Control Method of Flying Capacitor Converter Operated in Discontinuous Current Mode and Critical Current Mode

This paper proposes the control method of a three-level flying-capacitor converter (FCC), which is operated in discontinuous-current-mode (DCM) and critical current mode (CRM) to reduce the current ripple. An inverter connected to a single-phase grid requires a boost chopper and grid-connected inverter. In particular, reducing the volume of the boost inductor in the boost chopper is required. The DCM method is applied in order to minimize the volume of the boost inductor. However, the switching devices with high current rating are required due to a high current ripple when DCM is applied to a typical boost converter. In addition, FCC reduces the peak current of the boost inductor by the charging and discharging periods of the flying capacitor. In the proposed control method, FCC is designed to be operated in CRM at heavy load and DCM at light load. Due to the charging and discharging of the flying capacitor, the inductor current ripple in FCC is reduced compared to that of the typical boost converter with the same inductance. The validity of the proposed control method is confirmed with a 1-kW prototype. As a result, an efficiency of 96.2% is achieved at the 1-kW output power and the boost ratio of three.