Comparative Evaluation of Overload Capability and Rated Power Efficiency of 200V Si/GaN 7-Level FC 3-Φ Variable Speed Drive Inverter Systems

Abstract

Variable speed drive systems in e.g. robotics applications are challenged with discontinuous operation cycles and short-time overload current requirements of 2-3 times nominal load. As the motor itself constitutes a large thermal time constant compared to the semiconductor devices in the inverter, latter create a bottleneck for the increased losses during overload operation. Hence, special focus has to be laid on the inverter overload capability, preferably without overdimensioning the system. In this paper a transient thermal model of optimized cooling approaches for 200V GaN and Si packages is empirically deduced. The model is then used to design a 7-Level Flying Capacitor inverter (7L FCi) aiming for 99% efficiency at nominal load for facilitated motor integration and 3 times overload capability. Consequently, the number of parallel switches, switching frequencies and the volume of passive components such as the flying capacitors and the output filter inductor is considered. In order to omit oversizing the inverter and the output inductor for overload operation, an unorthodox way of increasing the switching frequency during overload is proposed. It is concluded, that the small chip size of the 200V GaN devices compared the 200V Si devices poses additional challenges when dealing with overload operation.