This paper gives a practical example of the design of an off-line switching power supply.
Factors governing the choice of a discontinuous flyback topology are discussed. The design
uses a pulsed-width modulation (PWM) control scheme implemented with a Unitrode
UC3840 IC. This chip's voltage-feed-forward feature is used to achieve improved output
regulation. The paper discusses closing the control loop to achieve both stability and
adequate dynamic regulation, and provides guidelines for transformer design and component selection.
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The circuit developed herein operates from a l17V (::!::15%), 60 Hz line and meets the
I. Output voltages:
a. +5V, :!::5%: 2.5A-5A
Ripple voltage: 50mV p-p maximum
b. +12V, :!::3%: IA-2.9A
Ripple voltage: lOOmV p-p maximum
2. Fificiency: 70% minimum
3. Line isolation: 3750V
These objectives are met by using a flyback converter topology with a MOSFET power
switch operating at 80kHz. The design features primary side control.
I. SELECTION OF FLYBACK TOPOLOGY
The flyback, when compared to other switching regulator topologies, has several cost and
1. For output power levels less than -150W, the design of the power transformer
(coupled inductor) is relatively simple.
2. Assembly costs for the flyback regulator are low due to a low overall component count.
In particular, only one magnetic element (i.e., the transformer) is employed as no
inductors are used in the output filters.
3. Output rectifier BY requirements are low, since they do not need to block voltages
which in other topologies are developed across the fIlter inductor.
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