For any voltage above 600 V DC, it is usually recommended to use 900 V~1200 V SiC MOSFETs instead of GaN HEMTs, given presently commercial GaN HEMTs have the maximum voltage rating up to 650 V. This paper is an attempt of employing 650 V E-mode GaN HEMTs to build a three-level bidirectional DC/DC converter, with the input as an 800V battery, the output as the ~400 V DC grid and half of switches working at the hard-switching mode. The active balancing control instead of using clamping diodes in the conventional neutral point clamping (NPC) topology is adopted, aiming at higher efficiency. Simulation based on the double-pulse-test results shows that such design strategy with GaN has a better efficiency over the conventional 1200-V SiC MOSFET based Buck/Boost converter. Two bottom-cooled GaN HEMTs are in parallel to enhance the power capability and efficiency, which requires the special focus on the parasitic parameters. The effects of parasitics especially the stray inductance in the current commutating loop and the gate-drive loop during switching transitions have been comprehensively analyzed in this paper. Experimental results under 800V bus voltage are presented to verify the proposed solution.

In this paper, a new zero voltage zero current transition (ZVZCT) boost converter using coupled inductors is proposed. The feature of this topology is: the main switch can be switched on with ZVZCT, and off with zero voltage switching (ZVS), without any voltage and current stress. The auxiliary switch is turned on with zero current switching (ZCS). Other semiconductor devices can also achieve soft switching (SS). Due to the existence of the coupling effect, the current impact of the auxiliary switch is completely suppressed when it is turned off. Lower input current and output voltage in the interval of the switch-off for the auxiliary switch are just equivalent to running at a low power level, which is an additional advantage of the coupled inductor ZVZCT solution. Another advantage of the topology is that the soft switching condition of the main switch for each specific load can be effectively optimized just by setting the ratio of coupling inductance corresponding to the circuit design and application load. Thus, the topology design can be improved to adapt to different load types. A 100 kHz experimental prototype has been verified, different load for 1 kW and 0.4 kW optimization comparison has been presented, and the case without optimization is also observed.