Datasheet

k x (V
OUT
+ V
F
)
D =
V
IN
V
IN
V
OUT
C
IN
Q
H
Q
L
D
L
D
H
T
1
D
1
D
2
L
O
C
O
k:1
LM5015
SNVS538C –NOVEMBER 2007–REVISED APRIL 2013 www.ti.com
Figure 16. Two-Switch Forward Converter Topology
Figure 17 shows the two operating modes of the Two-Switch Forward converter. During operation, the two
MOSFETS are turned ON and OFF simultaneously. The output voltage is regulated by modulating the duty cycle
of the MOSFETs. The relationship between the input voltage V
IN
, output voltage V
OUT
, duty cycle D, rectifier
diode forward drop V
F
, and transformer turns ratio k is defined by the following equation.
(3)
When the MOSFETS are turned ON, as shown in Figure 5a, the input voltage is applied to the power transformer
primary. The power transformer is magnetized by the application of the input voltage, and power flows to the
secondary side circuit via transformer coupling. When the MOSFETS are turned OFF, as shown in Figure 5b, the
power flow to the primary is cut off. The residual magnetizing inductance of the power transformer reverses the
voltage across the primary winding and forces the two clamp diodes D
H
and D
L
to conduct. This effectively
clamps the MOSFETs voltage to the input voltage, and applies the input voltage in reversed polarity to the power
transformer primary winding to demagnetize and reset the transformer.
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