Three-Level Isolated Direct AC–AC Converter and its Closed-Loop Control Strategy

Abstract

The isolated direct AC-AC converter has the advantages of high power density and long life time, however, suffering from the high cost and relatively low efficiency. In this paper, a three-level isolated direct resonant AC-AC converter is proposed, thus the voltage stress of power switches reduce to half of the input voltage amplitude and as a result, the voltage rating of the power switches is reduced. A hybrid modulation strategy for the proposed converter is proposed to realize the AC-AC conversion and reduce the switching power loss. Furthermore, in order to obtain a good control of the output voltage based on the proposed converter, the closed-loop control strategy of the output voltage combining a linear controller with the repetitive controller is proposed and the detailed parameters design rule is discussed. The proposed control strategy not only realizes the sinusoidal output voltage waveforms and good dynamic performance, but also effectively eliminates the effect of input voltage harmonics. The corresponding proposed topology, modulation and closed-loop control strategy are all verified by the detailed experimental results.