AN4137 Datasheet, PDF(1/20 Page) Fairchild Semiconductor

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AN4137 Datasheet, PDF (1/20 Pages) Fairchild Semiconductor – Design Guidelines

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Application Note AN4137

Design Guidelines for Off-line Flyback Converters

Using Fairchild Power Switch (FPS)

Abstract

This paper presents practical design guidelines for off-line

flyback converters employing FPS (Fairchild Power

Switch). Switched mode power supply (SMPS) design is

inherently a time consuming job requiring many trade-offs

and iterations with a large number of design variables.

The step-by-step design procedure described in this paper

helps engineers to design SMPS easily. In order to make the

design process more efficient, a software design tool, FPS

design assistant that contains all the equations described in

this paper is also provided. The design procedure is verified

through experimental prototype converter.

Bridge

rectifier

diode

AC line

VDC

CDC

Rsn

Csn Vsn

Dsn

FPS

Drain 1

GND 2

FB Vcc

Ra Da

H11A817A

DR(n)

LP(n)

NS(n)

CO(n)

CP(n)

DR1

LP1

NS1

CO1

CP1

H11A817A

Rbias

VO(n)

VO1

KA431

RF CF

Figure 1. Basic Off-line Flyback Converter Using FPS

1. Introduction

Figure 1 shows the schematic of the basic off-line flyback

converter using FPS, which also serves as the reference

circuit for the design process described in this paper.

Because the MOSFET and PWM controller together with

various additional circuits are integrated into a single

package, the design of SMPS is much easier than the discrete

MOSFET and PWM controller solution. This paper provides

a step-by-step design procedure for a FPS based off-line

flyback converter, which includes designing the transformer

and output filter, selecting the components and closing the

feedback loop. The design procedure described herein is

general enough to be applied to various applications. The

design procedure presented in this paper is also imple-

mented in a software design tool (FPS design assistant) to

enable the engineer finish their SMPS design in a short time.

In the appendix, a step-by-step design example using the

software tool is provided. An experimental flyback converter

from the design example has been built and tested to show

the validity of the design procedure.

Rev. 1.3.0

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