double line frequency output 23]. ripple. If the output capacitance does not sufficiently smooth the output ripple, the high and low amplitude current flowing through the LEDs generates light flicker [5,21–23]. 2.2. Ripple Free Stage 2.2. Ripple Free Stage Figure 3 shows the output ripple eliminator of the proposed LED converter for alle- Figure 3 shows the output ripple eliminator of the proposed LED converter for alle- viating light flicker. An output ripple eliminator is a kind of common collector amplifier viating light flicker. An output ripple eliminator is a kind of common collector amplifier using an NPN Darlington configuration, which is one of the basic amplifier topologies, as using an NPN Darlington configuration, which is one of the basic amplifier topologies, as a voltage follower. It consists of 𝑅 , 𝐶 , and a Darlington transistor 𝑄 [24,25]. 𝐸 𝐸 a voltage follower. It consists of R , C , and a Darlington transistor Q [24,25]. E E OUT Collector Emitter LED V V CB BE Power Base LED OUT Stage LED C V E B R = LED LED Figure 3. The output ripple eliminator. Figure 3. The output ripple eliminator. Since the bias voltage of the base is immediately transferred to the LED anode, the Since the bias voltage of the base is immediately transferred to the LED anode, the impedance should be carefully chosen to find the optimized LED operation point in terms impedance should be carefully chosen to find the optimized LED operation point in terms of power losses and light flicker elimination. of power losses and light flicker elimination. Figure 4 shows conceptual waveforms of the proposed output ripple eliminator. To Figure 4 shows conceptual waveforms of the proposed output ripple eliminator. achieve a high power factor of the proposed LED converter, the output waveform inevi- To achieve a high power factor of the proposed LED converter, the output waveform tably contains a large ripple with double line frequency. This double frequency ripple is inevitably contains a large ripple with double line frequency. This double frequency ripple is passed through the power stage and transferred to the ripple removal stage. Since the output voltage V is constant with output ripple, it can be expressed as: V (t) = V + V (t) (6) O O_dc O_ac … … 𝑐𝑜𝑠 𝑖𝑛 𝑖𝑛 𝑖𝑛 𝑐𝑜𝑠 𝑠𝑖𝑛 𝑖𝑛 𝑖𝑛 𝑠𝑖𝑛 𝑖𝑛 𝑖𝑛 𝑠𝑖𝑛 Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 13 Appl. Sci. 2021, 11, 421 5 of 14 passed through the power stage and transferred to the ripple removal stage. Since the output voltage 𝑉 is constant with output ripple, it can be expressed as: (6) where V and V (t) are the DC component and AC component with double line O_dc O_ac 𝑉 (𝑡 ) = 𝑉 + 𝑉 (𝑡 ) 𝑂 𝑂 _ 𝑂 _ frequency ripple, respectively. Therefore, the base bias voltage of the Darlington pair has where 𝑉 and 𝑉 (𝑡 ) are the DC component and AC component with double line 𝑂 _ 𝑂 _ the same frequency ripple in steady state and the instantaneous value of bias voltage can frequency ripple, respectively. Therefore, the base bias voltage of the Darlington pair has be expressed as: the same frequency ripple in steady state and the instantaneous value of bias voltage can be expressed as: V (t) = V + V (t) = (V V ) + [V (t) V (t)] (7) B B_dc B_ac O_dc CB_dc O_ac CB_ac (7) 𝑉 (𝑡 ) = 𝑉 + 𝑉 (𝑡 ) = (𝑉 − 𝑉 ) + [𝑉 (𝑡 ) − 𝑉 (𝑡 )] 𝐵 𝐵 _ 𝐵 _ 𝑂 _ _ 𝑂 _ _ where V and V (t) are the DC component and AC component of the collector- CB_dc CB_ac where 𝑉 and 𝑉 (𝑡 ) are the DC component and AC component of the collector- _ 𝐶 𝐵 _ base forward voltage, 𝑎𝑐 respectively. Considering the base-emitter forward voltage in the base forward voltage, respectively. Considering the base-emitter forward voltage in the Darlington input stage, V applied across LEDs can be obtains as: LED Darlington input stage, 𝑉 applied across LEDs can be obtains as: 𝐿𝐸𝐷 V (t) = (V V ) + [V (t) V (t)] (8) LED B_dc BE_dc B_ac BE_ac (8) ( ) ( ) ( ) 𝑉 𝑡 = (𝑉 − 𝑉 ) + [𝑉 𝑡 − 𝑉 𝑡 ] 𝐿𝐸𝐷 𝐵 _ _ 𝐵 _ _ where V is the DC value, and V (t) is the AC value. Using Kirchhoff’s voltage law, BE_dc BE_ac where 𝑉 is the DC value, and 𝑉 (𝑡 ) is the AC value. Using Kirchhoff’s voltage _ _ the AC component of bias voltage V t is determined from X , Z and V t , ( ) ( ) B_ac E E O_ac ( ) law, the AC component of bias voltage 𝑉 𝑡 is determined from 𝑋 , 𝑍 and 𝑉 (𝑡 ), 𝐵 _ 𝐸 𝐸 𝑂 _ V (t) = V (t) (9) B_ac O_ac 𝐸 (9) 𝑉 (𝑡 ) = ∙ 𝑉 (𝑡 ) 𝐵 _ 𝑂 _ 2 2 2 2 where Z = √ R + X is the total impedance, and X = 1/(4p f C ) is the impedance where 𝑍 = 𝑅 + 𝑋 is the total impedance, and 𝑋 = 1/(4𝜋 𝑓 ∙ 𝐶 ) is the impedance E E E E L E 𝐸 𝐸 𝐸 𝐸 𝐿 𝐸 of C . If the base-emitter forward voltage V components are negligible, from Equations of 𝐶 . If the base-emitter forward voltage 𝑉 components are negligible, from Equations E BE (8) and (9), the LED voltage can be derived as: (8) and (9), the LED voltage can be derived as: V (t) = (V V ) + p V (t) (10(10) ) LED O_dc CB_dc O_ac 𝑉 (𝑡 ) = (𝑉 − 𝑉 ) + ∙ 𝑉 (𝑡 ) 𝐿𝐸𝐷 𝑂 _ _ 2 2 𝑂 _ R + X 2 2 E E 𝑅 + 𝑋 𝐸 𝐸 Equation (10) shows that the output ripple eliminator can provide a DC value to the LEDs Equation (10) shows that the output ripple eliminator can provide a DC value to the LEDs by adjusting the AC value to nearly zero, which means light flicker completely disappears by adjusting the AC value to nearly zero, which means light flicker completely disappears from the LED tube. from the LED tube. V AC component O_dc O_ac DC component CB CB_dc CB_ac AC component DC component LED AC Line One Cycle Figure 4. Conceptual waveforms of the output ripple eliminator. Figure 4. Conceptual waveforms of the output ripple eliminator. 3. Design Parameters 3. Design Parameters Key parameters and components were selected for the converter. Table 1 shows the Key parameters and components were selected for the converter. Table 1 shows the specifications of the proposed LED converter. It operates at 100–240 VAC and is designed specifications of the proposed LED converter. It operates at 100-240 VAC and is designed to cover 10% input variation. The nominal power consumption is 10 W with 10% to cover ± 10% input variation. The nominal power consumption is 10 W with ±10% toler- tolerance. The target variation of the output voltage and current are 106 V  4% and ance. The target variation of the output voltage and current are 106 V ± 4% and 87 mA ± 87 mA  6%, respectively. 6%, respectively. 𝑎𝑐 𝑑𝑐 𝐶𝐵 𝑑𝑐 𝐵𝐸 𝑎𝑐 𝑎𝑐 𝑎𝑐 𝑎𝑐 𝑎𝑐 𝐵𝐸 𝑑𝑐 𝐵𝐸 𝑎𝑐 𝐵𝐸 𝑎𝑐 𝑑𝑐 𝐵𝐸 𝑑𝑐 𝑑𝑐 𝐶𝐵 𝑎𝑐 𝐶𝐵 𝑎𝑐 𝑑𝑐 𝐶𝐵 𝑑𝑐 𝑎𝑐 𝑑𝑐 𝑎𝑐 𝑑𝑐 𝑎𝑐 𝑑𝑐 Appl. Sci. 2021, 11, 421 6 of 14 Table 1. Specifications of the proposed LED converter. 10 W LED Converter Item MIN TYP MAX 100 90 100 - Input Voltage (VAC) 240 - 240 264 Input Current (mA) 34.1 (@90 VAC) - 122.2 (@264 VAC) Input Frequency (Hz) 50 - 60 Input Power (W) 9.0 10.0 11.0 Output Voltage (V) 102 106 112 Output Current (mA) 81.8 87.0 92.2 3.1. Determining the Inductance: L In a discontinuous current mode(DCM) single-stage buck-boost converter, the switch- ing cycle time (T = 1/ f ) can be expressed as [5]: SW SW T = T + T + T (11) SW ON_S ON_D I D LE where T is the switch turn-on time, T is the diode on-time and T is the time, ON_S ON_D I D LE in which no current flows in the inductor. Since T cannot be zero to ensure DCM I D LE operation under all conditions, D and D should satisfy D + D < 1. Therefore, from D D S S the volt-second balance law, the inequality for D becomes: D < (12) V + V I N O where D is the duty ratio of the switch turn-on time (= T /T ) and D is the S SW ON_S D duty ratio of the diode turn-on time (= T /T ). The input current is induced as SW ON_D D T V /2L from the buck-boost relationship, which is substituted into the power sw I N conversion relationship (V  I = h  V  I ) and rearranged for the inductance L: O O e f f I N I N e f f V V O I N L <   (13) 2 f I V + V sw O I N O where f is the switching frequency. From Equation (13), the minimum inductance can SW be determined at V , V and f = 70 k Hz. I N_M I N O_M AX SW_M AX 0.9 112 90 L <  = 1.55 m H (14) 2 70 k 92.2 m 90 + 112 Considering the size of inductor and margin, an inductance of 1.38 mH should be chosen and manufactured with an EE1616 core. 3.2. Determining the Output Capacitance: C A small output ripple requires a large output capacitance; however, the higher output capacitance, the bigger the capacitor size. Therefore, a proper percent flicker specifica- tion without an output ripple eliminator should be considered to determine the output capacitance while considering voltage stress and size. The output capacitance C has the following relationship [5,24]: I 2 I C_PP LED C   (15) 4p f V 4p f N DV L L C_PP S LED_Device Appl. Sci. 2021, 11, 421 7 of 14 where I is the peak-to-peak current of the output capacitor, which is approximately C_PP twice the average LED current I . Here, DV is the one LED peak-to-peak, LED LED_Device which varies with the LED current and N is the number of LEDs in series. Referring to Equation (15), the minimum output capacitance can be estimated with I of 92.2 mA LED_M AX and f of 50 Hz. In addition, to keep the light flicker around 10% without output ripple L_M I N eliminator, DV of 95 mV should be used according to V-I characteristics [26]. LED_Device 2 92.2 m C > = 88.3 F (16) 4 3.14 50 35 95 m For practical design for a T8 LED tube, C should be chosen as 160 V/100 F with dimensions of 12  25 mm (D  H). 3.3. Determining the Power Device: S, D Both the voltage and current stress should be calculated to select an appropriate device. Since the voltage stress of switch is V + V in a buck-boost converter topology, I N O the maximum voltage stress is determined at V and V . Therefore, the voltage I N_M AX O_M AX stress of the switch can be calculated as: p p V = 2V + V = 2264 + 112 = 485 V (17) S_M AX I N_M AX O_M AX From Table 1, when an input voltage of 90 VAC is provided to an 11 W system, the maximum RMS current stress of the switch when considering the expected power efficiency is: P 11 I N_M AX I = = = 136 m A (18) S_R MS_M AX V h 90 0.9 I N_M I N e f f The voltage stress of the diode is the same as V and the maximum average S_M AX current of the diode is: 2 I 2 92.2 m O_M AX I = = = 419 m A (19) D_AV G_M AX D 0.44 Therefore, an 800 V/2.5 A MOSFET switch (STP3NK80) and 1000 V/1 A diode (US1M) were selected to consider the component stress and margin with heat dissipation. 