AN4134 Datasheet, PDF (1/14 Pages) Fairchild Semiconductor – Design Guidelines for Off-line Forward Converters

www.fairchildsemi.com
Application Note AN4134
Design Guidelines for Off-line Forward Converters
Using Fairchild Power Switch (FPS^TM)
Abstract
This paper presents practical design guidelines for off-line
forward converter employing FPS (Fairchild Power Switch).
Switched mode power supply (SMPS) design is inherently a
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
helps the engineers to design a 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.
Bridge
rectifier
diode
C_L2
+
2C_DC
Non-doubled
V_DC
Doubled
2C_DC
-
Line Filter
C_L1
R_L1
NTC
AC line
Fuse
Reset Circuit
D_R2
Np
N_S2
D_F2
L₂
C_O2
L_p2
C_p2
V_O2
FPS
Drain
Vcc
FB GND
C_B
R_a
D_a
C_a
N_a
D_R1
N_S1
D_F1
R_d
L₁
C_O1
817A
L_p1
C_p1
V_O1
R_bias
R₁
KA431
R_{F CF}
R₂
Figure 1. Basic Off-line Forward Converter Using FPS
1. Introduction
Due to circuit simplicity, the forward converter has been
widely used for low to medium power conversion
applications. Figure 1 shows the schematic of the basic off-
line forward converter using FPS, which also serves as the
reference circuit for the design procedure 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 step-by-step design procedure for an
FPS based off-line forward converter, which includes
©2003 Fairchild Semiconductor Corporation
transformer
design, reset circuit design, output filter design, component
selection 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 implemented in a software design tool (FPS
design assistant) to enable the engineer to finish SMPS
design in a short time. In the appendix, a step-by-step design
example using the software tool is provided.
Rev. 1.0.0