WITH THE development of high brightness light emitting

1410 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 23, NO. 3, MAY 2008 Quasi-Active Power Factor Correction Circuit 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) are likely to be used for general lighting applications due to their high efficiency and longer life. The paper presents a quasi-active power factor corrector (PFC) for driving a string of HB LEDs. The singlestage PFC circuit has a high efficiency, and it does not increase the voltage/current stress on the active switch used in the switching converter due to PFC. The circuit has two operating modes based on the input voltage level and its features, like power factor correction and power balance, are explained. The experimental results obtained on a prototype converter along with waveforms are presented. Index Terms Driver, light emitting diodes (LEDs), power factor correction (PFC), pulse width modulation, switching converter. I. INTRODUCTION WITH THE development of high brightness light emitting diode (HB LED) technology, the output light efficiency of power LEDs has increased over 100 lumens/w [1]. The HB LED can be used as a solid state light source in general lighting applications. In addition to high efficiency, it has no mercury content and has a longer life. In the future, the power LED is likely to replace the existing lighting sources like the incandescent lamp and fluorescent lamp. For lamp drivers in general lighting applications, there are several regulations, e.g., harmonic limits on the input AC current have to meet Class C regulations for output power over 25 W [2]. Since the incandescent lamp is basically a resistor, it is easy to meet the requirement. For a fluorescent lamp, there are several power factor correction (PFC) circuits used in fluorescent lamp drivers or ballasts. It is the power factor correction circuit that makes the fluorescent ballast to meet Class C regulation. In general lighting applications, including fluorescent and HB-LED, power factor correction can be achieved using either a passive circuit or an active circuit. It is difficult to achieve a higher power factor and lower THD with a passive PFC which uses only inductors and capacitors, or with a variable inductive Manuscript received May 9, 2007; revised October 22, 2007. This paper was presented at the Applied Power Electronics Conference, Anaheim, CA, February 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: subbaraya.yuvarajan@ndsu. edu). 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 a low THD and a high power factor. Single-switch AC-DC-DC converters with power factor correction combines boost PFC and forward or flyback converters [4], [5] and with load current feedback power control [6]. Active input current shaper is another solution of single switch AC-DC-DC converter with power factor correction function [7]. In general, the use of two power stages is a good way to implement power factor correction and to balance the input and output powers but it increases the cost. Single power stage with charge pump PFC has been used in the fluorescent AC-DC-AC ballast. For a single power stage AC-DC-DC converter with PFC, it is hard to balance the input and output powers [8] [11]. Also, there are high voltage and current stresses on the power components. A HB-LED driver (AC-DC-DC Converter) draws power from AC mains and supplies a DC current to the LED string. The driver needs a DC-DC converter to convert the input voltage into a DC current source and it limits the effectiveness of a charge pump. A single-power-stage AC-DC-DC converter with PFC is one candidate for HB LED drivers. The use of a single power stage increases the stress on the switch in the DC-DC converter due to input current and PFC voltage, and there is a power balance problem. This paper presents a quasi-active PFC scheme assisted by a power converter feeding a string of HB LEDs [12]. In the proposed quasi-active PFC scheme which precedes a driving power stage, a passive circuit is used to implement power factor correction function. The input current or PFC voltage stress is not added on to the active switch used in the following power (DC/DC) converter. It is the passive circuit implementing PFC function that increases the reliability and lowers the cost. It is the following power converter driving the passive PFC circuit that makes the size of the components in the passive circuit small. There are two operating modes in the circuit in which it is easy to balance the input and output powers. The principle of operation of the proposed HB-LED driver is explained and experimental results are presented. II. BASIC OPERATION AND MODES The basic quasi-active PFC circuit is shown in Fig. 1. It consists of a high frequency coupled inductor (C_Inductor), three valley-fill diodes ( and ), two DC bulk capacitors ( and ), and a resonant capacitor. The PFC supplies a discontinuous power load, such as, a buck, a buck-boost, a forward, or a flyback converter. In the present application, a buck-converter controlling the current through a set of HB LEDs constitutes the load. The operation of the PFC circuit falls under two working (operating) modes: (a) direct-feed mode (occurs when the instantaneous input line voltage is higher than the voltage of 0885-8993/$25.00 2008 IEEE

