Single-stage driver for supplying high-power light-emitting-diodes with universal utility-line input voltages

Transcription

1 Single-stage driver for supplying high-power light-emitting-diodes with universal utility-line input voltages C.-A. Cheng H.-L. Cheng F.-L. Yang C.-W. Ku Department of Electrical Engineering, I-Shou University, Kaohsiung City 84001, Taiwan Abstract: The conventional two-stage non-isolated driver for high-power light-emitting diodes (LEDs) is composed of a powerfactor-correction (PFC) converter with a non-isolated flyback converter for regulating the output voltage/current for supplying LEDs. However, large numbers of circuit components are required and the total efficiency is limited because of two-stage power conversion. This study proposes a high-power LEDs driver based on a single-stage topology. The presented driver combines a buck-boost PFC converter with a non-isolated flyback converter with two output windings connected in series into a single-stage power-conversion circuit. The proposed driver offers high power factor, high efficiency, low totalharmonics-distortion (THD) of input utility-line current, cost-effectiveness and a constant-output voltage along with limitedoutput-current control scheme. Finally, a prototype driver is developed and tested in order to supply a 100 W LED light bar module with universal utility-line input voltages for indoor or outdoor general lighting applications. Operational analysis, design equations, a design example and experimental results provided by examining three LED light bar modules demonstrate the feasibility of the proposed circuit. 1 Introduction Light-emitting diodes (LEDs) are attractive solid-state lighting sources nowadays. LEDs are different from traditional incandescent lighting sources, which use filaments to make heat radiation, and fluorescent lamps, which use gaseous discharging [1 4]. The beneficial features of LED lighting sources are their compact size, long lifetime, energy savings, improved lighting efficiency, good colour rendering, lack of mercury, non-polluting quality and harmlessness to our environment. A LED has a long lifetime (typically more than h) and low maintenance costs [4]. Moreover, LEDs include almost all visible light and various colour temperatures [5 9]. Owing to many superior characteristics of LEDs, they are favourable for commercial lighting, background lighting, displays and decorative lighting applications [10 14]. LEDs are a major trend for modern lighting, and their lighting applications are getting more and more closely related to our daily lives [15 17]. Among LED lighting products, the LED light bar module is popular and is being used for such a wide variety of purposes. The advantageous features of LED light bar modules are their modular design, easy-to-assemble arrays comprised of dozens of LEDs or configured of several LED modules and that it is simple to combine LEDs of different colours, such as white, blue, red and green. The LED light bar modules not only allow the users to easily access individual components, but also to replace them when they are not functioning properly; thus, they are suitable for use in general lighting, advertising boards, garden landscaping lighting, emergency lighting, backlighting, street-lighting applications and so on [18]. The conventional two-stage non-isolated power driver for supplying a high-power LED light bar module with universal input utility-line voltages is shown in Fig. 1 [19]; it consists of a boost converter as the first stage (including filter inductor L in and capacitor C in, full-bridge rectified diodes D 1 D 4, a boost inductor L b, a power switch S b,a power diode D b, two capacitors C rec and C o1 ) for power factor correction (PFC) and a non-isolated flyback converter with two output windings connected in series as second stage (including input capacitor C o1, power switch S f, transformer T r with a magnetic inductor L m, two power diodes D f1 and D f2, two output capacitors C o2 and C o3, along with the LEDs) for regulating output voltage/current and lowering the voltage stresses in the output capacitors and diodes. In addition, the advantages of the boost converter utilised in the conventional non-isolated power driver are its simple topology, non-floating power switch, and easy-to-design driver circuit. Also, the inductor current is equal to the input current. Fig. 2 shows another two-stage non-isolated power driver for supplying a high-power LED light bar module with universal input utility-line voltages [19]; this version consists of a buck-boost converter as the first stage for PFC and a non-isolated flyback converter with two output windings connected in series as second stage for adjusting output voltage/current. However, the output voltage level of

