Balancing Limits of Flying Capacitor Voltages in Coupled Inductor FCML Converters

Abstract

Ensuring that the flying capacitor voltages are balanced is critical to the operation of flying capacitor multilevel (FCML) converters. This paper systematically explores robust voltage balancing of FCML converters using coupled inductors. Coupled inductors are shown to balance FCML converters with any number of levels or even number of phases, with some singularities. Balancing is improved with an even number of phases, levels, and higher coupling coefficients. The two-phase, three-level FCML converter yields the strongest and most reliable benefit from coupled inductor balancing. Coupled inductors with a modest coupling ratio can also improve balancing. A piece-wise linear model of the converter is developed to show that coupled inductor balancing is effective in intrinsically unbalanced con-verters. The theoretical predictions are verified using simulations and experimental results.