MIC2172_05 Datasheet, PDF (13/17 Pages) Micrel Semiconductor – 100kHz 1.25A Switching Regulators

MIC2172/3172
Then:
δ ≥ ^{2 × 1.25}
1.25 × 4
δ ≥ 0.5 (50%) Use 0.55.
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 ≤}
Where:
V_CE F_CE ± V_IN(max)
V_SEC
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:
a ≤ ^{65 × 0.8 ± 6.0}
5. 6
a ≤ 8.2143
Next, calculate the maximum primary inductance required to
store the needed output energy with a power switch duty
cycle of 55%.
Micrel, Inc.
(10)
L_PRI ≤
0.5 f_SW V_IN(min)2 T_ON2
P_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:
( ) ^{0.5 × 1× 105 × 4.02 5.5 × 10-6 2}
L_PRI ≤
1. 25
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_SEC2 T_OFF2
P_OUT
Where:
L_SEC = maximum secondary inductance
T_OFF = power switch off time
Then:
( ) ^{0.5 × 1× 105 × 5.62 × 4.5 × 10-6 2}
L_SEC ≤
1. 25
L_SEC ≤ 25.4µH
V_IN
4V to 6V
C1
R4*
22µF
Enable
Shutdown
R3
1k
V_IN
EN
V_SW
MIC3172
COMP _GND FB
P1 P2 S
C2
1µF
V_OUT
T1
5V, 0.25A
C3*
D2
1N5818
R1
C4 _3.74k
D1*
470µF _1%
1:1.25
L_PRI = 100µH
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