5508 ieee transactions on power electronics, vol. 29,...

5508 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 29, NO. 10, OCTOBER 2014

Feed-Forward Scheme for an ElectrolyticCapacitor-Less AC/DC LED Driver to Reduce

Output Current RippleYang Yang, Xinbo Ruan, Senior Member, IEEE, Li Zhang, Student Member, IEEE, Jiexiu He, and Zhihong Ye

Abstract—In order to achieve high-efficiency, high-power-factor,high-reliability, and low-cost, a flicker-free electrolytic capacitor-less single-phase ac/dc light emitting diode (LED) driver isinvestigated in this paper. This driver is composed of a power-factor-correction (PFC) converter and a bidirectional converter.The bidirectional converter is used to absorb the second harmoniccomponent in the output current of the PFC converter, thus pro-ducing a pure dc output to drive the LEDs. The spectrum of theoutput capacitor voltage of the bidirectional converter is analyzed,indicating that the output capacitor voltage contains harmoniccomponents at multiples of twice the line frequency apart from thedc component and second harmonic component. A feed-forwardcontrol scheme with a series of calculation operation is proposedto obtain the desired modulation signal, which contains the corre-sponding harmonic components, to ensure the bidirectional con-verter fully absorb the second harmonic current in the output ofthe PFC converter. Finally, a 33.6 W prototype is fabricated andtested in the lab, and the experiment results show that the proposedcontrol scheme greatly reduces the ripple of the LED driving cur-rent.

Index Terms—AC/DC LED driver, bidirectional converter, feed-forward control scheme, flicker, light emitting diode (LED), powerfactor correction (PFC).

I. INTRODUCTION

L IGHTING is responsible for about 20% of the global elec-tricity consumption, which motivates to develop energy-

efficient and environmental-friendly light source [1]. As thefourth-generation light source following incandescent lamps,fluorescent lamps and high-intensity discharge lamps, lightemitting diodes (LEDs) has the potential virtues of high effi-ciency, long lifetime, small volume, high brightness, rich color,etc., and it has been widely used in indoor and outdoor light-ing, backlighting of LCD panels, healthcare, and transporta-

Manuscript received July 8, 2013; revised October 11, 2013; acceptedNovember 18, 2013. Date of current version May 30, 2014. This work wassupported by Lite-On Technology Corp. Recommended for publication by As-sociate Editor R.-L. Lin.

Y. Yang, X. Ruan, L. Zhang, and J. He are with the Aero-Power Sci-techCenter, College of Automation Engineering, Nanjing University of Aeronau-tics and Astronautics, Nanjing 210016, China (e-mail: [email protected]; [email protected]; [email protected]; [email protected]).

Z. Ye is with Power SBG ATD-NJ R&D Center, Lite-On Technology Corp.,Nanjing 210019, China (e-mail: [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TPEL.2013.2293353

tion [2]–[5]. LED drivers can be realized in various approaches,such as resistor-based current limiter, linear regulator, chargepump, and switching-mode power supply. Compared with otherapproaches, the switching-mode power supply can realize con-stant current control and dimming function with high efficiency.Therefore, it is preferred for the high-power LED driver [6].

For single-phase LED drivers, power factor correction (PFC)technique must be incorporated to achieve high input powerfactor to meet relevant harmonic standards, for example,IEC61000-3-2 [7]. If the input power factor is unity, the in-put current is a pure sinusoidal waveform that is in phase withthe input voltage. Thus, the input power pulsates at twice theline frequency [8]. The LED driver needs to output a constantpower for avoiding the flicker phenomenon. Therefore, a storagecapacitor with large capacitance has to be employed to balancethe instantaneous power difference between the pulsating inputpower and the constant output power. The electrolytic capac-itor is preferred to be used as the storage capacitor. However,the lifetime of electrolytic capacitors is usually about 10 000 h.It is relatively shorter than the lifetime of the LED, which isgenerally about 80 000−100 000 h [9]. Hence, the electrolyticcapacitor is the main component that limits the lifetime of LEDdrivers.

