Design Idea DI-67 ® TOPSwitch-GX Isolated, Power Factor Corrected (PFC), 17 W LED Driver Application Device Power Output Input Voltage Output Current Topology LED Arrays TOP246F 17.6 W max 108-132 VAC 60 Hz 700 mA (16-24 V) Flyback The DC input capacitor (C3, 100 nF) was sized so that its voltage approaches zero at the AC input zero crossings (see the middle waveform of Figure 2). Design Highlights • • • • • • 700 mA of output current regulated to within ± 5% Power factor: > 0.98, THD: ≤ 9.6% No output current overshoot at power-on Temperature range: -40 °C to +80 °C Harmonics comply with IEC61000-3-2, Edition 2.1 Conducted EMI complies with CISPR-22 B A 220 µF capacitor (C6) was chosen for the bias supply to minimize the 120 Hz ripple current into the current source (Q1), which provides control current to U1. The output rectifier (D9) must be rated for an average of 3.5 A. Capacitor C7’s value (680 µF) sets the magnitude of the output ripple current to 600 mA peak to peak, at 120 Hz. Operation This low-cost, TOP246F based PFC LED Driver takes advantage of built-in TOPSwitch-GX features. This supply is current (not voltage) regulated, except in the case of no-load. Without a load, the output voltage is limited to about 30 V (max), by R6 and VR2. Configured as a flyback converter, this circuit operates in the discontinuous conduction mode. It can deliver an average of 700 mA (1 A at the peak of output ripple) over a 16 V to 24 V range, which makes it ideal for driving high current LED arrays, such as the Luxeon 12 UP LED Ring. Resistors R7, R8, R9 and U2’s LED set the 700 mA average current limit. The U2 phototransistor drives the current source (Q1) and the PFC-loop filter capacitor (C8, 100 µF). Capacitor C8 was sized to provide the low loop bandwidth necessary for high power-factor. R4 is the power-off discharge path for C6 and C8. The CONTROL pin bypass capacitor (C5, 1 µF) is just large enough to allow smooth start-up of the output load current, and yet small enough to prevent output current overshoot. A larger value of C5 would increase start-up delay time. C4 2.2 nF Y CAP D1-D4 IN4004 L2 150 uH VR1 P6KE200A 700 mA 16-24 V T1 1 C3 100 nF 2 3 1 4 L1 C2 100 nF X2 CAP C7 680 uF 35 V R6 100 Ω 5% 7 5 6 2 D5 UF4005 4 D8 UF4002 C6 220 uF 25 V 3 R4 10 kΩ 5% R2 499 Ω 1% R3 1.00 kΩ 1% 27 mH C1 100 nF X2 CAP RV1 150 V F1 T1A 250 V L PI-3746-110303 Q1 PN2907A D6 UF4005 D TOPSwitch-GX U1 CONTROL TOP246F L C S X D7 1N4148 R5 3.01 kΩ 1% VR2 1N5254B R7 100 Ω 1% F R1 10.0 kΩ 1% 108-132 VAC 60 Hz DI-67 + D9 BYV28-200 8 C5 1.0 uF C8 100 uF 25 V R8 1.50 Ω, 1% U2 PC81711NSZ N Figure 1. Isolated Constant Current PFC LED Driver. www.powerint.com RETURN R9 10.0 Ω, 1% October 2003 DI-67 Key Design Points TRANSFORMER PARAMETERS • To have high power factor, a constant duty factor must be maintained over the 8.33 ms half cycle period. Therefore, the bias supply voltage and the U1 CONTROL pin current must remain extremely constant. The values of C6 and C8 must be chosen accordingly. • Decreasing the value of C8 will reduce the turn-on delay time, but will also degrade the power factor. • Because low-cost is the goal of this low-loop-gain design, the tolerance of the output current depends on the CTR of the opto-coupler and the (unregulated) value of the bias voltage. The restricted AC input voltage range allows using the forward (not flyback) configuration for the bias winding. If the AC input voltage range is extended, voltage regulation of the bias supply circuit will be required. EF-20 or Equivalent ALG of 1570 nH/T2 Core Miles-Platts EF0700 EF20 8 Pin Horizontal Bobbin Winding Details Primary: 70T, 2 layers, 29 AWG Secondary: 13T, 2 x 32 AWG Triple Insulated Wire Bias: 9T, 2 x 23 AWG Winding Order (pin numbers) Primary: 2-5, tape Secondary: 8-7, tape Primary: 5-1, tape Bias: 3-4, tape 3 layers Primary Inductance 350 µH ±10% Primary Resonant Frequency 2.0 MHz (Min) 10 µH (Max) PI-3747-110403 Leakage Inductance 0.5 A/Div. 0A Table 1. Transformer Construction Information. 100 V/Div. 0.5 A/Div. 0A 0 25 50 Time (ms) +5% Upper Specification Limit 720 700 680 LED Current must be maintained within manufacturer's specified temperature range. -5% Lower Specification Limit 660 -40 Figure 2. PFC LED Driver Waveforms. Top: Input Current at 120 VAC 60 Hz. Middle: C3 Capacitor Voltage. Bottom: Output Current (with 20 V LED Load). PI-3748-110303 0V Average Output Current (mA) Voltage (V) 740 -20 0 20 40 60 80 Ambient Temperature (°C) Figure 3. PFC Driver Current vs. Temperature. For the latest updates, visit www.powerint.com Power Integrations reserves the right to make changes to its products at any time to improve reliability or manufacturability. Power Integrations does not assume any liability arising from the use of any device or circuit described herein, nor does it convey any license under its patent rights or the rights of others. The products and applications illustrated herein may be covered by one or more U.S. and foreign patents or potentially by pending U.S. and foreign patent applications assigned to Power Integrations. A complete list of Power Integrations' patents may be found at www.powerint.com. The PI logo, TOPSwitch, TinySwitch, LinkSwitch and EcoSmart are registered trademarks of Power Integrations. PI Expert and DPA-Switch are trademarks of Power Integrations. Copyright 2003, Power Integrations Power Integrations MAIN PHONE NUMBER +1 408-414-9200 A 10/03 5245 Hellyer Avenue APPLICATIONS HOTLINE +1 408-414-9660 APPLICATIONS FAX +1 408-414-9760 www.powerint.com San Jose, California 95138 For a complete listing of worldwide sales offices, please visit www.powerint.com