In one-quadrant choppers, the average DC output voltage is usually kept at a desired level, as there are
fluctuations in input voltage and output load. These choppers operate only in first quadrant of v–iplane.
In fact, output and input voltages and currents are always positive. Therefore, these converters are called
one-quadrant choppers.
One method of controlling the output voltage employs switching at a constant frequency, i.e., a constant
switching time period (T=t
on+t
off), and adjusting the on-duration of the switch to control the average
output voltage. In this method, which is called pulse-width modulation (PWM), the switch duty ratio d
is defined as the ratio of the on-duration to the switching time period.