2 Fig. 1 Traditional two-stage non-isolated power driver (consists of a boost converter and a non-isolated flyback converter with two output windings) for providing a high-power LED light bar module Fig. 2 Another two-stage non-isolated power driver (consists of a buck-boost converter and a non-isolated flyback converter with two output windings) for supplying a high-power LED light bar module the capacitor C o1 is quite high (typically V), and the voltage stresses of power diode D b and main power switch S b are relatively high, as shown in Figs. 1 and 2. In addition, large numbers of circuit components are required and the total efficiency is limited because of the above-mentioned two-stage power conversion. In response to these challenges, a single-stage power conversion for providing LEDs with high power factor (HPF) and compact size is a preferable solution [20 23]. This paper develops a single-stage power driver for supplying a high-power-rated LED light bar module for indoor or outdoor general purpose lighting applications with HPF, high efficiency, reduced voltage stress on the power switch, cost-effectiveness and constant output voltage along with limited output current control scheme. Design guidelines, a design example and experimental results of a prototype 100 W LED driver for testing three LED light bar modules with universal utility-line voltages are demonstrated. 2 Descriptions of proposed driver for high-power LEDs Referring to the two-stage power driver circuit for LEDs shown in Fig. 2, both the power switch S b in the buckboost PFC converter and the switch S f in the non-isolated flyback converter are operated at high frequency with a pulse-width-modulation (PWM) control scheme and share a common node. As a result, the two-stage configuration shown in Fig. 2 can be merged into a single-stage topology by integrating a power switch. The proposed power driver for supplying a high-power LED light bar module, along with a control integrated-circuit (IC) that uses a peakcurrent control scheme for achieving input current shaping as well as a voltage-current regulator for obtaining constant output voltage control, and limitation of output current, is shown in Fig. 3; the driver integrates a buck-boost PFC converter with a non-isolated flyback converter with two output windings connected in series into a single-stage power converter that possesses both buck-boost-type and flyback-type operations and includes an inductor L in, four capacitors C in, C rec, C f and C b, a power switch S b, six diodes D 1, D 2, D 3, D 4, D f and D b, a transformer T r with a magnetic inductor L m, three resistors R v1, R v2 and R c and the LEDs. As shown in Fig. 3, a voltage current regulator is utilised to control the output voltage and the output current of the LEDs. The voltage-current regulator senses the output voltage, denoted as V sv, through resistors R v1 and R v2, and simultaneously senses the output current, denoted as V sc, through the resistor R c. The output signal of the voltage current regulator feeds into the positive terminal of an error amplifier along with a compensation network and then compares with an internal reference voltage V ref (typically 2.5 V); thus it is able to generate a DC-level voltage V c, which is proportional to the difference between them. The signal V c coming from the error amplifier is fed into the multiplier and is multiplied by a portion of the rectified input utility-line voltage V rec. As a result, a rectified sinusoidal voltage V m, the envelope of which depends on the utility-line voltage waveform and the output level of the error signal, is obtained and fed into the comparator. Another input voltage signal V s of the comparator represents the sensed voltage v DS of the power switch coming from the current i DS multiplied by the currentsensing resistor. When voltage V s is equal to V m, the power switch is turned off. Therefore the envelope of peak value of the magnetic inductor current I Lm, pk is followed by a rectified sinusoidal period. The magnetic inductor L m discharges energy to the load during the power switch-off period. If the sensed current i Lm decreases to zero, the power switch will turn on again and another switching cycle will begin. Additionally, the presented LEDs power

3 Fig. 3 Proposed single-stage power driver for supplying a LED light bar module along with the control circuit driver offers HPF, high-efficiency, low-current THD, constant-voltage control, limited-output-current control and cost-effectiveness. 3 Analysis of the presented high-power LEDs driver In order to analyse the presented single-stage high-power LED driver, some assumptions are made and shown as follows: 1. All components used in the circuit are ideal. 2. The switching frequency is considered to be much higher than the line frequency. Thus, the sinusoidal input voltage can be considered to be constant for each switching period. 3. The single-stage power driver is designed to be operated in boundary-conduction mode (BCM). The input utility-line voltage v AC (t) is defined as v AC (t) 2VAC rms sin 2pf AC t (1) where V AC-rms is the input root-mean-square (rms) value of input utility-line voltage, and f AC is the utility-line frequency. The operation modes and principal switching waveforms for the proposed single-stage LEDs driver are shown in Figs. 4 and 5, respectively. Mode 1 (t 0 t, t 1 ): [Fig. 4a]: When the switch S b is turned on at t 0, the absolute value of the input utility-line voltage v AC (t) is applied to the magnetic inductance L m, and the magnetising current i Lm linearly increases. Since the power driver is designed to operate in BCM for achieving HPF, the magnetising current linearly increases from zero, and can be expressed as i Lm (t) v AC(t) L m (t t 0 ) (2) The diodes D b and D f are off because of being reverse-biased; therefore no current flows through the windings N b and N f. The current i Lm increases linearly to its peak value, denoted as I Lm, pk, according to the following equation I Lm,pk v AC(t) L m DT s (3) where D is duty cycle of the switch S b, and T s is the switching period. In addition, capacitors C f and C b provide energy to the LEDs. This mode finishes when switch S b turns off. Mode 2 (t 1 t, t 2 ): [Fig. 4b]: When S b is turned off at t 1, D b and D f turn on because of being forward-biased. Capacitors C b and C f equally share the DC output voltage; that means V Cb ¼ V Cf ¼ V DC /2. Thus, the drain-to-source voltage v DS is given by v DS (t) v AC (t) + V DC 2 (4)

