Wide ratio bidirectional DC-DC converters for supercapacitor storage applications

Abstract

This thesis presents new wide-ratio bidirectional hybrid DC-DC converters used in supercapacitor storage, frequently employed for enhancing the storage performances in microgrids or electric vehicles. The presented topologies are developed from unidirectional hybrid structures which are characterized by the use of switched cells consisting of identical capacitors or inductors switched between series and parallel connection with the help of diodes. By operating in this manner, the switching cells help achieve a wider (higher in step-up converters or lower in step-down converters) voltage conversion ratio, lower active switch stress, and smaller passive components. The bidirectional structures achieve the same benefits as the unidirectional topologies, with the added benefit of two quadrant operation. This work if focused on four converter topologies, from which three are proposed by the author. The studied topologies are described analytically, in order to obtain design equations and comparison metrics. Dynamic analysis is performed to achieve a better hardware design of converters, to assess their stability and to design current controllers. Digital simulations are used to aid the design and analysis of the studied converters. Experimental results are acquired for two hybrid topologies in applications with a power scale up to 5kW. In order to measure efficiency and test the influence of different switches, one topology was built using MOSFETs and GaN-FETs. To eliminate some disadvantages, an improved version is proposed for each of the studied topologies. Power sharing strategies for supercapacitor storage in microgrid applications are proposed and implemented with the help of a hybrid converter. Their operation is tested trough experimental and digital simulations.