Single-stage soft-switching ac-link AC-AC and DC-AC buck-boost converters with unrestricted load power factor

Soft-switching ac-link buck-boost converters, also known as ac-link universal power converters and partial resonant ac-link converters, have received remarkable attention during the last few years. These converters, which can be configured as dc-dc, dc-ac, ac-dc, or ac-ac (single-phase or multi-phase), are compact, reliable, and expected to offer longer lifetime compared to the other types of converters. These single-stage converters are an extension of the dc-dc buck-boost converters. However, several modifications are applied so that the current of the link inductor is alternating. By adding a small capacitor to the link and by modifying the control scheme the switches are all turned on at zero voltage, and they also have a soft turn-off. In this paper, it will be shown that the soft-switching ac-link three-phase ac-ac or dc-ac converters have some restrictions on the power factor of the load connected to them. Lower load power factors may damage the link capacitor as well as lead to hard switching of the switches. The range of the acceptable load power factor will be determined in this paper and a solution will be presented to overcome this problem. The proposed technique allows the converter to operate normally regardless of the load power factor value.