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A

9/03

DI-56

Table 1. Transformer Design Parameters.

Key Design Points

• For the nominal under-voltage set point V

R1 = (V

V

• For highest efficiency designs: use continuous conduction

mode operation designed at approximately 0.4 KRP;

minimize turns in the transformer and at this (19 W) power

level keep AC flux density (BM) <1500 Gauss; fully fill a

single layer for each winding to minimize leakage

inductance and maximize copper fill factor; if possible use

Schottky rectifying diodes (D20 and D30) with a low-

forward drop.

• The transformer primary is split in order to minimize

leakage inductance and thus obtain better cross-regulation.

Note: minimizing primary leakage inductance will improve

output cross-regulation at load extremes.

• The -12 V output is not directly sensed as part of the

regulation loop. Cross-regulation may be improved by

adding a second sense resistor to work in conjunction with

R10. Both resistors R10 and the second sense resistor

would be changed to 76 k

Ω each.

• Set resonant frequency of post-filter (L2, C22 or L3, C32)

beyond crossover frequency (typically 5% to 10% of

switching frequency).

• Good layout practices

- For length of +12 V secondary current loop from

transformer pin 8, diode D30 and capacitors C30, C31

and back to pin 7 of the transformer: ensure identical

path length for C30 and C31 to guarantee they equally

share the ripple current.

- The same is also true for the layout of the -12 V output.

• Choosing a larger DPA-Switch will increase efficiency at

low and medium input voltages.

TRANSFORMER PARAMETERS

Core Material

Bobbin

Winding Details

Winding Order &

Pin Numbers

Primary Inductance

Primary Resonant

Frequency

Leakage Inductance

Primary 7T + 7T, 2 x 29 AWG

Bias 5T, 1 x 36 AWG

+12 V 5T, 2 x 29 AWG

-12 V 5T, 2 x 29 AWG

22 µH ±25% (at 400 kHz)

Epcos P/N: P 14x8 N87, ungapped

8-pin P 14x8 surface mount bobbin

Primary-1 (4-NC), -12 V (6-5), Bias (2-3),

+12 V (8-7), Primary-2 (NC-1)

3.8 MHz (minimum)

0.75 µH (maximum)