DI-30
clamp/reset circuit to maintain drain voltage < 600 V and
DCMAX reduction to prevent transformer saturation.
• Maintain maximum flux density on transformer (T1)
< 2500 gauss.
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).
• 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.
During the OFF state, U3 and Q7 are off, allowing the X pin
tobepulledhighbythe+15VstandbysupplyviaR38andR12
andputtingtheTOPSwitch-GX intotheOFFstate.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.
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.
Key Design Points
Due to the complexity of this design it is not possible to include
alldetailsinthisDesignIdea. AnEngineeringPrototypeReport
(EPR) will be available for this design Dec. 2000. For updates
and all other information please refer to Power Integrations'
Web site.
• 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
D9
MBR2045
L4
20 µH
L5
0.5 µH
FROM MAIN
3.3 V
CONVERTER
L3
Mag Amp
6 Turns
R22
5 V WINDING
3 Ω
R23
R25
2.7 kΩ
R24
3.84 kΩ
1%
1 kΩ
Q2
C14
1200 µF
10 V
C15
1200 µF
10 V
TIP32
R27
390 Ω
C18
0.1 µF
50 V
R34
33 Ω
D11
UF4002
C21
1 µF
50 V
R26
10 kΩ
1%
D10
UF4002
U8
TL431
RTN
RTN
PI-3385-093002
Figure 2. Magamp for Independent 3.3 V Secondary Regulation.
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.
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