Micrel
FCE = safety derating factor (0.8 for most
MIC2172/3172
Then:
LPRI
commercial and industrial applications)
2
0.5×1×105 × 4.02
(
5.5×10−6
)
VIN(max) = maximum input voltage
≤
1.25
VSEC = transformer secondary voltage (VOUT + VF)
For the practical example:
LPRI ≤ 19.23µH
Use an 18µH primary inductance to overcome circuit
inefficiencies.
VCE = 65V max. for the MIC2172/3172
FCE = 0.8
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.
VSEC = 5.6V
Then:
65×0.8 6.0
5.6
a ≤
a ≤ 8.2143
2
2
0.5 fSW VSEC TOFF
LSEC
≤
(11)
Next, calculate the maximum primary inductance
required to store the needed output energy with a power
switch duty cycle of 55%.
POUT
Where:
SEC = maximum secondary inductance
TOFF = power switch off time
Then:
2
0.5 fSW
V
2 TON
L
IN(min)
LPRI
≤
(10)
POUT
Where:
PRI = maximum primary inductance
2
0.5×1×105 ×5.62 ×
(
4.5×10−6
)
L
LSEC
≤
1.25
fSW = device switching frequency (100kHz)
VIN(min) = minimum input voltage
TON = power switch on time
L
SEC ≤ 25.4µH
Figure 12. MIC3172 5V 0.25A Flyback Converter
M9999-041806
(408) 955-1690
April 2006
16
MICREL [ MICREL SEMICONDUCTOR ]
