Buck-Boost Converters
A Buck-Boost converter is a type of switched mode power supply that combines the principles of
the Buck Converter and the Boost converter in a single circuit. The Buck converter described in
Power Supplies produces a DC output in a range from 0V to just less than the input voltage. The
boost converter will produce an output voltage ranging from the same voltage as the input, to a
level much higher than the input.
In Fig. 1 the common components of the buck and boost circuits are combined. It shows a
simplified schematic of the buck-boost power stage with a drive circuit block
included. The power switch, Q1, is an n-channel MOSFET. The output diode is CR1. The
inductor, L, and capacitor, C, make up the effective output filter. The resistor, R, represents the
load seen by the power stage output.
Continuous Conduction Mode Analysis
In continuous conduction mode, the buck-boost converter assumes two states per switching
cycle. The ON State is when Q1 is ON and CR1 is OFF. The OFF State is when Q1 is OFF
and CR1 is ON. A simple linear circuit can represent each of the two states where the
switches in the circuit are replaced by their equivalent circuit during each state. The circuit
diagram for each of the two states is shown in Figure 2.
The duration of the ON state and OFF state are shown in Figure 3.
