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12/02

DI-30

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Figure 2. Magamp for Independent 3.3 V Secondary Regulation.

The components R12, C7, R38, Q7, C23, R37, U3, R106 and

D105 implement the remote ON/OFF drive circuit. During

the ON state, U3 and hence Q7 conduct, pulling the X pin to

SOURCE via resistor R12 (which sets the current limit).

During the OFF state, U3 and Q7 are off, allowing the X pin

to be pulled high by the +15 V standby supply via R38 and R12

and putting the TOPSwitch-GX into the OFF state. Components

R38 and D105 reduce device consumption to around 2mW by

supplying external current to the CONTROL pin from the

+15 V standby supply. Resistor R11 is required to allow

external bias feed.

Key Design Points

• The passive PFC inductors (LPFC1 and LPFC2) are

constrained by both thermal and efficiency requirements.

Design of these inductors is not covered in this Design Idea.

• Transformer reset: Use recommended Zener/capacitor

clamp/reset circuit to maintain drain voltage < 600 V and

DC

•Maintain maximum flux density on transformer (T1)

< 2500 gauss.

• Check for balanced currents on coupled inductor (L1) for

all load combinations.

• Use PI Expert (PIXls) Design Spreadsheet and refer to

Application Note AN-30 for details on designing forward

converters with TOPSwitch-GX.

This particular PC SFX 12 specification delivers a larger

proportion of power on the 12 V winding; efficiency would be

lower if that power were drawn from the 3.3 V output.

Due to the complexity of this design it is not possible to include

all details in this Design Idea. An Engineering Prototype Report

(EPR) will be available for this design Dec. 2000. For updates

and all other information please refer to Power Integrations'

Web site.

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 is a trademark of Power Integrations, Inc. ©Copyright 2002, Power Integrations, Inc.

3.3 V

RTN

RTN

PI-3385-093002

D11

UF4002

D10

UF4002

R22

3 Ω

Q2

TIP32

D9

MBR2045

L4

20 µH

L5

0.5 µH

R34

33 Ω

R27

390 Ω

R23

1 kΩ

R25

2.7 kΩ

C18

0.1 µF

50 V

R24

3.84 kΩ

1%

R26

10 kΩ

1%

U8

TL431

C21

1 µF

50 V

L3

Mag Amp

6 Turns

C14

1200 µF

10 V

C15

1200 µF

10 V

FROM MAIN

CONVERTER

5 V WINDING