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

MIC2171
Forward Converters
Micrel’s MIC2171 can be used in several circuit configura-
tions to generate an output voltage which is less than the input
voltage (buck or step-down topology). Figure 7 shows the
MIC2171 in a voltage step-down application. Because of the
internal architecture of these devices, more external compo-
nents 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 7. 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
Micrel
core is reset by the tertiary winding discharging T1’s peak
magnetizing current through D2.
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
50%, the MIC2172 has a maximum duty cycle capability of
90%. If 90% was required during operation (start-up and high
load currents), a complete reset of the transformer during the
off-time would require the voltage across the power switch to
be ten times the input voltage. This would limit the input
voltage to 6V or less for forward converter applications.
To prevent core saturation, the application given here uses a
duty cycle limiter consisting of Q1, C4 and R3. Whenever the
MIC2171 exceeds a duty cycle of 50%, T1’s reset winding
current turns Q1 on. This action reduces the duty cycle of the
MIC2171 until T1 is able to reset during each cycle.
T1
D3
1:1:1
1N5819 L1 100µH
V_IN
V_OUT
5V, 1A
12V
R4
D4
C5 _3.74k
R1*
C2*
1N5819
470µF _1%
C1
22µF
IN
SW
MIC2171
GND
FB
COMP
D1*
D2
1N5819
R2
1k
C3
1µF
Q1^† R3^†
C4^†
* Voltage clipper
† _{Duty cycle limiter}
Figure 7. MIC2171 Forward Converter
R5
1.24k
1%
4-12
1997