In order to extend the lifetime of LED drivers, the electrolyticcapacitor should be eliminated. There are several methods ofremoving the electrolytic capacitor. The first one is adoptingmagnetic components as the storage component instead of theelectrolytic capacitor [10], [11]. Unfortunately, magnetic com-ponents suffer from low energy storage density compared to thecapacitor, leading to a lower power density. Moreover, the sys-tem efficiency is relatively low due to the large magnetic coreloss and winding loss.

In public or road light systems, pulsating current can be usedto drive LEDs. By doing so, the output power will be pulsating.Consequently, the output power will be close to the pulsatinginput power, which allows much smaller storage capacitor tohandle the instantaneous power difference [15]–[18]. However,this approach is not suitable for applications where tight LEDpower control is required.

Another method is to reduce the storage capacitor by min-imizing the power difference to be handled by the capacitorbetween the input power and output power. In [12] and [13], thethird and fifth harmonics are injected into the input current toreduce the peak-to-average ratio of the input pulsating power.Similarly, intentionally increasing the voltage loop crossoverfrequency of the PFC converter to make the input current

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YANG et al.: FEED-FORWARD SCHEME FOR AN ELECTROLYTIC CAPACITOR-LESS AC/DC LED DRIVER 5509

Fig. 1. Circuit diagram of the flicker-free electrolytic capacitor-less ac/dc LEDdriver.

distorted also can attain this objective [14]. However, such meth-ods lead to a reduced input power factor.

It is known that a small capacitor with large voltage ripple canplay the same role as the energy storage component as a bulkyelectrolytic capacitor with small voltage ripple. Therefore, thestorage capacitor can be also reduced by intentionally increasingthe voltage ripple on the capacitor. In [19], a ripple port isinserted into the LED driver to handle the instantaneous powerdifference between the input power and the output power. As thevoltage ripple on the storage capacitor is intentionally designedto be quite large, the storage capacitor can be quite small, whichavoids the utilization of electrolytic capacitors. However, as theinput power under this approach is not directly transferred tothe load, the conversion efficiency of the proposed LED driveris not high. In [20], a bidirectional converter is proposed tobe equipped in parallel with the output capacitor to filter thesecond harmonic current in the output current of the single-phaserectifier, thus the output capacitor can be reduced. Furthermore,the storage capacitor in the bidirectional converter is also quitesmall as the voltage ripple on it is quite large. Similarly, a flicker-free electrolytic capacitor-less ac/dc LED driver is proposedin [21]. Fig. 1 shows the circuit diagram of this LED driver,which comprises an electrolytic capacitor-less PFC converter,a bidirectional converter and an LC filter composed of a filtercapacitor Co and a filter inductor Lo . The LC filter is used to filteronly the current harmonics at the switching frequency ratherthan the ac component pulsating at twice the line frequency(hereinafter referred to as second harmonic current), thus Co

can be very small and no electrolytic capacitors are needed.The bidirectional converter performs as an active power filter toabsorb the second harmonic current in the output current of thePFC converter i′o . Therefore, by controlling the input current ofthe bidirectional converter ib to be equal to the second harmoniccurrent in i′o , the output driving current io will be a pure dc one.Thus, the LEDs will operate without flicker. To avoid utilizingbulky electrolytic capacitor, the output capacitor in bidirectionalconverter Cdc is intentionally designed to have a large voltageripple.

