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Presentation
Student: Phi Nguyen (3631)
Date: 12/06/2003
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TRANSFORMER DESIGN
FOR FORWARD CONVERTERAND HALF BRIDGE FORWARD
CONVERTER
Agenda
1) Introduction
2) Circuit Illumination of DC to DC
3) Steps for Transformer design4) Example for Transformer design
Forward Converter
Half Bridge Converter(optional)
5) Advantage6) Conclusion
7) Question
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Introduction
- Transformer has two basic functions: to
provide electrical isolation and step up or step
down time varying voltage and current.
DC-to-DC converter enables the transformer to be
small.
- It can provide multiple output voltage.
- Throughout the design transformer, the coresize, optimum induction is selected and the
primary turns, secondary turns are calculated.
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CIRCUIT ILUMINATION OF DC-TO-DC FORWARD AND
HALF BRIDGE CONVERTER
In general, the transformer for the half bridge converter follows the
same general principal used for the forward converter.
Buck dc-dc converter: Vo=VdD
Forward & Half bridge conv.: Vo= Vd1
2
N
ND
Figure 1 - Forward converter
N1 N3 N2LmVd
D1
D2
D3
C1 Vo
S1
L1
Figure 2 - Half bridge forward converter
S1
S2
Vd
C1
C2
N1
N2
N2'
D1
D2
C Vo
Lx
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THE STEPS FOR TRANSFORMER DESIGN
Step 1: Selecting core size.From the basic of transmitted power and
the manufactures recommendation.
(Figure 2.2.2)
Step 2: Selecting optimum induction.
Use manufactures published chart.(Figure 2.9.1)
Step 3: Calculate primary turns.
Step 4: Calculate secondary turns.
2
1
2
1
N
N
V
V=
Note: Maximum duty ratio:
Dmax= 0.5 (ton=toff).
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EXAMPLE FOR TRANFORMER DESIGN
* Forward converter:
P = 100W at fsw 30 kHz.Input: Vi = 90 130 V (nominal 110V) 50-60Hz.
Output Vo and Io :
5V at 10 A (50W)
+12V at 2A (24W)
-12V at 2A (24W)
Total = 98W
-Core size: Use figure 2.2.2
At 100W E 42-15.
Core parameters: area Ae=181 mm
2
-Optimum induction: Use figure 2.9.1
At 100W and f = 30kHz.
Bopt=150mT (for push pull).
. Bm=250mT (for forward conv.)
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-Calculate primary turns:
Tsw=swf
1= 33s ton =
2
Tsw= 16.5 s
-Calculate primary voltage (V1):
Use the approximate conversion factor:
V1 = Vi x (1.3) x (1.9)
V1 = 110 x (1.3) x (1.9) = 272V
Min. primary turns:
Nmin =)2(mmA.(T)B
).(t.(V)V
em
on1 s= 100 turns
At V1 min = 90V in dc primary voltage
V1min= 90 x (1.3) x (1.9) = 222 V
V/turn =min
min1
N
V=
100
222= 2.22 V/turn
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-Calculate secondary turns:
N2 D2 C Vo
Lx
V2
Figure 3 - Single output application
Vo = V2 x Dmax V2 =maxD
Vo= 10V.
Choose V2 = 12V (2V drop D1 and Lx)
N2min = 22.2
12
= 5.4 turns. Choose N2 = 5.5 turns.
2
2
1
min1
N
V
N
V=
N1=V1min2
2
V
N= 102 turns.
Similarly, we have:
V2 (at 12V) = 2 V0 + Vdrop = 26 V N2 (at 12V) = 11.9 turns.
Choose N2 (at 12V) = 12 turns
(Confirm the result by Pexpt)
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Difference
Parameter Calculation PExpt % Difference
V1(V) 110 110 /
V2(V) 12 13.6 11.7%
N1(turn) 102 100 2%
N2(turn) 5.4 5.0 8%
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*Half bridge Forward converter:
This design is very similar to the method use for
the forward converter example.
Figure 2.11.1 - Typical core section graph
P = 100W at fs50 kHz.
Vi = 85 137 V (nominal 110V).
Vo = 5V and Io = 20 A (100W).
V1 = Vi x (1.3) x (1.9)V1min = 85 x (1.3) x (1.9) = 209V
V1nominal = 110 x (1.3) x (1.9) = 272V
V1max = 137 x (1.3) x (1.9) = 338V
-Core size: Use figure 2.11.1, at 100W .
We choose core type EC 41.Core parameter: core area Ae=120 mm
2
-Optimum induction: Use figure 2.9.1 at
100W and f=50kHz.
Bopt= 85 mT ( for P.P )
For half bridge, we choose the max. flux density
Bm=210mT.
[ Bm = (85 x 2) .nomV
V
1
max1
= 210mT ]
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Figure 4 - Multiple output application
N21
D3
D4 C2
L2
N22
D5
D6C3
L3
N2
D1
D2C1
L1
+12v
-12V
COMMON
+5v
N1
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CORE CONSIDERATION:
Related formulas:
Vd = N1dt
d; Vd = N1
ont
max
max = 2Bmax Ae =1
ond
N
t.V
(Bmax = 2B)
Bmax =sw.e.1
d
fA4N
V(ton =
swf
D; D = 0.5)
For forward converter:
Bmax < 0.5 (Bm Br) Bm = Bsat
-A large value of Bm allows Bmax large
result to smaller Ae. Therefore, smaller
core size.
-Higher switching frequency result in
smaller core size.
(However fsw> 100kHz result in a smaller value
ofBmax to be chosen to limit the core losses).
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Advantage
- Isolate the output from the input electrically.
- Smaller core size.
- Provide a multiple output voltage.
- Low cost.
The design for the DC power supplies is
very necessary.
*Conclusion
Transformer is designed in almost AC to DC
and DC-DC converter. It is an importantdevice in the DC power supply. Specially,
in dc-dc converter, transformer has a small
size and therefore a low cost.
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CONCLUSION:
Transformer is designed in almost AC to
DC and DC-DC converter. It is an
important device in the DC power supply.
Specially, in dc-dc converter, transformerhas a small size and therefore a low cost.
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A list of References:
1- Switch mode power supply handbook by Keith Billing, 2nd
Edition.Mc Graw Hill, 1999.
2- Power Electrics: Converter, Application and Design, Mohan,
Undeland and Robbin, Willey, 1989.
3- Introduction to Power Electronic by Daniel W. Hart, Prentice
Hall,
Inc. 1997.4- Lecture note Power Electric by Prof. D. Zhou, Ph. D. (2003).
5- Http://www.power
designers.com/infoweb/disg/converter.sht.
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