MIC2171_07 Datasheet, PDF (9/12 Pages) Micrel Semiconductor – 100kHz 2.5A Switching Regulator

Micrel, Inc.
For the practical example:
then:
V_CE = 65V max. for the MIC2171
F_CE = 0.8
V_SEC = 5.6V
a ≤ ^{65 × 0.8 − 6.0}
5.6
a ≤ 8.2 (N_PRI/N_SEC)
Next, calculate the maximum primary inductance
required to store the needed output energy with a power
switch duty cycle of 55%.
(10)
L _PRI
≥
0.5f_SW V_IN(min) ²T_ON2
P_OUT
where:
then:
L_PRI = maximum primary inductance
f_SW = device switching frequency (100kHz)
V_IN(min) = minimum input voltage
T_ON = power switch on time
L _PRI
≥
0.5 × 1× 10⁵
× (3.22)²
2.5
× (7.4 × 10 ^−6 )2
L_PRI ≥ 11.4µH
Use a 12µH 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.
(11)
L _SEC
≤
0.5f_SW V_SEC ²T_OFF ²
P_OUT
where:
then:
L_SEC = maximum secondary inductance
T_OFF = power switch off time
L _SEC
≤
0.5 × 1× 10⁵
× (5.41)²
2.5
× (2.6 × 10 ^−6 )²
L_SEC ≤ 7.9µH
Finally, recalculate the transformer turns ratio to insure
that it is less than the value earlier found in equation (9).
(12)
a ≤ ^LPRI
L _SEC
May 2007
MIC2171
then:
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 saturating the transformer core.
(13)
I_PEAK(pri)
=
V_IN(min) T_ON
L _PRI
so:
I_PEAK(pri)
=
3.22 × 7.6 × 10 ^−6
L _PRI
I_PEAK(pri) = 2.1A
Now find the minimum reverse voltage requirement for
the output rectifier. This rectifier must have an average
current rating greater than the maximum output current
of 0.5A.
(14)
V_BR
≥
V_IN(max) + (V_OUT a)
F_BR a
where:
V_BR = output rectifier maximum peak reverse
voltage rating
a = transformer turns ratio (1.2)
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.
Forward Converters
Micrel’s MIC2171 can be used in several circuit
configurations to generate an output voltage which is
less than the input voltage (buck or step-down topology).
Figure 6 shows the MIC2171 in a voltage step-down
application. Because of the internal architecture of these
devices, more external components are required to
implement a step-down regulator than with other devices
offered by Micrel (refer to the LM257x or MIC457x family
of buck switchers). However, for step-down conversion
requiring a transformer (forward), the MIC2171 is a good
choice.
A 12V to 5V step-down converter using transformer
isolation (forward) is shown in Figure 6. Unlike the
isolated flyback converter which stores energy in the
primary inductance during the controller’s on-time and
releases it to the load during the off-time, the forward
converter transfers energy to the output during the on-
9
M9999-051107