Ultrasparse AC-Link Converters

Soft-switching ac-link universal power converters, also called partial resonant converters and ac-link buck-boost converters, have received noticeable attention during the last few years. In these converters, each of the inputs and outputs can be dc, single-phase ac, or multiphase ac; therefore, they can be used for dc-dc, dc-ac, ac-dc, or ac-ac power conversion systems. The soft-switching ac-link universal power converters are compact, reliable, and expected to offer longer lifetime compared with the other types of converters. However, they require more switches, which make the control process more involved. The sparse ac-link buck-boost converters were proposed to partially solve this problem. The sparse configuration reduces the number of switches from 24 to 20 in a three-phase ac-ac configuration. This paper proposes a modified configuration, which further reduces the number of switches without changing the principles of operation. This converter, which is named ultrasparse ac-link buck-boost converter, reduces the number of switches from 24 to 16, in a three-phase ac-to-ac case, and from 20 to 10, in a dc-to-three-phase-ac configuration. The proposed converter is applicable to systems with unidirectional flow of power, such as PV and wind power generation systems. This converter is expected to offer higher reliability compared with the original converter and higher reliability and efficiency compared with the sparse configuration. Another important feature of this configuration is that it can be fabricated by insulated-gate bipolar transistor modules, which are more compact and more cost effective compared with discrete devices. This paper presents the principles of the operation of this configuration and compares the efficiency, the failure rate, and the current rating of the switches in the ultrasparse, sparse, and original configurations. Moreover, it evaluates the performance of the proposed converter through simulation and experiment.