Soft switching high frequency AC-link DC-DC buck-boost converters

This paper proposes a modified control scheme for bidirectional and unidirectional dc-dc parallel ac-link universal power converters, and studies their performance under both variable and fixed switching frequency. Moreover, this paper proposes two novel unidirectional dc-dc topologies to reduce the number of active switches while keeping the advantages of original configuration; i.e. high frequency ac link and soft switching. All the converters introduced here belong to a relatively new class of power converters called parallel ac-link universal power converters that can be configured as dc-dc, dc-ac, ac-dc, ac-ac, or hybrid converters. These converters are extensions of dc-dc buck boost converter. However, several modifications have been applied that results in high-frequency ac link inductor current with no dc component. At the same time, all switches benefit from the zero voltage turn-on and soft turn-off. This converter can offer galvanic isolation by adding a high frequency transformer to the link. Parallel ac-link universal converters have a complex control scheme; therefore, a microcontroller is usually used to control the switches; however, such digital control methods can limit the link frequency due to long execution time and delays. FPGAs may operate faster; however, they are costly. In this paper new control methods, which are implemented in analog domain, are investigated. The proposed control schemes allow having very high frequency ac link and consequently a compact converter. Moreover, the use of analog circuit instead of a microcontroller or FPGA reduces the cost of the system. Simulation and experimental results are given in this paper.