A High-Frequency Isolated AC-AC Converter with Independently Controlled Bipolar Buck-Boost Dual Outputs

In this paper, a new topology of a single-phase highfrequency isolated bipolar buck-boost AC-AC converter is proposed for application as a three-phase flexible AC-link (FACL) converter. The proposed converter comprises one input and two output AC ports, requiring only three discrete converters to implement a three-phase FACL converter with flexible magnitude and phase control. All three ports are electrically isolated through a three-winding high-frequency transformer, replacing the bulky and lossy power-frequency transformer used in existing FACL converters. Both output AC ports are decoupled in terms of voltage polarity and magnitude and can provide symmetric non-inverting and inverting buck-boost outputs, improving voltage magnitude and phase control and reducing design complexity. Salient features of the proposed converter include the suppression of input and output harmonics through second-order harmonic filters and bidirectional operation with inductive load support. The topology is implemented with standard full-bridge inverter modules, simplifying the implementation and commutation process. A dedicated switching strategy is developed for various operational scenarios required in FACL applications. The proposed switching strategy allows current sharing among primary-side switches during high-current stress intervals, relieving current stress and conduction losses. Detailed analysis of the proposed topology is provided, including component stresses and ripples, comparisons, and simulation results. Finally, hardware results from a laboratory-scale converter are presented to verify the advantages of the proposed topology.