Balancing Flying Capacitor Multilevel Converters with Coupled Inductors: Models, Analysis, and Limitations

This paper investigates the modeling, analysis, and design methods for balancing flying capacitor multilevel (FCML) converters using coupled inductors. Coupled inductors introduce multi-resonant dynamics to multiphase FCML converters. It synergizes with FCML converters by reducing inductor current ripple, reducing switch stress, and providing a strong and fast voltage and current balancing mechanism. Coupled inductor balancing is shown to be robust against many types of disturbances. A piece-wise linear (PWL) model and an energy-balance-based model are developed to show that coupled inductor balancing is effective in intrinsically unbalanced converters. Analytical and numerical methods are used to evaluate the effect of initial conditions, circuit parameters, and losses on the balancing behavior, and to show how coupled inductors in many cases balance the flying capacitors. The models and key principles are verified with simulations and experimental results.