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 each DC bulk capacitor). a) Direct-Feed Mode: As the PFC s output current is changed from zero to a final (fixed) value, the input line will directly feed energy to the load and the resonant capacitor through the rectifier and the primary winding of C_inductor [Fig. 2(a)]. The load current passing through stores energy which will be released to the resonant capacitor and the capacitors and through when it goes to zero [Fig. 2(b)]. During this time, the input source charges the resonant capacitor and the two DC bulk capacitors and through. Because the output voltage of the bridge rectifier is less than the sum of the voltages on capacitors and, the charging current through will decay. It is clear that during this mode, the capacitors and store part of the input energy and their voltages increase. b) Coupled-Boost Mode: Since the input AC voltage is lower than the voltage on and during this mode, the resonant capacitor releases its stored energy resulting in a reduction in its voltage when the load current is changed from zero to a final (fixed) value. When the voltage on becomes lower than that on and, the two capacitors will release the stored energy to the load and the resonant capacitor [Fig. 3(a)]. The energy release corresponds to currents flowing through windings and of the coupled inductor and the stored energy. The coupled inductors and also resonate with the capacitor. As the load current goes back to zero, the coupled inductors and continue to resonate with, whose voltage increases. 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 to the resonant capacitor, and. At the same time, the input power line will also directly feed energy to the capacitors, and, as shown in Fig. 3(b). It is clear that during this mode, the capacitors and release the stored energy to the load and have their voltage decrease. As shown in Fig. 1, the reflected load current is the current through the active switch in the following DC/DC converter or discontinuous current load. The current stress on the active switch is decided only by the output load current and it is independent of the input PFC current. The voltage stress on the active switch is determined by the maximum input voltage of the following DC/DC converter. The quasi-active PFC circuit has automatic voltage regulation that helps to keep the maximum DC bus voltage close to the amplitude of the input voltage for all load conditions. III. POWER FACTOR CORRECTION AND POWER BALANCE A. Power Factor Correction During the direct-feed mode, the current through is composed of two parts as shown in Fig. 4(a), the pulse current which is the reflected load current when the active switch of DC/DC converter turns on, and Fig. 4(b), the decaying current through. The slope of the decaying current is proportional to the difference between the input rectified AC voltage and the DC bus voltage. It is easy to see that the current through will decay slowly as the instantaneous rectified AC voltage approaches the DC bus voltage. It means that, as the rectified AC voltage gets close to the DC bus voltage, the average current through over one switching period will increase. The mathematical expression for the average input current during one switching period is given by where is the DC bus voltage; is the reflected load current; and are associated with, switching frequency, and. For a fixed switching frequency and a constant output (1) (2)

1412 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 23, NO. 3, MAY 2008 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. 6. Input current waveform for coupled-boost mode. Fig. 5. Plot of input current versus input voltage (d-f mode) current source, and are constants. The plot of versus is shown in Fig. 5. The input current varies linearly with the input voltage during the direct-feed mode, which means the input current follows the input voltage. During the coupled-boost mode, the input current is the current through which is decaying (Fig. 6). The average input current for a switching period is given by where is a constant associated with and switching frequency. The plot of versus is shown in Fig. 7. The relationship between the input current and the input voltage during the coupled-boost mode is almost linear, which means the input current follows the input voltage. Considering the plots shown in Figs. 5 and 7, it is seen that there is a linear relation between the input current and voltage that shows the inherent power factor correction function of the circuit. B. Power Balance The ratio between the time intervals for the direct feed and the coupled-boost mode depends on the ratio between the instantaneous input voltage and the voltage on or. In one AC cycle, as the voltage on and increases, the interval of (3) Fig. 7. Plot of input current versus input voltage (c-b mode). the coupled-boost mode increases, which means that the input power decreases. As the voltage on and deceases, the interval of coupled-boost mode decreases and the input power increases. Due to the automatic variation of the coupled-boost mode interval, it is easy to balance the input and output powers and make the maximum bus voltage closer to the amplitude of the input voltage for all load conditions. Suppose the output power decreases; the capacitors and will release less energy to the load during coupled-boost mode, which means the change (decrease) in the voltage on and is less. Thus, there is a reduction in the interval of the direct-feed mode and an increase in the interval of the coupledboost mode. This reduces the stored energy in the two DC bulk capacitors that balances the lower energy released by the bulk capacitors to the load. In the same way, the energy balance can be explained for the case of increasing output power. C. Design Considerations The quasi-active PFC circuit is a passive circuit driven by the discontinuous current pulses of the following DC-DC con-