3.4. Determining the Output Ripple Eliminator: R , C , Q E E The voltage and current relation is almost linear in driving region of LED IV curve [26], so the dynamic resistance of the LED device can be defined as: LED_Device R =  3.23 W (20) LED_Device LED_Device To achieve a percent flicker under 1%, the current variation D I of the LED LED_Device device should be within 1.4 mA according to the luminance-current characteristics [26]. Consequently, the allowed LED device voltage DV is 3.23  1.4 m = 4.5 mV. LED_Device Therefore, when the peak-to-peak output voltage DV (t) is 3.33 V, the bias voltage O_ac V (t) should be 157.5 mV in a series of 35 LEDs. From Equation (9), X /Z is obtained as: B_ac E E X DV 0.158 E B_ac p = = = 0.047 (21) 2 2 DV 3.33 R + X O_ac E E A small capacitor can be used to remove the low frequency ripple, and C can be implemented with a 100 V/1 F chip capacitor. Since the capacitor impedance X is 1.59 kW at 60 Hz, R should be: R = X  33 kW (22) E E 0.047 Appl. Sci. 2021, 11, 421 8 of 14 The output ripple eliminator greatly reduces the light flicker but at the same time, additional power loss occurs in Darlington transistor Q. The power loss of Q can be estimated as (V /2 + V ) I , where the V is peak-to-peak voltage across LED_PP BE LED LED_PP the LED array and I is the average current flowing to the LED array [24,27]. LED 35 95 m P = + 1.2  92.2 m = 264 mW (23) Q_M AX Considering Equation (23) and heat dissipation, an MJF6388 Darlington transistor is selected to maintain stable operation. 4. Experimental Results A 10 W prototype of the LED converter was designed to perform the experiments, as Appl. Sci. 2020, 10, x FOR PEER REVIEW 8 of 13 shown in Figure 5. The input voltage is 100–240 VAC and the line frequency is 50 Hz and 60 Hz. The output current is 87 mA, and output voltage is 106 V with 35 LEDs in series to 60 Hz. The output current is 87 mA, and output voltage is 106 V with 35 LEDs in series to satisfy the luminous uniformity. Table 2 shows the specifications of the key components. satisfy the luminous uniformity. Table 2 shows the specifications of the key components. Figure 5. (a) AC driving circuit, (b) DC driving circuit, (c) LED module with the output ripple Figure 5. (a) AC driving circuit, (b) DC driving circuit, (c) LED module with the output ripple eliminator. eliminator. Table 2. Specifications of key components. Table 2. Specifications of key components. Fuse 250 V, 1 A Fuse 250 V, 1 A Line Filter TOROIDAL, 31 uH, 4 A Line Filter TOROIDAL, 31 uH, 4 A X-capacitor 275 VAC, 100 nF X-capacitor 275 VAC, 100 nF AC Driving Circuit AC Driving Circuit Bridge BridgeDiode Diode VRRM VRRM= =1000 1000V V, , IO IO = =1 1A A Film Capacitor 600 V, 100 nF Film Capacitor 600 V, 100 nF PCB CEM-3, 1T, 0.5 oz, Single Layer PCB CEM-3, 1T, 0.5 oz, Single Layer Film Capacitor 630 V, 100 nF Film Capacitor 630 V, 100 nF Inductor EE1616, 1.38 mH ElectrInductor olytic Capacitor 400 EE1616, V, 221.38 uF, mH 105℃ VDSS = 800 V, ID = 2.5A, RDS(ON) < Electrolytic Capacitor 400 V, 22 uF, 105 C DC Driving Circuit FET 4.5Ω VDSS = 800 V, ID = 2.5A, DC Driving Circuit Dio FET de VRRM = 1000 V, IO = 1 A RDS(ON) < 4.5 W PCB FR-4, 1T, 0.5oz, Double Layer Diode VRRM = 1000 V, IO = 1 A Darlington Transistor NPN, VCE 100 V, IC = 10 A PCB FR-4, 1T, 0.5 oz, Double Layer Figure 6 shows the input voltage and current waveform of the proposed LED con- Darlington Transistor NPN, VCE 100 V, IC = 10 A verter. The input current waveform follows the input voltage, which means it operates properly for power factor correction. Since the PFC operation is not affected by the output Figure 6 shows the input voltage and current waveform of the proposed LED converter. ripple eliminator, there is no difference in the power factor between the reference con- The input current waveform follows the input voltage, which means it operates properly verter and the proposed converter. for power factor correction. Since the PFC operation is not affected by the output ripple eliminator , there is no difference in the power factor between the reference converter and the proposed converter. Appl. Sci. 2020, 10, x FOR PEER REVIEW 8 of 13 60 Hz. The output current is 87 mA, and output voltage is 106 V with 35 LEDs in series to satisfy the luminous uniformity. Table 2 shows the specifications of the key components. Figure 5. (a) AC driving circuit, (b) DC driving circuit, (c) LED module with the output ripple eliminator. Table 2. Specifications of key components. Fuse 250 V, 1 A Line Filter TOROIDAL, 31 uH, 4 A X-capacitor 275 VAC, 100 nF AC Driving Circuit Bridge Diode VRRM = 1000 V, IO = 1 A Film Capacitor 600 V, 100 nF PCB CEM-3, 1T, 0.5 oz, Single Layer Film Capacitor 630 V, 100 nF Inductor EE1616, 1.38 mH Electrolytic Capacitor 400 V, 22 uF, 105℃ VDSS = 800 V, ID = 2.5A, RDS(ON) < DC Driving Circuit FET 4.5Ω Diode VRRM = 1000 V, IO = 1 A PCB FR-4, 1T, 0.5oz, Double Layer Darlington Transistor NPN, VCE 100 V, IC = 10 A Figure 6 shows the input voltage and current waveform of the proposed LED con- verter. The input current waveform follows the input voltage, which means it operates properly for power factor correction. Since the PFC operation is not affected by the output ripple eliminator, there is no difference in the power factor between the reference con- Appl. Sci. 2021, 11, 421 9 of 14 verter and the proposed converter. Appl. Sci. 2020, 10, x FOR PEER REVIEW 9 of 13 Figure 6. Input voltage waveform (red) and input current waveform (blue) (a) at 100 VAC/60 Hz, Figure 6. Input voltage waveform (red) and input current waveform (blue) (a) at 100 VAC/60 Hz, (b) 100 VAC/50 Hz, (c) 240 VAC/50 Hz, (d) 240 VAC/50 Hz. (b) 100 VAC/50 Hz, (c) 240 VAC/50 Hz, (d) 240 VAC/50 Hz. Figure 7 shows the measured waveform of the output voltage, output current, and Figure 7 shows the measured waveform of the output voltage, output current, and light output at 240 VAC/50 Hz. The light output waveform measured with the photosen- light output at 240 VAC/50 Hz. The light output waveform measured with the photosen- sor amplifier contains a double line frequency ripple in the reference converter without sor amplifier contains a double line frequency ripple in the reference converter without an output ripple eliminator. However, with the proposed converter, the light flicker is an output ripple eliminator. However, with the proposed converter, the light flicker is completely removed and the result is almost a straight line. completely removed and the result is almost a straight line. Figure 7. Waveform of the output voltage (dark yellow), output current (blue) and light output Figure 7. Waveform of the output voltage (dark yellow), output current (blue) and light (green) of the (a) reference converter, (b) proposed converter. output (green) of the (a) reference converter, (b) proposed converter. Using Equation (1), the percent flicker can be calculated in each input condition. Using Equation (1), the percent flicker can be calculated in each input condition. Ta- Table 3 shows the calculated percent flicker of the reference and the proposed converter ble 3 shows the calculated percent flicker of the reference and the proposed converter which are 13.5% and 1.9%, respectively. When using the output ripple eliminator, the light which are 13.5% and 1.9%, respectively. When using the output ripple eliminator, the light flicker almost disappeared, and the percent flicker of the proposed converter with the flicker almost disappeared, and the percent flicker of the proposed converter with the out- output ripple eliminator was improved by 11.6% on average. put ripple eliminator was improved by 11.6% on average. Table 3. Comparison of percent flicker. Reference Proposed Input Light Output (mV) Percent Light Output (mV) Percent Flicker Flicker VAC Hz MAX AVG MIN MAX AVG MIN (%) (%) 50 450 393 336 14.5 401 393 385 2.0 60 443 394 345 12.5 402 395 388 1.8 50 451 394 337 14.5 401 393 385 2.0 60 443 394 345 12.4 400 393 386 1.8 Appl. Sci. 2021, 11, 421 10 of 14 Table 3. Comparison of percent flicker. Reference Proposed Input Percent Percent Light Output (mV) Light Output (mV) Flicker Flicker VAC Hz MAX AVG MIN MAX AVG MIN (%) (%) 50 450 393 336 14.5 401 393 385 2.0 60 443 394 345 12.5 402 395 388 1.8 50 451 394 337 14.5 401 393 385 2.0 60 443 394 345 12.4 400 393 386 1.8 Figure 8 shows power efficiency and efficiency difference according to the input voltage measured at 50 Hz. There is no big variation according to the line frequency. The power efficiency of the proposed converter is 2.6–2.9% lower than that of the reference, which means the output ripple eliminator consumes less than 2.6–2.9% of power. Although the total power increases 2.7% on average, the power efficiency is still more than 87% in 100–240 VAC, and the light flicker can be completely removed. Appl. Sci. 2020, 10, x FOR PEER REVIEW 10 of 13 Figure 8. Power efficiency and efficiency difference according to the input voltage. Figure 8. Power efficiency and efficiency difference according to the input voltage. Figure 9 shows the measured output current of the reference and proposed LED con- Figure 8 shows power efficiency and efficiency difference according to the input volt- verters, which was measured using an LED module with 35 LEDs in series. The measured age measured at 50 Hz. There is no big variation according to the line frequency. The data of the proposed LED converter is almost the same as that of