A High-Frequency PCB-Winding Transformer Design with Medium Voltage Insulation for Solid-State Transformer

A Solid-state transformer directly reduces medium voltage to low voltage (e.g. 400 V) with minimized power conversion stages. Insulation structure is the bottleneck of DC-DC module in SST from insulation effectiveness, manufacturing process, thermal management, and power density point of view. In this paper, a compact PCB-winding transformer structure is proposed to handle the medium voltage insulation by FR4 in PCB-winding. The primary side winding is built in PCB-winding and secondary side winding is still Litz wire for a lower loss. With semi-conductive shielding and stress grading layer design, the E-field can be restrained in the primary side PCB for a partial discharge free insulation. An arc section winding structure is proposed to reduce the high E-field inside the insulation layer to improve insulation performance. The layer-to-layer winding resistance and overall loss/footprint trade-off is analyzed for a low loss design. Finally, the design is demonstrated on an 800/400V, 15-kW, 200-kHz CLLC converter with 98.8% peak efficiency and 130W/in³ power density, whose transformer achieves partial discharge free up to 14.6kV.