When external supplied voltage
VS
is less than the desired regulated output voltage, the converter is
set to boost operation. In this mode, power switch
Q1
is on and
Q2
is off. Figure 9 shows the
simplified equivalent circuits of battery charging during boost operation. In Figure 9(a), transistor
Q3
is closed and
Q4
is open, allowing the supply voltage to charge the inductor. In Figure 9(b), transistor
Q4
is closed and transistor
Q3
is open to engage inductor discharge mode. Similar to buck operation,
the dynamic model for boost operation is expressed using the following equation