Although the proposed solutions in [20] and [21] can effec-tively reduce the second harmonic current in the output current,the magnitude of the second harmonic current can be still highif the voltage ripple on the storage capacitor is too large forthe sake of reducing the storage capacitor. In this paper, basedon the flicker-free electrolytic capacitor-less ac/dc LED driverpresented in [21], the spectrum of the output capacitor volt-age of the bidirectional converter is analyzed provided that the

second harmonic current is completely absorbed by the bidirec-tional converter. It is revealed that the output capacitor voltagecontains harmonic components at multiples of twice the line fre-quency apart from the dc and the second harmonic components.Therefore, the corresponding harmonic components should beintroduced into the modulation signal of the bidirectional con-verter for fully absorbing the second harmonic current in theoutput of the PFC converter. When the bidirectional converteradopts the traditional dual loop control, the modulation signalis obtained by the output of the current regulator. However, asthe output of the current regulator contains little harmonic com-ponents at multiples of twice the line frequency except for thesecond harmonic component, the second harmonic current in thePFC output current cannot be completely absorbed. Therefore,the ripple of the LED driving current can be relatively large,causing LED flicker. To tackle the issue, a feed-forward controlscheme is proposed to obtain the desired modulation signal, andthus reducing the LED driving current ripple and avoiding theLED flicker.

The paper is organized as follows. In Section II, the operat-ing principle of the flicker-free electrolytic capacitor-less ac/dcLED driver is briefly reviewed. After that, the feed-forwardcontrol scheme to reduce the LED driving current ripple is pro-posed in Section III. The experimental results are presented inSection IV, followed by the conclusions in Section V.

II. OPERATING PRINCIPLE OF FLICKER-FREE ELECTROLYTIC

CAPACITOR-LESS AC/DC LED DRIVER

The application of the flicker-free electrolytic capacitor-lessac/dc LED driver has been reported and discussed in detailin [21]. Here, we briefly reiterate its operating principle in orderto have a better understanding of our proposed feed-forwardcontrol scheme.

The input voltage is defined as

vin(t) = Vin sin ωint (1)

where Vin and ωin are the amplitude and the angular frequencyof the input voltage, respectively. Here, ωin = 2π/Tline , whereTline is the line period of the input voltage. When a unity powerfactor is achieved, the input current is a sinusoidal waveformthat is in phase with vin , i.e.,

iin (t) = Iin sin ωint (2)

where Iin is the amplitude of the input current. From (1) and(2), the instantaneous input power can be derived as

pin(t) = vin(t)iin (t) =VinIin

2(1 − cos 2ωint) . (3)

Assuming that the PFC converter is ideal with efficiency of100% and that the output voltage Vo across LEDs is a constantdc voltage, the instantaneous output power of the PFC converterwill be equal to the instantaneous input power. So, the outputcurrent of the PFC converter, i′o , is

i′o(t) =VinIin

2Vo(1 − cos 2ωint) = Io (1 − cos 2ωint) (4)




Fig. 2. Schematic diagram of bidirectional converter with control circuit.

where Io = VinIin /(2Vo ) is the average of output current. From(4), it is obvious that i′o contains a second harmonic currentwhich may cause LED flicker. So, for the purpose of avoidingthis phenomenon, a bidirectional converter can be introduced toperform as an active filter to provide the current flow path forthe second harmonic current. Fig. 2 shows the topology of thebidirectional buck/boost converter employed in [21], in whichthe output voltage of the PFC converter VC o is regarded as theinput voltage for the bidirectional buck/boost converter. As theLED driving current mainly contains dc component, the voltagedrop across the inductor Lo can be neglected. Therefore, VC o isapproximately equal to Vo .

A dual closed-loop control method is adopted for the bidirec-tional converter in [21]. vCd c

is the voltage of capacitor Cdc . Theaverage value of vCd c

is controlled by the outer voltage loop.The bandwidth of the outer voltage loop should be designed tobe low enough so that the output of the voltage regulator vo Gv isapproximately a dc value. ib ref is the second harmonic current,which is separated from the PFC output current i′o , as shown inFig. 1. The sum of ib ref and vo Gv is used as the input currentreference of ib . At steady state, the bidirectional converter op-erates as an active filter. Thus, the current reference of ib willnot contain a dc component. In other words, the dc componentof vo Gv is zero and the current reference of ib is almost equalto ib ref . From (4), ib ref is

ib ref (t) = −Io cos 2ωint. (5)

The inner current loop adopts an average current control,enforcing ib to track the current reference. Therefore, ib is

ib(t) = −Io cos 2ωint (6)

and the driving current io will be a purely dc Io accordingly.Fig. 3 shows the key waveforms of the LED driver, includingthe waveforms of the input voltage vin , the input current iin , theinput power pin , the output power Po , the PFC output currenti′o , the bidirectional converter input current ib , and LED drivingcurrent io .

