參數(shù)資料
型號: MIC2171
廠商: Micrel Semiconductor,Inc.
元件分類: 基準電壓源/電流源
英文描述: Isolated Flyback Switching Regulator with 9V Output
中文描述: 隔離反激式開關(guān)穩(wěn)壓9V輸出
文件頁數(shù): 9/10頁
文件大?。?/td> 80K
代理商: MIC2171
1997
4-11
MIC2171
Micrel
4
(10)
L
0.5 f
V
P
T
PRI
SW
IN(min)
2
ON
2
OUT
Where:
L
PRI
= maximum primary inductance
f
SW
= device switching frequency (100kHz)
V
IN(min)
= minimum input voltage
T
ON
= power switch on time
Then:
L
0.5
1 10
3.22
2.5
7.4 10
PRI
2
-62
×
×
)
×
L
PRI
11.4
μ
H
Use an 12
μ
H primary inductance to overcome circuit ineffi-
ciencies.
To complete the design the inductance value of the second-
ary is found which will guarantee that the energy stored in the
transformer during the power switch on time will be com-
pleted discharged into the output during the off-time. This is
necessary when operating in discontinuous-mode.
(11)
L
SEC
0.5 f
SW
V
SEC
2
T
OFF
2
P
OUT
Where:
L
SEC
= maximum secondary inductance
T
OFF
= power switch off time
Then:
L
0.5
1 10
5.41
2.5
2.6 10
SEC
-62
×
×
×
L
SEC
7.9
μ
H
Finally, recalculate the transformer turns ratio to insure that
it is less than the value earlier found in equation (9).
(12)
a
L
PRI
L
SEC
Then:
a
11.4
7.9 = 1.20
This ratio is less than the ratio calculated in equation (9).
When specifying the transformer it is necessary to know the
primary peak current which must be withstood without satu-
rating the transformer core.
(13)
I
PEAK(pri)
=
V
IN(min)
T
ON
L
PRI
So:
I
= 3.22
7.6 10
12 H
PEAK(pri)
-6
×
I
PEAK(pri)
= 2.1A
Now find the minimum reverse voltage requirement for the
output rectifier. This rectifier must have an average current
rating greater than the maximum output current of 0.5A.
(14)
V
BR
V
IN(max)
+ V
OUT
a
F
BR
a
)
Where:
V
BR
= output rectifier maximum peak
reverse voltage rating
a
= transformer turns ratio (1.2)
F
BR
= reverse voltage safety derating factor (0.8)
Then:
V
6.0 + 5.0
0.8
1.2
1.2
BR
×
)
×
V
BR
12.5V
A 1N5817 will safely handle voltage and current require-
ments in this example.
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