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Application Note AN4140
Transformer Design Consideration for off-line Flyback
Converters using Fairchild Power Switch (FPS
TM)
©2003 Fairchild Semiconductor Corporation
Rev. 1.0.0
1. Introduction
For flyback coverters, the transformer is the most important
factor that determines the performance such as the
efficiency, output regulation and EMI. Contrary to the
normal transformer, the flyback transformer is inherently an
inductor that provides energy storage, coupling and isolation
for the flyback converter. In the general transformer, the
current flows in both the primary and secondary winding at
the same time. However, in the flyback transformer, the
current flows only in the primary winding while the energy
in the core is charged and in the secondary winding while the
energy in the core is discharged. Usually gap is introduced
between the core to increase the energy storage capacity.
This paper presents practical design considerations of
transformers for off-line flyback converters employing
Fairchild Power Switch (FPS). In order to give insight to the
reader, practical design examples are also provided.
2. General Transformer design procedure
(1) Choose the proper core
Core type
:
Ferrite is the most widely used core material
for commercial SMPS (Switchied mode power supply)
applications. Various ferrite cores and bobbins are shown in
Figure 1. The type of the core should be chosen with regard
to system requirements including number of outputs,
physical height, cost and so on. Table 1 shows features and
typical application of various cores.
Figure 1. Ferrite core (TDK)
Table 1. Features and typical applications of various
cores
Core size
:
Actually, the initial selection of the core is
bound to be crude since there are too many variables. One
way to select the proper core is to refer to the manufacture's
core selection guide. If there is no proper reference, use the
table 2 as a starting point. The core recommended in table 1
is typical for the universal input range, 67kHz switching
frequency and 12V single output application. When the input
voltage range is 195-265 Vac (European input range) or the
switching frequency is higher than 67kHz, a smaller core can
be used. For an application with low voltage and/or multiple
outputs, usually a larger core should be used than
recommended in the table.
Table 2. Core quick selection table (For universal input
range, fs=67kHz and 12V single output)
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