The basic principle of pulse width modulation (PWM): the control mode is to control the on-off of the switching devices of the inverter circuit, so that a series of pulses with equal amplitude are obtained at the output end, and these pulses are used to replace the sine wave or the required waveform. That is, multiple pulses are generated in the half cycle of the output waveform, so that the equivalent voltage of each pulse is a sine waveform, and the output is smooth and low order harmonics are few. By modulating the width of each pulse according to certain rules, the output voltage of the inverter circuit and the output frequency can be changed.
For example, if the sine half wave waveform is divided into N equal parts, the sine half wave can be regarded as a waveform composed of N connected pulses. The width of these pulses is equal to π/n, but the amplitude is not equal. The top of the pulse is not a horizontal straight line, but a curve. The amplitude of each pulse changes according to the sinusoidal law. If the above pulse sequence is replaced by the same number of rectangular pulse sequences with equal amplitude but unequal width, so that the midpoint of the rectangular pulse coincides with the midpoint of the corresponding sine bisection, and the area (i.e. impulse) of the rectangular pulse and the corresponding sine part is equal, a group of pulse sequences can be obtained, which is called PWM waveform. It can be seen that the pulse width changes according to the sine law. According to the principle that impulse is equal and the effect is the same, PWM waveform and sine half wave are equivalent. For the negative half cycle of the sine wave, the PWM waveform can also be obtained by the same method.