Decoupling Control And Closed-Loop Design Of Multi-Level Converter

The Multi-Level (ML) converter has the advantages such as low voltage stress for power devices, smaller output filters and reduced harmonic content of the output voltage, it is very suitable for high voltage and high power conversion applications. The ML converter splits voltage constraints among series-connected switches through the flying capacitors, so keeping the flying capacitor voltages at proper value is very important.The ML converter is a multi-input and multi-output, strongly coupled nonlinearly system, its control elements (duty cycles) and the controlled elements (output voltage and flying capacitor voltages) of the ML converter are strongly coupled, so the system should be decoupled to multiple single-input and single-output sub-systems such that each output can be controlled independently.This paper builds the small-signal model of the ML converter, and decouples the converter into several independent single-input single-output systems with the control strategy proposed in [8], so the closed-loops for the output voltage and the flying capacitor voltages can be designed independently. Based on the small-signal model, the three-level converter is taken as the example to illustrate the design of the closed-loops. The validity of the small-signal model and the controlled strategy proposed in [8] are verified by the simulation and experiment results. The results show that the modeling and design of the closed-loops are valid and the flying capacitor voltages control is independent with the output voltage control.