Abstract: A variable polarity welding supply was developed based on ZVS-PWM(PWM:pulse width modulation) for improving overall efficiency and reliability.The design of components for achieving zero-voltage switching(ZVS) is described through the process of soft switching.By means of the second inverter based on hard-switching and the coincident-control between the first and the second inverter,the variable polarity output can be achieved,which duty ratio and output current can be adjusted independently.Finally,based on the simulation of the system by PSPICE,conditions for achieving soft-switching and shortages of the topology were described.It can be proved by the simulation that the zero-voltage switching can be achieved reliably under a given conditions of the load.