Quasi-Active Power Factor Correction Circuit for Hb Led Driver

1410 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 23, NO. 3, MAY 2008

Quasi-Active Power Factor CorrectionCircuit for HB LED Driver

Kening Zhou, Jian Guo Zhang, Subbaraya Yuvarajan, Senior Member, IEEE, and Da Feng Weng

Abstract—High brightness light emitting diodes (HB LEDs) arelikely to be used for general lighting applications due to their highefficiency and longer life. The paper presents a quasi-active powerfactor corrector (PFC) for driving a string of HB LEDs. The single-stage PFC circuit has a high efficiency, and it does not increase thevoltage/current stress on the active switch used in the switchingconverter due to PFC. The circuit has two operating modes basedon the input voltage level and its features, like power factor correc-tion and power balance, are explained. The experimental resultsobtained on a prototype converter along with waveforms are pre-sented.

Index Terms—Driver, light emitting diodes (LEDs), power factorcorrection (PFC), pulse width modulation, switching converter.

I. INTRODUCTION

WITH THE development of high brightness light emittingdiode (HB LED) technology, the output light efficiency

of power LEDs has increased over 100 lumens/W [1]. The HBLED can be used as a solid state light source in general lightingapplications. In addition to high efficiency, it has no mercurycontent and has a longer life. In the future, the power LED islikely to replace the existing lighting sources like the incandes-cent lamp and fluorescent lamp.

For lamp drivers in general lighting applications, there areseveral regulations, e.g., harmonic limits on the input AC currenthave to meet Class C regulations for output power over 25 W [2].Since the incandescent lamp is basically a resistor, it is easy tomeet the requirement. For a fluorescent lamp, there are severalpower factor correction (PFC) circuits used in fluorescent lampdrivers or ballasts. It is the power factor correction circuit thatmakes the fluorescent ballast to meet Class C regulation.

In general lighting applications, including fluorescent andHB-LED, power factor correction can be achieved using eithera passive circuit or an active circuit. It is difficult to achieve ahigher power factor and lower THD with a passive PFC whichuses only inductors and capacitors, or with a variable inductive

Manuscript received May 9, 2007; revised October 22, 2007. This paper waspresented at the Applied Power Electronics Conference, Anaheim, CA, Feb-ruary 25-March 1, 2007. Recommended for publication by Associate Editor J.M. Alonso.

K. Zhou is with the Zhejiang University of Science and Technology, Zhejiang,China.

J. G. Zhang is with the ZhejiangUniversity, Hangzhou, Zhejiang, China.S. Yuvarajan is with the Department of Electrical Engineering, North Dakota

State University, Fargo, ND 58105 USA (e-mail: [email protected]).

D. F. Weng is with MAXIM Integrated Products, Sunnyvale, CA 94086 USA.Color versions of one or more of the figures in this paper are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TPEL.2008.921184

filter [3]. The active PFC on the other hand can provide alow THD and a high power factor. Single-switch AC-DC-DCconverters with power factor correction combines boost PFCand forward or flyback converters [4], [5] and with load currentfeedback power control [6]. Active input current shaper isanother solution of single switch AC-DC-DC converter withpower factor correction function [7].

In general, the use of two power stages is a good way to imple-ment power factor correction and to balance the input and outputpowers but it increases the cost. Single power stage with chargepump PFC has been used in the fluorescent AC-DC-AC ballast.For a single power stage AC-DC-DC converter with PFC, it ishard to balance the input and output powers [8]–[11]. Also, thereare high voltage and current stresses on the power components.A HB-LED driver (AC-DC-DC Converter) draws power fromAC mains and supplies a DC current to the LED string. Thedriver needs a DC-DC converter to convert the input voltage intoa DC current source and it limits the effectiveness of a chargepump. A single-power-stage AC-DC-DC converter with PFC isone candidate for HB LED drivers. The use of a single powerstage increases the stress on the switch in the DC-DC converterdue to input current and PFC voltage, and there is a power bal-ance problem.

This paper presents a quasi-active PFC scheme assisted bya power converter feeding a string of HB LEDs [12]. In theproposed quasi-active PFC scheme which precedes a drivingpower stage, a passive circuit is used to implement power factorcorrection function. The input current or PFC voltage stress isnot added on to the active switch used in the following power(DC/DC) converter. It is the passive circuit implementing PFCfunction that increases the reliability and lowers the cost. It is thefollowing power converter driving the passive PFC circuit thatmakes the size of the components in the passive circuit small.There are two operating modes in the circuit in which it is easy tobalance the input and output powers. The principle of operationof the proposed HB-LED driver is explained and experimentalresults are presented.

