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 has shown that in FCML converters, capacitor voltages are unobservable over certain ranges of duty ratios depending on the specific converter level count. This work derives the minimum number and placement of differential capacitor voltage sensors required to ensure capacitor voltage observability over the entire conversion ratio range, particularly applicable to inverter applications. Experimental results employing industry-relevant hardware verify capacitor voltage observability with the proposed approach and demonstrate improved capacitor voltage estimation compared to the previously proposed solutions for retaining observability.
