Soft-Switching AC-Link Three-Phase AC-AC Buck-Boost Converter

In this paper, the soft-switching ac-link ac-ac buck-boost converter will be studied in more detail. This single-stage converter, which is, in essence, an extension of the dc-dc buck-boost converter, can be an excellent alternative to dc-link converters. Being a buck-boost converter, this converter is capable of both stepping-up and stepping-down the voltage. The link current and voltage are both alternating, and their frequency can be as high as permitted by the switches and the sampling time of the microcontroller. This eliminates the need for dc inductors or dc electrolytic capacitors, and the main energy storage element is an ac inductor (L). Moreover, in this converter, galvanic isolation can be provided by adding a single-phase high-frequency transformer to the link. Therefore, the proposed converter is expected to be more compact compared to the conventional dc-link converter. The other advantage of this converter is the soft switching of the switches, which is feasible by adding a small capacitor (C) to the link. In this paper, the design and analysis of this converter will be studied in detail. In order to accurately analyze this converter, the effect of the LC link resonance on the performance of the converter will be studied. This analysis helps in evaluating the performance of the converter at low power levels when the resonating time of the LC link is not negligible. Using this analysis, the link peak current and the link frequency may be calculated at any point of operation. The accuracy of this method is verified through simulations and experiments. Detailed comparison of the proposed converter with the dc-link converter will be also presented in this paper. It will be shown that, despite having more switches, the current rating of the switches is lower in this converter. Moreover, the efficiencies of the two converters will be compared. Finally, the performance of the soft-switching ac-link ac-ac buck-boost converter is experimentally evaluated in this paper. It will be shown that the converter has the possibility of changing both the frequency and the voltage. Both step-up and step-down operations will be verified through experiments.