Analysis and cell-level experimental verification of a 25 kW all-SiC isolated front end 6.6 kV/400 V AC-DC solid-state transformer

Solid-state transformers (SSTs) could serve as interfaces between a medium-voltage (MV) AC grid and a low-voltage (LV) DC load or source, i. e., could be employed in applications with power supply character such as traction auxiliary supplies or rack-level power supplies in future datacenters. For handling the high input-side AC voltage and output side current, SSTs are typically realized as input-series output-parallel (ISOP) arrangements of multiple converter cells, whereby each cell comprises a medium-frequency isolation stage. This paper presents such a multi-cell 25 kW all-SiC MVAC-LVDC SST (6.6 kV AC to 400 V DC) based on the isolated front end (IFE) approach, which is an interesting alternative to the isolated back end (IBE) configuration mainly discussed in literature so far. The IFE concept is briefly explained, the main component stresses are derived, and a converter cell prototype is designed and tested. The 5 kW prototype cell features a power density of 1.5 kW/l (24.6 W/in3) and a measured peak efficiency of 97.5%. This is significantly higher than previously published data for 'FEbased SSTs, and in the same range as what has been reported recently for industrial IBE-based SSTs. Thus, this paper confirms that the 'FE approach can be a feasible and interesting alternative for realizing MVAC-LVDC SST systems with low complexity.