II. BASIC OPERATION AND MODES

The basic quasi-active PFC circuit is shown in Fig. 1. It con-sists of a high frequency coupled inductor (C_Inductor), threevalley-fill diodes ( and ), two DC bulk capacitors (and ), and a resonant capacitor . The PFC supplies a dis-continuous power load, such as, a buck, a buck-boost, a forward,or a flyback converter. In the present application, a buck-con-verter controlling the current through a set of HB LEDs consti-tutes the load. The operation of the PFC circuit falls under twoworking (operating) modes: (a) direct-feed mode (occurs whenthe instantaneous input line voltage is higher than the voltage of

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ZHOU et al.: QUASI-ACTIVE POWER FACTOR CORRECTION CIRCUIT FOR HB LED DRIVER 1411

Fig. 1. Basic quasi-active PFC circuit.

Fig. 2. Equivalent circuits for direct-feed mode: (a) output current increasing;(b) output current decreasing.

each DC bulk capacitor and ) and (b) coupled-boost mode(occurs when the input voltage is lower than the voltage of eachDC bulk capacitor).

a) Direct-Feed Mode: As the PFC’s output current ischanged from zero to a final (fixed) value, the input line willdirectly feed energy to the load and the resonant capacitorthrough the rectifier and the primary winding of C_inductor[Fig. 2(a)]. The load current passing through stores en-ergy which will be released to the resonant capacitor andthe capacitors and through when it goes to zero[Fig. 2(b)]. During this time, the input source charges theresonant capacitor and the two DC bulk capacitors and

through . Because the output voltage of the bridgerectifier is less than the sum of the voltages on capacitorsand , the charging current through will decay. It is clearthat during this mode, the capacitors and store part ofthe input energy and their voltages increase.

b) Coupled-Boost Mode: Since the input AC voltage islower than the voltage on and during this mode, the reso-nant capacitor releases its stored energy resulting in a reduc-tion in its voltage when the load current is changed from zero to afinal (fixed) value. When the voltage on becomes lower thanthat on and , the two capacitors will release the stored en-ergy to the load and the resonant capacitor [Fig. 3(a)]. Theenergy release corresponds to currents flowing through wind-ings and of the coupled inductor and the stored energy.The coupled inductors and also resonate with the capac-itor .

As the load current goes back to zero, the coupled induc-tors and continue to resonate with , whose voltageincreases. As the voltage across reflected to the secondary

is less the voltage across and , diodes and turn off,and the stored magnetic energy in and is transferred to

. The inductor will release the stored magnetic energy tothe resonant capacitor , and . At the same time, theinput power line will also directly feed energy to the capacitors

, and , as shown in Fig. 3(b). It is clear that during thismode, the capacitors and release the stored energy to theload and have their voltage decrease.

As shown in Fig. 1, the reflected load current is the currentthrough the active switch in the following DC/DC converteror discontinuous current load. The current stress on the activeswitch is decided only by the output load current and it is inde-pendent of the input PFC current. The voltage stress on the ac-tive switch is determined by the maximum input voltage of thefollowing DC/DC converter. The quasi-active PFC circuit hasautomatic voltage regulation that helps to keep the maximumDC bus voltage close to the amplitude of the input voltage forall load conditions.

III. POWER FACTOR CORRECTION AND POWER BALANCE

A. Power Factor Correction

During the direct-feed mode, the current through is com-posed of two parts as shown in Fig. 4(a), the pulse current whichis the reflected load current when the active switch of DC/DCconverter turns on, and Fig. 4(b), the decaying current through

. The slope of the decaying current is proportional to the dif-ference between the input rectified AC voltage and the DC busvoltage. It is easy to see that the current through will decayslowly as the instantaneous rectified AC voltage approaches theDC bus voltage. It means that, as the rectified AC voltage getsclose to the DC bus voltage, the average current through overone switching period will increase.

The mathematical expression for the average input currentduring one switching period is given by

(1)

(2)

where is the DC bus voltage; is the reflected load cur-rent; and are associated with , switching frequency ,and . For a fixed switching frequency and a constant output


Fig. 3. Equivalent circuits for coupled-boost mode: (a) I increasing; (b) I decreasing (higher and lower reflected voltages).

Fig. 4. Input current waveform for direct-feed mode.

Fig. 5. Plot of input current versus input voltage (d-f mode)

current source , and are constants. The plot ofversus is shown in Fig. 5. The input current varies lin-early with the input voltage during the direct-feed mode, whichmeans the input current follows the input voltage. During thecoupled-boost mode, the input current is the current through

which is decaying (Fig. 6). The average input current for aswitching period is given by

(3)

where is a constant associated with and switching fre-quency . The plot of versus is shown in Fig. 7.The relationship between the input current and the input voltageduring the coupled-boost mode is almost linear, which meansthe input current follows the input voltage. Considering the plotsshown in Figs. 5 and 7, it is seen that there is a linear relationbetween the input current and voltage that shows the inherentpower factor correction function of the circuit.

