Analysis of Steady-State Balancing in the Flying Capacitor Multilevel Converter Considering Capacitor Voltage Ripple

The flying capacitor multilevel (FCML) converter has demonstrated impressive power densities and high efficiencies. In pursuit of increased power density, the flying capacitance is often reduced. This work investigates the effects of flying capacitor sizing on the steady-state behavior of the inductor current and flying capacitor voltages. A piecewise parabolic model is developed to include flying capacitor voltage ripple in circuit operation and provide analytical insights. The capacitor voltage ripple, often overlooked in standard analysis of the converter, leads to flying capacitor voltage imbalance, with the worst balancing at nominal duty cycles. Additionally, the flying capacitor voltage ripple creates harmonics in the inductor current ripple appearing at multiples of the switching frequency as opposed to multiples of the effective switching frequency as typically modeled. The results from the model are experimentally validated with a 4-level FCML converter prototype and show excellent matching.