Balancing Multiphase FCML Converters with Coupled Inductors: Modeling, Analysis, Limitations

This paper investigates the modeling, analysis, and design methods for passively balancing flying capacitor multilevel (FCML) converters using coupled inductors. Coupled inductors synergize with FCML converters by reducing inductor current ripple, reducing switch stress, and, as proven in this paper, by providing flying capacitor voltage balancing. This enables FCML topologies to be scaled well to larger systems. This paper proves that coupled inductors can solve the unbalancing problem in many FCML converters. Moreover, tools are developed to thoroughly explain and quantify coupled inductor balancing, allowing general design guidelines to be offered for robust coupled inductor FCML converters. Finally, this paper derives the limitations of coupled inductor balancing with respect to the number of phases, levels, and the required coupling ratio. The key principles of coupled inductor FCML balancing in steady-state are demonstrated with a systematic theoretical framework and extensive experimental and simulation results.