A Small-Signal Charge Model for Active Balancing of Flying Capacitor Multilevel Converters

This article presents methods for modeling capacitor voltage dynamics in the Flying Capacitor Multilevel (FCML) converter and closed-loop control techniques for regulating the capacitor voltages to their nominal balanced values. A small-signal plant model is derived by studying the open-loop capacitor voltage dynamics using a linear-ripple model of the inductor current. Perturbations to the switching signals acting as control inputs are defined, and their impacts on the average currents flowing into the flying capacitors are identified. Unlike prior works based on averaging, the proposed plant models accurately capture the impact of the inductor current ripple on the average capacitor currents and yield valid models of capacitor voltage dynamics at light-load conditions. Using the proposed control inputs, the small-signal system is shown to be completely controllable, and a state-feedback controller is presented to damp the natural capacitor voltage oscillation. Plant models and controller structures are presented for all three operating modes of the four-level FCML converter. Experimental measurements of a hardware prototype verify the presented open- and closed-loop models. The capacitor voltage responses to supply voltage step-transients significantly improve with the proposed active balancing controller across the full load range.