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DI-57

Table 1. Transformer Design Parameters.

The bias winding is rectified and filtered by D1 and C23 and

provides operating power to the DPA-Switch. The regulation

is fed back from the secondary through the opto-transistor of

U2.

Key Design Points

• For the nominal under-voltage set point V

R1 = (V

V

• For highest efficiency designs: use continuous conduction

mode operation, this design uses approximately 0.5 KRP;

minimize turns in the transformer and at this (60 W) power

level, keep AC flux density BM < 1100 Gauss; fully fill a

single layer for each winding to minimize leakage

inductance and maximize copper fill factor; if possible use

a Schottky rectifier diode (D2) with a low-forward drop.

• The transformer primary is split in order to minimize

leakage inductance to obtain better efficiency.

• Tighter output voltage tolerance is possible by using a

TL431 (and associated components) in place of the Zener

diode VR2.

• Follow good layout practices:

- For length of the secondary current loop from transformer

pins 9 & 10, diode D2 and capacitors C10 to C18 and

back to pins 6 & 7 of the transformer: ensure identical

path length for each of the capacitors C10 to C18 to

ensure they equally share the ripple current.

TRANSFORMER PARAMETERS

Core Material

Bobbin

Winding Details

Primary-1 (4-NC), Bias (1-5), + 12 V

(9, 10-6, 7), Primary-2 (NC-2)

Winding Order and

Pin Numbers

Primary Inductance

Primary Resonant

Frequency

Leakage Inductance

Primary 8T + 8T, 2 x 24 AWG

Bias 5T, 1 x 34 AWG

+12 V 5T, 4 x 27 AWG

3.0 MHz (minimum)

Ferroxcubed P/N: EFD25 3F3

10-pin EFD25 surface mount bobbin

21 µH ±25% (at 400 kHz)

1 µH (maximum)

For the latest updates, visit our Web site: www.powerint.com

Power Integrations reserves the right to make changes to its products at any time to improve reliability or manufacturability.

Power Integrations does not assume any liability arising from the use of any device or circuit described herein, nor does it convey any license

under its patent rights or the rights of others. The products and applications illustrated herein (including circuits external to the products and

transformer construction) may be covered by one or more U.S. and foreign patents or potentially by pending U.S. and foreign patent

applications assigned to Power Integrations. A complete list of Power Integrations’ patents may be found at www.powerint.com.

The PI Logo, TOPSwitch, TinySwitch, LinkSwitch and EcoSmart are registered trademarks of Power Integrations, Inc.

PI Expert and DPA-Switch are trademarks of Power Integrations, Inc. ©Copyright 2003, Power Integrations, Inc.

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