Abstract: A great change of load occurs by freewheeling diode turning on and turning off in freewheeling circuit, it acts great quality of di/dt on main circuit.Voltage surge happens at the action of transformer leakage, induction of snubber and circuit induction, and harms IGBT by high voltage, and influences IGBT reliable work.If freewheel diode is in state of turning off, over high voltage will not produce, and it is good switching trace.Match model of IGBT U_ce and U_ce voltage equation is derived during turning off, and it shows that the less transformer leakage, the less voltage surge.According to main circuit parameter, R is calculated in good switching trace, and capacitance is calculated at the requirement of resistance power in snubber circuit and switching trace, and model is testified by the experiment.Reliability of main circuit is improved in full bridge inverter.