Improved switching loss calculation in neutral point clamped inverter via waveforms linearization

An improved switching loss calculation method for IGBT switching on, switching off and the diode reverse recovery energy losses in neutral point clamped inverter is proposed in this paper. A model of linear instantaneous switching on and off waveforms is presented to approximate actual switching process. The IGBT switching on and off duration, the changing rates of current di/dt and collector-emitter voltage dv/dt all can be calculated at different load currents with the given IGBT parameters. Besides, the proposed method is apparently different from the linear waveform method used before which assuming switching time to be a constant value. In addition, compared with the traditional mathematical equation that uses measured data to fit a formula, the equation proposed here is based on physical model and it can indicate the relationship between gate drive resistor, the IGBT capacitance, trans-conductance and switching process. An inverter employing a switching pattern as sinusoidal PWM (SPWM) was designed and experiment was carried out to test the switching waveforms under different output currents. Switching loss under different currents was also measured and compared with calculated value using mathematical equation proposed in this paper.