AND8039 Datasheet, PDF (1/12 Pages) ON Semiconductor – The One-Transistor Forward Converter

AND8039/D
The One-Transistor
Forward Converter
Prepared by: Marty Brown
http://onsemi.com
APPLICATION NOTE
INTRODUCTION
The one–transistor forward converter is the most
elementary form of transformer–isolated buck converter. It
is typically used in off–line applications in the 100–300 watt
region. This application note illustrates the approach one
would take to design a high DC input voltage, one–transistor
+V_IN
forward converter. With additional modifications, it could
be made work as a 110 VAC off–line power supply.
Description of Operation
A simplified schematic of a one–transistor forward
converter can be seen in Figure 1.
L_O
D1
+V_OUT
+
C_IN
RESET
WINDING
I_SW
+
D2
C_OUT
GND
GND
CONTROL
+
V_SW
–
Figure 1. Simplified Schematic of a One Transistor Forward Converter
One can see a transformer has been placed between the
input voltage and a buck converter output stage. The power
switch (SW) is used to create a rectangular voltage
waveform whose amplitude is the input voltage and its duty
cycle is the controllable variable. The transformer provides
both a step–up or down function and a safety dielectric
isolation between the input line and the output load.
The major restriction of this topology is the maximum
duty cycle must be about 50 percent. Whenever a core is
driven in a unidirectional fashion, that is, current only being
driven from one direction into the primary, the core must be
reset. Magnetization energy which serves only to reorient
the magnetic domains within the core must be emptied, or
else the core will “walk–up” to saturation after a few cycles.
To do this, one needs to reset the core. Resetting is done by
drawing current from a winding during the period when the
transformer is unloaded, that is, when the power switch and
rectifiers are not conducting. Any winding can provide the
reset function, but the higher the voltage on the winding, the
quicker the core will reset. Typically, this is the primary
winding or a separate reset winding of equal turns to the
primary. Current from the reset winding can then be returned
to the input capacitor and reused during the next cycle of
operation.
The typical switch voltage and current can be seen in
Figure 2. When the power switch is ON, the switch sees the
output filter inductor’s current reflected by through the
transformer. The amplitude of the primary current is the
output rectifier current times turns ratio of the transformer
(N1/N2) plus a small amount of magnetization current.
During the power switch OFF time, the switch voltage “flys”
up to about twice the input voltage. During this time, the
reset winding begins to output magnetization current back
to the input capacitor.
© Semiconductor Components Industries, LLC, 2000
1
November, 2000 – Rev. 0
Publication Order Number:
AND8039/D