ZVS Analysis and a Design Method for Unidirectional Medium-Voltage LLC-DCX With High Step-Up Ratio

The high step-up ratio in a medium-voltage dc transformer (DCX) amplifies the referred parasitic capacitance from the secondary side, resulting in an altered resonance mode. This letter analyzes the parasitic effect on the zero-voltage switching (ZVS) condition under rated load in an LLC-DCX. A rule of thumb is proposed to determine the main parameters in the system, including the magnetizing inductance, resonant inductance, and switching frequency. It is suggested to increase the resonant inductance to fully charge the equivalent parasitic capacitance (C-eq) prior to the deadtime for secure ZVS. The proposed design no longer requires to lower the magnetizing inductance and can adapt to a wide range of insulation design of the transformer core and windings. Scaled experimental results are presented to validate the analysis.