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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

2. Fairchild Power Switch(FPS) Built

In Protection Circuits

Since a Fairchild Power Switch (FPS)’s built in protection

circuits do not require additional, external components,

reliability is increased without increasing cost. Note that the

protective circuitry can completely stop the SMPS operation

(latch mode protection) until the power is turned off and on

again, and can make the control voltage restart above the

ULVO level should the latch be released below ULVO

(Auto Restart Mode protection).

2.1 Output Overload Protection

An overload is any load greater than the load defined as

normal for operation. This is not a short circuit. The

Fairchild Power Switch (FPS) overload protection

determines whether the overload is true or merely transient.

Only a true overload will trigger the overload protection.

When the Fairchild Power Switch (FPS) senses an overload,

it waits for a specified time. If the overload is still present

after this, it is considered a true overload and the device

shuts down.

The Fairchild Power Switch (FPS) has a current control that

prohibits current flow above a set maximum, which means

the maximum input power is limited at any given voltage.

Therefore, if the output load tries to draw more than this

level, Vo (Figure 9) drops below the set voltage and LM431

can draw only a given minimum current. As a result, the opto

coupler secondary current drops to almost zero. Almost the

entire current flow at the node is from the Fairchild Power

Switch (FPS) 1mA current source. Hence, the internal 3kΩ

resistor (2.5R + R = 3KΩ) moves Vfb to 3V. From this point

on, however, the 5µA current source starts to charge Cfb,

and, because the opto coupler secondary current is almost

zero, Vfb continues to increase. When Vfb reaches 7.5V, the

Fairchild Power Switch (FPS) shuts down.

2uA 1mA

Vck Fairchild Power SwiStchP(FSPS)

Vo

Vfb

#4

OSC.

Cfb

D1

D2

22.R5R

Vfb*

R

S

Q

R

KLMA443311

Rsense

Ioffset

Sense

7.5V

Thermal

Shutdown

Reset

R

Q

S

Shutdown

Figure 9. Fairchild Power Switch(FPS) overload protection circuit.

The shutdown delay interval is therefore determined by Cfb.

When Cfb is 10nF, the shutdown delay t2 (see Figure 10b) is

about 9mS. With Cfb at 0.1µF, t2 is about 90ms. Such delay

intervals do not allow the typical transients observed to shut

down the Fairchild Power Switch (FPS). Note that, if a

longer delay is needed, Cfb cannot be made arbitrarily large

because it is important in determining the dynamic response

of the SMPS.

When a large value of Cfb is necessary, a series connected

capacitor and Zener diode can be connected across Cfb as

shown in Figure 10a. The combination works in this way:

When Vfb is below 3V, the low valued Cfb allows the SMPS

to have a good dynamic response. When Vfb is above 3.9V,

the high valued Cd extends the delay time to the desired

shutdown point. Of course, where transients are insignificant

and good dynamic response is not required, then eliminate

Cd and the Zener, thereby eliminating their added costs.

6

©2002 Fairchild Semiconductor Corporation

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