Analysis of conventional snubber circuits for PWM inverters using bipolar transistors

The ideal behaviour of conventional capacitive and inductive snubbers for bipolar transistors in PWM inverters is described in terms of the equivalent circuits that are operational during the switch-on and switch-off sequences. These equivalent circuits are analysed so that all currents and voltages during switching conditions can be predicted analytically, and the describing equations are tabulated. From these results, the losses in the snubber resistors are calculated for each switching event. The analysis is extended to cover the case in which the inverter is carrying sinusoidal current, so that average snubber resistor dissipation is predicted analytically. Theoretical results are compared with experimental results and SPICE circuit simulation. The way in which stray circuit inductances cause departures from the ideal conditions is also discussed