3kV/42kW Pump-back Test for Medium Voltage DC Transformer with Series-Connected SiC MOSFETs

The medium voltage DC transformer (DCX) plays a crucial role in DC grids. To increase the power density of medium voltage DCX, series-connected SiC MOSFETs are utilized to construct a DCX prototype. Active clamping modules (ACMs) with a corresponding voltage balancing strategy are employed to achieve even voltage sharing among series-connected SiC MOSFETs while minimizing the induced power loss. A critical aspect is the ACM capacitor energy recovery during deadtime; with an uncontrolled rectifier, this process degrades DCX load regulation. Consequently, this paper employs an active rectifier, substantially improving load regulation. To validate the voltage balancing control under higher voltage and power conditions, a pump-back test topology with 2DCXs and an additional boost converter is employed, keeping both the forward and reverse LLC converters behaving like a DCX. Experimental results demonstrate excellent voltage sharing and a peak efficiency of 98.6%.