Synergetic Control of a Monolithic-Bidirectional-GaN-Transistor-Based Three-Phase Current DC-Link AC–AC Converter

Variable speed drives (VSDs) operating from a threephase mains are typically realized as ac–ac voltage dc-link/source converters (VSCs). Compact VSCs using wide-bandgap (WBG) semiconductors often require LC output filters to protect the motor from high dv/dt of the switched voltage. Ac-ac current dclink/source converters (CSCs) featuring a back-to-back connection of a current-source rectifier (CSR) and current-source inverter (CSI) by a shared dc-link inductor inherently provide continuous motor voltages. This paper introduces the loss-optimal operating mode of ac–ac CSCs: using the minimum possible and hence time-varying dc-link current (the maximum absolute value of the CSR and CSI phase currents) minimizes the conduction losses and either the CSR or the CSI operates with 2/3-PWM (one phase terminal clamped to the dc-link inductor while only the remaining two phase currents are synthesized with PWM), reducing switching losses. The respective other stage employs PWM in all three phases (3/3-PWM) and shapes the dc-link current. A proposed synergetic control method realizes loss-optimal operation for the entire output voltage and current range of the ac-ac CSC, with motor voltages lower and higher than the grid voltage, and ensures smooth transitions between operating points. Experimental results of a 1.4 kW, 200 V (line-to-line rms) ac–ac CSC demonstrator using first-generation 600 V, 140 m${\mathrm{m}}\mathrm{\Omega }$ monolithic bidirectional GaN transistors, grid-side and motor-side EMI filters, and a switching frequency of 72 kHz confirm the loss optimal operation (measured nominal ac-ac efficiency increases by 0.3% to 97% compared to conventional operation; part-load efficiency gains of up to 1% are observed) and seamless transitions between operating points.