reference, which shows power efficiency of the proposed converter is 2.6–2.9% lower than that of the reference, that the output current regulation is hardly affected by the output ripple eliminator. The av- which means the output ripple eliminator consumes less than 2.6–2.9% of power. Alt- erage output current and its standard deviation were 87.1 mA and 0.19 mA for the proposed hough the total power increases 2.7% on average, the power efficiency is still more than LED converter at 60 Hz, respectively. The positive and negative maximum deviations from 87% in 100–240 VAC, and the light flicker can be completely removed. the average output current were measured as +0.3 mA (0.34%) and 0.2 mA (0.23%) at Figure 9 shows the measured output current of the reference and proposed LED con- 60 Hz. The variation of the output current is less than 1%, which is great performance, verters, which was measured using an LED module with 35 LEDs in series. The measured since the power consumption tolerance of the LED tube is usually 10%. data of the proposed LED converter is almost the same as that of reference, which shows that the output current regulation is hardly affected by the output ripple eliminator. The average output current and its standard deviation were 87.1 mA and 0.19 mA for the pro- posed LED converter at 60 Hz, respectively. The positive and negative maximum devia- tions from the average output current were measured as + 0.3 mA (0.34%) and –0.2 mA (0.23%) at 60 Hz. The variation of the output current is less than 1%, which is great per- formance, since the power consumption tolerance of the LED tube is usually ± 10%. Figure 9. Output current according to the input voltage. Figure 10 shows the measured power factor in 100–240 VAC at 60 Hz. There is no large difference between the reference and proposed LED converters, which means the variation of the power factor is not affected by the output ripple eliminator. Due to the Appl. Sci. 2021, 11, 421 11 of 14 Appl. Sci. 2020, 10, x FOR PEER REVIEW 11 of 14 60Hz_Reference 60Hz_Proposed 100.0 95.0 90.0 87.3 87.3 87.3 87 87.1 87.2 87.2 87.2 87.2 87.2 87.1 86.9 86.9 86.9 86.9 87.2 87.2 87.2 87.2 85.0 87 87.1 87.1 87.1 87.1 87.1 87 86.8 86.8 86.8 86.9 80.0 75.0 70.0 65.0 60.0 100 120 140 160 180 200 220 240 Input Voltage [VAC] Figure 9. Output current according to the input voltage. Figure 9. Output current according to the input voltage. Figure 10 shows the measured power factor in 100–240 VAC at 60 Hz. There is no large difference between the reference and proposed LED converters, which means the Appl. Sci. 2020, 10, x FOR PEER REVIEW 11 of 13 Figure 10 shows the measured power factor in 100–240 VAC at 60 Hz. There is no variation of the power factor is not affected by the output ripple eliminator. Due to the large difference between the reference and proposed LED converters, which means the influence of the input current harmonics, it slightly decreases as the input voltage increases. influence of the input current harmonics, it slightly decreases as the input voltage in- variation of the power factor is not affected by the output ripple eliminator. Due to the Nevertheless, the power factor is still higher than 0.84 under all input voltages. creases. Nevertheless, the power factor is still higher than 0.84 under all input voltages. influence of the input current harmonics, it slightly decreases as the input voltage in- creases. Nevertheless, the power factor is still higher than 0.84 under all input voltages. Figure 10. PF at 60 Hz. Figure 10. PF at 60 Hz. Figure 10. PF at 60 Hz. Figur Figure e 11 11shows showsthe the THD THDof ofthe the pr pr oposed oposedLED LED co converter nverterat at 5 500Hz Hzand and60 60Hz, Hz,which which is less than 14.3% under all input voltages. The values of THD at 100 VAC are 8.52% and is less than 14.3% under all input voltages. The values of THD at 100 VAC are 8.52% and Figure 11 shows the THD of the proposed LED converter at 50 Hz and 60 Hz, which 8.36% at 50 Hz and 60 Hz, respectively. At 240 VAC, the values are 11.44% at 50 Hz and 8.36% at 50 Hz and 60 Hz, respectively. At 240 VAC, the values are 11.44% at 50 Hz and is less than 14.3% under all input voltages. The values of THD at 100 VAC are 8.52% and 14.26% at 60 Hz, respectively. The measured results are slightly high at 60 Hz, but they are 14.26% at 60 Hz, respectively. The measured results are slightly high at 60 Hz, but they 8.36% at 50 Hz and 60 Hz, respectively. At 240 VAC, the values are 11.44% at 50 Hz and very suitable for a high-quality LED tube. are very suitable for a high-quality LED tube. 14.26% at 60 Hz, respectively. The measured results are slightly high at 60 Hz, but they are very suitable for a high-quality LED tube. Figure 11. Total harmonic distortion (THD) according to the input voltage. Figure 12 shows the measured results of the harmonic current at 100 VAC at 50 Hz and 60 Hz. The maximum permissible harmonic current from the IEC 61000-3-2 Class D standard for less than 25 W of lighting is presented with a gray bar [28,29]. The harmonic current is markedly lower than the IEC standards. Output Current [mA] Appl. Sci. 2020, 10, x FOR PEER REVIEW 12 of 14 Appl. Sci. 2021, 11, 421 12 of 14 50Hz_Proposed 60Hz_Proposed Appl. Sci. 2020, 10, x FOR PEER REVIEW 12 of 14 50Hz_Proposed 60Hz_Proposed 20 14 18 12 16 10 14 8 12 6 10 4 8 2 6 0 100 120 140 160 180 200 220 240 Input Voltage [VAC] Figure 11. Total harmonic distortion (THD) according to the input voltage. 100 120 140 160 180 200 220 240 Input Voltage [VAC] Figure 12 shows the measured results of the harmonic current at 100 VAC at 50 Hz Figure 11. Total harmonic distortion (THD) according to the input voltage. and 60 Hz. The maximum permissible harmonic current from the IEC 61000-3-2 Class D Figure 11. Total harmonic distortion (THD) according to the input voltage. standard for less than 25 W of lighting is presented with a gray bar [28,29]. The harmonic Figure 12 shows the measured results of the harmonic current at 100 VAC at 50 Hz current is markedly lower than the IEC standards. Figure 12 shows the measured results of the harmonic current at 100 VAC at 50 Hz and 60 Hz. The maximum permissible harmonic current from the IEC 61000-3-2 Class D and 60 Hz. The maximum permissible harmonic current from the IEC 61000-3-2 Class D standard for less than 25 W of lighting is presented with a gray bar [28,29]. The harmonic standard for less than 25 W of lighting is presented with a gray bar [28,29]. The harmonic current is markedly lower than the IEC standards. current is markedly lower than the IEC standards. Limits for IEC61000-3-2 Class D 100VAC_50Hz_Reference 100VAC_60Hz_Reference 3.00 20.00 2.70 18.00 Limits for IEC61000-3-2 Class D 100VAC_50Hz_Reference 100VAC_60Hz_Reference 2.40 16.00 3. 2.00 10 20. 14.00 00 2. 1.70 80 18. 12.00 00 1.50 10.00 2.40 16.00 1.20 8.00 2.10 14.00 0.90 6.00 1.80 12.00 1. 0.50 60 10. 4.00 00 1. 0.20 30 8. 2.00 00 0.00 0.00 0.90 6.00 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 0.60 4.00 Harmonic Order 0.30 2.00 Figure 12. Harmonic current at 100 VAC 50/60 Hz. 0.00 0.00 Figure 12. Harmonic current at 100 VAC 50/60 Hz. 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 5. Conclusions Harmonic Order 5. Conclusions In this study, an AC/DC LED converter has been proposed to remove the flicker of a T8In LED this study, tube. The anpr Aoposed C/DC LLED ED co converter nverter ha uses s bee an n pr output oposed ripple to reeliminato move the r fli and cker a o SSBB f a Figure 12. Harmonic current at 100 VAC 50/60 Hz. converter topology for the input voltage 100–240 VAC. A 10 W prototype was designed T8 LED tube. The proposed LED converter uses an output ripple eliminator and a SSBB to verify the high performance. The calculated percent was 1.9% at all input conditions. converter topology for the input voltage 100–240 VAC. A 10 W prototype was designed 5. Conclusions The power efficiency is lower than that of a conventional converter by 2.7% at 100–240 VAC, to verify the high performance. The calculated percent was 1.9% at all input conditions. In this study, an AC/DC LED converter has been proposed to remove the flicker of a but it is still high (>87% and even 89% at 220 VAC). The experimental results represented The power efficiency is lower than that of a conventional converter by 2.7% at 100–240 T8 LED tube. The proposed LED converter uses an output ripple eliminator and a SSBB the LED output current regulation as less than 0.92% at 100–240 VAC and the LED converter VAC, but it is still high (>87% and even 89% at 220 VAC). The experimental results repre- converter topology for the input voltage 100–240 VAC. A 10 W prototype was designed has a high power factor (>0.84) and low total harmonic distortion (<14.3%). Moreover, sented the LED output current regulation as less than 0.92% at 100–240 VAC and the LED to verify the high performance. The calculated percent was 1.9% at all input conditions. the harmonic current of the LED converter reaches the IEC 61000-3-2 Class D standard at converter has a high power factor (>0.84) and low total harmonic distortion (<14.3%). The power efficiency is lower than that of a conventional converter by 2.7% at 100–240 100 VAC and 240 VAC input voltages for high-quality LED converters. Moreover, the harmonic current of the LED converter reaches the IEC 61000-3-2 Class D VAC, but it is still high (>87% and even 89% at 220 VAC). The experimental results repre- standard at 100 VAC and 240 VAC input voltages for high-quality LED converters. sented the LED output current regulation as less than 0.92% at 100–240 VAC and the LED converter has a high power factor (>0.84) and low total harmonic distortion (<14.3%). Moreover, the harmonic current of the LED converter reaches the IEC 61000-3-2 Class D standard at 100 VAC and 240 VAC input voltages for high-quality LED converters. Total Harmonic T Dis ott al or Har tionm [on %] ic Distortion [%] Maximum Pe Ma rm xiim ssum ible Pe rmissible Harmornic C Ha urre rmnt orni [A] c Current [A] 8.521 8.521 4.74 4.74 8.364 8.364 4.73 4.73 3.02 3.02 8.181 8.181 2.99 2.99 8.163 8.163 2.51 2.51 2.52 2.52 7.986 7.986 8.17 8.17 2.44 2.44 2.39 2.39 7.832 7.832 2.26 2.26 7.978 7.978 2.25 2.25 2.22 2.22 7.693 7.693 2.23 2.23 7.899 7.899 2.23 2.23 2.21 2.21 7.53 7.53 2.05 2.05 7.401 7.401 2.04 2.04 7.164 7.164 1.91 1.91 1.89 1.89 7.511 7.511 1.76 1.76 6.913 6.913 1.76 1.76 7.218 7.218 1.66 1.66 1.59 1.59 6.939 6.939 1.35 1.35 7.668 7.668 1.40 1.40 1.22 1.22 7.041 7.041 1.16 1.16 8.491 8.491 0.97 0.97 7.676 0.92 0.92 7.676 9.305 9.305 0.75 0.75 0.74 0.74 8.441 8.441 0.54 0.54 10.425 10.425 0.48 0.48 0.35 0.35 9.344 9.344 0.36 0.36 11.634 11.634 0.20 0.20 0.11 0.11 11.568 11.568 0.07 0.07 11.936 11.936 0.05 0.05 11.443 11.443 14.264 14.264 Harmonic C Ha urre rm nt o ni [m c A] Current [mA] Appl. 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A New Circuit Design of AC/DC Converter for T8 LED Tube

