Investigation of modularity in DC-DC converters for solid state transformers

This paper investigates the potential for size reduction of a Dual Active Bridge (DAB) DC-DC converter for a solid state transformer (SST) by implementing the converter in a highly modular manner. It is shown that when IGBT switches are used, a high degree of modularization of the DAB results in lower efficiency and increased overall transformer weight. However if the degree of modularization is increased to the point where the module level voltages and currents allow the use of MOSFET switches, then efficiency comparable to that achievable using IGBTs can be achieved. More importantly with MOSFETs used as switches the switching frequency can be increased and the size of the transformer reduced. It is calculated that in the case of a 110 kW, DAB converter implemented using 4 modules, switching at 4 kHz the overall weight of transformers required is 400 kg. This compares with 112 kg for the same DAB implemented using 56 modules all switching at 30 kHz with MOSFET switches. In the first case the efficiency is calculated to be 99.1% compared to 97.9% in the second case.