PWM
Initialism of pulsewidth modulation.
PWM: switching a signal on and off to control average power
Pulse width modulation is a control technique that rapidly switches a power signal between on and off states. The ratio of on-time to total cycle time determines the average voltage or current delivered to a load. A PWM signal running at 100 kHz with a 75 percent duty cycle, for instance, spends three-quarters of each cycle energized and one-quarter de-energized, yielding an average output of 75 percent of the supply voltage.
The frequency and duty cycle are the two critical parameters. Frequency must be high enough that the load experiences a steady average value rather than flickering; most industrial applications use frequencies between 4 kHz and 20 kHz to stay above audible range and minimize switching losses. Duty cycle, expressed as a percentage, is what the control system actually varies to regulate power delivery.
DC motor speed control is the most common application: varying the duty cycle changes average voltage to the motor winding without large resistive losses. LED brightness control uses the same principle; human eyes integrate the on-off flicker into perceived brightness. Power supplies and Class D audio amplifiers rely on PWM to achieve high efficiency by keeping switching devices either fully on or fully off, minimizing heat dissipation during transitions.
The technique requires a switching device, typically a MOSFET or IGBT, capable of rapid on-off cycles with minimal energy waste. The control circuit must generate a reference signal (usually a ramp or sawtooth wave) and compare it against a command signal; wherever the command exceeds the ramp, the switch closes. This comparison naturally produces PWM output as frequency and duty cycle track the input.
PWM controllers are found in motor drives, LED drivers, switching power supplies, and industrial inverters. Their efficiency advantage over linear regulators and rheostats makes them essential for applications requiring tight thermal control or battery operation. The switching noise they generate requires careful filtering and PCB layout to prevent electromagnetic interference.