1. POWER ELECTRONICS R & D LABORATORY DESIGN OF FORWARD CONVERTER WITH LCD SNUBBER Presented by Laszlo Huber November, 1999 Chungli, Taiwan

2. POWER ELECTRONICS R & D LABORATORY EXAMPLE DPS-200PP-76 1. SPECIFICATIONS –Line voltage: 90-265 V, 47-63 Hz –Outputs: 5 V / 1.5-20 A 12 V / 0.2-8 A 3.3 V / 0-20 A -5 V / 0-0.3 A -12V / 0-0.4 A 5 Vaux / 5-720 mA P omax = 200 W (combined 5V/3.3V P omax = 120 W) P omin = 9.9 W (~ 5%) –Hold-up time: 10 ms 2. TOPOLOGY Multi-output forward converter with non-dissipative (LCD) snubber and voltage-doubler rectifier (w/o PFC) 3. MAJOR DESIGN COMPONENTS –Bulk capacitors C B1,2 –Primary-side switch Q –Secondary-side diodes –Forward Transformer T –Forward inductors

3. POWER ELECTRONICS R & D LABORATORY 4. BULK CAPACITORS C B1,2 At low-line range With

4. POWER ELECTRONICS R & D LABORATORY 5. V Bmin WITH HOLD-UP TIME Const. power load Forward converter should be designed for bulk-voltage range:

5. POWER ELECTRONICS R & D LABORATORY 6. FORWARD CONVERTER TRANSFER FUNCTION Derivation of forward-converter transfer function from forward-inductor flux-balance (CCM):

6. POWER ELECTRONICS R & D LABORATORY 7. COMPARISON OF DIFFERENT RESET TECHNIQUES FOR FORWARD TRANSFORMER –Reset winding –RCD clamp –Active clamp –LCD snubber 7.1 FORWARD CONVERTER WITH RESET WINDING Transformer flux balance: Maximum switch voltage: Usually:

7. POWER ELECTRONICS R & D LABORATORY

8. POWER ELECTRONICS R & D LABORATORY 7.2 FORWARD CONVERTER WITH RCD CLAMP Clamp voltage V cl  f (V B ) ! Goal: 600-V MOSFET From transformer flux balance: Turns ratio (@ 5V output) Sec-side diodes:

9. POWER ELECTRONICS R & D LABORATORY 7.3 FORWARD CONVERTER WITH ACTIVE CLAMP Clamp voltage Goal: 600- V MOSFET From transformer flux balance: Design approach:

10. POWER ELECTRONICS R & D LABORATORY Derivation of D max + D min = 1 Substitute V clmax from (1) and V clmin from (2) into (3): From the forward-converter transfer function Combining (6) and (7): (1) (2) (3) (4) (5) (6) (7)

11. POWER ELECTRONICS R & D LABORATORY Turns ratio (@ 5V output) Sec-side diodes: Advantages of active-clamp reset: 25 - 35 V Schottky Diode — larger N lower current stress on prim.-side lower voltage stress on sec.-side — transformer operates in I and III quadrant From Faraday’s Law: — ZVS (Zero-voltage-svitching) efficiency higher switching frequency — lower EM I smaller core ( A c )