Comparative Evaluation of Three-Phase Three-Level GaN and Seven-Level Si Flying Capacitor Inverters for Integrated Motor Drives Considering Overload Operation

Integrated Motor Drives (IMDs) are gaining popularity in industrial Variable Speed Drive (VSD) applications, thanks to their more compact realization and simpler installation. However, mission profiles of, e.g., servo applications, demand overload torques of two to three times the nominal value during several seconds, which is thermally challenging for the power electronics. Accordingly, high efficiency and power density of the inverter are of paramount importance for motor integration. Multi-Level Flying Capacitor inverters (ML-FCis) benefit from a reduced output filter volume and improved switching and onstate performance of low-voltage devices for increasing number of levels N, whereas the PCB overhead and gate drive volume increases. As shown in this paper, considering the performance of commercially available devices, the optimal number of levels for a 7.5kW IMD with an 800V DC link is N = 7 for Si and N = 3 for GaN semiconductor technology. A simple design procedure allows to dimension the 7L Si and 3L GaN FCi for 99% efficiency at nominal operation, while providing a high short-term overload torque (three times the nominal torque) also for low inverter output frequencies. A phase module of the 3L GaN FCi is realized as a hardware demonstrator of the IMD, whereby a detailed thermal model specifies its overload operating range, including, e.g., manufacturing tolerances of the setup and limited heatspreding in the power semiconductor cooling baseplate. The experimental analysis of the demonstrator verifies an efficiency of 98.94% and the practically required overload capability.