A 7 kV/400 V isolated DC-DC converter utilizing 6.5 kV and 3.3 kV SiC MOSFETs is proposed for medium-voltage energy storage systems. The topology integrates a neutral point clamped (NPC) stage with modular multilevel converter (MMC) structure, achieving 12:1 voltage step-down before transformer isolation. The NPC stage operates at 1 kHz with 6.5 kV devices, while MMC submodules switch at 20 kHz using 3.3 kV devices, enabling 40 kHz transformer operation through frequency multiplication. Capacitor voltage self-balancing is achieved through a novel switching scheme, and power flow is regulated through phase-shift control between primary and secondary voltages of transformer. Zero-voltage switching is realized for MMC submodules during voltage transitions. Simulation results validate the design, and experimental characterization of SiC devices demonstrates switching performance with dv/dt up to 40 V/ns.