Applied Sciences , Volume 11 (1) – Jan 4, 2021

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Multidisciplinary Digital Publishing Institute
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2076-3417
DOI
10.3390/app11010421
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Abstract

applied sciences Article 1 2 , Sunghwan Kim and Haiyoung Jung * Electrical Engineering Department, Inha University, 100 Inharo, Nam-gu Incheon 22212, Korea; saint1119@naver.com Fire and Disaster Prevention Department, Semyung University, 65 Semyung-ro, Jecheon-si, Chungcheongbuk-do 27136, Korea * Correspondence: hyjung@semyung.ac.kr; Tel.: +82-43-649-1695 Abstract: This study is about an improved high-quality light-emitting diode (LED) converter for a T8 LED tube. The converter is separated into the AC driving circuit and DC driving circuit. Also, the LED tube was applied with an output ripple eliminator for the optical performance. The AC driving circuit and DC driving circuit are assembled at the end of the LED tube in a G13 base and a G13 base dummy, respectively, and the output ripple eliminator is located on an LED PCB. The proposed LED converter is founded on a SSBB (single-stage buck-boost) converter topology and was designed for 10 W operation for a 600 mm T8 LED tube. The light waveform of the LED tube was measured by a photosensor. The waveform had almost no ripple and was the same as a straight line. The average calculated percent flicker of the proposed LED converter was an average of 1.9% at 100 and 240 VAC, 50 and 60 Hz. The proposed converter has lower power efficiency than a conventional converter by 2.7% at 100–240 VAC, but it still has high power efficiency (>87%). The measurement results represent that the LED output current regulation is below 0.92% at 100–240 VAC and the converter obtains the power factor more than 0.84 and the total harmonic distortion is less than 14.3%. All of the current harmonics reach the IEC 61000-3-2 Class D standards for high-quality LED converters. Keywords: high circuit efficiency; photosensor; power factor correction; SSBB converter topology; ripple Citation: Kim, S.; Jung, H. A New Circuit Design of AC/DC Converter 1. Introduction for T8 LED Tube. Appl. Sci. 2021, 11, LEDs have a lot of characteristics such as high luminous efficacy, energy saving 421. https://doi.org/10.3390/app properties and long lifetimes. These advantages have allowed LED lighting to replace other types of lighting quickly in the marketplace. Furthermore, as regulations for the use of traditional lighting such as incandescent lamps and florescent lamps become increasingly Received: 25 November 2020 stronger, the market share of LED lighting is expected to increase rapidly [1–3]. Accepted: 18 December 2020 An LED lighting product generally consists of various mechanical parts, an LED Published: 4 January 2021 module and an LED converter. An LED converter should provide stable and accurate current to the LED module to achieve good optical performance, since the luminance Publisher’s Note: MDPI stays neu- variation of LEDs depends on the variation of the current supplied to LEDs. In addition, the tral with regard to jurisdictional clai- LED converter requires high power efficiency, high power factor (PF), low total harmonic ms in published maps and institutio- distortion (THD), low total cost, and low light flicker [4,5]. nal affiliations. The light quality of LED lighting is mainly determined by the driving method and the key components of the LED converter. Due to the low circuit complexity and low cost, a single-stage power factor correction (PFC) driving method is usually used in many Copyright: © 2021 by the authors. Li- types of low-power LED lightings, including LED tubes. However, the output current censee MDPI, Basel, Switzerland. inevitably has a double line frequency ripple and light flicker is generated by variation This article is an open access article of the luminance. Various studies show that low-frequency light flicker caused by large distributed under the terms and con- output current ripple can adversely affect human health and cause headaches, visual ditions of the Creative Commons At- fatigue and epileptic attacks [6–9]. tribution (CC BY) license (https:// Figure 1 shows a block diagram of a typical single-stage buck-boost LED converter, creativecommons.org/licenses/by/ which is composed of many function blocks. An AC voltage with 50/60 Hz is supplied to a 4.0/). Appl. Sci. 2021, 11, 421. https://doi.org/10.3390/app11010421 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 421 2 of 14 Appl. Sci. 2020, 10, x FOR PEER REVIEW 2 of 13 Figure 1 shows a block diagram of a typical single-stage buck-boost LED converter, which is composed of many function blocks. An AC voltage with 50/60 Hz is supplied to bridge rectifier through a line filter, and a full-wave rectified sinusoidal voltage is supplied a bridge rectifier through a line filter, and a full-wave rectified sinusoidal voltage is sup- to the DC link capacitor C . Switching is conducted for power factor correction to LINK plied to the DC link capacitor CLINK. Switching is conducted for power factor correction to obtain high power factor, and smoothing is performed to reduce the output ripple and obtain high power factor, and smoothing is performed to reduce the output ripple and control the output current [5]. control the output current [5]. AC Line Filter Rectifier Inductor Capacitor LINK Voltage Sensing Controller Switch Current Sensing Figure 1. The block diagram of typical single-stage buck-boost light-emitting diode (LED) con- Figure 1. The block diagram of typical single-stage buck-boost light-emitting diode (LED) converter. verter. The smoothing to remove the ripple is not perfect, so the single-stage PFC driving The smoothing to remove the ripple is not perfect, so the single-stage PFC driving method always makes a low-frequency output ripple, and output current varies between a method always makes a low-frequency output ripple, and output current varies between maximum peak and minimum peak [10]. If the light has cyclic variation in the amplitude, a maximum peak and minimum peak [10]. If the light has cyclic variation in the ampli- percent flicker is very useful to represent the level of light flicker, which is defined as [11]: tude, percent flicker is very useful to represent the level of light flicker, which is defined A B as [11]: Percent Flicker (%) =  100 (1) A + B 𝐴 − 𝐵 (1) where A and B are the maxi Pemum rcent and Flickminimum er (%) = luminance, × 100 respectively. The Equation (1) 𝐴 + 𝐵 shows that the low percent flicker represents good performance. Several methods can where A and B are the maximum and minimum luminance, respectively. The Equation be considered to improve the light flicker caused by the double line frequency output (1) shows that the low percent flicker represents good performance. Several methods can ripple [12–15]. One of the obvious ways is to increase the link capacitor C ; however, DC be considered to improve the light flicker caused by the double line frequency output regulatory requirements for PF and THD performance may not be met. Therefore, this ripple [12–15]. One of the obvious ways is to increase the link capacitor CDC; however, method is usually suitable for non-regulated low power LED lighting. Another method is to regulatory requirements for PF and THD performance may not be met. Therefore, this increase the output capacitance for the smoothing operation. However, a large electrolytic method is usually suitable for non-regulated low power LED lighting. Another method is capacitor is needed to remove the output ripple completely, which increases the system size to in and crea cost. se thTher e output c efore, a this paci is ta not ncesuitable for the sm foro small othinlighting g operati appli on. Ho cations wever with , a lsmall arge el cir ec cuit tro- space. lytic ca V pa arious citor converter is needed topologies to remove th have e outpu been t pr rippl oposed e com to plrete emove ly, wh curr ich ent incr ripple eases [th 16 e –sy 18s- ]. Item si n [16],za e fa li n cd k e co r-f st re . e Ther LED efc oo re n,v th erits er is no com t sui pose ta dbo le f P fo FrC sm fly ab lla li ck gh cti on nv g a erppl ter a ica nd tia on bs wi idire th ct i sm ona al ll b ci urc ck ui /b t o sp os atce co . n Va ve ri rt o eus r to co re nd ve uc rte e tr hto e c pol urro egi ntes rip hp alv ee w ba ee s n p rpr opo opo sed sed to t ro ed re uc m eo th ve e c cur urr re en ntt rri ip pp ple le . T [1 h6 e– o 1u 8t]p . u In t c [ u 1r6 r] e,n a t w fli acker vefo- rfm ree is L alE m D osco t fn la v ter , w ter hi le co hm igpo h p sed ow eo rfe PF fficC ien flcyb y ia sck ma