23Basic Switching Circuits
Figure 2.9 shows a half bridge converter. This circuit is a high voltage equivalent of a
TTL totem-pole output.
The switches conduct alternately, which produces a bipolar
voltage across the transformer primary. This requires that we have a full wave rectiﬁ er for
the output. A clamp winding is not necessary since the opposite phase output diode will
allow the current to ﬂ ow in the secondary winding. We can add freewheeling diodes to
the primary to control the voltage present on the secondary when the switches open. The
capacitors provide a voltage divider that sets one end of the primary winding to one-half
the input voltage. These capacitors are almost always part of the input DC power supply,
so they perform the dual functions of voltage divider and input charge reservoir.
Figure 2.10 shows a full bridge converter. This design uses four switches to alternate the
direction of current through the core.
Figure 2.11 shows a push-pull converter. The switches open and close 180 degrees out of
phase, just as in a class B push-pull audio ampliﬁ er. Push-pull converters are rarely used
in off-line supplies because they require high voltage transistors and it is very difﬁ cult
to control the ﬂ ux in the transformer. Modern current mode PWM controllers have made
using push-pull circuits practical in low voltage circuits.
2.7 Synchronous Rectiﬁ cation
In all of the circuits we have reviewed in this chapter, we have used diodes as voltage-
controlled switches. When they are reverse biased, they act as open switches. When they
are forward biased, they act as closed switches. Power MOSFETs also work as switches.
Figure 2.9 : Idealized half bridge converter
When the gate to source voltage is sufﬁ
cient to turn on a MOSFET, current can ﬂ ow in
either direction through the transistor. Power MOSFETs that are used as switches can
have on resistance of 0.01 ohm or less. A Schottky diode that is conducting 5 A will drop
approximately 0.4 V and dissipate 2 W. A power MOSFET with 0.01 ohm on resistance
Figure 2.10 : Idealized full bridge converter
Figure 2.11 : Idealized push-pull converter