MIC2172_06 Datasheet, PDF (16/20 Pages) Micrel Semiconductor – 100kHz 1.25A Switching Regulators

Micrel
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%.
L_PRI
≤
0.5
f_SW
V_IN(min)2
P_OUT
T_ON2
(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
MIC2172/3172
Then:
( ) _LPRI
≤
0.5 ×1×10⁵
× 4.0²
1.25
5.5 ×10^−6
2
LPRI ≤ 19.23µH
Use an 18µH primary inductance to overcome circuit
inefficiencies.
To complete the design the inductance value of the
secondary is found which will guarantee that the energy
stored in the transformer during the power switch on
time will be completed discharged into the output during
the off-time. This is necessary when operating in
discontinuous-mode.
L_SEC
≤
0.5
f_SW
V_SEC2
P_OUT
T_OFF2
(11)
Where:
L_SEC = maximum secondary inductance
T_OFF = power switch off time
Then:
( ) _LSEC
≤
0.5 ×1×10⁵
× 5.6² ×
1.25
4.5 ×10^−6
2
L_SEC ≤ 25.4µH
April 2006
Figure 12. MIC3172 5V 0.25A Flyback Converter
16
M9999-041806
[phone]