A single-stage AC-AC solid-state transformer with ZVS operation

Abstract

Solid-state transformer (SST) is an attractive concept, which provides galvanic isolation and voltage scaling by means of a medium-frequency link. In this paper, a single-stage alternative current (AC)-AC SST solution without bulky energy storage elements is presented. The front-end rectifier (FER) and rear-end inverter (REI) of the SST works at line-frequency, while the intermediate LLC series resonant converter (SRC) fulfils the tasks of galvanic isolating and regulating the output voltage. To achieve high efficiency and bidirectional power flow capability of the SST, the zero voltage switching (ZVS) condition of the LLC SRC is comprehensively analysed, and the parameters design method of the LLC SRC is developed, where the resonant currents are constructed by applying the same gating signals to the primary-side and secondary-side bridges and the dead-time is properly selected. Consequently, the presented SST has the merits of high conversion efficiency, naturally bidirectional power flow capability, potentially high power density and high reliability. The experimental results on a 1.5 kW SST prototype show that sinusoidal input and output current, ZVS operation over the full load range are achieved, and the peak efficiency is 97.63%. The experimental results verify the functionality and effectiveness of the developed methods.