Application Note RT8462 Buck-Boost Design for MR16 LED Applications with High Power Factor Introduction To Replace AC-driven halogen lighting by LEDs based on a switching LED driver with high power factor, the following design of RT8462 Buck-Boost application was developed to fulfill the requirement. Because an LED driver with constant (DC) LED current has very poor power factor, we need to create an LED driver with input current in phase with the input voltage. When used with a 12VACrms source from electronic transformer and a rectifier bridge, the power factor is slightly under 98% using a halogen bulb. For this resistive load request with electronic transformer, the RT8462 Buck-Boost circuit operates in a constant output power mode. In this design, it is minimized the current spikes coming out of an electronic transformer to less than 6A. The LED MR16 solutions exhibit spikes and exceed a transformer’s reliability and operating lifetime. Features: No Electrolytic Capacitor necessary for Lamp life-time. Lower input surge current with electronic transformer. Compatible with different electronic transformers as many as possible. High Power Factor >90% with electronic transformer. Output Current Accuracy:±3%. High efficiency >70% with electronic transformer. Adjustable Switching frequency for Small Components. Inherent open-circuit LED protection. The circuit in Figure 1 is an RT8462 AC LED driver with 96% power factor. The topology is buck-boost mode since the wide-ranging input of 0V to 17V (12Vrms) and the wide ranging AN001 www.richtek.com Application Note 4-LED string voltage of 9V to 14V crossover each other. The 12VAC(rms) input is first rectified by D3-D6 to create a 120Hz 0V to 18V supply. The RT8462 works best when the Vcc pin is held above 7V and driving the power MOSFET M1 with proper gate voltage. LED current foldback with the ACTL pin voltage is used to create a high power factor LED driver. The maximum LED current is set by R4 at 680mA, but the ACTL pin monitors the VIN voltage and shapes the LED current waveform to match VIN. A light soft-start capacitor allows the RT8462 to get started up quickly and keep the power factor high. With the ACTL pin folding back the LED current at low VIN, startup is not harsh and inrush currents do not affect the high power factor. Figure 1. RT8462 Buck-Boost Mode LED Driver with 96% Power Factor. In this application circuit, the LED string current and voltage waveforms at 120Hz. The approximate output LED power was multiplied to measure the average LED current and the average LED voltage. Figure 2 shows the 120Hz waveforms generated by the RT8462 buck-boost mode AC LED driver in Figure 1. V_VIN, I_IN and I_LED are shown – all at 120Hz. AN001 www.richtek.com Application Note The 120Hz is a high-enough frequency to not be perceived by the human eye. The I_IN and V_VIN is IN-PHASE operating and the power factor test result achieve to 96%. CH1 : V_ACTL CH2 : I_LED CH3 : I_IN CH4 : V_VIN Figure 2. 120Hz AC LED current and voltage waveforms delivering ~4W of LED power. Generally, the electronic transformers modulated the input AC voltage with a frequency of 30 kHz to150 kHz. This waveform is step-down from 120V or 230V (typical) to 12VAC with a transformer. The electronic transformer is a self -driven resonant half bridge converter and has a minimal load for function properly. The electronic transformers will cause visible flicker from the MR16 lamp, and induce some switching noise at the circuit. In This compatibility of the electronic transformers and application circuit could also cause reliability issues with the lamps. AN001 www.richtek.com Application Note PCB Schematic L2 2 ACTL L1 22uH/2A-5A C1 1 ACL R6 100K 2 LED- D4 1.8uH/2A-5A PMEG4020 1 D3 PMEG4020 VIN 1uF/25V C7 4.7uF/25V D2 ACN VOUT D5 PMEG4020 R7 10K C3 R8 GATE 0R R2 M1 36N06 G1 2 420K 3 1 LED+ R4 470mohm PMEG4020 2 D D6 PMEG4020 S 4.7uF/25V 16 15 14 R9 2 GATE NC ISP 10K 1 2 3 4 NC RT8462 U1 5 SS 1 2 1 10nF R1 50mohm R5 2 C4 VC ISN AC TL EN 10K 8 SR14 GND OVP C6 1uF 10K 1 9 R10 ISW 2 D1 VIN VCC D CTL 10 RSET 6 11 OVP GND R3 10K ACTL 12 GBIAS C2 4.7uF/25V 7 2 1 13 1 OVP C5 3.3nF PCB Layout AN001 www.richtek.com Application Note EVB Outline Bill of Materials Reference Qty. Part Number Description Package High Voltage Multi-Topology WQFN-16L 3x3 Manufacture U1 1 RT8462 C2,C3,Cout 3 GRM32ER71H475KA88 4.7uF 1210 Murata C1,C6 2 GRM32ER72A105KA35 1uF 1210 Murata C5 1 GRM188R71H333K 3.3nF/16V/X7R 0603 Murata C4 1 GRM188R71H103K 10nF/50V/X7R 0603 Murata 1 PMEG4020 Schottky Barrier Rectifiers DO-214 NXP L1 1 NR8040T220M 22uH, 8*8 TAIYO YUDEN L2 1 NR4020T20M 2.0uH, 4*4 TAIYO YUDEN M1 1 FPD36N06N N-Channel MOSFET TO-252 IPS R1 1 CS12FTER50 50mohm 2512 Viking R4 1 CS12FTER470 470mohm 2512 Viking R6 2 WR06X1003F 100k 0603 Walsin 4 WR06X1002F 10K 0603 Walsin 1 WR06X4203F 420K 0603 Walsin D2,D3,D4,D5, D6 R3,R5,R7,R9, R10 R2 AN001 LED Driver RichTek www.richtek.com Application Note The RT8462 buck-boost application compared the performance of each reference design when powered by a 12VAC source and real electronic transformer in the Table 1. Power Peak Factor Inrush(A) 9.07 0.96 1.12 80.67 0.544 10.38 0.94 1.92 69.18 13.2 0.544 10.11 0.96 2.80 71.42 LIGHTECH LET 60 120VAC 13.3 0.547 10.16 0.96 2.24 71.21 HATCH RS12-80M 120VAC 13.3 0.544 10.96 0.96 3.40 66.01 Power Source INPUT VOUT(V) IOUT(A) Input Power(W) AC-POWER 12VAC 13.4 0.546 PHILIPS ET-E60 220VAC 13.2 EUROFASE 0084 220VAC Efficiency(%) Table 1. The performance of 12VAC source and real electronic transformer PHILIPS ET-E60 EUROFASE 0084 LIGHTECH LET 60 HATCH RS12-80M Table 2. The electronic transformer were tested with theRT8462 designs Listed in Table1 AN001 www.richtek.com Application Note The following figures show voltage and current waveforms designated by the RT8462 buck-boost application with the electronic transformer used was the PHILIPS ET-E60. CH1 : V_LED CH2 : V_VIN CH3 : I_ILED CH4 : I_VIN Figure 3. LED current and voltage waveforms delivering by electronic transformer ET-E60. For optimized the compatibility of LED lamp with different electronic transformer, place bigger input capacitor C1 will achieve this better matching result. In the other hand, this input capacitor will cause higher input inrush current and worse power factor. For dedicated design of electronic transformer, RT8462 buck-boost is the best solution which implement the compactable and high-performance with changing the compensation components. AN001 www.richtek.com Application Note More Information For more information, please find the related datasheet or application notes from Richtek website: http://www.richtek.com/LED/ Richtek Technology Corporation Headquarter 5F, No. 20, Taiyuen Street, Chupei City Hsinchu, Taiwan, R.O.C. Tel: (8863)5526789 Fax: (8863)5526611 AN001 www.richtek.com