co inn ta v ier neter d [ 1a 6n ,1 d 7 ]a . However, due to the high circuit complexity, it cannot be applied to applications such bidirectional buck/boost converter to reduce the current ripple was proposed to reduce as T8 LED tubes. A multiplexing ripple cancellation was proposed by adding a ripple the current ripple. The output current waveform is almost flat, while high power effi- cancellation unit to achieve flicker-free operation in [18]. Although this solution is very ciency is maintained [16,17]. competitive, it is still too large to mount inside a G13 base with a diameter of 28 mm However, due to the high circuit complexity, it cannot be applied to applications such for low-power LED tubes [19]. In this paper, an LED converter with an output ripple as T8 LED tubes. A multiplexing ripple cancellation was proposed by adding a ripple eliminator is proposed to remove the double line frequency flicker of a T8 LED tube, while cancellation unit to achieve flicker-free operation in [18]. Although this solution is very providing high power efficiency, high PF, low THD and precise output current regulation. competitive, it is still too large to mount inside a G13 base with a diameter of 28 mm for Due to the addition of the output ripple eliminator, the power efficiency has been slightly low-power LED tubes [19]. In this paper, an LED converter with an output ripple elimi- reduced by an average of 2.7%. Nevertheless, the power efficiency is still more than 87% nator is proposed to remove the double line frequency flicker of a T8 LED tube, while and light flicker is almost removed. The proposed LED converter is separated into AC and providing high power efficiency, high PF, low THD and precise output current regulation. DC driving circuit and designed to mount in a G13 base and G13 base dummy. Because of Due to the addition of the output ripple eliminator, the power efficiency has been slightly the size limitations, we propose a practical method for product manufacturing by locating reduced by an average of 2.7%. Nevertheless, the power efficiency is still more than 87% the output ripple eliminator on the LED module. In Section 2, the theory of the output and light flicker is almost removed. The proposed LED converter is separated into AC and ripple eliminator operation is explained. In Section 3, design specifications of the proposed DC driving circuit and designed to mount in a G13 base and G13 base dummy. Because LED converter are represented in detail. In Section 4, a 10 W prototype is introduced and of the size limitations, we propose a practical method for product manufacturing by lo- experimental results are discussed. Finally, conclusions are given in Section 5. cating the output ripple eliminator on the LED module. In Section 2, the theory of the output ripple eliminator operation is explained. In Section 3, design specifications of the + Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 13 proposed LED converter are represented in detail. In Section 4, a 10 W prototype is intro- Appl. Sci. 2021, 11, 421 3 of 14 duced and experimental results are discussed. Finally, conclusions are given in Section 5. 2. The Proposed LED Converter for a T8 LED Tube 2. The Proposed LED Converter for a T8 LED Tube 2.1. Power Stage 2.1. Power Stage Figure 2a shows the overall stages of the proposed LED converter, which consists of a Figure 2a shows the overall stages of the proposed LED converter, which consists of power stage for constant current control and a ripple remove stage to reduce the output a power stage for constant current control and a ripple remove stage to reduce the output ripple. The power stage is physically divided into two driving circuit that are electrically ripple. The power stage is physically divided into two driving circuit that are electrically linked though the LED module [5]. Figure 2b shows a simplified circuit diagram of the linked though the LED module [5]. Figure 2b shows a simplified circuit diagram of the proposed LED converter with separated driving circuit for a T8 LED tube. proposed LED converter with separated driving circuit for a T8 LED tube. AC Driving Part LF CX VA VDC BD N C LINK1 GND (a) Power Stage Ripple free Stage AC Rectifier PFC & DC/DC Ripple Eliminator PFC Controller DC Driving Part LED– VDC L1 C CLINK2 LED+ CONTROLLER D GND (b) Figure 2. (a) Overall stages of the proposed LED converter; (b) simplified circuit diagram of AC and Figure 2. (a) Overall stages of the proposed LED converter; (b) simplified circuit diagram of AC DC driving circuit. and DC driving circuit. The proposed LED converter employs a single-stage buck-boost converter topology The proposed LED converter employs a single-stage buck-boost converter topology and consists of an AC driving circuit for rectification and a DC driving circuit for constant and consists of an AC driving circuit for rectification and a DC driving circuit for constant current control. The AC driving circuit consists of various filter components (CX, LF) for current control. The AC driving circuit consists of various filter components (CX, LF) for high frequency suppression, a full bridge rectifier (BD) and a DC link capacitor (C ). LINK1 high frequency suppression, a full bridge rectifier (BD) and a DC link capacitor (C LINK1). The DC driving circuit includes another DC link capacitor (C ), a power switch for LINK2 The DC driving circuit includes another DC link capacitor (CLINK2), a power switch for energy transfer (S), an inductor (L), a diode (D) and an output capacitor (C ) [5,20]. The full energy transfer (S), an inductor (L), a diode (D) and an output capacitor (C O) [5,20]. The bridge rectifier generates a positive full wave sinusoidal voltage from the AC line and full bridge rectifier generates a positive full wave sinusoidal voltage from the AC line and supplies it to the DC link capacitor in the AC driving circuit. The inductor is magnetized supplies it to the DC link capacitor in the AC driving circuit. The inductor is magnetized and stores rectified input energy from the AC driving circuit when the switch S turns on. and stores rectified input energy from the AC driving circuit when the switch S turns on. During the turn-off time, the inductor is demagnetized, and stored energy is trans- ferred to the output capacitor. In the single-stage driving method, the input voltage can be defined as: v (t) = V sin(2p f t) (2) I N in L + Appl. Sci. 2020, 10, x FOR PEER REVIEW 4 of 13 During the turn-off time, the inductor is demagnetized, and stored energy is trans- ferred to the output capacitor. In the single-stage driving method, the input voltage can Appl. Sci. 2021, 11, 421 4 of 14 be defined as: (2) ( ) 𝑣 (𝑡 ) = 𝑉 ∙ 2𝜋 𝑓 𝑡 𝐼𝑁 𝐿 where V is the amplitude of the input voltage and f is the line frequency of the input where 𝑉in is the amplitude of the input voltage and L 𝑓 is the line frequency of the input voltage. Since the controller is operated for power factor correction, the input current voltage. Since the controller is operated for power factor correction, the input current waveform follows the input voltage waveform. Therefore, the input current is defined as: waveform follows the input voltage waveform. Therefore, the input current is defined as: (3) i (t) = I sin(2p f t) (3) I N in L 𝑖 (𝑡 ) = 𝐼 ∙ (2𝜋 𝑓 𝑡 ) 𝐼𝑁 𝐿 where 𝐼 is the peak value of the input current. From Equations (1) and (2), the instanta- where I is the peak value of the input current. From Equations (1) and (2), the instanta- in neous output power can be expressed as [5,21]: neous output power can be expressed as [5,21]: (4) ( ) ( ) ( ) [ ( )] 𝑝 𝑡 = 𝜂 ∙ 𝑝 𝑡 = 𝜂 ∙ 𝑉 ∙ 𝐼 ∙ 2𝜋 𝑓 𝑡 = 𝑃 ∙ 1 − 4𝜋 𝑓 𝑡 𝑂𝑈𝑇 𝑓𝑓𝑒 𝐼𝑁 𝑓𝑓𝑒 𝑖𝑛 𝐿 𝑜 𝐿 p (t) = h  p (t) = h  V  I  sin (2p f t) = P [1 cos(4p f t)] (4) where 𝜂 is the expected power efficiency and 𝜂 ∙ 𝑉 ∙ 𝐼 o ⁄2is substituted with the OU T e f f I N e f f in in L L 𝑓𝑓𝑒 𝑓𝑓𝑒 amplitude of the output power 𝑃 . When 𝑉 is the output voltage, the instantaneous out- 𝑜 𝑜 where h is the expected power efficiency and h  V  I /2 is substituted with the e f f e f f in in put current can be expressed as: amplitude of the output power P . When V is the output voltage, the instantaneous output o o (5) current can be expressed as: 𝑖 (𝑡 ) = 𝐼 ∙ [1 − (4𝜋 𝑓 𝑡 )] 𝑈𝑇𝑂 𝑜 𝐿 where 𝐼 is the amplitude of the output current. The equation (5) shows that the single- i (t) = I [1 cos(4p f t)] (5) OU T L stage power factor correction driving method always contains double line frequency out- put ripple. If the output capacitance does not sufficiently smooth the output ripple, the where I is the amplitude of the output current. The equation (5) shows that the single-stage high and low amplitude current flowing through the LEDs generates light flicker [5,21– power factor correction driving method always contains double line frequency output 23]. ripple. If the output capacitance does not sufficiently smooth the output ripple, the high and low amplitude current flowing through the LEDs generates light flicker [5,21–23]. 2.2. Ripple Free Stage 2.2. Ripple Free Stage Figure 3 shows the output ripple eliminator of the proposed LED converter for alle- Figure 3 shows the output ripple eliminator of the proposed LED converter for alle- viating light flicker. An output ripple eliminator is a kind of common collector amplifier viating light flicker. An output ripple eliminator is a kind of common collector amplifier using an NPN Darlington configuration, which is one of the basic amplifier topologies, as using an NPN Darlington configuration, which is one of the basic amplifier topologies, as a voltage follower. It consists of 𝑅 , 𝐶 , and a Darlington transistor 𝑄 [24,25]. 