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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

2.3 Fast Protection Without Delay

It was mentioned above that the Fairchild Power

Switch(FPS) shutdown capability is associated with a delay,

to allow normal transients to occur without shutting down

the system. In effect, this restricts the protection range. If

fast protection is required, then additional circuitry is called

for. Figure 12 shows how it’s done. A transistor is used to

force the feedback photodiode current to increase causing

the primary side Vfb to be forced below 0.3V. Therefore the

Fairchild Power Switch (FPS) stops switching. Depending

on the magnitude of the photodiode current, the protection

n:1

Vin

V in

Vsn

can be made to operate sufficiently fast. In such a case, when

the transistor is turned off, the protection shifts to auto restart

mode for normal operation. It is also possible to use this

circuit as an output enable circuit. Fast latch mode protection

can be implemented by adding a photocoupler. Thus, when

the output terminal latch mode transistor turns on, a large

current flows through the photodiode PC2. A large current

therefore flows through the primary phototransistor, which

increases Vfb rapidly. This executes fast latch mode

protection with no time delay.

Vo

PC1

PC1

FSairPchSild Power Switch(FPS)

1

5

Co n tro l

IC

2

4

Vfb

3

Vcc

Rsd

PC2

Ccc Vtx

Cfb

Vo

PC2

n *:1

LKMA443131

B urst Mo de

Shut Down

Latch Mode

Shut Down

Figure 12. A fast protection circuit without a shutdown delay.

2.4 Overvoltage Protection

The Fairchild Power Switch (FPS) has a self protection

feature that operates even when faults exist in the feedback

path. These could include an open or short circuit. On the

primary side, if the feedback terminal is short circuited, then

the voltage on it is zero and hence the Fairchild Power

Switch (FPS) is unable to start switching.

If the feedback path open circuits, the protection circuit

operates as though a secondary overload is present. Further,

if the feedback terminal looks open due say, to some fault in

the primary feedback circuit, the primary side could switch

at the set maximum current level until the protection circuit

operates. This causes the secondary voltage to become much

greater than the rated voltage. Note that in such a case if

there were no protection circuit, the fuse could blow or, more

serious, a fire could start.

It is possible that, without a regulator, devices directly

connected to the secondary output could be destroyed.

Instead, however, the Fairchild Power Switch (FPS) over

voltage protection circuit operates. Since VCC is propor-

tional to the output, in an over voltage situation it will also

increase. In the Fairchild Power Switch (FPS), the protection

circuit operates when Vcc exceeds 25V. Therefore in normal

operation Vcc must be set below 25V.

©2002 Fairchild Semiconductor Corporation

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