MIC2172/3172
Then:
2
×
1.25
δ
≥
1.25
×
4
δ ≥
0.5 (50%) Use 0.55.
Micrel, Inc.
0.5 f
SW
V
IN(min)2
T
ON2
L
PRI
≤
P
OUT
(10)
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:
0.5
×
1× 10
5
×
4.0
2
5.5
×
10
-6
1.25
The slightly higher duty cycle value is used to overcome
circuit inefficiencies. A few iterations of equation (8) may be
required if the duty cycle is found to be greater than 50%.
Calculate the maximum transformer turns ratio
a,
or
N
PRI
/N
SEC
, that will guarantee safe operation of the MIC2172/
3172 power switch.
(9)
a
≤
V
CE
F
CE
± V
IN(max)
V
SEC
L
PRI
≤
(
)
2
Where:
a
= transformer maximum turns ratio
V
CE
= power switch collector to emitter
maximum voltage
F
CE
= safety derating factor (0.8 for most
commercial and industrial applications)
V
IN(max)
= maximum input voltage
V
SEC
= transformer secondary voltage (V
OUT
+ V
F
)
For the practical example:
V
CE
= 65V max. for the MIC2172/3172
F
CE
= 0.8
V
SEC
= 5.6V
Then:
65
×
0.8 ± 6.0
a
≤
5.6
a
≤
8.2143
L
PRI
≤
19.23µH
Use an 18µH 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:
0.5
×
1× 10
5
×
5.6
2
×
4.5
×
10
-6
1.25
L
SEC
≤
(
)
2
Next, calculate the maximum primary inductance required to
store the needed output energy with a power switch duty
cycle of 55%.
L
SEC
≤
25.4µH
V
IN
4V to 6V
T1
R4*
C3*
D2
1N5818
C4
470µF
V
OUT
5V, 0.25A
R1
3.74k
1%
C1
22µF
V
IN
EN
MIC3172
D1*
V
SW
1:1.25
L
PRI
= 100µH
Enable
Shutdown
R3
1k
COMP GND
FB
P1 P2 S
C2
1µF
R2
1.24k
1%
* Optional voltage clipper (may be req’d if T1 leakage inductance too high)
Figure 12. MIC3172 5V 0.25A Flyback Converter
April 2005
13
M9999-042205
MICREL [ MICREL SEMICONDUCTOR ]
