19Basic Switching Circuits
2.4 Inverting Boost Converter
Figure 2.5 shows the circuit of an ideal inverting boost converter. The switch and voltage
source provide current to charge the inductor with energy while the switch is closed.
While the inductor is charging, the current in the load is supplied by the capacitor because
the diode is reverse biased. When the switch opens, the current in the inductor continues
to fl ow, but now the inductor current forward biases the diode and fl ows through the load
circuit. Since one side of the inductor is tied to the common point, the current fl ow when
the switch opens causes a negative output voltage.
When the regulator is at steady-state, the output voltage is determined by Eq. (2-10) for
continuous mode operation. Just as in the positive boost converter, the output voltage will
be larger in magnitude than (or equal to) the input voltage.
out in
(Duty Cycle)/( Duty Cycle) 1 (2-10)
2.5 Buck-Boost Converter
If we add an additional switch and an additional diode to the boost converter as in Figure
2.6 , we can create a buck-boost converter that will allow us to create a positive voltage
that is either above or below the input voltage. Both switches close and open at the same
time in this circuit. Again, the inductor is charged while the switches are closed and
energy is delivered to the load when the switches open, just as it is in the boost converter.
Figure 2.5 : Idealized inverting boost converter

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