POWER ELECTRONICS R D LABORATORY DESIGN OF FORWARD

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

POWER ELECTRONICS R & D LABORATORY EXAMPLE DPS-200 PP-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 -12 V / 0 -0. 4 A 5 Vaux / 5 -720 m. A Pomax = 200 W (combined 5 V/3. 3 V Pomax = 120 W) Pomin = 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 CB 1, 2 Primary-side switch Q Secondary-side diodes Forward Transformer T Forward inductors

POWER ELECTRONICS R & D LABORATORY 4. BULK CAPACITORS CB 1, 2 At low-line range With

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

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

POWER ELECTRONICS R & D LABORATORY 7. COMPARISON OF DIFFERENT RESET TECHNIQUES FORWARD TRANSFORMER – Reset winding – RCD clamp – Active clamp – LCD snubber 7. 1 FORWARD CONVERTER WITH RESET WINDING Transformer flux balance: Maximum switch voltage: Usually:

POWER ELECTRONICS R & D LABORATORY

POWER ELECTRONICS R & D LABORATORY 7. 2 FORWARD CONVERTER WITH RCD CLAMP Clamp voltage Vcl f (VB) ! Goal: 600 -V MOSFET From transformer flux balance: Turns ratio (@ 5 V output) Sec-side diodes:

POWER ELECTRONICS R & D LABORATORY 7. 3 FORWARD CONVERTER WITH ACTIVE CLAMP Clamp voltage Goal: 600 - V MOSFET From transformer flux balance: Design approach:

POWER ELECTRONICS R & D LABORATORY Derivation of Dmax + Dmin = 1 (1) (2) (3) Substitute Vclmax from (1) and Vclmin from (2) into (3): (4) (5) (6) From the forward-converter transfer function (7) Combining (6) and (7):

POWER ELECTRONICS R & D LABORATORY Turns ratio (@ 5 V output) Sec-side diodes: 25 - 35 V Schottky Diode Advantages of active-clamp reset: — 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: smaller core ( Ac ) — ZVS (Zero-voltage-svitching) efficiency higher switching frequency — lower EM I