Single-Phase Isolated Bidirectional Bipolar Output Buck-Boost AC–AC Converter With Safe-Commutation

This article develops a new single-phase high-frequency transformer (HFT) isolated bidirectional buck-boost AC–AC converter. The proposed topology comprises bidirectional AC switches and passive components and is free of diodes. The proposed topology can provide a symmetric noninverting and inverting buck-boost output with a flexible voltage gain of ±ND/(1-D), providing two degrees of freedom for output voltage control by the HFT turns ratioNand dynamic duty ratioD. The proposed converter integrates a two-winding HFT, providing electrical isolation with better magnetic utilization in comparison with tapped/three-winding transformers, while eliminating the bulky line-frequency transformer for reduction in system size, weight, and cost. The proposed converter utilizes input and output second-order LC filters, providing continuous and low ripple input and output currents. The operation of the proposed converter is reversible, providing bidirectional power flow and support for nonunity power factor loads. A proposed safe-commutation switching strategy is developed for bidirectional AC switches, which provides continuous inductor current path during PWM dead-time, resisting switch voltage overshoots. Moreover, only two switches conduct at high frequency in the proposed modulation, reducing the switching loss. A detailed circuit operation and analysis is provided, along with component design guidelines and loss analysis. The comparisons highlight the advantages of the proposed converter, followed by experimental results on a laboratory-built prototype verifying the circuit operation and features.