Sinusoidal pulse-width modulation (SPWM)

Sinusoidal pulse-width modulation is a control technique used in power electronics to make a switching circuit produce an output that closely approximates a sine wave. Ordinary pulse-width modulation switches a load on and off and varies the duty cycle to set an average level; SPWM extends this idea by continuously varying the duty cycle along a sinusoidal pattern. The result is a train of pulses whose average, after filtering, follows a smooth sine wave, which is exactly what is needed to generate alternating current from a direct-current source.

The pulses are generated by comparing a low-frequency sinusoidal reference, representing the desired output, with a much higher-frequency triangular carrier. Where the sine is greater than the triangle, the switch is on; elsewhere it is off. As the sine rises and falls, the resulting pulses grow wider near its peaks and narrower near its zero crossings, so the moving average reproduces the sine. The high carrier frequency pushes the unwanted switching harmonics far above the wanted signal, where they are easy to filter out.

SPWM is widely applied in inverters, which convert DC into AC, and in variable-speed motor drives, where it lets a circuit synthesise an AC waveform of adjustable frequency and amplitude. By driving an H-bridge or three-phase bridge with sinusoidal PWM, designers obtain clean, controllable AC output with high efficiency, since the switches still operate fully on or fully off.

Frequently asked questions

How does SPWM differ from ordinary PWM?

Ordinary PWM holds a fixed duty cycle to set a constant average level, while SPWM varies the duty cycle continuously along a sinusoidal pattern so the averaged output forms a sine wave.

How are SPWM pulses generated?

A sinusoidal reference is compared with a high-frequency triangular carrier. The switch is on where the sine exceeds the triangle, producing pulses that are wider near the sine's peaks.

Where is SPWM used?

It is used in inverters that convert DC to AC and in variable-speed motor drives, where it synthesises an AC waveform of adjustable frequency and amplitude.