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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

APPLICATION NOTE

n

Vi=10 0V

V i= 400 V

M e rits & de m e rits

V DS

V DS

nVO=67V

V i=1 0 0V

0

n VO =6 7 V

V i=4 0 0V

VD

1

V i/n =7 5 V

0

V O= 50 V

V D S=1 67V

V D=1 25 V

V DS

n VO =1 0 0 V

V i=1 0 0V

0

2

VD

V i/n =5 0 V

V O= 50 V

0

0

VD

V i/n =2 6 7 V

0

V O= 50 V

VDS=46 7V

VD=31 7V

V DS

n VO =1 0 0 V

V i=4 0 0V

0

VD

V i/n =2 0 0 V

-V oltage applied to sw itch device is

lo w

-E ffective current in sw itching device

and prim ary w inding is low

-V oltage applied to rectifier diode is

h ig h

-O utput volta ge ripple is sm all

-C ontrol m ust be done through short

turn-on tim e

n:1

VLm

VD

Vi

VDS

- Inte rm e dia te D e sign M e thod

V D S=2 00V

V D=1 00 V

V DS

0

V i=1 0 0V

n VO =1 5 0 V

3

VD

V i/n =3 3 V

0

V O= 50 V

V D S=2 50V

VD= 83V

V O= 50 V

0

VDS=50 0V

VD=25 0V

V DS

n VO =1 5 0 V

V i=4 0 0V

0

VD

0

V i/n =1 3 3 V

V O= 50 V

VDS=55 0V

VD=18 3V

-V oltage applied to sw itch device is

h ig h

-E ffective current in sw itching device

and prim ary w inding is high

-V oltage applied to rectifier diode is

lo w

-O utput volta ge ripple is large l

-C ontrol m ust be done through long

turn-on tim e

Figure 18. The current and voltage ratings required on the primary side switching device and the secondary side

rectifier diode depend on the turns ratio (n) selected.

4.3.2.3 Designer’s Choice

As is clear from the above discussion, DCM operation can

be advantageous, in terms of cost and efficiency, if the input

is small and it is required to precisely control the input power

through the primary side switch (MOSFET) current.

On the other hand, for large inputs and where switching turn

on loss could be a major problem, a CCM design would be

more advantageous. In conclusion, therefore, the system

designer must decide between the two modes according to

which mode best fits the characteristics of the system being

designed.

16

©2002 Fairchild Semiconductor Corporation

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