Why use Pulse-Width Modulation (PWM)?
Switch-mode converters employ a power semiconductor switch (often a MOSFET) to drive a magnetic element (transformer or inductor) whose rectified output produces a dc voltage. Pulse-width modulation (PWM) is used to control the switch output power, by varying its ON time vs OFF time. The ratio of ON time to the switching period time is the duty cycle, and is determined by the regulator feedback. Like any regulator, a switch-mode converter uses feedback to maintain its target output voltage in response to load current changes. Because the switching action causes power dissipation in the switch to be low (little current when voltage is present, and small voltage when handling high current), power conversion efficiencies exceeding 90% are common, about twice that of a linear regulator. Figure 1 shows three different variations of the PWM duty cycle: 10%, 50%, and 90%.