From (6), the instantaneous input power of the bidirectionalconverter is

pbb(t) = Voib(t) = −VoIo cos 2ωint = −Po cos 2ωint (7)

Fig. 3. Key waveforms of the flicker-free electrolytic capacitor-less ac/dc LEDdriver.

Fig. 4. Key waveforms of the bidirectional converter.

Fig. 4 shows the key waveforms of the bidirectional buck/boost converter, including the waveforms of the instantaneousinput power pbb , the input current ib , and the output capacitorvoltage vCd c

. As seen, Cdc is charged from the time Tline /8 to3Tline /8 with vCd c

increasing from VCd c min to VCd c max . Thecharging energy of Cdc from Tline /8 to 3Tline /8 is

ΔECd c(t) =

∫ t

T l in e /8pbb (t)dt

=∫ t

T l in e /8(−Po cos 2ωint)dt

=Po

ωinsin2

(ωint − π

4

). (8)

ΔECd ccan be also expressed as

ΔECd c(t) =

12Cdcv

2Cd c

(t) − 12CdcV

2Cd c min . (9)

From (8) and (9), we have

12Cdcv

2Cd c

(t) − 12CdcV

2Cd c min =

Po

ωinsin2

(ωint − π

4

). (10)




From (10), the voltage across the bidirectional converter ca-pacitor can be expressed as

vCd c(t) =

√2Po sin2 (ωint − (π/4))

ωinCdc+ V 2

Cd c min . (11)

Substituting t = 3Tline /8 into (11), the maximum voltage ofthe capacitor Cdc can be derived as

VCd c max =√

2Po

ωinCdc+ V 2

Cd c min . (12)

The average voltage of Cdc can be approximated as

VCd c=

VCd c min + VCd c max

2

=12

(VCd c min +

√2Po

ωinCdc+ V 2

Cd c min

). (13)

From (11) and (13), we have

vCd c(t) =

√−Po sin 2ωint

ωinCdc+

(Po

2ωinCdcVCd c

)2

+ V 2Cd c

.

(14)It is obvious from (14) that vCd c

contains not only dc com-ponent and the second harmonic component but also other har-monic components.

III. FEED-FORWARD CONTROL SCHEME OF THE

BIDIRECTIONAL CONVERTER

A. Cause of Current Ripple in the LED Driving Current

As the inductor current of the buck/boost converter is bidirec-tional, the converter will operate in continuous current conduc-tion mode. Thus, the relationship between the input and outputvoltage can be expressed as

dbb (t) = 1 − VC o

vCd c(t)

. (15)

Substituting (14) and VC o = Vo into (15) leads to

dbb(t)=1− Vo√−

(Po sin 2ωint

ωinCdc

)+

(Po

2ωinCdcVCd c

)2

+V 2Cd c

.

(16)

Conducting Fourier transformation on (16), we have

dbb(t) =a0

2+

∞∑m=1

(am cos 2mωint + bm sin 2mωint) (17)

where

a0 =4

Tline

∫ T l i n e /2

0dbb(t)dt (18)

am =4

Tline

∫ T l i n e /2

0dbb(t) cos 2mωintdt (19)

bm =4

Tline

∫ T l i n e /2

0dbb(t) sin 2mωintdt. (20)

Fig. 5. Amplitude of mth harmonic in the duty cycle of the bidirectionalconverter under different output capacitor.

