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AN701 Datasheet(PDF) 10 Page - Vishay Siliconix

Part No. AN701
Description  reduce the size of energy storage components
Download  19 Pages
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Maker  VISHAY [Vishay Siliconix]
Homepage  http://www.vishay.com
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AN701 Datasheet(HTML) 10 Page - Vishay Siliconix

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AN701
Vishay Siliconix
www.vishay.com
10
Document Number: 70575
16-Jan-01
Figure 17. Marcon 10
mF 25 V, Capacitance Versus
Temperature
10
9
8
7
25
35
45
55
65
75
11
Temperature (
_C)
Figure 18. Marcon 10
mF 25 V, Capacitance Change
with Voltage
11
10
9
8
0
1
2
3
4
5
12
67
89
10
11
Voltage (V)
DESIGN EXAMPLE: A 15W, 500KHZ, 48V/5V DCTODC CONVERTER
Resonant Reset Forward Converter
Most forward converters are designed using a clamp circuit.
While at low frequencies this technique may be acceptable, at
high frequencies it becomes unnecessary: the parasitic
elements of the circuit will reset the transformer flux
automatically, provided a few precautions are taken.
It has been shown that [1] the resonant reset concept is
dominated by the parasitic capacitance of the MOSFET and
the magnetizing inductance of the transformer. Yet the
capacitance of the output diode should also be considered.
The
correct
equivalent
circuit
of
the
converter
the
approximates to Figure 19.
During the off time, D2 is conducting and CD1 appears
connected across the primary of the transformer, in parallel
with LMAG. The leakage inductance has a small and
insignificant effect on the waveform—as the primary current
has ceased flowing—and the only remaining current is the
current that is charging COUT.
Figure 19 Resonant Reset Forward Converter
Lout
Cout
Cout
Vds
Q1
Cin
CAP
Lmag
N = 1
CD1
D1
D2
In effect, the magnetizing inductance of the transformer forms
a parallel tuned circuit across the transformer and resonates
at a frequency determined by the parasitic elements. The reset
period needs to be short enough to allow full reset of the core,
before the next switching interval occurs. This will be governed
by the selection of the MOSFET and the Schottky diode.
Component Selection
The following information is supplied in order to help designer
select correct components for use with the Si9114A.
Vishay Siliconix does not necessarily recommend or approve
these components for specific applications. Designers should
contact manufacturers directly to obtain correct and current
data sheets.
Capacitor Selection
As stated previously, ceramic capacitors are a good choice
when operating at high frequency, due to the extremely low
ESR, and high reliability, and long operating lifetimes.
In the design example, the required size of capacitors was
defined as follows:
Input capacitor:
A 15-W output converter with 85% efficiency will require
15/0
@85 = 17.65 W of input power. Assuming that operation
at nominal conditions is 48 V, with duty cycle of
δ=0.376
(measured), the switching current will be governed by the
size of the output inductor (Figure 20).
Figure 20
lA
lDC


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