MIC2171 Datasheet, PDF (9/10 Pages) Micrel Semiconductor – 100kHz 2.5A Switching Regulator Preliminary Information

MIC2171
Micrel
(10)
L_PRI ≥
0.5 f_SW V_IN(min)2 T_ON2
P_OUT
Where:
(12) a ≤
Then:
L_PRI
L_SEC
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 ×} (^3.22)^{2 × 7.4 × 10-6 2}
L_PRI ≥
2.5
a ≤ ^11.4 = 1.20
7.9
This ratio is less than the ratio calculated in equation (9).
When specifying the transformer it is necessary to know the
primary peak current which must be withstood without satu-
rating the transformer core.
(13)
I_PEAK(pri) =
V_IN(min) T_ON
L_PRI
L_PRI ≥ 11.4µH
So:
Use an 12µH primary inductance to overcome circuit ineffi-
ciencies.
To complete the design the inductance value of the second-
I_PEAK(pri) =
3.22 × 7.6 × 10^-6
12µH
ary is found which will guarantee that the energy stored in the
I_PEAK(pri) = 2.1A
transformer during the power switch on time will be com- _{Now find the minimum reverse voltage requirement for the}
pleted discharged into the output during the off-time. This is _{output rectifier. This rectifier must have an average current}
necessary when operating in discontinuous-mode.
rating greater than the maximum output current of 0.5A.
(11)
L_SEC ≤
0. 5
f_SW
V_SEC2
T
2
OFF
P_OUT
(14)
( ) _{VBR ≥}
V_IN(max) + V_OUT a
F_BR a
4
Where:
Where:
L_SEC = maximum secondary inductance
T_OFF = power switch off time
Then:
V_BR = output rectifier maximum peak
reverse voltage rating
a = transformer turns ratio (1.2)
( ) ^{0.5 × 1× 105 ×} (^5.41)^{2 × 2.6 × 10-6 2}
L_SEC ≤
2.5
L_SEC ≤ 7.9µH
Finally, recalculate the transformer turns ratio to insure that
it is less than the value earlier found in equation (9).
F_BR = reverse voltage safety derating factor (0.8)
Then:
V_BR ≥
6.0 + (5.0 × 1.2)
0.8 × 1.2
V_BR ≥ 12.5V
A 1N5817 will safely handle voltage and current require-
ments in this example.
1997
4-11