Commutation in a High Power IGBT Based Current Source Inverter

This paper presents an investigation of the effect of the commutation (overlap) period on the value of the output current and on the stability of a current source inverter (CSI). The current source inverter employs insulated gate bipolar transistors (IGBTs) as switches. A mathematical formula is developed to account for the overlap period and to explore the effects of various parameters on the deviation of the output current from the demand current. Also, this paper shows that the CSI during commutation is subject to oscillations due to diode reverse recovery and due to low output impedance of the "Master" switch. The effect of the commutation loop elements and the stray inductances are explored. The oscillations are shown in both simulation and experimental results. These oscillations necessitate the need for small RC snubbers, whose effect is also explored in analysis and experimentation. It will be shown that the snubbers not only dampen oscillations but they also achieve voltage sharing between series IGBTs or series diodes in the "Slave" or "Off" switches. The design guidelines for improving the performance of the CSI will be set.