Physics-based modelling and experimental characterisation of parasitic turn-on in IGBTs

As power electronic engineers increase the switching speed of voltage source converters for the purpose of higher power density, the dI/dt and dV/dt across the power semiconductors increases as well. A well-known adverse consequence of high dV/dt is parasitic turn-on of the power device in the same phase leg as the device being triggered. This causes a short circuit with high shoot-through current, high instantaneous power dissipation and possibly device degradation and destruction. It is critical for converter designers to be able to accurately predict this phenomenon through diagnostic and predictive modelling. In this paper, a physics-based device and circuit model is presented together with experimental results on parasitic turn-on of IGBTs in voltage source converters. Because the model is physics based, it produces more accurate results compared with compact circuit models like SPICE and other circuit models that use lumped parameters. The discharge of the Miller capacitance is simulated as a voltage dependent depletion capacitance and an oxide capacitance as opposed to a lumped capacitor. The model presented accurately simulates IGBT tail currents, PiN diode reverse recovery and the non-linear miller capacitance all of which cannot be solved by lumped parameter compact models. This is due to the fact that the IGBT current in the model is calculated using the Fourier series based re-construction of the ambipolar diffusion equation and the miller capacitances are calculated using fundamental device physics equations. This paper presents a physics-based device and circuit model for parasitic turn-on in silicon IGBTs by numerically modelling the minority carrier distribution profile in the drift region. The model is able to accurately replicate the transient waveforms by avoiding the use of lumped parameters normally used in compact models.