Therefore, the amplitude of the mth harmonic is

Dm =√

a2m + b2

m . (21)

Substituting Vo = 48 V, Io = 700 mA, Po = 33.6 W, Tline =0.02 s, and VCd c

= 160 V into (21), the amplitude of the mthharmonic in the duty cycle of the bidirectional converter underdifferent output capacitor Cdc can be plotted in Fig. 5. Since Po

and VCd care constant, the dc component of the duty cycle does

not change as Cdc varies. When Cdc is decreased, the voltageripple on the output capacitor will increase. Thus, the amplitudeof the mth harmonic in duty cycle dbb(t) will increase accord-ingly. When the bidirectional converter adopts conventional dualloop control, the modulation signal is the output of the currentregulator, and it is compared to the carrier, producing the dutycycle dbb(t). As mentioned ‘earlier, the bandwidth of the outervoltage loop is relatively low, the output of the voltage regulatorvo Gv is approximately a dc value with little second harmonicand harmonics at multiples of twice the line frequency. So, theinput current reference of the bidirectional converter, which isthe sum of ib ref and vo Gv , will mainly contain the secondharmonic ripple, little dc component and little other harmoniccomponents. Thus, the bidirectional converter with such modu-lation signal can not obtain the required harmonic componentsin vCd c

(t) as expressed in (14). As a result, the input current ofthe bidirectional converter will not accurately track the secondharmonic current in the PFC output current. So, the current rip-ple will occur in the LED driving current, causing LED flicker.

B. Proposed Feed-Forward Control Scheme

In a half-line cycle, if the duty cycle of the bidirectionalconverter varies as (16), vCd c

will be the waveform as shownin Fig. 4. Accordingly, the input current of the bidirectionalconverter ib will track the second harmonic current in the outputcurrent of the PFC without steady-state error.

The two power switches of the bidirectional converter operatein a complementary manner. For simplicity, we choose the dutycycle of Q2 as the controlled object. From (16), the duty cycleof Q2 is

d′bb(t) = 1 − dbb (t)

=Vo√

−Po sin 2ωint

ωinCdc+

(Po

2ωinCdcVCd c

)2

+V 2Cd c

. (22)




Fig. 6. Feed-forward control schematic diagram of the bidirectional converter.

Fig. 7. Realization circuit for feed-forward control scheme of the bidirectional converter.

The duty cycle is obtained by comparing the modulation sig-nal with the sawtooth carrier signal. So, the needed modulationsignal is VM times the duty ratio, i.e.,

vm (t) = d′bb (t) VM

=VoVM√

−(

Po sin 2ωint

ωinCdc

)+

(Po

2ωinCdcVCd c

)2

+V 2Cd c

(23)

where VM is the amplitude of the sawtooth signal. Equation (23)can be rewritten as

vm (t) =1√

−k1Io sin 2ωint + k2(24)

where

k1 =1

ωinCdcVoV 2M

(25)

k2 =

(Po

2ωinCdcVCd c

)2

+ V 2Cd c

V 2o V 2

M

. (26)

When Vo, Cdc , ωin , and VCd care selected, k1 is a constant

and k2 is a dc component as a function of Po .Fig. 6 shows the feed-forward control schematic diagram

of the bidirectional converter according to (24)–(26). A feed-forward path and a calculation circuit for producing the modu-lation signal are introduced in the dual loop control circuit.

C. Design Procedure of the Proposed Feed-Forward ControlScheme

Based on Fig. 6, Fig. 7 presents the control circuit of theproposed feed-forward control scheme, which contains a voltageregulator, a current regulator, a −90◦ phase-shift circuit, anadder, a division circuit, and a square rooting circuit.

The voltage across the output capacitor of the bidirectionalconverter vCd c

is sensed by R1 and R2 , as shown in Fig. 2. Inthis paper, VCd c

= 160 V and Vref = 2.55 V, so we choose R1 =620 kΩ and R2 = 10 kΩ.

