A novel bridgeless flying capacitor multilevel rectifier

A novel transformerless three-phase unidirectional bridgeless flying capacitor (UBFC) rectifier for medium-voltage and high-voltage applications is presented in this paper. The rectifier consists of three single-phase rectifiers in wye connection; the DC outputs of the three single-phase rectifiers are connected in parallel to provide a high common DC-link bus, which makes it convenient to connect with neutral point clamped (NPC) or flying capacitor (FC) multilevel inverters. To avoid phase-to-phase current circulation, an additional inductor and diode are added to each single-phase rectifier. Compared with conventional bidirectional multilevel converters, only half IGBTs are used, the voltage stress is lower, and the line-frequency transformers are no longer required, which makes the proposed rectifier has a series of advantages such as reduced cost, simplified control complexity, increased reliability and improved overall efficiency. The basic operating principle of the proposed rectifier in continuous conduction mode is discussed; an improved carrier-based level-shifted pulse-width modulation (PWM) method integrated with voltage balancing and the double closed-loop control strategy are proposed to achieve more voltage levels and keep the flying capacitors in balance. The feasibility of the proposed rectifier is verified by the simulation and experimental results.