Abstract: By using the magnetizing inductance of transformer, resonant capacitor and additional inductance,a halfbridge LLC resonant welding power supply with constant output voltage was implemented.Through careful analysis and design of relevant parameters (such as the series resonant inductor Lr,series resonant capacitor Cr,and the resonant inductor Lm paralleled with transformer etc) in the resonant tank,the ZVS turn-on can be achieved for primary side power switch,and the ZCS turn-off can be achieved for secondary rectifier diodes over the entire operation region.Hence,the loss and the interference can be decreased,the efficiency can be increased,the switching frequency can be higher so that the weight and the volume of the inductance, the capacitance and the transformer,and so on,can be decreased dramatically,and the dynamic behavior can be improved.The operation principle was also discussed briefly,a 2.5kW half-bridge LLC welding power supply was constructed and the experimental results were presented.