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Fairchild Semiconductor - Transformer Design Consideration

Numéro de référence AN-4140
Description Transformer Design Consideration
Fabricant Fairchild Semiconductor 
Logo Fairchild Semiconductor 





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AN-4140 fiche technique
www.fairchildsemi.com
AN-4140
Transformer Design Consideration for Offline Flyback
Converters Using Fairchild Power Switch (FPS)
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.
Core Features
EE EI -Low cost
EFD
EPC
EER
PQ
-Low profile
-Large winding window area
-Various bobbins for multiple
output
-Large cross sectional area
-Relatively expensive
Typical Applications
Aux. power
Battery charger
LCD Monitor
CRT monitor, C-TV
DVDP, STB
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.
Output
Power
0-10W
10-20W
EI core
EI12.5
EI16
EI19
EI22
EE core EPC core EER core
EE8
EE10
EE13
EE16
EE19
EPC10
EPC13
EPC17
EPC19
20-30W
30-50W
50-70W
EI25
EI28 EI30
EI35
EE22
EE25
EE30
EPC25
EPC30
EER25.5
EER28
EER28L
70-100W EI40
100-150W EI50
150-200W EI60
EE35
EE40
EE50
EE60
EER35
EER40
EER42
EER49
Table 2. Core quick selection table (For universal input range,
fs=67kHz and 12V single output)
Figure 1. Ferrite core (TDK)
©2003 Fairchild Semiconductor Corporation
AN4140
Rev. 1.0.0
APPLICATION NOTE
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