MIC2171 Micrel Semiconductor, MIC2171 Datasheet - Page 9

• Manufacturer Part Number: MIC2171
• Description: 100kHz 2.5A Switching Regulator Preliminary Information
• Manufacturer: Micrel Semiconductor

MIC2171
(10)
Where:
Then:
Use an 12 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)
Where:
Then:
Finally, recalculate the transformer turns ratio to insure that
it is less than the value earlier found in equation (9).
1997
L
f
V
T
L
L
T
L
L
L
L
L
SW
PRI
PRI
SEC
SEC
ON
OFF
PRI
IN(min)
PRI
SEC
SEC
= device switching frequency (100kHz)
= power switch on time
= maximum primary inductance
= maximum secondary inductance
= power switch off time
11.4 H
7.9 H
= minimum input voltage
0.5 f
0.5
0.5 f
0.5
SW
SW
1 10
1 10
V
P
P
IN(min)
V
OUT
OUT
SEC
5
5
2
2
T
T
3.22
OFF
ON
2.5
5.41
2.5
2
2
2
2
7.4 10
2.6 10
-6 2
-6 2
4-11
(12)
Then:
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)
So:
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)
Where:
Then:
A 1N5817 will safely handle voltage and current require-
ments in this example.
I
V
a = transformer turns ratio (1.2)
F
V
I
I
a
a
V
V
PEAK(pri)
PEAK(pri)
PEAK(pri)
BR
BR
BR
BR
BR
= reverse voltage safety derating factor (0.8)
= output rectifier maximum peak
reverse voltage rating
12.5V
11.4
L
L
7.9
6.0 + 5.0
V
SEC
PRI
= 2.1A
IN(max)
=
=
0.8
= 1.20
3.22
V
IN(min)
F
BR
L
+ V
PRI
1.2
12 H
a
7.6 10
T
1.2
OUT
ON
a
-6
Micrel
4