Experimental Comparison of Three-Phase AC-AC Monolithic Bidirectional GaN Transistor-Based Current DC-Link Converter and GaN Transistor-Based Voltage DC-Link Converter

Grid-connected variable speed drives (VSDs) are ac-ac converters that ideally provide smooth sinusoidal output voltages for compatibility with standard motors, avoiding over-voltage and/or EMI issues if long motor cables are used. Such ac-ac VSDs are either realized as a voltage-source converter (VSC) with an output filter or as a current-source converter (CSC) which inherently provides smooth motor voltages. CSCs require switching devices with bipolar voltage blocking capability, leading to a significant disadvantage regarding chip area usage compared to VSCs. The recent availability of 600V GaN monolithic bidirectional power transistors (M-BDSs) removes this disadvantage and therefore motivates an in-depth comparison of a VSC and a CSC with identical system specifications (1.4 kW, 200V line-to-line-rms, 97% nominal efficiency, conducted EMI limits on grid and motor side) and roughly equal chip area usage as the CSC employs first-generation 140mΩ GaN M-BDSs. The presented 600V-GaN-based ac-ac VSC and the CSC demonstrators achieve similar power densities of 1.7 kW/dm³ and 1.8 kW/dm³, respectively, and almost equal nominal efficiencies of 97%. The CSC shows generally higher part-load efficiency, which reflects in weighted efficiencies of 95.7% for the VSC and 96.6% for the CSC considering the NEMA Power Index mission profile for variable-speed centrifugal systems like pumps, fans, or compressors. EMI pre-compliance tests confirm that both demonstrators meet the same conducted EMI limits, and radiated EMI pre-compliance testing of the CSC indicates the effectiveness of the motor-side EMI filter even with inexpensive unshielded motor cables.