Quasi square wave operation of modular multilevel converter based dual active bridge DC–DC converter with inductor energy recovery

The modular multilevel DC–DC transformer (MMDCT) provides a reliable solution to overcome the challenges of conventional dual‐active‐bridge converters in terms of power semiconductors ratings and dv/dt stress on transformer coupling windings. An alternative modulation method, quasi‐square‐wave, was proposed to reduce the cell capacitance of modular multilevel bridges. However the application of quasi‐square‐wave modulation is found to result in underdamped switching transients and losses when resetting energy stored in arm inductance. This paper presents a detailed transient analysis of MMDCT arm insertion based on an equivalent circuit, which contributes to a more accurate component sizing and gives voltage estimation for individual half‐bridge submodules. Additionally, a revised switching sequence is proposed to recover this inductor energy and lower the oscillation‐related losses. Simulated and experimental results from a scaled test rig of MMDCT are implemented and validate the proposed component sizing and switching sequence, indicating that the converter efficiency can be improved under revised switching sequence. Finally, a silicon carbide based, high‐frequency MMDCT is proposed and simulated. This paper presents a comprehensive component design technique for modular multilevel converter based dual‐active‐bridge DC–DC converter (MMDCT). In addition, this paper proposes a revised switching sequence that recovers inductor energy, interrupts voltage and current oscillation during arm insertion, and enhances the conversion efficiency for high‐frequency MMDCT operations. This advancement enables the use of MMDCT at higher switching frequencies, where silicon carbide is ideally suited for this application.