duty cycle

The fraction of each switching period that a switch spends turned on, like the share of a minute a strobe light is actually glowing. Written D between 0 and 1: a 30% duty cycle means on for 3 µs and off for 7 µs in a 10 µs period. It is the single master knob of nearly every switching converter.

Because the switch only delivers energy while it's on, the average output tracks D almost linearly. A buck converter outputs V_in × D, so feeding 12 V at D = 0.42 yields about 5 V. A controller constantly nudges D up or down to hold the output steady as the load and input wander — that adjustment is the whole job of a PWM control loop.