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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

1. Block Diagram and Basic Operation

of a Fairchild Power Switch(FPS)

IC current is less than 300µA. IC operation starts when CCC

(Figure 2) charges to 15V. Because only a small current (<1

1.1 Block Diagram

mA) is allowed to flow in through the resistor during normal

Figure 1 presents a block diagram of a Fairchild Power

Switch (FPS). It can be divided into several large, functional

sections: under voltage lockout circuitry (UVLO); reference

voltage; oscillator (OSC); pulse width modulation (PWM)

operation this technique reduces the current dissipation in

the SMPS start up resistor.

DC

LINK

Rg

Ccc

block; protection circuits; and gate driving circuits.

Vcc

1.2 Under Voltage Lockout (UVLO)

A Fairchild Power Switch (FPS) under voltage lockout

(UVLO) circuitry (Figure 2) guarantees stable operation of

the IC’s control circuit by stopping and starting it as a

function of the value of VCC (Figure 3). The tur off and turn

on voltage thresholds are fixed internally at 10V and 15V,

respectively. Therefore the UVLO circuitry turns off the

control circuit when VCC is lower than 10V and starts it

when VCC is higher than 15V. Once the control circuit starts

operating, VCC must drop below the 10V level for the

UVLO to stop the circuit again. Before switching starts, the

3

Power On

Reset

+

Latch

Comp ara to r

-

6V

Fairchild Power

SSwiPtchS(FPS)

In te rn al Bia s

1 5V /1 0 V

Vz

UV L

5V

V re f

O Good Logic

Figure 2. Detail of the undervoltage lockout (UVLO)

circuitry in a Fairchild Power Switch. The gate

operating circuit holds in a low state during UVLO,

thereby maintaining the SenseFET at turnoff.

Icc

[mA]

20

Power On

Reset Range

0.3

6V 10V 15V

Vc

Vz [[VcV]]

Figure 3. Fairchild Power Switch(FPS) control circuit status vs. Vcc.

1.3 Feedback Control Circuit

The Fairchild Power Switch(FPS) control IC uses a current

mode PWM and operates such that MOSFET current is

proportional to the feedback voltage Vfb. This limits the

MOSFET current at every cycle. It also offers other

advantages, such as a well regulated SMPS output voltage

with input voltage changes. This method of control also

works successfully in SMPSs used for monitors, which may

have a broad range of synchronizing frequencies to deal

with. As shown in Figure 4, the Fairchild Power Switch

(FPS) oscillator turns on the MOSFET. The feedback

comparator operates to turn it off again, when the MOSFET

current reaches a set value proportional to Vfb. The

MOSFET turn off operation is as follows:(1) the internal

(R+2.5R) voltage divider sets the voltage fed back to one

input of the feedback comparator at Vfb /3.5; (2) a current

proportional to the drain current flows to the MOSFET sense

terminal making Vsense proportional to the drain current;

and, (3) when Vsense becomes greater than Vfb, the output of

the feedback comparator goes high, turning off the

MOSFET. Figure 4 also shows that the circuit is designed to

use an opto isolator in the feedback loop. This is appropriate

for an off line design where input to output

2

©2002 Fairchild Semiconductor Corporation

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