13.8kV/400kW Solid State Transformer DC Fast Charger (SST-DCFC) Based on 15kV SiC AC Switch

This paper proposes a novel medium voltage (MV) solid state transformer (SST) based DC fast charger (SST-DCFC). The SST-DCFC utilizes a patent pending AC/DC topology which includes a series resonant converter (SRC) stage and 15kV SiC AC switch. The SRC addresses challenges in medium voltage SST by achieving soft switching for the MV switches across wide voltage and load ranges. The topology is analyzed through both time domain analysis and first harmonic approximation to provide useful equations for circuit design. A novel 15 kV SiC AC switch based on series-connection of the 1700 V SiC MOSFETs is further developed to enable a simple half bridge implementation on the MV side. 30 kVdc insulation and 8.0 kVrms partial discharge (PD) tests have been demonstrated to prove the feasibility for 13.8 kVac three-phase four-wire connection. A single-phase SST-DCFC prototype operating at 8.0 kVac to 800 Vdc is developed. A 7.2 kVac to 520 Vac experiment with a constant load of 50 kW is also carried out to verify the feasibility of the proposed SST-DCFC. The single-phase SST-DCFC prototype features a power density of 25 W/inch³ and a measured peak efficiency of 97.6%.