Comparative Evaluation of Gate Driver and LC-Filter Based dv/dt-Limitation for SiC-Based Motor-Integrated Variable Speed Drive Inverters

Compared to state-of-the-art IGBTs, SiC power semiconductors allow to achieve ever higher system efficiencies and higher power densities in next-generation Variable Speed Drives (VSDs), thanks to their smaller relative chip size, ohmic on-state characteristic and lower specific switching losses resulting in a smaller switching-stage footprint and lower heat sink as well as DC-link capacitor volumes. However, the high slew rate of the switching transitions, an inherent consequence of the low switching losses, represents a major challenge and potentially results in lifetime degrading unequal voltage distribution across the motor windings and bearing currents. This work analytically and experimentally compares different means for dv/dt-limitation, namely, a conventional passive LC-dv/dt-filter and a Gate Driver (GD)-based approach based on increased GD resistances in combination with explicit Miller capacitors, at the example of a 10 kW industrial motor-integrated VSD. For a state-of-the-art dv/dt-limitation of up to 6 V/ns the LC-filter shows lower losses compared to the GD-based limitation. The latter, however, has a higher part-load efficiency and/or lower losses compared to the (roughly) load independent losses in the LC-filter resulting from the dissipation of the energy stored in the filter capacitor within each switching cycle, beneficial for light loads, e.g., < 40 % of rated output power. Next-generation motors with reinforced insulation allow a dv/dt-limitation of up to 15 V/ns. In this case, the GD-based limitation shows lower losses in the whole operating range, since they directly scale with the now smaller overlap of voltage and current resulting from the faster switching transitions. Considering a state-of-the-art motor, finally, a hardware demonstrator of a three-phase VSD employing an LC-filter to limit the dv/dt to 5.6 V/ns is realized, which achieves a full inverter stage power density of 30 kW/dm(3 )(497 W/in(3)) and an inverter efficiency of > 99 %.