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.