FAIRCHILD Design Guidelines for Offline Forward Converters using FPS

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Application Note AN4134 Design Guidelines for Off-line Forward Converters Using Fairchild Power Switch (FPSTM) Abstract

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

This paper presents practical design guidelines for off-line

make the design process more efficient, a software design

forward converter employing FPS (Fairchild Power Switch).

tool, FPS d esign as sistant, that contains all the equations

Switched mode power supply (SMPS) design is inherently a

described in this paper, is also provided.

time consuming job requiring many trade-offs and iteration with a large number of design variables. The step-by-step design procedure described in this paper

L2 Lp2

Bridge 2CDC Np NS2

Reset Circuit VO2

rectifier Cp2

Non-doubled CO2

diode

Doubled

FPS Lp1

2CDC DR1

Drain

Ra Da

Vcc VO1

DF1 CO1 Cp1

GNDFB

Ca Na

Line Filter Rd

Rbias

CL1 817A

CB R1

RF CF

Fuse KA431

AC line

Figure 1. Basic Off-line Forward Converter Using FPS

1. Introduction transformer

Due to circuit simplicity, the forward converter has been

design, reset circuit design, output filter design, component

widely used for low to medium power conversion

selection and closing the feedback loop. The design

applications. Figure 1 shows the schematic of the basic off-

procedure described herein is general enough to be applied

line forward converter using FPS, which also serves as the

to various applications. The design procedure presented in

reference circuit for the design procedure described in this

this paper is also implemented in a software design tool (FPS

paper. Because the MOSFET and PWM controller together

design assistant) to enable the engineer to finish SMPS

with various additional circuits are integrated into a single

design in a short time. In the appendix, a step-by-step design

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

example using the software tool is provided.

MOSFET and PWM controller solution. This paper provides step-by-step design procedure for an FPS based off-line forward converter, which includes

Rev. 1.0.0

©2003 Fairchild Semiconductor Corporation