lm5015mhx National Semiconductor Corporation, lm5015mhx Datasheet - Page 12

• Manufacturer Part Number: lm5015mhx
• Description: High Voltage Monolithic Two-switch Forward Dc-dc Regulator
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Information
The following information is intended to provide guidelines for
the power supply designer using the LM5015.
TWO-SWITCH FORWARD TOPOLOGY
Two-Switch Forward converter, like the conventional Single-
Switch Forward converter, is derived from the Buck converter
topology. The main difference between a Forward converter
and a Buck is that a power transformer is introduced in the
forward converter. The transformer realizes the input-output
isolation, and the turns ratio provides a means to optimize the
duty cycle for the particular input and output voltage require-
ments of the application.
The Two-Switch Forward converter employs two power MOS-
FET switches instead of the one switch of the Single-Switch
Forward converter. However the two-switch approach offers
two major advantages over its single-switch counterpart:
1.
The voltage across the power MOSFET switches in a
Two-Switch Forward converter is clamped to the input
voltage, allowing the input voltage range to approach the
FIGURE 4. Two-Switch Forward Converter Topology
12
2.
Figure 4 illustrates the Two-Switch Forward converter topol-
ogy. The power circuit consists of an input capacitor C
MOSFET switches Q
a power transformer T
output inductor L
LM5015 integrates both Q
Forward converter can be realized without a need for discrete
power MOSFETs. With a slightly higher cost, the two rectifier
diodes D
former can be replaced with synchronous rectifier MOSFETs
to improve efficiency in applications with relatively low output
voltage.
rating of the MOSFETs. Whereas, the maximum
operating voltage of a Single-Switch Forward converter
is typically limited to half the MOSFET voltage rating.
The power transformer of a Two-Switch Forward
converter is simpler, and hence costs less, than that of a
Single-Switch Forward converter, because the Two-
Switch converter transformer eliminates the tertiary reset
winding that is normally required in the Single-Switch
converter.
1
and D
2
O
on the secondary side of the power trans-
, and an output capacitor C
H
and Q
1
, two rectifier diodes D
H
and Q
L
, two clamp diodes D
L
, a low cost Two-Switch
30034619
1
o
. Since the
and D
H
and D
IN
2
, two
, an
L
,