Electronic Thesis and Dissertation Repository


Doctor of Philosophy


Electrical and Computer Engineering


Gerry Moschopoulos


Conventional three-phase ac-dc converters have two converter stages. They have a front-end converter that converts the input ac voltage into an intermediate dc bus voltage and a second, back-end converter that converts this dc bus voltage into the desired isolated dc output voltage. The front-end converter also performs power factor correction (PFC) and shapes the three-phase input currents so that they are nearly sinusoidal and in phase with the three-phase input voltages. This allows the ac power source to be used in the most efficient manner.

The front-end ac-dc converter is typically implemented with six switches while the back-end dc-dc converter is typically implemented with a four switch dc-dc full-bridge topology. Power electronic researchers have been motivated to try to reduce the number of switches that are used in the conventional two-stage approach in order to reduce cost and simplify the overall ac-dc converter. There are two general approaches to doing this: This first approach is to reduce the number of switches in the front-end ac-dc converter. The second approach is to combine the ac-dc converter and the dc-dc converter in a single converter so that the overall ac-dc converter can be implemented in a single converter stage that can simultaneously perform ac-dc power conversion with PFC and dc-dc power conversion.

The main focus of this thesis is on new power converter topologies that convert a three-phase ac input voltage into an isolated dc output voltage with a reduced number of switches. In the thesis, a new family of reduced switch front-end converter topologies is proposed, an example converter from this new family is selected for further study and a modified version of this topology is studied as well. In addition to these front-end converters, two new three-phase ac-dc single-stage converters are proposed and their properties and characteristics are compared. For each new converter that is investigated in detail, its modes of operation are explained, its steady-state characteristics are determined by mathematical analysis, and the results of the analysis are used to develop a design procedure that can be used to select key components. The design procedure of each new converter is demonstrated with an example that was used in the implementation of an experimental prototype that confirmed the feasibility of the converter.

The thesis concludes by presenting that have been reached as a result of the work that was performed, stating its main contributions to the power electronics literature and suggesting future research that can be done based on the thesis work.