𝐸 𝐸 a voltage follower. It consists of R , C , and a Darlington transistor Q [24,25]. E E OUT Collector Emitter LED V V CB BE Power Base LED OUT Stage LED C V E B R = LED LED Figure 3. The output ripple eliminator. Figure 3. The output ripple eliminator. Since the bias voltage of the base is immediately transferred to the LED anode, the Since the bias voltage of the base is immediately transferred to the LED anode, the impedance should be carefully chosen to find the optimized LED operation point in terms impedance should be carefully chosen to find the optimized LED operation point in terms of power losses and light flicker elimination. of power losses and light flicker elimination. Figure 4 shows conceptual waveforms of the proposed output ripple eliminator. To Figure 4 shows conceptual waveforms of the proposed output ripple eliminator. achieve a high power factor of the proposed LED converter, the output waveform inevi- To achieve a high power factor of the proposed LED converter, the output waveform tably contains a large ripple with double line frequency. This double frequency ripple is inevitably contains a large ripple with double line frequency. This double frequency ripple is passed through the power stage and transferred to the ripple removal stage. Since the output voltage V is constant with output ripple, it can be expressed as: V (t) = V + V (t) (6) O O_dc O_ac … … 𝑐𝑜𝑠 𝑖𝑛 𝑖𝑛 𝑖𝑛 𝑐𝑜𝑠 𝑠𝑖𝑛 𝑖𝑛 𝑖𝑛 𝑠𝑖𝑛 𝑖𝑛 𝑖𝑛 𝑠𝑖𝑛 Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 13 Appl. Sci. 2021, 11, 421 5 of 14 passed through the power stage and transferred to the ripple removal stage. Since the output voltage 𝑉 is constant with output ripple, it can be expressed as: (6) where V and V (t) are the DC component and AC component with double line O_dc O_ac 𝑉 (𝑡 ) = 𝑉 + 𝑉 (𝑡 ) 𝑂 𝑂 _ 𝑂 _ frequency ripple, respectively. Therefore, the base bias voltage of the Darlington pair has where 𝑉 and 𝑉 (𝑡 ) are the DC component and AC component with double line 𝑂 _ 𝑂 _ the same frequency ripple in steady state and the instantaneous value of bias voltage can frequency ripple, respectively. Therefore, the base bias voltage of the Darlington pair has be expressed as: the same frequency ripple in steady state and the instantaneous value of bias voltage can be expressed as: V (t) = V + V (t) = (V V ) + [V (t) V (t)] (7) B B_dc B_ac O_dc CB_dc O_ac CB_ac (7) 𝑉 (𝑡 ) = 𝑉 + 𝑉 (𝑡 ) = (𝑉 − 𝑉 ) + [𝑉 (𝑡 ) − 𝑉 (𝑡 )] 𝐵 𝐵 _ 𝐵 _ 𝑂 _ _ 𝑂 _ _ where V and V (t) are the DC component and AC component of the collector- CB_dc CB_ac where 𝑉 and 𝑉 (𝑡 ) are the DC component and AC component of the collector- _ 𝐶 𝐵 _ base forward voltage, 𝑎𝑐 respectively. Considering the base-emitter forward voltage in the base forward voltage, respectively. Considering the base-emitter forward voltage in the Darlington input stage, V applied across LEDs can be obtains as: LED Darlington input stage, 𝑉 applied across LEDs can be obtains as: 𝐿𝐸𝐷 V (t) = (V V ) + [V (t) V (t)] (8) LED B_dc BE_dc B_ac BE_ac (8) ( ) ( ) ( ) 𝑉 𝑡 = (𝑉 − 𝑉 ) + [𝑉 𝑡 − 𝑉 𝑡 ] 𝐿𝐸𝐷 𝐵 _ _ 𝐵 _ _ where V is the DC value, and V (t) is the AC value. Using Kirchhoff’s voltage law, BE_dc BE_ac where 𝑉 is the DC value, and 𝑉 (𝑡 ) is the AC value. Using Kirchhoff’s voltage _ _ the AC component of bias voltage V t is determined from X , Z and V t , ( ) ( ) B_ac E E O_ac ( ) law, the AC component of bias voltage 𝑉 𝑡 is determined from 𝑋 , 𝑍 and 𝑉 (𝑡 ), 𝐵 _ 𝐸 𝐸 𝑂 _ V (t) = V (t) (9) B_ac O_ac 𝐸 (9) 𝑉 (𝑡 ) = ∙ 𝑉 (𝑡 ) 𝐵 _ 𝑂 _ 2 2 2 2 where Z = √ R + X is the total impedance, and X = 1/(4p f C ) is the impedance where 𝑍 = 𝑅 + 𝑋 is the total impedance, and 𝑋 = 1/(4𝜋 𝑓 ∙ 𝐶 ) is the impedance E E E E L E 𝐸 𝐸 𝐸 𝐸 𝐿 𝐸 of C . If the base-emitter forward voltage V components are negligible, from Equations of 𝐶 . If the base-emitter forward voltage 𝑉 components are negligible, from Equations E BE (8) and (9), the LED voltage can be derived as: (8) and (9), the LED voltage can be derived as: V (t) = (V V ) + p V (t) (10(10) ) LED O_dc CB_dc O_ac 𝑉 (𝑡 ) = (𝑉 − 𝑉 ) + ∙ 𝑉 (𝑡 ) 𝐿𝐸𝐷 𝑂 _ _ 2 2 𝑂 _ R + X 2 2 E E 𝑅 + 𝑋 𝐸 𝐸 Equation (10) shows that the output ripple eliminator can provide a DC value to the LEDs Equation (10) shows that the output ripple eliminator can provide a DC value to the LEDs by adjusting the AC value to nearly zero, which means light flicker completely disappears by adjusting the AC value to nearly zero, which means light flicker completely disappears from the LED tube. from the LED tube. V AC component O_dc O_ac DC component CB CB_dc CB_ac AC component DC component LED AC Line One Cycle Figure 4. Conceptual waveforms of the output ripple eliminator. Figure 4. Conceptual waveforms of the output ripple eliminator. 3. Design Parameters 3. Design Parameters Key parameters and components were selected for the converter. Table 1 shows the Key parameters and components were selected for the converter. Table 1 shows the specifications of the proposed LED converter. It operates at 100–240 VAC and is designed specifications of the proposed LED converter. It operates at 100-240 VAC and is designed to cover 10% input variation. The nominal power consumption is 10 W with 10% to cover ± 10% input variation. The nominal power consumption is 10 W with ±10% toler- tolerance. The target variation of the output voltage and current are 106 V  4% and ance. The target variation of the output voltage and current are 106 V ± 4% and 87 mA ± 87 mA  6%, respectively. 6%, respectively. 𝑎𝑐 𝑑𝑐 𝐶𝐵 𝑑𝑐 𝐵𝐸 𝑎𝑐 𝑎𝑐 𝑎𝑐 𝑎𝑐 𝑎𝑐 𝐵𝐸 𝑑𝑐 𝐵𝐸 𝑎𝑐 𝐵𝐸 𝑎𝑐 𝑑𝑐 𝐵𝐸 𝑑𝑐 𝑑𝑐 𝐶𝐵 𝑎𝑐 𝐶𝐵 𝑎𝑐 𝑑𝑐 𝐶𝐵 𝑑𝑐 𝑎𝑐 𝑑𝑐 𝑎𝑐 𝑑𝑐 𝑎𝑐 𝑑𝑐 Appl. Sci. 2021, 11, 421 6 of 14 Table 1. Specifications of the proposed LED converter. 10 W LED Converter Item MIN TYP MAX 100 90 100 - Input Voltage (VAC) 240 - 240 264 Input Current (mA) 34.1 (@90 VAC) - 122.2 (@264 VAC) Input Frequency (Hz) 50 - 60 Input Power (W) 9.0 10.0 11.0 Output Voltage (V) 102 106 112 Output Current (mA) 81.8 87.0 92.2 3.1. Determining the Inductance: L In a discontinuous current mode(DCM) single-stage buck-boost converter, the switch- ing cycle time (T = 1/ f ) can be expressed as [5]: SW SW T = T + T + T (11) SW ON_S ON_D I D LE where T is the switch turn-on time, T is the diode on-time and T is the time, ON_S ON_D I D LE in which no current flows in the inductor. Since T cannot be zero to ensure DCM I D LE operation under all conditions, D and D should satisfy D + D < 1. Therefore, from D D S S the volt-second balance law, the inequality for D becomes: D < (12) V + V I N O where D is the duty ratio of the switch turn-on time (= T /T ) and D is the S SW ON_S D duty ratio of the diode turn-on time (= T /T ). The input current is induced as SW ON_D D T V /2L from the buck-boost relationship, which is substituted into the power sw I N conversion relationship (V  I = h  V  I ) and rearranged for the inductance L: O O e f f I N I N e f f V V O I N L <   (13) 2 f I V + V sw O I N O where f is the switching frequency. From Equation (13), the minimum inductance can SW be determined at V , V and f = 70 k Hz. I N_M I N O_M AX SW_M AX 0.9 112 90 L <  = 1.55 m H (14) 2 70 k 92.2 m 90 + 112 Considering the size of inductor and margin, an inductance of 1.38 mH should be chosen and manufactured with an EE1616 core. 3.2. Determining the Output Capacitance: C A small output ripple requires a large output capacitance; however, the higher output capacitance, the bigger the capacitor size. Therefore, a proper percent flicker specifica- tion without an output ripple eliminator should be considered to determine the output capacitance while considering voltage stress and size. The output capacitance C has the following relationship [5,24]: I 2 I C_PP LED C   (15) 4p f V 4p f N DV L L C_PP S LED_Device Appl. Sci. 2021, 11, 421 7 of 14 where I is the peak-to-peak current of the output capacitor, which is approximately C_PP twice the average LED current I . Here, DV is the one LED peak-to-peak, LED LED_Device which varies with the LED current and N is the number of LEDs in series. Referring to Equation (15), the minimum output capacitance can be estimated with I of 92.2 mA LED_M AX and f of 50 Hz. In addition, to keep the light flicker around 10% without output ripple L_M I N eliminator, DV of 95 mV should be used according to V-I characteristics [26]. LED_Device 2 92.2 m C > = 88.3 F (16) 4 3.14 50 35 95 m For practical design for a T8 LED tube, C should be chosen as 160 V/100 F with dimensions of 12  25 mm (D  H). 3.3. Determining the Power Device: S, D Both the voltage and current stress should be calculated to select an appropriate device. Since the voltage stress of switch is V + V in a buck-boost converter topology, I N O the maximum voltage stress is determined at V and V . Therefore, the voltage I N_M AX O_M AX stress of the switch can be calculated as: p p V = 2V + V = 2264 + 112 = 485 V (17) S_M AX I N_M AX O_M AX From Table 1, when an input voltage of 90 VAC is provided to an 11 W system, the maximum RMS current stress of the switch when considering the expected power efficiency is: P 11 I N_M AX I = = = 136 m A (18) S_R MS_M AX V h 90 0.9 I N_M I N e f f The voltage stress of the diode is the same as V and the maximum average S_M AX current of the diode is: 2 I 2 92.2 m O_M AX I = = = 419 m A (19) D_AV G_M AX D 0.44 Therefore, an 800 V/2.5 A MOSFET switch (STP3NK80) and 1000 V/1 A diode (US1M) were selected to consider the component stress and margin with heat dissipation. 