Latest dual-gate (2G) monolithic bidirectional (MB) gallium nitride (GaN) enhancement-mode field effect transistors (e-FETs) enable a performance breakthrough of current DC-link inverters, e.g., in terms of power conversion efficiency, power density, cost and complexity. In fact, a single 2G MB GaN e-FET can replace the two anti-series connected conventional power semiconductors required in this inverter topology, realizing a four-quadrant (AC) switch with bidirectional voltage blocking capability and allowing controlled bidirectional current flow. Furthermore, as shown in this paper in case of three-phase (3-Φ) buck-boost (bB) current source inverter (CSI) systems comprising a DC-link current impressing buck-type DC/DC input stage and a subsequent boost-type 3-Φ current DC-link inverter output stage, a variable DC-link current control strategy, based on aSynergetic Control concept, can be applied to significantly reduce the switching losses occurring in the 3-Φ inverter. This strategy is denominated two-third pulse-width modulation (2/3-PWM), since by properly shaping the DC-link current with the input stage, the desired 3-Φ sinusoidal load phase currents can be generated by switching, in each switching period, only two out of the three phases of the output stage. Based on comprehensive circuit simulations and analytical calculations, a detailed explanation of the developed modulation and control schemes in different operating conditions is provided, and the reduction of losses enabled by 2/3-PWM is confirmed. Next, the seamless transition of the 3-Φ bB CSI system from 2/3-PWM to conventional 3/3-PWM is demonstrated. Finally, a 3.3 kW 3-Φ bB CSI system, applying 2/3-PWM and employing research samples of 2G MB GaN e-FETs in the 3-Φ inverter, is estimated to achieve an efficiency of 98.4% and a power density of 18 kW/dm 3 (295 W/in 3 ) at a switching frequency of 140 kHz. Index Terms-Dual-gate (2G) monolithic bidirectional (MB) gallium nitride enhancement-mode field-effect transistors (e-FETS), three-phase buck-boost current source inverter system, variable DC-link current control. in 2001, all inElectronic Engineering. In 1991, he joined Panasonic Corporation, Osaka, Japan, where he was engaged in the research and development of GaAs HFETs, HBTs, and GaAs MMICs. Since 2003, is involved in the research on GaN-based semiconductor devices. He authored or coauthored more than 60 technical papers in international journals and conference proceedings. He submitted more than 60 patents on semiconductor devices and on their process technologies. His current interest is on GaN-based bidirectional switches and on their applications.

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Solid-state transformers (SSTs) are employing compact high-power medium-voltage (MV) medium-frequency transformers (MFTs) and enable the power transfer between galvanically isolated DC systems. Considering a typical SST isolation stage, i.e., a 166 kW unregulated series-resonant DC-DC converter acting as a DC transformer (DCX) with equal input and output MV DC voltages of 7 kV (1:1-DCX), we derive component-level and system-level performance limits of aircooled realizations with either an air-core transformer (ACT) or with a magnetic-core transformer (MCT). We describe the design of two fully rated MFT prototypes in detail and provide a comprehensive experimental characterization to validate the derived performance limits, including also the dielectric losses and the analysis of stray magnetic fields. The realized ACT and MCT prototypes achieve measured efficiencies of 99.5 % and 99.7 % at gravimetric power densities of 16.5 kW/kg and 6.7 kW/kg, respectively. Considering 10 kV SiC MOSFETs, calculated system-level efficiencies of the ACT-based and MCTbased 1:1-DCX result in 99.0 % and 99.2 % at the nominal operating point, with similar part-load efficiency characteristics. The paper concludes with an application-oriented qualitative evaluation of the two concepts.

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