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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

isolation is required. Cfb improves the noise characteristics.

If the control IC incorporates an error amp however, as in

Fairchild’s KA3842B/3B/4B/5B current mode PWM

controller ICs for SMPSs then a resistor and capacitor are

required to provide the feedback to the error amp. This

would provided the same functions as those provided by the

circuit of Figure 4, e.g., fine control of the output voltage

through Vfb. Similarly, other appropriate devices here are

Fairchild’s LM431/TL431/KA431 series of three terminal

shunt regulators. These have a very sharp turn on

characteristic much like a Zener diode and are widely used in

SMPS secondary side error amplifiers.

Vfb

#4

Cfb

Vss

#5

Precision

Current Source

2uA 1mA

5V

2.5R

Vfb*

R

Vck

OSC.

On

Off

S

Q

R

Css

Vsense

Reset

Rsense

Fairchild Power SwSitcPhS(FPS)

Idrain

#1

Ioffset

#2

Sense

Isense

Figure 4. Fairchild Power Switch(FPS) feedback circuit appropriate for off line SMPS use (current mode PWM).

1.4 Example Fairchild Power Switch (FPS)

Control Circuit

Figure 5 shows two approaches to control feedback with a

Fairchild Power Switch (FPS). The design in Figure 5a uses

the LM431 regulator, and that of Figure 5b uses a Zener diode.

Even though zenor diode approach is cost effective, the output

regulation is relatively poor.

In Figure 5a, C1 together with R1 produce high frequency

pole formed by the internal 3.5kΩ resistor and Cfb in the

compensation networrk. R4 limits the maximum current of the

photodiode to 2.3mA [(12V - 2.5V - 2V) /3.3KΩ, where 2.5V

is the LM431's saturation voltage and 2 V is the photodiode's

voltage drop]. Cfb should be determined by considering the

shutdown delay time (see Section 2.1). In Figure 5b, R3 sets a

fixed current to the Zener diode to stabilize its voltage.

©2002 Fairchild Semiconductor Corporation

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