Abstract: A spot welding inverter power have been introduce.According to the characteristic of spot welding,the ordinary full bridge zero-voltage switch pulse width modulation direct current/direct current convert using auxiliary network has been modified in order to minimize the primary duty ration loss.The parameters of the lag bridge and the auxiliary network have been optimized by programme.The transfer function of main circuit was constructed completely.A double-loop control method has been used,so the output current can accurately follow the programmable reference,and the magnetic flux imbalance is avoided.