MIC2172/3172
Then:
Micrel
2
2
0.5 f
V
T
SW IN(min)
ON
L
≤
(10)
PRI
2 × 1. 2 5
P
OUT
δ ≥
1.25 × 4
Where:
δ ≥ 0.5 (50%) Use 0.55.
L
= maximum primary inductance
PRI
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%.
f
= device switching frequency (100kHz)
= minimum input voltage
SW
V
T
IN(min)
= power switch on time
Calculate the maximum transformer turns ratio a, or
ON
N
/N
,thatwillguaranteesafeoperationoftheMIC2172/
Then:
PRI SEC
3172 power switch.
2
5
2
-6
0.5 × 1×10 × 4.0 5.5 × 10
(
)
V
F
– V
L
≤
CE CE
IN(max)
PRI
(9)
1. 2 5
a ≤
V
SEC
L
≤ 19.23µH
PRI
Where:
a = transformer maximum turns ratio
Use an 18µH primary inductance to overcome circuit ineffi-
ciencies.
V
= power switch collector to emitter
maximum voltage
CE
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.
F
= safety derating factor (0.8 for most
commercial and industrial applications)
CE
V
= maximum input voltage
IN(max)
V
= transformer secondary voltage (V
+ V )
OUT F
SEC
2
2
4
0.5 fSW VSEC TOFF
For the practical example:
LSEC
Where:
≤
(11)
POUT
V
F
= 65V max. for the MIC2172/3172
CE
= 0.8
CE
V
= 5.6V
L
= maximum secondary inductance
= power switch off time
SEC
SEC
Then:
T
OFF
Then:
65 × 0.8 – 6.0
a ≤
5.6
2
0.5 × 1×105 × 5.62 × 4.5 × 10-6
(
)
a ≤ 8.2143
LSEC
≤
1. 2 5
Next, calculate the maximum primary inductance required to
store the needed output energy with a power switch duty
cycle of 55%.
L
≤ 25.4µH
SEC
VOUT
5V, 0.25A
VIN
4V to 6V
T1
D2
1N5818
C1
22µF
R4*
C3*
R1
C4
470µF
3.74k
1%
D1*
VIN
Enable
VSW
EN
Shutdown
1:1.25
PRI = 100µH
L
MIC3172
COMP
FB
GND
R2
1.24k
1%
R3
1k
P1 P2
S
C2
1µF
* Optional voltage clipper (may be req’d if T1 leakage inductance too high)
Figure 12. MIC3172 5V 0.25A Flyback Converter
1997
4-25
MICREL [ MICREL SEMICONDUCTOR ]
