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AN-4140 の電気的特性と機能

AN-4140のメーカーはFairchild Semiconductorです、この部品の機能は「Transformer Design Consideration」です。


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部品番号 AN-4140
部品説明 Transformer Design Consideration
メーカ Fairchild Semiconductor
ロゴ Fairchild Semiconductor ロゴ 




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AN-4140 Datasheet, AN-4140 PDF,ピン配置, 機能
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

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AN-4140 pdf, ピン配列
where VDCmin is the minimum DC input voltage, Dmax is the
maximum duty cycle, Pin is the maximum input power fs is the
switching frequency of the FPS device and KRF is the ripple factor.
Once Lm is determined, the maximum peak current and RMS
current of the MOSFET in normal operation are obtained as
(3) Determine the number of turns for each output
Figure 6 shows the simplified diagram of the transformer, whrere
Vo1 stands for the reference output that is regulated by the feedback
control while Vo(n) stands for the n-th output.
First, determine the turns ratio (n) between the primary side and the
feedback controlled secondary side as a reference.
rms-rms where-where and-and in-in
With the chosen core, the minimum number of turns for the
transformer primary side to avoid the core saturation is given
by
tums-tums over-over sat-sat
where Lm is the primary side inductance, Iover is the FPS
pulse-by-pulse current limit level, Ae is the cross-sectional area of
the core and Bsat is the saturation flux density in tesla.
If the pulse-by-pulse current limit level of FPS is larger than the
peak drain current of the power supply design, it may result in
excessive transformer size since Iover is used in determining the
minimum primary side turns as shown in equation (6). Therefore, it
is required to choose a FPS with proper current limit specifications
or to adjust the peak drain current close to Iover by increasing the
ripple factor as shown in Figure 5. It is reasonable to design Idspeak to
be 70-80% of Iover considering the transient response and tolerance
of Iover.
where Np and Ns1 are the number of turns for primary side and
reference output, respectively, Vo1 is the output voltage and VF1 is
the diode (DR1) forward voltage drop of the reference output that is
regulated by the feedback control.
Then, determine the proper integer for Ns1 so that the resulting Np is
larger than Npmin obtained from equation (6).
The number of turns for the other output (n-th output) is determined
as
turns-turns
The number of turns for Vcc winding is determined as
where Vcc* is the nominal value of the supply voltage of the FPS
device, and VFa is the forward voltage drop of Da as defined in
Figure 6. Since Vcc increases as the output load increases, it is
proper to set Vcc* as Vcc start voltage (refer to the data sheet) to
avoid triggering the over voltage protection during normal
operation.
Pulse-by-pulse current limit of FPS (Iover)
Increasing ripple factor (KRF)
Decreasing pirmary side Inductance (Lm)
Figure 5. Adjustment peak drain current
Figure 6. Simplified diagram of the transformer


3Pages


AN-4140 電子部品, 半導体
Secondary windings with only a few turns should be spaced across
the width of the bobbin window instead of being bunched together,
in order to maximize coupling to the primary. Using multiple
parallel strands of wire is an additional technique of increasing the
fill factor and coupling of a winding with few turns as shown in
Figure 10.
Figure 11. Stacked winding on other winding
Figure 13. Sandwich winding
Figure 12. Stacked winding on other output
(3) Minimization of Leakage Inductance
The winding order in a transformer has a large effect on the
leakage inductance. In a multiple output transformer, the secondary
with the highest output power should be placed closest to the
primary for the best coupling and lowest leakage. The most
common and effective way to minimize the leakage inductance is a
sandwich winding as shown in Figure 13.
(4) Transformer shielding
A major source of common mode EMI in Switched Mode Power
Supply (SMPS) is the parasitic capacitances coupled to the
switching devices. The MOSFET drain voltage drives capacitive
current through various parasitic capacitances. Some portion of
these capacitive currents flow into the neutral line that is connected
to the earth ground and observed as common mode noise. By using
an electrostatic separation shield between the windings (at primary
winding side, or at secondary winding side, or both), the common
mode signal is effectively "shorted" to the ground and the
capacitive current is reduced. When properly designed, such
shielding can dramatically reduce the conducted and radiated
emissions and susceptibility. By using this technique, the size of
EMI filter can be reduced. The shield can be easily implemented
using copper foil or tightly wound wire. The shield should be
virtually grounded to a quiescent point such as primary side DC
link, primary ground or secondary ground.
Figure 14 shows a shielding example, which allows the removal of
the Y-capacitor that is commonly used to reduce common mode
EMI. As can be seen, shields are used not only on the bottom but
also on the top of the primary winding in order to cancel the
coupling of parasitic capaci-tances. Figure 15 also shows the
detailed shielding construction.

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部品番号部品説明メーカ
AN-4140

Transformer Design Consideration

Fairchild Semiconductor
Fairchild Semiconductor


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