On the Role of Switch Output Capacitance on Passive Balancing within the Flying Capacitor Multilevel Converter

Abstract

The Flying Capacitor Multi-Level (FCML) con-verter is heralded as enabling the utilization of low-voltage switches within a high-voltage converter by evenly distributing the voltage stress on a series string of switches through the use of “flying” capacitors. However, this advantage of the converter requires that the flying capacitors do not deviate too far from their ideal voltage distribution regardless of transients, load disturbances, or other parasitics to ensure the switches are not overvolted among other concerns. Previous works have identified certain theoretic combinations of duty cycle and level count where the flying capacitor voltages do not naturally balance and instead diverge. Although seemingly problematic, in practice this behavior is not observed in practical implementations. To resolve this discrepancy between FCML theory and experiment, we analyze the impact of switch output capacitance on the flying capacitor voltage balancing dynamics, and illustrate this capacitance has a naturally balancing effect.