In industrial applications, many loads have strict requirements for the output characteristics of inverters. In addition to variable frequency and adjustable voltage, the output voltage fundamental wave should be as large as possible and the harmonic content should be as small as possible. A square wave output inverter composed of thyristor components without self turn-off capability generally adopts multiple or multi-level measures to make the output waveform show a stepped change, close to a sinusoidal waveform. This measure makes the circuit structure more complex, but the cost is relatively high and the effect is not satisfactory. The inverter is composed of self closing devices for high-frequency on/off switch control. The output amplitude is equal and the width changes according to the sine law of the pulse sequence voltage. Through modulation control, the output voltage eliminates low order harmonics, greatly improving the output characteristics of the inverter. This inverter circuit is a Pulse Width Modulation (PWM) type inverter。