Analog control of AC link universal power converters: The key to very high frequency AC link conversion systems

The ac-link universal power converters have been proposed as an alternative to the conventional power converters. These converters, which can be configured as dc-ac, ac-dc, dc-dc, ac-ac and hybrid power conversion systems, offer numerous advantages over the other types of converters, including higher power density. The link current and voltage in these converters are alternating with no dc component, and this eliminates the need for dc electrolytic capacitors or large dc inductors. Despite being single-stage, these converters can change the level of voltage or frequency in a wide range. Moreover, regardless of the input or output form or their number of phases, galvanic isolation can be provided by a single-phase high frequency transformer. The other advantage of these converters is that they can benefit from the soft switching. Therefore, the switching losses are negligible, and there is no need for adding the snubber circuits. The soft-switching feature of the universal power converters allows the switching frequency, which is proportional to the link frequency, to be increased without affecting the efficiency, and this can further increase the power density. However, the complicated control of these converters, which necessitates using microcontrollers or FPGAs, has limited the maximum link frequency. This paper proposes a simple analog circuit for implementing the closed-loop control as well as a new open-loop control scheme for these converters. This allows significant increase in the frequency of operation at a low cost, and leads to very high power density universal power converters. The proposed control scheme is evaluated in this paper.