Modular Multilevel Converters

Cascaded H-bridge (CHB) based electric vehicle chargers are widely recognized for their modularity, scalability, and robust fault-tolerant capability. Such converters are often modulated using level-shifted carrier-based PWM (LSPWM) schemes due to computational simplicity and ease of implemen- tation, but results in significant power imbalance among the modules. This power imbalance can be mitigated by reassigning carriers to modules, called Carrier-Reassignment PWM (CR- PWM). A First-In-First-Out (FIFO) based CRPWM is very easy to implement and provides some improvement over LSPWM, but fails to achieve perfect power balance. This paper proposes a novel carrier-reassignment scheme specifically designed for a 17-level CHB stage with eight reassignments per fundamental cycle, resulting in near-perfect power balance among modules. The redistribution of power also results in improvements in semiconductor losses - both conduction and switching loss. Loss models for the semiconductor devices were created through hardware double-pulse tests, SPICE and PLECS models, and integrated with MATLAB/Simulink to validate the proposed PWM scheme.