99% Efficient 10 kV SiC-Based 7 kV/400 V DC Transformer for Future Data Centers

The power supply chain of data centers from the medium voltage (MV) utility grid down to the chip-level voltage consists of many series connected power conversion stages and accordingly shows a relatively low efficiency. Solid-state transformers (SSTs) could improve the efficiency by substantially reducing the number of power conversion stages and/or directly interfacing the MV ac grid to a 400 V dc bus, from where server racks with a power consumption of several tens of kilowatts could be supplied by individual SSTs. The recent development of SiC MOSFETs with a blocking voltage of 10 kV enables the realization of a simple and, hence, highly reliable two-stage SST topology, consisting of an ac/dc power factor correction rectifier and a subsequent isolated dc/dc converter. In this context, an isolated 25 kW, 48 kHz, 7 kV to 400 V series resonant dc/dc converter based on 10 kV SiC MOSFETs is realized and tested in this paper. To achieve zero voltage switching of all MOSFETs, a special modulation scheme to actively control the amount of the switched magnetizing current on the MV- and low voltage-sides is implemented. Furthermore, the design of all main components and, especially, the electrical insulation of the employed medium-frequency transformer are discussed in detail. Calorimetric efficiency measurements show that a full-load efficiency of 99.0% is achieved, while the power density reaches 3.8 kW/L (63 W/in
³
).