A state machine decoder for phase disposition pulse width modulation of three-phase coupled-inductor semi-bridge converters

Coupled-inductor semi-bridge converters, assembled from magnetically linked pairs of complementary unidirectional phase legs, can achieve a substantially improved harmonic output voltage performance with the same number of silicon devices as traditional converters. However, they must be operated so as to ensure that a continuous common-mode current always flows through each coupled-inductor, to ensure each phase leg can always produce the required switched output voltage. This is particularly challenging for a three-phase coupled-inductor bridge controlled by PD modulation, since there is no direct correlation between the modulator output requirements and the individual phase leg switching device commands. This paper presents a novel strategy to control each phase leg´s common-mode inductor current under PD modulation, by introducing a square wave offset signal into the modulator reference command, and then passing the commanded output of the modulator through a post processing state machine to correctly switch each semi-bridge phase leg to maintain the required DC common-mode current. The strategy has been verified by detailed simulation studies and matching experimental results.