B. Power Balance

The ratio between the time intervals for the direct feed andthe coupled-boost mode depends on the ratio between the in-stantaneous input voltage and the voltage on or . In oneAC cycle, as the voltage on and increases, the interval of

Fig. 6. Input current waveform for coupled-boost mode.

Fig. 7. Plot of input current versus input voltage (c-b mode).

the coupled-boost mode increases, which means that the inputpower decreases. As the voltage on and deceases, theinterval of coupled-boost mode decreases and the input powerincreases. Due to the automatic variation of the coupled-boostmode interval, it is easy to balance the input and output powersand make the maximum bus voltage closer to the amplitude ofthe input voltage for all load conditions.

Suppose the output power decreases; the capacitors andwill release less energy to the load during coupled-boost

mode, which means the change (decrease) in the voltage onand is less. Thus, there is a reduction in the interval of thedirect-feed mode and an increase in the interval of the coupled-boost mode. This reduces the stored energy in the two DC bulkcapacitors that balances the lower energy released by the bulkcapacitors to the load. In the same way, the energy balance canbe explained for the case of increasing output power.

C. Design Considerations

The quasi-active PFC circuit is a passive circuit driven bythe discontinuous current pulses of the following DC-DC con-


Fig. 8. HB LED driver with quasi-active power factor correction circuit.

Fig. 9. Picture of the HB LED driver with quasi-active PFC demo board.

verter (Fig. 8). To achieve the highest possible efficiency, thecurrent in the coupled inductor under rated load should becontinuous during most of the direct-feed interval and discon-tinuous during most of the coupled-boost mode interval. In thisway, the current in is continuous most of the time and itsamplitude is low thereby avoiding the use of a costly differen-tial inductor. The value of the inductor is determined by theswitching frequency and the value of the inductor in the down-stream DC-DC converter. When the reflected load current stepsup from zero to a certain level, the current in the inductor of thefollowing DC-DC converter increases and the inductor storesthe energy. The value of has very little effect on the currentthrough the inductor in the DC-DC converter and the energystored. This means that the value of the coupled inductorshould be smaller than that of the inductor in the DC-DC con-verter.

During the direct-feed interval, the coupled inductor transfersthe input energy into the two DC bulk capacitors, which retainenough energy to supply output power during the coupled-boostmode interval. It is clear that, for a given inductor current, ahigher inductor value will increase the energy stored in the DCcapacitors. Based on the previously mentioned requirements,the value of inductor can be chosen to be between half andone-fourth that of the inductor in the DC-DC converter, thatis

(4)

where to 1/2.The turns-ratio of the coupled inductor is chosen as

(5)

where and ) are the number of turns on theprimary , secondary , and the third winding ofthe coupled inductor. By choosing a turns-ratio over 2, the deadtime of the input current can be made zero. The values of the twobulk capacitors and depend on the the DC bus capacitor

that is used in the regular AC-DC power supply, and theyshould be double the value of the capacitor as

.The values of and can be made larger to make sure that

the voltage ripple is less then 10%. In the circuit, the resonant ca-pacitor controls the instant at which the coupled-boost modestarts. If the value of is zero, the coupled-boost mode willprevail until a current is set up in . The basic requirementis to have the coupled-boost mode operation only when the in-stantaneous ac input voltage is lower than the voltage on thecapacitors and . Based on this requirement, the value ofthe reflected load current, and the value of the coupled inductor

, the value of can be determined.

IV. EXPERIMENTAL RESULTS

The quasi-active PFC circuit is used as a HB LED driverfor an output power of 30 W, as shown in Fig. 8. Ten 3-WLEDs from Cree (XLamp 7090 XL Series LED) are used asthe load. The system efficiency is measured to be 88.05% (fora maximum load of 0.74 A at 40 V at V AC and

W). The key components are: uH,uF/200 V, and uH. The picture of

the demo board is shown in Fig. 9. The waveforms of the inputAC voltage and current obtained are shown in Fig. 10. A digitalpower analyzer was used to obtain the harmonics in the input


Fig. 10. Waveforms of input voltage and current for the quasi-active PFC demo board.

TABLE IHARMONIC IN THE INPUT CURRENT ALONG WITH CLASS C LIMITS

current and the power factor. The harmonics present in the cur-rent along with the Class C limits are given in Table I.

The THD is measured as 9.32% and the input power factoris measured as 0.99. It is seen that the waveform of the inputcurrent is much closer to the sinusoidal waveform of the inputcurrent of an active-boost PFC converter and it meets Class Cregulation.

V. CONCLUSION

A quasi-active PFC topology which can be used to driveHB LEDs is presented in this paper. It has no active switchin the PFC section, and the whole quasi-active PFC sectionis driven by the discontinuous input current of the followingDC/DC power stage which actually supplies the HB LEDs.


