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AN4105

Design Considerations for Switched Mode Power Supplies Using A Fairchild Power Switch (FPS) in a Flyback Converter

Fairchild Semiconductor 

AN4105

V tx

Vin

V in

V sn

Fairchild Power Switch(FPS)

SPS

C o n tro l

5

IC

n :1

1

2

4

V fb

3

V cc

Cfb Ccc

Rsd

V tx

n *:1

(a) F lyback conv erter

n *V sn /n

Vo

n *V o

0

APPLICATION NOTE

LM431

K A 4 31

No Rsd

Low Rsd

L a rg e R sd

V cc

1 0V

7 .5V

n *V in /n

(b )

The voltage

F layback

Vtx waveform

conv erter

of

c

oNnBtar ondl l

eVrCCv(tohlet aregcetifieadn

VdNBR)

depending on Rsd

sd dependen t

V cc

(1 )

(2 )

(3 )

S h u tdo wn

UVLO

S h u tdo wn

3V

V fb

0V

Cvcc

t

O u tp ut sh o rt

UV L O S h u tdo wn

(C ) V cc and V fb w aveform s depending on the relative size of C vc c at output short

Figure 11. Operation of the SMPS flyback converter’s output short circuit protection (latch mode ).

8

©2002 Fairchild Semiconductor Corporation

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