The voltage regulator is composed of resistors R3 , R4 , andcapacitor C1 . Here, Gv (s) is designed to be 4.3 × 10−3 +5/s, so R3 = 100 kΩ, R4 = 43 kΩ, C1 = 2 μF. The currentregulator is composed of resistors R14 , R15 and capacitor C3 .Here, Gi(s) is designed to be 1 + 3.03× 104 /s, so R14 = 10 kΩ,R15 = 10 kΩ, C3 = 3.3 nF.

The −90◦ phase-shift circuit is composed of resistorsR9−R11 and capacitor C2 . From (5), −Iosin2ωint in (24) canbe obtained from shifting ib ref by −90◦ through the phase-shiftcircuit. Therefore, to have −90◦ phase shift, R9 and C2 shouldsatisfy

1R9C2

= 2ωin (27)

where ωin is the angular frequency of the input voltage, i.e.,ωin = 2π·50 rad/s. We choose R9 = 31.2 kΩ and C2 = 51 nF.To make the phase-shift circuit have a unity gain, R10 is set tobe equal to R11 . Here, we choose R10 = R11 = 10 kΩ.




Fig. 8. Photograph of the prototype.

The adder is composed of resistors R16−R20 . According to(25), k1 = 1.57. So, we could choose R16 = R17 = R18 = R20= 10 kΩ, and R19 = 2.74 kΩ.

The output of adder vx is the sum of −k1Iosin2ωint andvo Gc . In order to obtain the modulation signal vm as presentedin (24), vx should further go through the division circuit and thesquare rooting circuit. The relationship between the input signaland output signal of the division circuit can be written as

vy = − vzR24

km vxR23(28)

where vz = 1 V and km is the inherent coefficient of multiplier.When km = 1, we can choose R23 = R24 = 10 kΩ.

The relationship between the input signal and output signalof square rooting circuit can be written as

vm =√

− vyR22

km R21. (29)

Here, km = 1, and R21 = R22 = 10 kΩ.

IV. EXPERIMENTAL VERIFICATION

A. Prototype Design

To verify the validity of the proposed feed-forward controlscheme in reducing the LED driving current ripple, a 33.6 Wflicker-free electrolytic capacitor-less ac/dc LED driver proto-type consisting of a flyback PFC converter and a buck/boostbidirectional converter is constructed in the lab. Fig. 8 showsthe photo of the prototype. The flyback PFC converter operatesin discontinuous current mode (DCM). The DCM flyback PFCconverter can automatically achieve unity power factor when theduty cycle of the converter keeps constant in a half-line cycle.Fig. 9 shows the control schematic diagram of the DCM flybackPFC converter. Here, an average current control is adopted toregulate the output flux of LEDs. The current of the secondaryside diode is is sensed by a current transformer circuit consist-ing of TC T ,Dct , and Rct . After that, the RC filter comprisingRf 1 and Cf 1 is used to filter the high-frequency harmonics.The sensed current signal and current reference Io ref are sentto the current regulator Gf c . By comparing the output signalof Gf c with the sawtooth carrier signal, the driving signal ofswitch Qf is obtained. The buck/boost bidirectional converter

Fig. 9. Control schematic diagram of the flyback PFC converter.

TABLE IPROTOTYPE SPECIFICATIONS

is connected in parallel with the output capacitor of the flybackPFC converter and ib ref is the output of the band-pass filter, asshown in Fig. 9.

The specifications of the prototype are given in Table I. Theload consists of two strings of LEDs connected in parallel. Eachstring has 13 LEDs connected in series and they draw a currentof 0.35 A. Each LED has a voltage drop of 3.7 V.

