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7/03

DI-52

Table 1. Transformer Construction Information.

Figure 2. Efficiency vs. Output Power.

conduction path for the output inductor (L4) current when the

transformer reset is complete.

Key Design Points

• Transformer core reset is critical in this design. MOSFET

gate loading will affect the transformer-reset waveform.

Capacitors C20, C22 and C

reset. Choose values to ensure sufficient reset at low line

and safe maximum drain voltage at high line. Also use

300 kHz operation for longest reset time.

• Capacitors C20 and C22 will capacitively drive MOSFET

gate capacitances C

C22 should be chosen to ensure that gate drive voltage

attains turn-on threshold of MOSFET (Vg

conditions (low line for forward MOSFET).

• Reduce transformer leakage inductance by filling each

winding layer across the entire width of the bobbin.

Table 2. L4 Output Inductor Design Parameters.

02

3

4

15

Pout (W)

70

60

50

80

90

100

36 VDC

48 VDC

72 VDC

TRANSFORMER PARAMETERS

Core Material

Bobbin

Winding Details

Bias (2-5), Primary-1 (4-NC),

12 V (9,10-6,7), Primary-2 (NC-1)

Winding Order and

Pin Numbers

Primary Inductance

Primary Resonant

Frequency

Leakage Inductance

Primary 5T + 5T, 4 x 26 AWG

Bias 5T, 1 x 30 AWG

12 V 6T, 4 x 26 AWG

3.8 MHz (minimum)

Ferroxcube P/N: EFD25, ungapped

10-pin EFD25 surface mount bobbin

Pin (1-4): 190 µH ±25% @ 300 kHz

1 µH (maximum)

INDUCTOR PARAMETERS

Core Material

Bobbin

Winding Details

Inductance

Pin (4,5-9,10): 40 µH ±10%

@ 300 kHz

Main (4,5-9,10)

Main 18T, 3 x 24 AWG

10-pin EFD20 surface mount bobbin

Ferroxcube P/N: EFD20-3F3

gap for inductance required

Winding Order and

Pin Numbers