Extremely sparse parallel AC-link universal power converters

Universal power converters are a relatively new class of power converters that can be configured as dc-dc, dc-ac, ac-dc, and ac-ac. These converters that may be classified as parallel and series offer numerous merits over other types of power converters. The parallel ac link universal converter is an extension of a buck-boost converter in which the current of the inductor is alternating, the input and output can have any number of phases and the switches benefit from the soft switching. The main limitation of the parallel ac-link universal power converter is its large number of switches. A three-phase ac-ac configuration requires 12 bidirectional switches that are realized by 24 unidirectional switches. Several modifications, including Sparse and Ultra-Sparse parallel ac-link universal power converters, have been proposed to reduce the number of switches while maintaining the principles of the operation and hence the advantages of this converter. The number of switches in the three-phase ac-ac sparse and ultra-sparse configurations are reduced to 20 and 16, respectively. Another modified configuration, called soft-switched transformerless inverter, was proposed for photovoltaic application. This paper proposes a number of topologies based on the parallel ac-link universal power converters with significantly reduced number of switches. The first category of the proposed topologies, which is an extension of the soft-switched transformerless inverter, reduces the number of switches in a bidirectional three-phase ac-ac configuration to 16. The other topology, which is an extension of the ultra-sparse configuration, reduces the number of switches in the unidirectional ac-ac configuration to 10. This topology can offer galvanic isolation with only a single-phase high frequency transformer. This paper presents the principles of the operation of the proposed topologies and evaluates them through simulation and experiment.