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Chapter 6
Again, we start with the compensation values from the data sheet and will change them
based on results in the lab
. No slope compensation is necessary, since duty cycle is
limited to 50%.
The large current pulses on the input will require very low ESR to maintain the voltage at
the control IC. Selecting input capacitors equal to the output capacitors will provide the
necessary low ripple. The very large input current pulses may make a forward converter a
better choice for this application.
6.6 Forward Converter Circuits
A forward converter is a single switch converter that uses a transformer to transfer energy
from the primary circuit to the secondary circuits. Energy fl ows from the primary to the
secondary while the switch is conducting current. Figure 6.15 shows a representative
circuit for a forward converter. A voltage clamp is necessary for a forward converter
because all transformer current stops when the switch turns off. The clamp provides a
path for the current in the magnetizing inductance of the transformer and the leakage
inductance. In the fl yback circuit, the current fl ow in the secondary provides a path for the
ux of the core when the switch opens; the clamp is only necessary to reduce stress on
the switch from leakage inductances.
Any of the clamp circuits in Figure 6.10 can be applied to the forward converter. The
clamp circuits will have a voltage that is controlled by the secondary voltage when used
in a fl yback circuit because of the requirement that V / N is equal for all windings. This is
not true for the forward converter. The clamp winding in Figure 6.10 (a) guarantees that
the switch voltage is twice the input voltage while the magnetizing current ramps down.
Circuits B and C will have varying voltages depending on the amount of energy that is
dissipated in the resistor. You must exercise care in designing the maximum duty cycle,
transformer magnetizing inductance, and RC time constant when using Circuits B and
C to ensure you do not exceed the voltage rating of the switch. Notice that circuit C is
L1
C1
Clamp Diode
V1
V
FB
SW
V
in
GND
Control IC
Figure 6.15 : Representative circuit for a forward converter

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