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AN4105

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

Fairchild Semiconductor 

APPLICATION NOTE

AN4105

Fairchild Power Switch(FPS) V

SPS

2uA 1mA

7.5V

Vo

Vfb

#

Cf 4

Cd

b

D1 D2

Vfb*

3 . 9V

3V

KLMA443311

Vz=3.9V

7 . 5V

0

t1 t2

Tim e Constant

t3

2uA = Cfb*0.9V/t2

t

S h u t do wn

= 3.5R*Cfb

2uA = Cd*3.6V/t3

(a)

<Fairch<ildSPPowSerloSnwgitcDhe(FlaPySe)dloSnghDutedlaoywend S>hutdown>

(b)

Figure 10. Long delay shutdown.

2.2 Output Short Circuit Protection

When the SMPS’s output terminal is short circuited the input

current is at maximum. The output power however, is not a

maximum. This is because there can be no load voltage since

the load is a short circuit. Under such conditions the output

short circuit protection operates as follows.

When the output load short circuits a relatively large

transformer winding in the SMPS, the Fairchild Power

Switch(FPS)’s MOSFET current becomes much greater than

Ipeak because the inductor’s magnetic core is unable to reset.

This is due to low transformer coil voltage at turn off. This

occurs because the current remaining in the inductor during

the Fairchild Power Switch(FPS)’s minimum turn on time

cannot decrease by that amount during the remaining turn off

time. Even though it is a large current, it does not unduly

stress the MOSFET but does greatly stress the transformer’s

secondary coils and the secondary side rectifiers.

In most flyback or forward converters the controller gets its

power from a small secondary bias winding in the

transformer. Furthermore, this controller voltage is propor-

tional to the output voltage (see circuit of Figure 11a). This

is fairly straightforward because, for a flyback converter, the

coil voltage when the switch turns off is proportional to the

output voltage.

For a forward converter, the transformer average voltage at

turn off is proportional to the output voltage. For a flyback

converter SMPS, the output short circuit protection circuit

can be operated in either latch mode or auto restart mode.

The change in Vcc as a function of Rsd is shown in Figure

11b. Rsd is shown in Figure 11a. When Rsd is zero, Vcc

reaches the maximum value of Vtx (see lower primary

winding, Figure 11a), i.e., n'Vsn/n, which is proportional to

the maximum transformer current. For this case, if the output

short circuits, Vcc increases and, after a specified delay, the

protection circuit operates, entering the latch mode.

However, when Rsd is made sufficiently large, Vcc can

become smaller than n'Vo. Therefore, if the output short

circuits, Vcc drops, but, if Ccc (see Figure 11a) is sufficiently

large, Vcc stays at a level higher than the UVLO's lower

threshold voltage (10V) until Vfb reaches 7.5V (Figure 11c-

2) and latch mode protection starts. In contrast, if Ccc is

small enough Vcc approaches the UVLO's low threshold

before Vfb reaches 7.5V (Figure 11c-3), and the UVLO

operates instead of the protection circuit, stopping the device

switching. In such a case, if Vcc exceeds the ULVO's upper

threshold voltage (15V), auto restart operates again. Rsd,

Ccc, and Cfb have affects even at start up and power down, so

their values must be decided upon carefully. Our

experiments have indicated that 10-20W is most appropriate

for Rsd.

©2002 Fairchild Semiconductor Corporation

7

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