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DI-37

B

7/04

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Table 1. Transformer Design Parameters.

TRANSFORMER PARAMETERS

Core Material

Epcos P/N: P 14 x 8 N87,

ungapped

Bobbin

8-pin P 14 x 8 surface mount

bobbin

Winding Details Primary 10T + 10T, 1 x 26 AWG

3.3 V, 4 x 26 AWG

Winding Order

and Pin Numbers

Primary-1 (4-NC), 3.3 V (5-6),

Primary-2 (NC-1)

Primary

Inductance

600 µH ±25% at 400 kHz

Primary Resonant

Frequency

3.8 MHz (minimum)

Leakage

Inductance

1 µH (maximum)

Key Design Points

• Transformer core reset is critical in this design. MOSFET

gate loading will affect the transformer reset waveform.

Capacitor C

MOSFET such that C

line and safe maximum drain voltage at high line.

• Choose synchronous rectifier MOSFETs which have both

low R

Q

• Reduce transformer leakage inductance by filling each

winding layer across the entire width of the bobbin.

• Choose a low-drop Schottky diode (such as Vishay

SL44 – V

• Choose a larger DPA-Switch to increase efficiency at low

and medium input voltages.

INDUCTOR PARAMETERS

Core Material

Epcos P/N: P 14 x 8 N87,

gap for inductor

Bobbin

8-pin P 14 x 8 surface mount

bobbin

Winding Details Main 10T, 2 x 24 AWG

Bias 30T, 1 x 32 AWG

Winding Order

and Pin Numbers

Main (7,8-5,6)

Bias (1-2)

Inductance

Pin (7,8-5,6): 16 µH ±10% at

400 kHz

Table 2. Inductor Design Parameters.