The quasi-active PFC section is subjected to the switchingfrequency of the following DC/DC converter. Hence, its effi-ciency and reliability are higher and the size and cost are lower.The test results on a prototype shows that the total harmonicdistortion (THD) is below 10% and the efficiency is also high.

REFERENCES

[1] HB LED Market Outlook detailed at Industry Conf. [Online]. Avail-able: http://www.ledjournal.com/led_newsletter_5-06.htm#hb

[2] M. OLeary, “Plug in safeguard AC power line quality,” EDN, pp.57–66, Mar. 18, 2004.

[3] W. H. Wolfle and G. Hurley, “Quasi-active power factor correction witha variable inductive filter: Theory, design, and practice,” IEEE Trans.Power Electron., vol. 18, no. 1, pp. 248–255, Jan. 2000.

[4] H. Wei, I. Batarseh, G. Zhu, and P. Kornetzky, “A single-switchAD-DC converter with power factor correction,” IEEE Trans. PowerElectron., vol. 15, no. 3, pp. 421–430, May 2000.

[5] R. Redl, L. Balogh, and N. O. Sokal, “A new family of single-stageisolated power-factor correctors with fast regulation of the outputvoltage,” in Proc. IEEE Power Electronics Specialists Conf., 1994, pp.1137–1144.

[6] Q. Zhao, M. Xu, F. C. Lee, and J. Qian, “Single-switch parallel powerfactor correction AC-DC converters with inherent load current feed-back,” IEEE Trans. Power Electron., vol. 19, no. 4, pp. 928–936, Jul.2004.

[7] J. Sebastian, A. Femandez, P. Villegas, M. Hemando, and J. Prieto,“New topologies of active input current shapers to allow AC-to-DCconverters to comply with the IEC-1000-3-2,” IEEE Trans. Power Elec-tron., vol. 17, no. 4, pp. 493–501, Jul. 2002.

[8] M. H. L. Chow, K. W. Siu, C. K. Tse, and Y.-S. Lee, “A novel methodfor elimination of line-current harmonics in single-stage PFC switchingregulators,” IEEE Trans. Power Electron., vol. 13, no. 1, pp. 75–83, Jan.1998.

[9] M. H. L. Chow, Y. S. Lee, and C. K. Tse, “Single-stage single-switchisolated PFC regulator with unity power factor, fast transient responseand low voltage stress,” in Proc. IEEE Power Electronics SpecialistsConf., 1998, pp. 1422–1428.

[10] M. M. Jovanovic, D. M. Tsang, and F. C. Lee, “Reduction of voltagestress in integrated high-quality rectifier-regulators by variable fre-quency control,” in Proc. IEEE Applied Power Electronics Conf.,1994, pp. 569–575.

[11] C. Qiao and K. M. Smedley, “A topology survey of single-stage powerfactor corrector with a boost type input-current-shaper,” IEEE Trans.Power Electron., vol. 16, no. 3, pp. 360–368, May 2001.

[12] D. F. Weng, “Quasi-active power factor correction circuit for switchingpower supply,” U.S. Patent #6 909 622, Jun. 2005.

Kening Zhou received the B.S. and M.S. degreesin electrical engineering from Zhejiang University,Hangzhou, China, in 1982 and 1998, respectively.

He is presently an Associate Professor at theZhejiang University of Science and Technology,Zhejiang, China. His main study area includeselectric-measurement-control technology and powerelectronics technology.

Jian Guo Zhang received the Associate degreein optic engineering from Zhejiang University,Hangzhou, China, in 1999.

His main study area includes computer networkand digital and power electronic technology.

Subbaraya Yuvarajan (SM’84) received theM.Tech. and Ph.D. degrees from the Indian Instituteof Technology, Chennai, Madras, India, in 1969 and1981, respectively.

He was with the Department of Electrical Engi-neering, PSG College of Technology, from 1969 to1974, and with the Indian Institute of Technologyfrom 1974 to 1983. He joined the Department ofElectrical and Computer Engineering, North DakotaState University, Fargo, in 1983, where his currentlya Professor. His research interests include high-per-

formance power supplies and power conversion for renewable energy sourceslike PV power and PEM fuel cell.

Da Feng Weng received the B.S. and M.E. degreesin electrical engineering from Zhejiang University,Hangzhou, China, in 1982 and 1985, respectively,and the Ph.D. degree in electrical engineering fromNorth Dakota State University, Fargo, in 1995.

He has worked in several lighting and semicon-ductor industries, including Magne Tek, MatsushitaElectric Works, Philips Research, Analog devices,Intersil, and he is currently with MAXIM IntegratedProducts. His research includes power electronicstopology and control.

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Quasi-Active Power Factor Correction Circuit for Hb Led Driver