4 Fig. 4 Operation modes of the proposed LEDs driver a Mode 1 b Mode 2 The magnetising current i Lm linearly decreases with a downslope of V DC /2L m, and can be expressed as i Lm (t) V DC 2L m (t t 1 ) (5) The magnetising inductor L m discharges energy to the capacitors C b and C f as well as LEDs through the diodes D b and D f during the off-time interval. This mode finishes when power switch S b turns on again at time interval t 2 and the operational mode returns to Mode 1 for the next switching cycle. 4 Design guidelines of key components 4.1 Design equation of magnetic inductor L m Fig. 6 shows the illustrated waveform for the magnetic inductor current i Lm under the control scheme that has constant T on and variable T S. The peak value of input current is given by 2 Pin 2 Pout I AC pk (6) V AC rms, min hv AC rms, min where V AC-rms, min is the minimum value of the input utilityline rms voltage; h is the estimated circuit efficiency, and

5 Fig. 5 Principal waveforms for the proposed LED driver Referring to Fig. 6, the difference in amplitude between peak and low levels of current, indicated as DI, is obtained by the following equation DI v AC T L on 2 2 VAC rms V DC T m 2L off (8) m where T on and T off are turn-on and turn-off time of the power switch, respectively. In order to design the magnetic inductor under BCM, the turn-on time of the power switch can be expressed by Fig. 6 Illustrated waveform of magnetic inductor current i Lm under constant T on and variable T S control scheme T on (2pf AC t) L mi Lm pk sin(2pf AC t) 2 VAC rms, min sin(2pf AC t) (9) the relationship between input power P in and output power P out is P out ¼ hp in. The peak value of magnetic inductor current can be expressed by I Lm pk 2I AC pk 2 2Pout (7) hv AC rms, min While sin(2pf AC t) is equal to unity, the maximum level of T on, indicated as T on-max, is given by T on max L m I Lm pk (10) 2 VAC rms, min

6 The turn-off time of the power switch can be expressed as 2L m I Lm pk sin(2pf AC t) T off (2pf AC t) 2 (11) 2VAC rms, min sin(2pf AC t) V DC The switching frequency f sw of the power switch is shown in the following equation (see (12)) Rearranging (12), the magnetic inductor L m operated in BCM can be expressed as L m V AC rms, 2 ( min 2 ) 2VAC rms, min sin(2pf AC t) V DC ( 2f sw min P in 4 ) 2VAC rms, min V DC (13) where f sw-min is the minimum switching frequency of the power switch. 4.2 Description of DC output voltage V DC Referring to Fig. 3, the DC output voltage V DC can be expressed as V DC ( 1 + N ) f 2VAC rms, min D max (14) N b 1 D max where N b and N f, respectively, are the primary-side and secondary-side windings of the transformer T r, and D max is the maximum duty cycle of the power switch S b. 4.3 Design equation for turns-tatio n From (14), the definition of turns-ratio n of transformer T r can be determined by dividing windings N f by N b, and can be expressed as n N f N b (1 D max)v DC 1 (15) 2 VAC rms, min D max 4.4 Design equations for voltage sensing resistors R v1 and R v2 Referring to Fig. 3 and using a voltage divider operation, the voltage V sv is given by R v2 V sv V DC (16) R v1 + R v2 Rearranging (16) and pre-choosing an appropriate resistor R v1, the voltage sensing resistor R v2 can be determined by R v2 R v1v sv V DC V sv (17) 4.5 Design equation for current sensing resistor R c In Fig. 3 and using Ohm s law, the current sensing resistor can be expressed as R c V sc I DC, max (18) where I DC, max is the allowed maximum output current of the voltage current regulator. 5 Design example A prototype driver for supplying a high-power-rated LED light bar module has been built and tested to meet the following specifications. 5.1 Input specifications 1. input utility-line voltage range: v AC rms ¼ V 2. input utility-line frequency: f AC ¼ 60 Hz 3. minimum input voltage: v AC rms, min ¼ 85 V 4. output power: P out ¼ 100 W 5. output voltage: V DC ¼ 200 V 6. output current: I DC ¼ 0.5 A 7. minimum switching frequency f sw min :10kHz 8. maximum duty cycle D max : estimated efficiency h: 90% 5.2 Determining peak value of input current and inductor current According to (6) and (7), the peak value of input current I in-pk and magnetic inductor current I Lm-pk are, respectively given by and 2 Pout I AC pk hv AC rms, min A I Lm pk 2I AC pk A 5.3 Determining the magnetic inductor L m From (13), the magnetic inductor L m can be determined by L m V 2 ( in rms, min 2 ) 2Vin rms,min sin(2pf AC t) V ( DC 2f sw min P in 4 ) 2Vin rms,min V DC 85 2 ( 2 ) ( 2 10k(100/0.9) 4 ) 468 mh f sw (2pf AC t) T on (2pf AC t) + T off (2pf AC t) V 2 ( AC rms, min 2 ) 2VAC rms, min sin(2pf AC t) V DC ( 2L m P in 2 2VAC rms, min sin(2pf AC t) V DC + 2 ) 2 VAC rms, min (12)

