
TOPSwitch-GX
Forward
Design Methodology
Application Note AN-30
Introduction
The single-ended forward converter topology is often the best
solution for AC-DC applications that require higher powers and
higher output currents than are practical from flyback converters.
The forward converter extends the power capability of
TOPSwitch-GX
 to greater than 200 W for high current outputs.
The feature set of 
TOPSwitch-GX
 offers the following advantages
in single-ended forward designs:
 Built-in soft-start
 Built-in under-voltage lockout
 Built-in adjustable current limit
 Programmable duty cycle reduction to limit duty cycle
excursion at high line and transient load conditions
 Higher efficiency (typically >70%)
 Very good light-load efficiency
 Voltage mode control for simpler loop designs with
magnetic amplifier post-regulators
 Built-in remote on-off
 Low component count
 Improved EMI
Scope
This application note is for engineers designing an AC-DC
power supply using 
TOPSwitch-GX
 in a single-ended forward
converter. It addresses single input voltage 230 VAC or doubled
115 VAC input, but does not address universal input (85 V to
265 V) designs.  The document highlights design parameters
that are fundamental to the use of 
TOPSwitch-GX
 in a single-
ended forward converter.  It offers a procedure to compute
transformer turns, output inductance and other design
parameters.  This procedure enables designers to build an
operational prototype in the shortest possible time.  Refinement
of the prototype hardware after bench evaluation will lead to a
final design.
The design methodology presented here is sufficiently general
to cover a variety of single-ended forward designs, including
power supplies for personal computers.  It provides for multiple
outputs with coupled inductors, independent multiple outputs,
and outputs with both linear or magnetic amplifier post
regulators.
December 2002
U1
D
L
S
F
C
AC
INPUT
V
O
+
+
–
–
R
A
Non-Doubled
Doubled
Clamp
Diode
Snubber
Output
Inductor
Output
Capacitor
R
C
C
VS
R
D
TOPSwitch-GX
R
B
Under-Voltage
Lockout Sense
PI-2817-121201
FEEDBACK
CIRCUIT
CONTROL
Primary
Clamp
Circuit
TL431 with
Frequency
Compensation
V
DB
V
DB
+
+
–
+
–
V
Z
–
2C
IN
2C
IN
Bias
Figure 1.  Typical Configuration of TOPSwitch-GX in a Single-Ended Forward Converter.