Topologies and Control Strategies of Cascaded Bridgeless Multilevel Rectifiers

This paper presents a new unidirectional cascaded bridgeless multilevel rectifier (CBR) well suited for serving as the rectifier stage of a transformerless cascaded multilevel converter. Compared to a traditional cascaded H-bridge rectifier, the proposed CBR utilizes fewer fully controlled switching devices by replacing the fully controlled H-bridge modules with bridgeless PFC modules, thus simplifying the control circuits, increasing the system reliability, and cutting down the implementation expenses. However, when a single-phase CBR is operating under a unity power factor, the input current will be severely distorted because of the unidirectional power flow property of the bridgeless PFC modules. Therefore, aiming at achieving a satisfactory current quality and power factor simultaneously, an improved control strategy and a revised topology for the single-phase CBR are provided as two effective solutions. Specifically, for the single-phase CBR under the improved control, the limitation of the maximum value of the boost inductance considering an acceptable power factor is derived. Besides, the method of determining the configuration of the modified single-phase CBR is given based on the steady-state analysis. In addition, for the three-phase CBR, the reason that the traditional control strategy is capable of realizing unity power factor rectification while mitigating input current zero-crossing distortion is analyzed. Finally, simulations in the MATLAB/Simulink environment and experimental results from scaled-down laboratory prototypes are provided to verify the proposed theories.