7 5.4 Determining the turns-ratio n Referring to (15), the turns-ratio n is given by n N f N b (1 D max)v DC 1 2 VAC rms, min D max (1 0.5) In addition, the turns-ratio n is selected as Determining the voltage sensing resistors R v1 and R v2 From (17) with a V sv of 2.5 V, and the resistor R v1 is appropriately chosen to be 10 kv, the resistor R v2 is given by R v2 R v1 V sv 10k 2.5 V DC V sv V 5.6 Determining the current sensing resistor R c According to (18) with a V sc of 200 mv and an I DC, max of 0.5 A, the resistor R c is given by R c 6 Experimental results V sc 200m I DC, max V A prototype driver with universal input utility-line voltage ranges for supplying a 100 W LED light bar module (eight high-power-rated LEDs in series connection and rated power of 12.5 W for each LED) has been built and tested in this paper, and Table 1 displays the specifications of the utilised high-power-rated LED light bar module. The key parameters in the proposed driver for high-power LED are presented in Table 2. The measured gate-driving signal v GS of power switch S b, magnetic inductor current i Lm and their zoomed-in waveforms are shown in Fig. 7. Table 1 Specifications of the utilised high-power-rated LED light bar module Items Table 2 Parameters Key Parameters in the proposed high-power LED driver filter inductor (L in ) filter capacitor (C in ) magnetic inductor (L m ) power switch (S b ) power diode (D f, D b ) capacitors (C f,c b) resistors (R v1, R v2, R c ) Values Values each LED rated voltage 25 V rated current 0.5 A rated power 12.5 W eight LEDs in series connection output voltage 200 V output current 0.5 A output power 100 W 3mH 100 nf/630 V 0.47 mh SPP20N60S5 BYT56M 100 mf/160 V 10kV, 126 V, 0.4 V The measured input utility-line voltage v AC (from 110 V-rms) and current i AC of the presented driver for supplying the first LED light bar module is shown in Fig. 8. Fig. 9 depicts the measured output voltage V DC and current I DC of the power driver for supplying the first LED light bar module at an input voltage of 110 V. The rms values of output voltage and current of three LED light bar modules are approximately 200 V and 0.5 A, respectively. Table 3 presents the measured output voltage V DC and current I DC of the power driver for supplying three experimental LED light bar modules. In addition, the output voltage ripple of three LED light bar modules are 7.2, 8.2 and 7.3%, respectively; the output-current ripple of three LED light bar modules are 11.4, 13.6 and 11%, respectively. Fig. 10 shows the measured input utility-line current harmonics of the presented driver for supplying three LED light bar modules, and compares them with the IEC Class C standards as determined by utilising a power analyser. In addition, all of the measured current harmonics Fig. 7 Measured gate-driving signal v GS (10 V/div) and magnetic inductor current i Lm (2 A/div), Time: 2 ms/div; Along with their zoomed-in waveforms (10 V/div, 1 A/div, 20 ms/div)