3.4. Determining the Output Ripple Eliminator: R , C , Q E E The voltage and current relation is almost linear in driving region of LED IV curve [26], so the dynamic resistance of the LED device can be defined as: LED_Device R =  3.23 W (20) LED_Device LED_Device To achieve a percent flicker under 1%, the current variation D I of the LED LED_Device device should be within 1.4 mA according to the luminance-current characteristics [26]. Consequently, the allowed LED device voltage DV is 3.23  1.4 m = 4.5 mV. LED_Device Therefore, when the peak-to-peak output voltage DV (t) is 3.33 V, the bias voltage O_ac V (t) should be 157.5 mV in a series of 35 LEDs. From Equation (9), X /Z is obtained as: B_ac E E X DV 0.158 E B_ac p = = = 0.047 (21) 2 2 DV 3.33 R + X O_ac E E A small capacitor can be used to remove the low frequency ripple, and C can be implemented with a 100 V/1 F chip capacitor. Since the capacitor impedance X is 1.59 kW at 60 Hz, R should be: R = X  33 kW (22) E E 0.047 Appl. Sci. 2021, 11, 421 8 of 14 The output ripple eliminator greatly reduces the light flicker but at the same time, additional power loss occurs in Darlington transistor Q. The power loss of Q can be estimated as (V /2 + V ) I , where the V is peak-to-peak voltage across LED_PP BE LED LED_PP the LED array and I is the average current flowing to the LED array [24,27]. LED 35 95 m P = + 1.2  92.2 m = 264 mW (23) Q_M AX Considering Equation (23) and heat dissipation, an MJF6388 Darlington transistor is selected to maintain stable operation. 4. Experimental Results A 10 W prototype of the LED converter was designed to perform the experiments, as Appl. Sci. 2020, 10, x FOR PEER REVIEW 8 of 13 shown in Figure 5. The input voltage is 100–240 VAC and the line frequency is 50 Hz and 60 Hz. The output current is 87 mA, and output voltage is 106 V with 35 LEDs in series to 60 Hz. The output current is 87 mA, and output voltage is 106 V with 35 LEDs in series to satisfy the luminous uniformity. Table 2 shows the specifications of the key components. satisfy the luminous uniformity. Table 2 shows the specifications of the key components. Figure 5. (a) AC driving circuit, (b) DC driving circuit, (c) LED module with the output ripple Figure 5. (a) AC driving circuit, (b) DC driving circuit, (c) LED module with the output ripple eliminator. eliminator. Table 2. Specifications of key components. Table 2. Specifications of key components. Fuse 250 V, 1 A Fuse 250 V, 1 A Line Filter TOROIDAL, 31 uH, 4 A Line Filter TOROIDAL, 31 uH, 4 A X-capacitor 275 VAC, 100 nF X-capacitor 275 VAC, 100 nF AC Driving Circuit AC Driving Circuit Bridge BridgeDiode Diode VRRM VRRM= =1000 1000V V, , IO IO = =1 1A A Film Capacitor 600 V, 100 nF Film Capacitor 600 V, 100 nF PCB CEM-3, 1T, 0.5 oz, Single Layer PCB CEM-3, 1T, 0.5 oz, Single Layer Film Capacitor 630 V, 100 nF Film Capacitor 630 V, 100 nF Inductor EE1616, 1.38 mH ElectrInductor olytic Capacitor 400 EE1616, V, 221.38 uF, mH 105℃ VDSS = 800 V, ID = 2.5A, RDS(ON) < Electrolytic Capacitor 400 V, 22 uF, 105 C DC Driving Circuit FET 4.5Ω VDSS = 800 V, ID = 2.5A, DC Driving Circuit Dio FET de VRRM = 1000 V, IO = 1 A RDS(ON) < 4.5 W PCB FR-4, 1T, 0.5oz, Double Layer Diode VRRM = 1000 V, IO = 1 A Darlington Transistor NPN, VCE 100 V, IC = 10 A PCB FR-4, 1T, 0.5 oz, Double Layer Figure 6 shows the input voltage and current waveform of the proposed LED con- Darlington Transistor NPN, VCE 100 V, IC = 10 A verter. The input current waveform follows the input voltage, which means it operates properly for power factor correction. Since the PFC operation is not affected by the output Figure 6 shows the input voltage and current waveform of the proposed LED converter. ripple eliminator, there is no difference in the power factor between the reference con- The input current waveform follows the input voltage, which means it operates properly verter and the proposed converter. for power factor correction. Since the PFC operation is not affected by the output ripple eliminator , there is no difference in the power factor between the reference converter and the proposed converter. Appl. Sci. 2020, 10, x FOR PEER REVIEW 8 of 13 60 Hz. The output current is 87 mA, and output voltage is 106 V with 35 LEDs in series to satisfy the luminous uniformity. Table 2 shows the specifications of the key components. Figure 5. (a) AC driving circuit, (b) DC driving circuit, (c) LED module with the output ripple eliminator. Table 2. Specifications of key components. Fuse 250 V, 1 A Line Filter TOROIDAL, 31 uH, 4 A X-capacitor 275 VAC, 100 nF AC Driving Circuit Bridge Diode VRRM = 1000 V, IO = 1 A Film Capacitor 600 V, 100 nF PCB CEM-3, 1T, 0.5 oz, Single Layer Film Capacitor 630 V, 100 nF Inductor EE1616, 1.38 mH Electrolytic Capacitor 400 V, 22 uF, 105℃ VDSS = 800 V, ID = 2.5A, RDS(ON) < DC Driving Circuit FET 4.5Ω Diode VRRM = 1000 V, IO = 1 A PCB FR-4, 1T, 0.5oz, Double Layer Darlington Transistor NPN, VCE 100 V, IC = 10 A Figure 6 shows the input voltage and current waveform of the proposed LED con- verter. The input current waveform follows the input voltage, which means it operates properly for power factor correction. Since the PFC operation is not affected by the output ripple eliminator, there is no difference in the power factor between the reference con- Appl. Sci. 2021, 11, 421 9 of 14 verter and the proposed converter. Appl. Sci. 2020, 10, x FOR PEER REVIEW 9 of 13 Figure 6. Input voltage waveform (red) and input current waveform (blue) (a) at 100 VAC/60 Hz, Figure 6. Input voltage waveform (red) and input current waveform (blue) (a) at 100 VAC/60 Hz, (b) 100 VAC/50 Hz, (c) 240 VAC/50 Hz, (d) 240 VAC/50 Hz. (b) 100 VAC/50 Hz, (c) 240 VAC/50 Hz, (d) 240 VAC/50 Hz. Figure 7 shows the measured waveform of the output voltage, output current, and Figure 7 shows the measured waveform of the output voltage, output current, and light output at 240 VAC/50 Hz. The light output waveform measured with the photosen- light output at 240 VAC/50 Hz. The light output waveform measured with the photosen- sor amplifier contains a double line frequency ripple in the reference converter without sor amplifier contains a double line frequency ripple in the reference converter without an output ripple eliminator. However, with the proposed converter, the light flicker is an output ripple eliminator. However, with the proposed converter, the light flicker is completely removed and the result is almost a straight line. completely removed and the result is almost a straight line. Figure 7. Waveform of the output voltage (dark yellow), output current (blue) and light output Figure 7. Waveform of the output voltage (dark yellow), output current (blue) and light (green) of the (a) reference converter, (b) proposed converter. output (green) of the (a) reference converter, (b) proposed converter. Using Equation (1), the percent flicker can be calculated in each input condition. Using Equation (1), the percent flicker can be calculated in each input condition. Ta- Table 3 shows the calculated percent flicker of the reference and the proposed converter ble 3 shows the calculated percent flicker of the reference and the proposed converter which are 13.5% and 1.9%, respectively. When using the output ripple eliminator, the light which are 13.5% and 1.9%, respectively. When using the output ripple eliminator, the light flicker almost disappeared, and the percent flicker of the proposed converter with the flicker almost disappeared, and the percent flicker of the proposed converter with the out- output ripple eliminator was improved by 11.6% on average. put ripple eliminator was improved by 11.6% on average. Table 3. Comparison of percent flicker. Reference Proposed Input Light Output (mV) Percent Light Output (mV) Percent Flicker Flicker VAC Hz MAX AVG MIN MAX AVG MIN (%) (%) 50 450 393 336 14.5 401 393 385 2.0 60 443 394 345 12.5 402 395 388 1.8 50 451 394 337 14.5 401 393 385 2.0 60 443 394 345 12.4 400 393 386 1.8 Appl. Sci. 2021, 11, 421 10 of 14 Table 3. Comparison of percent flicker. Reference Proposed Input Percent Percent Light Output (mV) Light Output (mV) Flicker Flicker VAC Hz MAX AVG MIN MAX AVG MIN (%) (%) 50 450 393 336 14.5 401 393 385 2.0 60 443 394 345 12.5 402 395 388 1.8 50 451 394 337 14.5 401 393 385 2.0 60 443 394 345 12.4 400 393 386 1.8 Figure 8 shows power efficiency and efficiency difference according to the input voltage measured at 50 Hz. There is no big variation according to the line frequency. The power efficiency of the proposed converter is 2.6–2.9% lower than that of the reference, which means the output ripple eliminator consumes less than 2.6–2.9% of power. Although the total power increases 2.7% on average, the power efficiency is still more than 87% in 100–240 VAC, and the light flicker can be completely removed. Appl. Sci. 2020, 10, x FOR PEER REVIEW 10 of 13 Figure 8. Power efficiency and efficiency difference according to the input voltage. Figure 8. Power efficiency and efficiency difference according to the input voltage. Figure 9 shows the measured output current of the reference and proposed LED con- Figure 8 shows power efficiency and efficiency difference according to the input volt- verters, which was measured using an LED module with 35 LEDs in series. The measured age measured at 50 Hz. There is no big variation according to the line frequency. The data of the proposed LED converter is almost the same as that of reference, which shows power efficiency of the proposed converter is 2.6–2.9% lower than that of the reference, that the output current regulation is hardly affected by the output ripple eliminator. The av- which means the output ripple eliminator consumes less than 2.6–2.9% of power. Alt- erage output current and its standard deviation were 87.1 mA and 0.19 mA for the proposed hough the total power increases 2.7% on average, the power efficiency is still more than LED converter at 60 Hz, respectively. The positive and negative maximum deviations from 87% in 100–240 VAC, and the light flicker can be completely removed. the average output current were measured as +0.3 mA (0.34%) and 0.2 mA (0.23%) at Figure 9 shows the measured output current of the reference and proposed LED con- 60 Hz. The variation of the output current is less than 1%, which is great performance, verters, which was measured using an LED module with 35 LEDs in series. The measured since the power consumption tolerance of the LED tube is usually 10%. data of the proposed