Comparative Evaluation of Three-Phase Three-Level Flying Capacitor and Stacked Polyphase Bridge GaN Inverter Systems for Integrated Motor Drives

This article presents a comprehensive comparative evaluation of a three-phase Three-Level (3L) Flying Capacitor Converter (FCC) and a Stacked Polyphase Bridge Inverter (SPBI), specifically a converter system formed by two Series-Stacked Two-Level three-phase Converters (2L-SSC), for the realization of a 7.5 kW Integrated Motor Drive (IMD) with a high short-term overload capability. The 2L-SSC requires a motor with two three-phase windings and a split DC-link, but uses standard six-switch, two-level transistor configurations. In contrast, the bridge legs of the 3L-FCC feature flying capacitors whose voltages must be actively balanced. Despite the 800 V DC-link voltage, both topologies employ the same set of 650 V GaN power transistors, i.e., the same total chip area, and if operated at the same switching frequency, show identical semiconductor losses. Electric Discharge Machining (EDM) damage of the motor bearings is a relevant issue caused by the common-mode (CM) voltages of the inverter stage. The high effective switching frequency of the 3L-FCC and the possibility of CM voltage canceling in the 2L-SSC facilitate mitigation of EDM by means of CM chokes, whereby a substantially smaller CM choke with lower losses suffices for the 2L-SSC; based on exemplary designs, the 2L-SSC features only about 75% of the total volume and 85% of the nominal losses of the 3L-FCC. If, alternatively, motor-friendliness is maximized by including DC-referenced sine-wave output filters, the 3L-FCC's higher effective switching frequency and the 2L-SSC's need for two sets of filters due to the dual-winding-set motor change the outcome. In this case, the 3L-FCC features only about 60% of the volume and only about 55% of the 2L-SSC's nominal losses.