Abstract: To meet the welding requirements of the new materials with low resistance,adigital powerfulmedium frequency inverter based resistance spotwelding power is designed with the maximum output current of 20 kA.In the power supply IGBT modules are used to compose the full-bridge converter and the bipolar limited soft switching PWM drive mode is adopted It realizes the zero-voltage zero-current switching(ZVZCS) and reduces the loss of power switch pipe.Control circuit takes digital signal processor as core,and contains the true RMS processing circu it and the incremental PlD control method.All these finally lead to precise control of the power pulse width modulation circuit.Experiment results demonstrate design of the main circu it is valid the control system is stable and reliable.