8 Fig. 8 Measured input utility-line voltage v AC (100 V/div) and current i AC (2 A/div) for first LED light bar module at an input voltage of 110 V; Time: 5 ms/div Fig. 9 Measured output voltage VDC (100 V/div) and current IDC (0.5 A/div) for first LED light bar module at an input voltage of 110 V; Time: 2 ms/div of the proposed driver for supplying three LED light bar modules meet the IEC standards requirements. Figs. 11 and 12 show measured PF and efficiency of the proposed driver for supplying three LED light bar modules under universal input utility-line voltages (from 85 to 265 V), respectively. In the first experimental LED light bar module at a utilityline rms voltage of 110 V, the measured PF and efficiency are 0.99 and 91.94%, respectively. Moreover, the highest and lowest measured efficiency are 92.7 and 88.43%; these occurred at a utility-line rms voltage of 135 and 265 V, respectively. The highest and lowest measured PF are and 0.89; these happened at a utility-line rms voltage of 85 and 265 V, respectively. In the second experimental LED light bar module at a utility-line rms voltage of 110 V, the measured PF and efficiency are and 92.07%, respectively. Additionally, the highest and lowest measured efficiency are 93.1 and 87.83%; these occurred at a utilityline rms voltage of 135 and 265 V, respectively. The highest and lowest measured PF are and 0.875; these happened at a utility-line rms voltage of 85 and 265 V, respectively. In the third experimental LED light bar module at a utility-line rms voltage of 110 V, the measured PF and efficiency are and 92.5%, respectively. In addition, the highest and lowest measured efficiency are 93.4 and 88.73%; these occurred at a utility-line rms voltage of 135 and 265 V, respectively. The highest and lowest measured PF are and 0.883; these happened at a utility-line rms voltage of 85 and 265 V, respectively. Fig. 13a shows a photo of three experimental LED light bar modules, taken by a digital camera. Figs. 13b d depict the photos of the prototype LED driver for respectively, providing three LED light bar modules (eight high-powerrated LEDs included in each module) after lighting up and operating at rated output power. Table 3 Measured output voltage and current for three LED light bar modules Parameters Output voltage, rms Output voltage, pk pk Output voltage ripple,% Output current, rms Output current, pk pk Output current ripple, % first LED light bar module 200 V 14.3 V A 57 ma 11.4 second LED light bar module 200 V 16.3 V A 68 ma 13.6 third LED light bar module 200 V 14.5 V A 55 ma 11 Fig. 10 Measured input utility-line current harmonics of the presented driver for supplying three LED light bar modules compared with IEC class C standards

9 Fig. 11 Measured power factor of the presented driver for supplying three LED light bar modules under universal input utility-line voltages Fig. 12 Measured efficiency of the presented driver for supplying three LED light bar modules under universal input utility-line voltages 7 Conclusion The proposed single-stage power driver for a high-power LED light bar module, integrating a buck-boost PFC converter with a non-isolated flyback converter with two output windings connected in series, with HPF and using a constant output voltage and limitation of output current control scheme, offers fewer power switches and greater cost-effectiveness than the conventional version. A prototype circuit has been built in order to provide a 100 W LED light bar module with universal input utility-line voltages. In addition, the measured outcomes of the proposed driver for supplying three LED light bar modules with universal input voltages are also included in this paper. The experimental results have demonstrated HPF (the highest PF is larger than at an input voltage of 85 V), high efficiency (the highest efficiency is larger than 92.7% at an input voltage of 135 V), low THD of utility-line current (the measured current THD is smaller than 14% at an input voltage of 110 V), constant output voltage and Fig. 13 Photos of three experimental LED light bar modules a Photo of the three experimental LED light bar module, and photos of the prototype LED driver for providing b First LED light bar module c Second LED light bar module, and d Third LED light bar module after lighting up and operating at rated output power limited output-current control for providing with three LED light bar modules; these results verify the functionality of the proposed LED driver.

