Analysis and Implementation of Minimum-Sensor Capacitor Voltage Estimators for Flying Capacitor Multilevel Converters

This work studies the estimation of capacitor voltages in the Flying Capacitor Multilevel (FCML) converter via direct measurements of only the input voltage, switched node voltage, and a minimum number of additional voltage sensors. Prior work developing capacitor voltage estimators for FCML converters has demonstrated that capacitor voltages are unobservable over specific ranges of duty ratios that depend on the converter level count. This work derives the minimum number of differential capacitor voltage sensors required to ensure capacitor voltage observability over the entire conversion ratio range and discusses where these sensors must be placed. Experimental results employing industry-standard digital signal processor hardware verify capacitor voltage observability with the proposed approach and demonstrate improved converter performance compared to the dithering solution previously proposed for retaining observability.