LED converter is almost the same as that of reference, which shows that the output current regulation is hardly affected by the output ripple eliminator. The average output current and its standard deviation were 87.1 mA and 0.19 mA for the pro- posed LED converter at 60 Hz, respectively. The positive and negative maximum devia- tions from the average output current were measured as + 0.3 mA (0.34%) and –0.2 mA (0.23%) at 60 Hz. The variation of the output current is less than 1%, which is great per- formance, since the power consumption tolerance of the LED tube is usually ± 10%. Figure 9. Output current according to the input voltage. Figure 10 shows the measured power factor in 100–240 VAC at 60 Hz. There is no large difference between the reference and proposed LED converters, which means the variation of the power factor is not affected by the output ripple eliminator. Due to the Appl. Sci. 2021, 11, 421 11 of 14 Appl. Sci. 2020, 10, x FOR PEER REVIEW 11 of 14 60Hz_Reference 60Hz_Proposed 100.0 95.0 90.0 87.3 87.3 87.3 87 87.1 87.2 87.2 87.2 87.2 87.2 87.1 86.9 86.9 86.9 86.9 87.2 87.2 87.2 87.2 85.0 87 87.1 87.1 87.1 87.1 87.1 87 86.8 86.8 86.8 86.9 80.0 75.0 70.0 65.0 60.0 100 120 140 160 180 200 220 240 Input Voltage [VAC] Figure 9. Output current according to the input voltage. Figure 9. Output current according to the input voltage. Figure 10 shows the measured power factor in 100–240 VAC at 60 Hz. There is no large difference between the reference and proposed LED converters, which means the Appl. Sci. 2020, 10, x FOR PEER REVIEW 11 of 13 Figure 10 shows the measured power factor in 100–240 VAC at 60 Hz. There is no variation of the power factor is not affected by the output ripple eliminator. Due to the large difference between the reference and proposed LED converters, which means the influence of the input current harmonics, it slightly decreases as the input voltage increases. influence of the input current harmonics, it slightly decreases as the input voltage in- variation of the power factor is not affected by the output ripple eliminator. Due to the Nevertheless, the power factor is still higher than 0.84 under all input voltages. creases. Nevertheless, the power factor is still higher than 0.84 under all input voltages. influence of the input current harmonics, it slightly decreases as the input voltage in- creases. Nevertheless, the power factor is still higher than 0.84 under all input voltages. Figure 10. PF at 60 Hz. Figure 10. PF at 60 Hz. Figure 10. PF at 60 Hz. Figur Figure e 11 11shows showsthe the THD THDof ofthe the pr pr oposed oposedLED LED co converter nverterat at 5 500Hz Hzand and60 60Hz, Hz,which which is less than 14.3% under all input voltages. The values of THD at 100 VAC are 8.52% and is less than 14.3% under all input voltages. The values of THD at 100 VAC are 8.52% and Figure 11 shows the THD of the proposed LED converter at 50 Hz and 60 Hz, which 8.36% at 50 Hz and 60 Hz, respectively. At 240 VAC, the values are 11.44% at 50 Hz and 8.36% at 50 Hz and 60 Hz, respectively. At 240 VAC, the values are 11.44% at 50 Hz and is less than 14.3% under all input voltages. The values of THD at 100 VAC are 8.52% and 14.26% at 60 Hz, respectively. The measured results are slightly high at 60 Hz, but they are 14.26% at 60 Hz, respectively. The measured results are slightly high at 60 Hz, but they 8.36% at 50 Hz and 60 Hz, respectively. At 240 VAC, the values are 11.44% at 50 Hz and very suitable for a high-quality LED tube. are very suitable for a high-quality LED tube. 14.26% at 60 Hz, respectively. The measured results are slightly high at 60 Hz, but they are very suitable for a high-quality LED tube. Figure 11. Total harmonic distortion (THD) according to the input voltage. Figure 12 shows the measured results of the harmonic current at 100 VAC at 50 Hz and 60 Hz. The maximum permissible harmonic current from the IEC 61000-3-2 Class D standard for less than 25 W of lighting is presented with a gray bar [28,29]. The harmonic current is markedly lower than the IEC standards. Output Current [mA] Appl. Sci. 2020, 10, x FOR PEER REVIEW 12 of 14 Appl. Sci. 2021, 11, 421 12 of 14 50Hz_Proposed 60Hz_Proposed Appl. Sci. 2020, 10, x FOR PEER REVIEW 12 of 14 50Hz_Proposed 60Hz_Proposed 20 14 18 12 16 10 14 8 12 6 10 4 8 2 6 0 100 120 140 160 180 200 220 240 Input Voltage [VAC] Figure 11. Total harmonic distortion (THD) according to the input voltage. 100 120 140 160 180 200 220 240 Input Voltage [VAC] Figure 12 shows the measured results of the harmonic current at 100 VAC at 50 Hz Figure 11. Total harmonic distortion (THD) according to the input voltage. and 60 Hz. The maximum permissible harmonic current from the IEC 61000-3-2 Class D Figure 11. Total harmonic distortion (THD) according to the input voltage. standard for less than 25 W of lighting is presented with a gray bar [28,29]. The harmonic Figure 12 shows the measured results of the harmonic current at 100 VAC at 50 Hz current is markedly lower than the IEC standards. Figure 12 shows the measured results of the harmonic current at 100 VAC at 50 Hz and 60 Hz. The maximum permissible harmonic current from the IEC 61000-3-2 Class D and 60 Hz. The maximum permissible harmonic current from the IEC 61000-3-2 Class D standard for less than 25 W of lighting is presented with a gray bar [28,29]. The harmonic standard for less than 25 W of lighting is presented with a gray bar [28,29]. The harmonic current is markedly lower than the IEC standards. current is markedly lower than the IEC standards. Limits for IEC61000-3-2 Class D 100VAC_50Hz_Reference 100VAC_60Hz_Reference 3.00 20.00 2.70 18.00 Limits for IEC61000-3-2 Class D 100VAC_50Hz_Reference 100VAC_60Hz_Reference 2.40 16.00 3. 2.00 10 20. 14.00 00 2. 1.70 80 18. 12.00 00 1.50 10.00 2.40 16.00 1.20 8.00 2.10 14.00 0.90 6.00 1.80 12.00 1. 0.50 60 10. 4.00 00 1. 0.20 30 8. 2.00 00 0.00 0.00 0.90 6.00 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 0.60 4.00 Harmonic Order 0.30 2.00 Figure 12. Harmonic current at 100 VAC 50/60 Hz. 0.00 0.00 Figure 12. Harmonic current at 100 VAC 50/60 Hz. 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 5. Conclusions Harmonic Order 5. Conclusions In this study, an AC/DC LED converter has been proposed to remove the flicker of a T8In LED this study, tube. The anpr Aoposed C/DC LLED ED co converter nverter ha uses s bee an n pr output oposed ripple to reeliminato move the r fli and cker a o SSBB f a Figure 12. Harmonic current at 100 VAC 50/60 Hz. converter topology for the input voltage 100–240 VAC. A 10 W prototype was designed T8 LED tube. The proposed LED converter uses an output ripple eliminator and a SSBB to verify the high performance. The calculated percent was 1.9% at all input conditions. converter topology for the input voltage 100–240 VAC. A 10 W prototype was designed 5. Conclusions The power efficiency is lower than that of a conventional converter by 2.7% at 100–240 VAC, to verify the high performance. The calculated percent was 1.9% at all input conditions. In this study, an AC/DC LED converter has been proposed to remove the flicker of a but it is still high (>87% and even 89% at 220 VAC). The experimental results represented The power efficiency is lower than that of a conventional converter by 2.7% at 100–240 T8 LED tube. The proposed LED converter uses an output ripple eliminator and a SSBB the LED output current regulation as less than 0.92% at 100–240 VAC and the LED converter VAC, but it is still high (>87% and even 89% at 220 VAC). The experimental results repre- converter topology for the input voltage 100–240 VAC. A 10 W prototype was designed has a high power factor (>0.84) and low total harmonic distortion (<14.3%). Moreover, sented the LED output current regulation as less than 0.92% at 100–240 VAC and the LED to verify the high performance. The calculated percent was 1.9% at all input conditions. the harmonic current of the LED converter reaches the IEC 61000-3-2 Class D standard at converter has a high power factor (>0.84) and low total harmonic distortion (<14.3%). The power efficiency is lower than that of a conventional converter by 2.7% at 100–240 100 VAC and 240 VAC input voltages for high-quality LED converters. Moreover, the harmonic current of the LED converter reaches the IEC 61000-3-2 Class D VAC, but it is still high (>87% and even 89% at 220 VAC). The experimental results repre- standard at 100 VAC and 240 VAC input voltages for high-quality LED converters. sented the LED output current regulation as less than 0.92% at 100–240 VAC and the LED converter has a high power factor (>0.84) and low total harmonic distortion (<14.3%). Moreover, the harmonic current of the LED converter reaches the IEC 61000-3-2 Class D standard at 100 VAC and 240 VAC input voltages for high-quality LED converters. Total Harmonic T Dis ott al or Har tionm [on %] ic Distortion [%] Maximum Pe Ma rm xiim ssum ible Pe rmissible Harmornic C Ha urre rmnt orni [A] c Current [A] 8.521 8.521 4.74 4.74 8.364 8.364 4.73 4.73 3.02 3.02 8.181 8.181 2.99 2.99 8.163 8.163 2.51 2.51 2.52 2.52 7.986 7.986 8.17 8.17 2.44 2.44 2.39 2.39 7.832 7.832 2.26 2.26 7.978 7.978 2.25 2.25 2.22 2.22 7.693 7.693 2.23 2.23 7.899 7.899 2.23 2.23 2.21 2.21 7.53 7.53 2.05 2.05 7.401 7.401 2.04 2.04 7.164 7.164 1.91 1.91 1.89 1.89 7.511 7.511 1.76 1.76 6.913 6.913 1.76 1.76 7.218 7.218 1.66 1.66 1.59 1.59 6.939 6.939 1.35 1.35 7.668 7.668 1.40 1.40 1.22 1.22 7.041 7.041 1.16 1.16 8.491 8.491 0.97 0.97 7.676 0.92 0.92 7.676 9.305 9.305 0.75 0.75 0.74 0.74 8.441 8.441 0.54 0.54 10.425 10.425 0.48 0.48 0.35 0.35 9.344 9.344 0.36 0.36 11.634 11.634 0.20 0.20 0.11 0.11 11.568 11.568 0.07 0.07 11.936 11.936 0.05 0.05 11.443 11.443 14.264 14.264 Harmonic C Ha urre rm nt o ni [m c A] Current [mA] Appl. 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