MIC2172/3172
Micrel
Finally, recalculate the transformer turns ratio to insure that
it is less than the value earlier found in equation (9).
a = transformer turns ratio (0.8)
F
= reverse voltage safety derating factor (0.8)
BR
Then:
L
PRI
a ≤
(12)
L
6.0 + 5.0 × 0.8
(
)
SEC
V
≥
BR
0.8 × 0.8
Then:
V
≥ 15.625V
BR
1. 8 × 10-5
2.54 × 10-5
A 1N5817 will safely handle voltage and current require-
ments in this example.
a ≤
Forward Converters
a ≤ 0.84 Use 0.8 (same as 1:1.25).
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.
Micrel’s MIC2172/3172 can be used in several circuit con-
figurations to generate an output voltage which is less than
the input voltage (buck or step-down topology). Figure 13
shows the MIC3172 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 LM457x family of buck switchers). However,
for step-down conversion requiring a transformer (forward),
the MIC2172/3172 is a good choice.
V
T
IN(min) ON
I
=
(13)
So:
PEAK(pri)
L
PRI
-6
4.0 × 5.5 × 10
18µH
I
=
PEAK(pri)
A 12V to 5V step-down converter using transformer isolation
(forward) is shown in figure 14. 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-time, using the off-time to reset the
transformer core. In the application shown, the transformer
core is reset by the tertiary winding discharging T1’s peak
magnetizing current through D2.
I
= 1.22A
PEAK(pri)
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 0.25A.
VIN(max) + V
a
(
)
OUT
VBR
Where:
≥
(14)
FBR
a
For most forward converters the duty cycle is limited to 50%,
allowing the transformer flux to reset with only two times the
input voltage appearing across the power switch. Although
during normal operation this circuit’s duty cycle is well below
V
= output rectifier maximum peak
reverse voltage rating
BR
VIN
D1
1N4148
VIN
VSW
EN
C2
D3
MIC3172
2.2µF
1N4148
R3†
COMP
FB
GND
P1 P2
C1*
100µF
3.7k
R3
470
S
R2†
1.2k
C4
1µF
R4
10Ω
C3
1µF
L1
100µH
C5
5V, 0.1A to 1A
(ILOAD > 100mA)
D2
330µF
* Locate near MIC2172/3172 when supply leads > 2"
†
R3/R2 sets output voltage
Figure 13. Step-Down or Buck Converter
4-26
1997
MICREL [ MICREL SEMICONDUCTOR ]
