Hysteresis current regulation of three phase flying capacitor inverter with balanced capacitor voltages

This paper presents a new hysteresis current regulation strategy for a three-phase three-level flying capacitor multilevel inverter, that uses the zero-crossing of the averaged phase leg output voltages to select the switched output level for each inverter phase leg. Consequently each phase leg requires only one hysteresis comparator to generate its switching signals, which significantly improves its performance compared to multiple band strategies. Furthermore, since the phase leg average voltage is an estimation of the load back-emf, the same averaged signal can be used to vary the regulator hysteresis bands to maintain a constant switching frequency and achieve better harmonic performance. A state machine is then used to implement active balancing to maintain balanced capacitor voltages. Finally, the paper uses the phase leg switch states to calculate the interacting current between the phases. This current is subtracted from the measured phase leg currents before making the switching decision, to avoid interaction between the phase legs.