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參數(shù)資料
| 型號(hào)： | AN701 |
| 廠商： | Vishay Intertechnology,Inc. |
| 英文描述： | Designing High-Frequency DC-to-DC Converters With the Si9114A Switchmode Controller |
| 中文描述： | 設(shè)計(jì)高頻型DC - DC轉(zhuǎn)換器隨著Si9114A開關(guān)模式控制器 |
| 文件頁數(shù)： | 10/20頁 |
| 文件大?。?/td> | 1113K |
| 代理商： | AN701 |
AN701
Vishay Siliconix
www.vishay.com
10
Document Number: 70575
16-Jan-01
Figure 17.
Marcon 10 F 25 V, Capacitance Versus
Temperature
10
9
8
7
25
35
45
55
65
75
11
C
F
Temperature ( C)
Figure 18.
Marcon 10 F 25 V, Capacitance Change
with Voltage
11
10
9
8
0
1
2
3
4
5
12
6
7
8
9
10
11
Voltage (V)
C
F
)$$*&"
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, D
2
is conducting and C
D1
appears
connected across the primary of the transformer, in parallel
with L
MAG
. 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 C
OUT
.
Figure 19
Resonant Reset Forward Converter
L
out
C
out
C
out
V
ds
Q1
C
CAP
L
mag
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
l
A
l
DC
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