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PDF AN-4140 Data sheet ( Hoja de datos )
| Número de pieza | AN-4140 | |
| Descripción | Transformer Design Consideration | |
| Fabricantes | Fairchild Semiconductor | |
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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
http://www.Datasheet4U.com
1 page  
- Vcc operating range : As mentioned above, Vcc voltage is
influenced by the snubber capacitor voltage. Since the snubber
capacitor voltage changes according to drain current, Vcc voltage
can go above its operating range triggering OVP in normal
operation. In that case, Vcc winding should be placed closest to the
reference output winding that is regulated by feedback control and
far from the primary side winding as shown in Figure 9.
Na (Vcc winding)-Na (Vcc winding)
Ns1 (Reference output)-Ns1 (Reference output)
Figure 9. Winding sequence to reduce Vcc variation
- Control scheme : In the case of primary side regulation, the
output voltages should follow the Vcc voltage tightly for a good
output regulation. Therefore, Vcc winding should be placed close
to the secondary windings to maximize the coupling of the Vcc
winding with the secondary windings. Meanwhile, Vcc winding
should be placed far from primary winding to minimize coupling to
the primary. In the case of secondary side regulation, the Vcc
winding can be placed between the primary and secondary or on
the outermost position.
(c) Secondary side winding
When it comes to a transformer with multiple outputs, the highest
output power winding should be placed closest to the primary side
winding, to reduce leakage inductance and to maximize energy
transfer efficiency. If a secondary side winding has relatively few
turns, the winding should be spaced to traverse the entire width of
the winding area for improved coupling. Using multiple parallel
strands of wire will also help to increase the fill factor and coupling
for the secondary windings with few turns as shown in Figure 10.
To maximize the load regulation, the winding of the output with
tight regulation requirement should be placed closest to the
winding of the reference output that is regulated by the feedback
control.
Secondary winding (4 turns)
Secondary winding (3 strands, 4 turns)
Figure 10. Multiple parallel strands winding
(2) Winding method
-Stacked winding on other winding: A common technique for
winding multiple outputs with the same polarity sharing a common
ground is to stack the secondary windings instead of winding each
output winding separately, as shown in Figure 11. This approach
will improve the load regulation of the stacked outputs and reduce
the total number of secondary turns. The windings for the lowest
voltage output provide the return and part of the winding turns for
the next higher voltage output. The turns of both the lowest output
and the next higher output provide turns for succeeding outputs.
The wire for each output must be sized to accommodate its output
current plus the sum of the output currents of all the output stacked
on top of it.
-Stacked winding on other output: If a transformer has a very
high voltage and low current output, the winding can be stacked on
the lower voltage output as shown in Figure 12. This approach
provides better regulation and reduced diode voltage stress for the
stacked output. The wire and rectifier diode for each output must
be sized to accommodate its output current plus the sum of the
output currents of all the output stacked on top of it.
5 Page | ||
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| PDF Descargar | [ Datasheet AN-4140.PDF ] | |
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