B. Experimental Results

Figs. 10 and 11 show the experimental waveforms withoutfeed-forward control scheme at full load and half load underdifferent input voltages, respectively, in which vin is the inputvoltage, i′o is the output current of the flyback PFC converter,ib is the input current of the buck/boost bidirectional converter,io is the LED driving current, and vCd c

is the output voltage ofthe bidirectional converter. It can be seen that, when the feed-forward control scheme is not employed, the peak-to-peak valueof LED driving current is about 22% and 18% of the averageoutput current at full load and half load, respectively, which willcause LED flicker phenomenon.

Figs. 12 and 13 show the experimental waveforms with feed-forward control scheme at full load and half load under differentinput voltages, respectively. It can be seen that, when the feed-forward control scheme is incorporated, the peak-to-peak valueof LED driving current is about 8% and 6% of the averageoutput current at full load and half load, respectively. Comparedto the experimental results in Figs. 10 and 11, the ripple of the




Fig. 10. Experimental waveforms without feed-forward scheme at full load under input voltage of: (a) 90 V, (b) 220 V, and (c) 264 V.

Fig. 11. Experimental waveforms without feed-forward scheme at half load under input voltage of: (a) 90 V, (b) 220 V, and (c) 264 V.

Fig. 12. Experimental waveforms with feed-forward scheme at full load under input voltage of: (a) 90 V, (b) 220 V, and (c) 264 V.

Fig. 13. Experimental waveforms with feed-forward scheme at half load under input voltage of: (a) 90 V, (b) 220 V, and (c) 264 V.




Fig. 14. Spectrums of LED driving current with and without feed-forward scheme: (a) full load and (b) half load.

Fig. 15. LED lighting at full load: (a) without feed-forward control scheme and (b) with feed-forward control scheme.

LED driving current is greatly reduced, which implies that theproposed feed-forward control scheme can effectively reducethe LED driving current ripple, thus preventing LEDs fromflickering.

Fig. 14 presents spectrums of the LED driving current io withand without feed-forward control scheme at full load and halfload, respectively. It can be found that the harmonic compo-nents in the LED driving current are evidently reduced whenthe proposed feed-forward control scheme is adopted.

Fig. 15 shows the photos of the LED lighting with and withoutthe feed-forward control scheme at full load. It is clear thatthe LED flicker is prevented when the proposed feed-forwardcontrol scheme is adopted, which verifies the effectiveness ofthe proposed control scheme.

Fig. 16 shows the measured efficiency curves of the LEDdriver under different input voltages. As seen, the efficiency ofthe LED driver without bidirectional converter is around 90.5%.After the bidirectional converter is introduced, the efficiency de-creases to about 87%. The reason is that the bidirectional con-verter performs as an active filter and only handles the reactivepower. Hence, there will be conduction loss and switching lossin the power switches as well as core loss and winding loss in theinductor. Since the proposed feed-forward control scheme canmake the bidirectional converter absorb all the second harmoniccurrent in the output of the PFC converter, the conduction lossin the bidirectional converter will be slightly increased, whichdegrades the conversion efficiency a little.

Fig. 16. Measured efficiency curves of the LED driver.

V. CONCLUSION

The current ripple in the LED output current will raiseLED flicker problem. In this paper, a flicker-free electrolyticcapacitor-less ac/dc LED driver, which constitutes a flyback PFCconverter and a bidirectional converter, is studied. The spectrumof the output capacitor voltage of the bidirectional converter isanalyzed, indicating that the output capacitor voltage containsharmonics components at multiples of twice the line frequencyapart from dc component and the second harmonic component.Therefore, the modulation signal must involve the correspond-ing harmonic components so that the bidirectional converter can




fully absorb the second harmonic current in the output currentof the PFC converter. To attain the objective, this paper pro-poses a feed-forward control scheme and the required harmoniccomponents are introduced into the modulation wave signal bythe feed-forward path and a series of calculation operation. Thebidirectional converter under the proposed control scheme canentirely absorb the second harmonic current, which will effec-tively prevent the LED from flickering.

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Yang Yang was born in Shanxi Province, China, in1988. He received the B.S. and M.S. degrees in elec-trical engineering from Nanjing University of Aero-nautics and Astronautics, Nanjing, China, in 2010and 2013, respectively.

