What is the Purpose of Pulse Width Modulation (PWM)?
Switch-mode converters employ a power semiconductor switch (usually a MOSFET) to drive a magnetic element (transformer or inductor) whose rectified output produces a dc voltage. Efficiencies exceeding 90% are common, about twice that of a linear regulator. A switch-mode converter varies its dc output current in response to load changes. One widely-used approach is pulse width modulation (PWM) that controls the power switch output power by varying its ON and OFF times. The ratio of ON time to the switching period time is the duty cycle. Fig. 1 shows three different variations of the PWM duty cycle: 10%, 50%, and 90%. The duty cycle and power handling rarely have anything to do with each other. Instead, the duty cycle is adjusted to regulate the output voltage.