Comparative evaluation of isolated front end and isolated back end multi-cell SSTs

Solid-state transformers (SSTs) are power electronic interfaces between medium voltage (MV) and low voltage (LV) systems that provide galvanic isolation by means of medium frequency (MF) transformers, making them suitable for MVAC to LVDC conversion in environments where weight and volume constraints apply. This paper discusses an isolated front end (IFE) SST concept that allows to reduce the complexity and physical size of the MV side converter assemblies compared to the well-known isolated back end (IBE) SST topologies. The IFE approach performs the entire grid current and output voltage control on the LV side using standard non-isolated |AC|-DC boost converter stages. A generic comparison of the IFE and the IBE concepts reveals that the lower complexity of the IFE, e. g., a lower total MV blocking voltage requirement (number of cascaded cells), comes along with higher device RMS currents and hence slightly higher chip area requirements. On the other hand, a case study considering a 25kW, 6.6 kV AC to 400V DC SST shows advantages of the IFE in part-load operation due to lower switching and transformer core losses. This makes the IFE approach interesting for applications where MF isolation instead of low frequency isolation is required because of space and weight constraints (e. g., traction, subsea or aircraft environments), and where low system complexity is desirable.