Evaluation of switching performance of SiC devices in PWM inverter fed induction motor drives

Abstract

Double pulse test (DPT) is a widely accepted method to evaluate the switching characteristics of semiconductor switches, including SiC devices. However, the observed switching performance of SiC devices in a PWM inverter for induction motor drives (IMD) is almost always worse than the DPT characterization, with slower switching speed, more switching losses, and more serious parasitic ringing. This paper systematically investigates the factors that limit the SiC switching performance from both the motor side and inverter side, including the load characteristics of induction motor/power cable, two more phase-legs for the three-phase PWM inverter as compared to the DPT, and the parasitic capacitive coupling effect between power devices and heat sink. Based on the three-phase PWM inverter with 1200 V SiC MOSFETs, the test results show that the induction motor, especially with a relatively long power cable, will significantly impact the switching performance, leading to switching time increase by a factor of 2, switching loss increase up to 30%, and serious parasitic ringing with 1.5 μs duration as compared to that tested by DPT. In addition, the interactions among the three phase-legs cannot be ignored unless the decoupling capacitors are mounted close to each phase-leg to support the dc bus voltage during switching transients. Also, the coupling capacitance induced by the heat sink equivalently increases the junction capacitance of power devices. However, its influence on the switching behavior in the motor drives is small considering the relatively large capacitance of the motor load.