Enabling the use of lower current-rated SiC MOSFET devices in large-current power converters by paralleling multiple H-bridges in the sub-modules

This paper presents a new sub-module topology constructed by multiple parallel-connected SiC MOSFET H-bridges. Apart from the advantage of achieving high voltage applications with low voltage-rated devices as in conventional sub-modules in modular multilevel converters, the proposed multiple H-Bridge submodule also provides the freedom of degree in current rating. The structure and the operation principle of the proposed topology are introduced first, and followed by a detailed low-level control algorithm aimed at solving the unequal steady-state current issue resulting from on-state resistance mismatches of paralleled discrete SiC MOSFETs. A modular multilevel STATCOM model constructed by multiple H-Bridge submodules is built in MATLAB/Simulink. Simulation results show: 1) multiple H-bridge submodules not only allow active current sharing of SiC MOSFET devices without the requirement of auxiliary chokes or capacitors, but also allow voltage balancing of sub-module capacitors; 2) the proposed topology allows the same converter rating to be achieved with half the amount of submodules operating at approximately three times of PWM frequency, compared with Si IGBT submodules converter, while providing 40% power loss reduction; 3) the decoupled conduction paths of paralleled H-bridges see approximately 14.8 % switching loss reduction compare with conventional paralleled SiC MOSFET full bridge sub-module.