Natural Power Factor Correction and Soft Switching Design for a Single-Stage Bidirectional Dual Active Bridge AC-DC Converter

This article presents a natural power factor correction (PFC) modulation scheme for a single-stage, single-phase, resonant-type, bidirectional dual active bridge (DAB) ac-dc converter. The converter is composed of a cycloconverter cascaded with a full-bridge (FB) rectifier through a LC-series resonant circuit and a high-frequency transformer. The high- and low- frequency cyclic PWM modulation method applied in the cycloconverter results in half of the switching loss compared with the conventional PWM scheme. Moreover, a unit power factor can be implemented instead of requiring grid-side current loop and grid current sensor. In order to improve the converter efficiency, the extended phase shift (EPS) modulation is employed to ensure zero-current-switching (ZCS) for the grid-side cycloconverter and zero-voltage-switching (ZVS) for the dc-side FB rectifier. The phase shift is the unique variable to regulate the transferred power, so that the computational intensity can be significantly decreased. The proposed optimized scheme also offers the advantages of fewer power backflow and lower current stress compared with conventional PFC schemes based on the inner mode. The validity and feasibility of the proposed control scheme is confirmed through a laboratory prototype