Since March 2013, he has been with the Nari-Relays Electric Co., Nanjing. His current researchinterests include dc/dc converters, ac/dc converters,and power supplies for LED.

Xinbo Ruan (M’97–SM’02) was born in HubeiProvince, China, in 1970. He received the B.S. andPh.D. degrees in electrical engineering from NanjingUniversity of Aeronautics and Astronautics (NUAA),Nanjing, China, in 1991 and 1996, respectively.

In 1996, he joined the Faculty of Electrical En-gineering Teaching and Research Division, NUAA,where he became a Professor in the College of Au-tomation Engineering in 2002 and has been involvedin teaching and research in the field of power elec-tronics. From August to October 2007, he was a Re-

search Fellow in the Department of Electronic and Information Engineering,Hong Kong Polytechnic University, Hong Kong, China. Since March 2008,he has been also with the College of Electrical and Electronic Engineering,Huazhong University of Science and Technology, China. He is a Guest Pro-fessor with Beijing Jiaotong University, Beijing, China; Hefei University ofTechnology, Hefei, China; and Wuhan University, Wuhan, China. He is the au-thor or coauthor of four books and more than 100 technical papers published injournals and conferences. His current research interests include soft-switchingdc–dc converters, soft-switching inverters, power factor correction converters,modeling the converters, power electronics system integration, and renewableenergy generation system.

Dr. Ruan was a recipient of the Delta Scholarship by the Delta Environ-ment and Education Fund in 2003 and was a recipient of the Special AppointedProfessor of the Chang Jiang Scholars Program by the Ministry of Education,China, in 2007. From 2005 to 2013, he was the Vice President of the ChinaPower Supply Society, and since 2008, he has been a member of the TechnicalCommittee on Renewable Energy Systems within the IEEE Industrial Electron-ics Society. He has been an Associate Editor for the IEEE TRANSACTIONS

ON INDUSTRIAL ELECTRONICS and the IEEE JOURNAL OF EMERGING AND

SELECTED TOPICS ON POWER ELECTRONICS since 2011 and 2013, respectively.He is a Senior Member of the IEEE Power Electronics Society and the IEEEIndustrial Electronics Society.

Li Zhang (S’12) was born in Hubei Province, China,in 1988. He received the B.S. degrees in electricalengineering and automation, in 2011, from NanjingUniversity of Aeronautics and Astronautics (NUAA),Nanjing, China, where he is currently working towardthe Ph.D. degree in electrical engineering.

His current research interests include inverter con-trol and renewable energy generation systems.




Jiexiu He was born in Jiangsu Province, China, in1991. She received the B.S. degree in electrical engi-neering and automation, in 2013, from Nanjing Uni-versity of Aeronautics and Astronautics, Nanjing,China, where she is currently working toward theM.S. degree in electrical engineering.

Her current research interests include ac/dc con-verters and power supplies for LED.

Zhihong Ye was born in Zhejiang Province, China, in1969. He received the B.S. and M.S. degrees in elec-trical engineering from Tsinghua University, Beijing,China, in 1992 and 1994, respectively, and the Ph.D.degree from the Bradley Department of Electrical andComputing Engineering, Virginia Polytechnic Insti-tute and State University, Blacksburg, VA, USA, in2000.

From 2000 to 2005, he was with General ElectricGlobal Research Center, Niskayuna, NY, USA, as anElectrical Engineer. From 2005 to 2006, he was with

Dell as a Commodity Quality Manager. Since 2006, he has been with Lite-OnTechnology Corp., Nanjing, China, as the Director of Research and Develop-ment. His current research interests include high-density, high-efficiency powersupply for computing, communication and consumer electronics applications,digital control, power converter topologies and controls, soft-switching tech-niques, etc. He holds 17 US patents and has authored or coauthored more than30 technical papers in transactions and international conferences.



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