Closed-loop current control of multilevel converters formed by parallel complementary unidirectional phase-legs

This paper presents a dual-loop current regulation strategy for coupled-inductor multilevel converters. The basis of the strategy is to recognise that the current in the coupled-inductor can be decoupled into independent DC bias and AC load currents, which can then be regulated using separate feedback processes. This allows the system to enforce continuous conduction operation of the inverter phase legs by dynamically varying the DC bias current to track the AC load current. Design principles are presented to account for the different common- and differential-mode load dynamics, and to maximise the closed-loop bandwidths of both feedback loops. This ensures that the DC bias current can follow rapid load transient events. Matching simulation and experimental results obtained for a five-level flying-capacitor coupled-inductor inverter are presented to validate the proposed control strategy.