State Space Analysis of Flying Capacitor Multilevel DC-DC Converters for Capacitor Voltage Estimation

Flying capacitor multilevel (FCML) converters show great promise for a number of DC-DC applications, but are subject to problems with voltage balance on the flying capacitors. A variety of techniques have been developed to regulate the flying capacitor voltages, but there is still a need to accurately measure them and understand the limitations of balancing algorithms in general. This work proposes an accurate, robust, and flexible way to estimate flying capacitor voltage states through sampled measurements of the switching node voltage. A state space model which reveals the necessary conditions for observability and controllability is developed. Scenarios where estimation and balancing cannot be achieved are explored; importantly, the criterion developed herein points to simple ways to eliminate these scenarios. The results are verified with a 6-level FCML hardware prototype.