10 8 Acknowledgments This work was supported in part by the National Science Council of Taiwan (R.O.C.) under grant no. NSC E In addition, the authors would like to thank Bestec Power Electronics Co., Ltd., for their financial support in this project and Anteya Technology Corporation for providing the authors with high-power-rated light bar-type LED modules and their aluminum heat-dissipation sinks. 9 References 1 Zukauskas, A., Shur, M., Gaska, R.: Introduction to solid-state lighting (Wiley, 2002) 2 Schubert, E.F.: Light-emitting diodes (Cambridge University Press, 2006) 3 Sauerlander, G., Hente, D., Radermacher, H., Waffenschmidt, E., Jacobs, J.: Driver electronics for LEDs. Proc. IEEE 41th IAS Annual Meeting, 2006, pp ON Semiconductor: LED lighting solutions, 2010, pp Hui, S.Y.R., Qin, Y.X.: General photo-electro-thermal theory for lightemitting diodes (LED) systems, IEEE Trans. Power Electron., 2009, 24, (8), pp Chiu, H.J., Lo, Y.K., Chen, J.T., Cheng, S.J., Lin, C.Y., Mou, S.C.: A high-efficiency dimmable LED driver for low-power lighting applications, IEEE Trans. Ind. Electron., 2010, 57, (2), pp Hui, S.Y.R., Li, S.N., Tao, X.H., Chen, W., Ng, W.M.: A novel passive off-line LED driver with long lifetime, IEEE Trans. Power Electron., 2010, 25, (10), pp Sebastian, J., Lamar, D.G., Arias, M., Rodriguez, M., Hernando, M.M.: A very simple control strategy for power factor correctors driving highbrigtness LEDs. Proc. IEEE APEC 08, 2008, pp Zhou, K., Zhang, J.G., Yuvaraj, S., Weng, D.F.: Quasi-active power factor correction circuit for HB LED driver, IEEE Trans. Power Electron., 2008, 23, (3), pp Chen, Y.S., Liang, T.J., Chen, K.H., Juang, J.N.: Study and implementation of high frequency pulse LED driver with selfoscillating circuit. Proc. IEEE ISCAS 11, 2011, pp Chen, Y.J., Yang, W.C., Moo, C.S., Hsieh, Y.C.: A high efficiency driver for high-brightness white LED lamp. Proc. IEEE TENCON 2010, 2010, pp Long, X., Liao, R., Zhou, J.: Development of street lighting systembased novel high-brightness LED modules, IET Optoelectron., 2009, 3, (1), pp Chiu, H.J., Cheng, S.J.: LED backlight driving system for large-scale of LCD panels, IEEE Trans Ind. Electron., 2007, 54, (5), pp Wu, C.Y., Wu, T.F., Tsai, J.R., Chen, Y.M., Chen, C.C.: Multistring LED backlight driving system for LCD panels with color sequential display and area control, IEEE Trans. Ind. Electron., 2008, 55, (10), pp Qu, X., Wong, S.C., Tse, C.K., Ruan, X.: Isolated PFC pre-regulator for LED lamps. Proc. IEEE IECON 08, 2008, pp Ye, Z., Greenfeld, F., Liang, Z.: A topology study of single-phase offline AC/DC converters for high brightness white LED lighting with power factor pre-regulation and brightness dimmable feature. Proc. IEEE IECON 08, 2008, pp Qin, Y.X., Chung, H.S.H., Lin, D.Y., Hui, S.Y.R.: Current source ballast for high power lighting emitting diodes without electrolytic capacitor. Proc. IEEE IECON 08, 2008, pp Punjabi S.: The highly effective LED lightbar to serve various purposes, [Online]. Available at: html 19 Broeck, H., Sauerlander, G., Vendt, M.: Power driver topologies and control schemes for LEDs. Proc. IEEE APEC 07, 2007, pp Rico-Secades, M., Calleja, A.J., Cardesin, J., Ribas, J., Corominas, E.L., Alonso, J.M., Garcia, J.: Driver for high efficiency LED based on flyback stage with current mode control for emergency lighting system. Proc. IEEE 39th IAS Annual Meeting, 2004, pp Gacio, D., Alonso, J.M., Calleja, A.J., Garcia, J., Rico-Secades, M.: A univrsal-input single-stage high-power-factor power supply for HB- LEDs based on integrated buck-flyback converter, IEEE Trans. Ind. Electron., 2011, 58, (2), pp Cheng, S.J., Yan, Y.C., Chuang, C.C., Chiu, H.J., Lo, Y.K., Mou, S.C.: A single-stage high efficiency high power factor LED driver. Proc. IEEE VPPC 09, 2009, pp Shao, J.W.: Single stage offline LED driver. Proc. IEEE APEC 09, 2009, pp