Transformer Design Consideration
Description
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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.
Core Features EE EI -Low cost EFD EPC EER -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
PQ
Table 1.
Features and typical applications of various cores Core size: Actually, the initial selection of the core is bound to be
2.
General Transformer design procedure (1)
Choose the proper core Core type : Ferrite is the most widely used core material for
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...
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