Emerging non-isolated high-power-density DC/DC converters with high conversion ratios and their applications

Abstract

With the growing concern for the environment, applications such as electric vehicles, grid-connected systems for renewable energy generation, and energy harvesting systems are emerging. Such applications drive the growing demand for unidirectional/bidirectional DC/DC converters with high conversion ratios, with the desirable features of high power density, high efficiency, low cost, and high reliability.

One possible DC/DC converter solution for such applications is that based on a high-frequency resonant converter, which consists of a high-frequency resonant inverter that is cascaded with a rectifier. The zero-voltage-switching (ZVS) operation, absence of a coupled-inductor or transformer, and requirement of only one ground-ended active switch make this type of converter suitable for high-frequency operation with high efficiency. Conventionally, the inverter stage of this type of converter is designed to output a pure sinusoidal ac voltage. In this thesis, a harmonics-boosted resonant converter that is suitable for extremely high voltage gain is proposed. Its inverter stage is designed to contain selected voltage harmonics that significantly boost the amplitude of its output voltage. This greatly increases the overall voltage gain of the converter. The design guideline of this converter is provided. To apply this harmonics-boosted converter for energy harvesting of salinity gradient power, a feasibility study is presented based on (i) the use of reverse electrodialysis and (ii) the harmonics-boosted resonant converter. Various issues of this energy harvesting system, such as the maximum power extraction, prevention of harmonics injection, and electrical characteristics matching, are investigated. To apply the high-frequency resonant converter for high current applications, a multiphase-interleaved high-step-up DC/DC converter with a wide load range is proposed. Unlike the conventional high-frequency resonant converter in which operation is sensitive to the load resistance, the voltage gain variance of this proposed converter can be confined within a small value and ZVS operation can be maintained over a wide load range. A detailed mathematical analysis is conducted for systematizing its design.

Another potential DC/DC converter solution for the aforementioned applications is that based on the switched-capacitor (SC) converter. This type of converter is non-magnetic, small in size, and light in weight, and it achieves a high power density. In addition, with only active switches and capacitors, it is inherently suitable for bidirectional power flow application. For existing SC converters, modular or partial modular topology is realized. However, the investigation of fully modular converters (which have both a modularized power stage and control stage) based on the SC topologies is still relatively rare. In this thesis, an initial investigation on a fully modular converter based on an SC topology is presented. Here, the SC modules can be freely assembled to achieve different modular converters with various voltage gains and power ratings. To achieve the modular converter with a minimized number of modules, a house-of-card architecture is presented, where the number of parallel modules, being adapted to the input current, is reduced at the high-voltage low-current stage. With consideration of the equal current sharing issue, the current-sharing capability of the SC module is also investigated.