ZHOU et al.: QUASI-ACTIVE POWER FACTOR CORRECTION CIRCUIT FOR HB LED DRIVER 1413 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, the current in the coupled inductor under rated load should be continuous during most of the direct-feed interval and discontinuous during most of the coupled-boost mode interval. In this way, the current in is continuous most of the time and its amplitude is low thereby avoiding the use of a costly differential inductor. The value of the inductor is determined by the switching frequency and the value of the inductor in the downstream DC-DC converter. When the reflected load current steps up from zero to a certain level, the current in the inductor of the following DC-DC converter increases and the inductor stores the energy. The value of has very little effect on the current through the inductor in the DC-DC converter and the energy stored. This means that the value of the coupled inductor should be smaller than that of the inductor in the DC-DC converter. During the direct-feed interval, the coupled inductor transfers the input energy into the two DC bulk capacitors, which retain enough energy to supply output power during the coupled-boost mode interval. It is clear that, for a given inductor current, a higher inductor value will increase the energy stored in the DC capacitors. Based on the previously mentioned requirements, the value of inductor can be chosen to be between half and one-fourth that of the inductor in the DC-DC converter, that is where to 1/2. The turns-ratio of the coupled inductor is chosen as (4) (5) where and ) are the number of turns on the primary, secondary, and the third winding of the coupled inductor. By choosing a turns-ratio over 2, the dead time of the input current can be made zero. The values of the two bulk capacitors and depend on the the DC bus capacitor that is used in the regular AC-DC power supply, and they should 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 capacitor controls the instant at which the coupled-boost mode starts. If the value of is zero, the coupled-boost mode will prevail until a current is set up in. The basic requirement is to have the coupled-boost mode operation only when the instantaneous ac input voltage is lower than the voltage on the capacitors and. Based on this requirement, the value of the 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 driver for an output power of 30 W, as shown in Fig. 8. Ten 3-W LEDs from Cree (XLamp 7090 XL Series LED) are used as the load. The system efficiency is measured to be 88.05% (for a 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 input AC voltage and current obtained are shown in Fig. 10. A digital power analyzer was used to obtain the harmonics in the input

1414 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 23, NO. 3, MAY 2008 Fig. 10. Waveforms of input voltage and current for the quasi-active PFC demo board. TABLE I HARMONIC IN THE INPUT CURRENT ALONG WITH CLASS C LIMITS current and the power factor. The harmonics present in the current along with the Class C limits are given in Table I. The THD is measured as 9.32% and the input power factor is measured as 0.99. It is seen that the waveform of the input current is much closer to the sinusoidal waveform of the input current of an active-boost PFC converter and it meets Class C regulation. V. CONCLUSION A quasi-active PFC topology which can be used to drive HB LEDs is presented in this paper. It has no active switch in the PFC section, and the whole quasi-active PFC section is driven by the discontinuous input current of the following DC/DC power stage which actually supplies the HB LEDs.

ZHOU et al.: QUASI-ACTIVE POWER FACTOR CORRECTION CIRCUIT FOR HB LED DRIVER 1415 The quasi-active PFC section is subjected to the switching frequency of the following DC/DC converter. Hence, its efficiency and reliability are higher and the size and cost are lower. The test results on a prototype shows that the total harmonic distortion (THD) is below 10% and the efficiency is also high. Kening Zhou received the B.S. and M.S. degrees in electrical engineering from Zhejiang University, Hangzhou, China, in 1982 and 1998, respectively. He is presently an Associate Professor at the Zhejiang University of Science and Technology, Zhejiang, China. His main study area includes electric-measurement-control technology and power electronics technology. REFERENCES [1] HB LED Market Outlook detailed at Industry Conf. [Online]. Available: 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 with a 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-switch AD-DC converter with power factor correction, IEEE Trans. Power Electron., vol. 15, no. 3, pp. 421 430, May 2000. [5] R. Redl, L. Balogh, and N. O. Sokal, A new family of single-stage isolated power-factor correctors with fast regulation of the output voltage, 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 power factor correction AC-DC converters with inherent load current feedback, 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-DC converters to comply with the IEC-1000-3-2, IEEE Trans. Power Electron., 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 method for elimination of line-current harmonics in single-stage PFC switching regulators, 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-switch isolated PFC regulator with unity power factor, fast transient response and low voltage stress, in Proc. IEEE Power Electronics Specialists Conf., 1998, pp. 1422 1428. [10] M. M. Jovanovic, D. M. Tsang, and F. C. Lee, Reduction of voltage stress in integrated high-quality rectifier-regulators by variable frequency 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 power factor 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 switching power supply, U.S. Patent #6 909 622, Jun. 2005. Jian Guo Zhang received the Associate degree in optic engineering from Zhejiang University, Hangzhou, China, in 1999. His main study area includes computer network and digital and power electronic technology. Subbaraya Yuvarajan (SM 84) received the M.Tech. and Ph.D. degrees from the Indian Institute of Technology, Chennai, Madras, India, in 1969 and 1981, respectively. He was with the Department of Electrical Engineering, PSG College of Technology, from 1969 to 1974, and with the Indian Institute of Technology from 1974 to 1983. He joined the Department of Electrical and Computer Engineering, North Dakota State University, Fargo, in 1983, where his currently a Professor. His research interests include high-performance power supplies and power conversion for renewable energy sources like PV power and PEM fuel cell. Da Feng Weng received the B.S. and M.E. degrees in electrical engineering from Zhejiang University, Hangzhou, China, in 1982 and 1985, respectively, and the Ph.D. degree in electrical engineering from North Dakota State University, Fargo, in 1995. He has worked in several lighting and semiconductor industries, including Magne Tek, Matsushita Electric Works, Philips Research, Analog devices, Intersil, and he is currently with MAXIM Integrated Products. His research includes power electronics topology and control.