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參數(shù)資料
| 型號： | 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)： | 18/20頁 |
| 文件大小： | 1113K |
| 代理商： | AN701 |
AN701
Vishay Siliconix
www.vishay.com
18
Document Number: 70575
16-Jan-01
Appendix B
%*
The flyback magnetic component is often referred to as a
transformer, but should in fact be viewed as an inductor with
energy storage capability.
Two modes of operation are possible. In d
iscontinuous mode
,
all the energy is transferred to the output(s). In c
ontinuous
mode
, some energy remains in the inductor. Discontinuous
mode provides the advantages of a smaller inductor and the
absence of unexpected or difficult transfer modes in the control
mechanism. As energy transfer is complete, furthermore,
rectifier currents always fall to zero before reverse voltages are
applied. The result is lower rectifier switching losses.
For the sake of simplicity, it is recommended that only highly
experienced designers use continuous mode, further
information on which can be reviewed in some of the
publications listed in the reference section of this application
note. In this appendix, only the discontinuous mode is
explained.
Due to the discontinuous nature of the energy transfer
mechanism, flyback inductors are usually larger than the
transformers encountered in forward and other Buck
topologies, where only voltage transformation
—
and no
energy storage
—
is performed.
To design a Flyback
Inductor
, the following information must be
known:
F
sw
operating switching frequency (usually 20 to
500 kHz)
maximum duty cycle (usually 50% in
Vishay Siliconix products)
target efficiency (usually 0.75 to 0.85)
minimum input voltage used
total output power
output voltage(s) required.
D
max
n
V
MIN
P
OUT
V
OUT
In this case, a design for a 24-V (16- to 30-V), 5-V, 2-A (10 W)
inductor will be demonstrated.
The first step is to determine the minimum primary inductance
from:
L
pmin
V
in min
x t
on max
2
x f
sw
x
2 x P
out
L
pmin
(18 IV x 2 s)
2
x 250 kHz x 0.75
2 x 10 W
L
pmin
12.2 H
12 H
The next step is to determine the peak primary current. For a
given inductor, the applied voltage will be:
V
Ldi
dt
Re-arranging yields:
i
pk
V
in min
x t
on max
L
pmax
i
pk
18 V x 2 s
12 H
3 A
The R.M.S. value for a triangular waveform can be used to
calculate the power that will be dissipated in the MOSFET:
i
ms
i
pk
x
max
3
i
ms
3 A x
0.5
3
1.22 A
1.2 A
To calculate the number of turns required for the 5-V output the
following quotation can be used:
N
S
N
p
V
O
V
F
(1
–
D
max
)
V
inmin
D
max
in this case, for D
max
= 0.5 and V
F
= 0.6 V for a 5 V output, the
secondary turns will be:
N
S
N
p
(5 V
0.6 V)(1
–
0.5)
16 V x 0.5
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