Active Voltage Balancing in Flying Capacitor Multi-Level Converters With Valley Current Detection and Constant Effective Duty Cycle Control

One of the challenges of utilizing flying capacitor multi-level (FCML) converters is the flying capacitor balancing. Poorly balanced flying capacitors increase the switch voltage stress, which is detrimental to performance and device rating requirements. Previously, valley current detection was shown as a potential method to balance flying capacitors, but the method suffers from poor flying capacitor balancing performance at light load. Here, we investigate the light load conditions that lead to poor balancing and propose a new method, constant effective duty cycle (CEDC) compensation, which provides active balancing for the full load range of the converter. The proposed method is validated with a 4-level FCML experimental prototype, demonstrating excellent flying capacitor balancing over all load ranges, across the full duty cycles range and with multiple induced flying capacitor imbalances.