Decentralised Control for Balancing the Cell-Voltages of a High Conversion Ratio Flying Capacitor Multilevel Converter
Résumé
This paper presents a decentralised control strategy for the balancing of the cell-voltages of a Flying Capacitor Multilevel Converter implementing a large number of cells to address high conversion ratio. The originality of this work is that no reference is required for regulating the capacitor voltages leading to an auto-balance capability. It is composed of several local cell-voltage balancing controllers and a global load-current regulator. For theoretical analysis purpose a change of state-variables is proposed leading to a new model of the converter. Then the study of the decentralised balancing method is performed based on a modal analysis of the system. A decoupling between the output current regulation and the cell-voltage balancing loops is proposed, then observed. The synthesis of the controllers, guaranteeing the stability of the system within a large bandwidth for each mode, is obtained. To address fault-tolerance requirement, this decentralised control strategy provides an interesting ability of auto-reconfiguration allowing to change the number of active cells during operation. Simulations of separated modal responses show the stability and bandwidth of the control loops for DC/DC 500V to 16V high conversion ratio application. Experimental results performed on a 5-cell multilevel converter demonstrate the validity of the proposed control method.