AN4105 Datasheet, PDF (1/22 Pages) Fairchild Semiconductor – Design Considerations for Switched Mode Power Supplies Using A Fairchild Power Switch (FPS) in a Flyback Converter

www.fairchildsemi.com
Application Note AN4105
Design Considerations for Switched Mode Power
Supplies Using A Fairchild Power Switch (FPS) in a
Flyback Converter
Introduction
Flyback switched mode power supplies (SMPS) are among
the most frequently used power circuits in household and
consumer electronics. The basic function of an SMPS is to
supply regulated power to the load on the secondary, or
output side. An SMPS typically incorporates a power
transformer, secondary-side rectifier diodes, a switching
semiconductor device with control IC, and peripheral
circuitry. If the level of integration of the switching and
control circuitry is not high enough, then additional, separate
circuits will be required to accommodate all functions. Such
additional components raise the overall SMPS cost and not
uncommonly reduce reliability.
Fairchild Power Switches are highly-integrated ICs for
power supply applications. They combine a high-voltage
power MOSFET (SenseFET) and pulse width modulation
(PWM) based control IC in one package. Moreover, they
provide enhanced IC functionality, thereby minimizing the
number of additional components needed in an SMPS.
Fairchild Power Switch (SPS) ICs are widely used in the
power circuits of a variety of equipment, such as color TVs,
printers, PCs, monitors, battery chargers and ac adapters.
They typically incorporate a variety of enhanced protection
functions and they permit a much reduced power
consumption in standby modes.
This application note considers the three major functional
blocks of an SMPS: Fairchild Power Switch (SPS) , flyback
converter, and transformer. It discusses a variety of issues
important to their design and use in the overall SMPS.
#3 Vcc
#4
Feedback
#5
Soft Start
& Sync.
6 .3V
2uA 1mA
Sync.
OSC.
3 2V
Vck
UVLO
Voltage
Ref.
2 . 5R
R
S
Q
R
Voffset
LEB
5V
Rsense
Sense
Reset
7 .5V
Thermal
Protection
OVP
Reset
S
Q
R
Control IC
#1
Drain
#2
Source
GND
Sense
FET
Figure1. Internal block diagram of a Fairchild Power Switch(FPS).
©2002 Fairchild Semiconductor Corporation
Rev. 1.0.2