Hybrid electric vehicle (HEV) and alternative energy have received growing attention nowadays, due to their contribution to the sustainable development. In these systems, power electronic circuits play a key role in bridging over the differences between sources and loads. The thesis investigates power converters to interface the energy sources and to prolong the life of the critical energy storage devices.The boost converter in commercial HEV powertrains meets obstacles to upgrading the rating while downsizing the converter. A four-level dc-dc converter, and its special case―a 3X dc-dc converter that operates at three discrete voltage ratios, can overcome the drawbacks by dramatically reducing the inductance requirement. The operating principles, the current ripple, the power loss analysis and a clamping circuit are introduced. The concept is verified by the experimental results from two 30-kW and 55-kW prototypes.Yet, for other applications like FC, PV and TEG, it is beyond the ability of the 3X dc-dc converter to attain high boost gain. Thus, a new switched-capacitor dc-dc converter is proposed. The component cost comparison and the power loss analysis demonstrate its many features such as low component power rating and count, low capacitance requirement, light weight and high efficiency. The features are validated by a 450-W prototype.For the needs of converting the dc power to ac and vice versa, a family of transformer based impedance source (trans-Z-source) inverters is proposed. They possess the buck-boost functionality and the reliable operation against shoot-through/open-circuit fault, which are unavailable in traditional voltage source and current source inverters. Compared to the existing Z-source inverters, they exhibit the increased voltage gain and reduced voltage stress in the voltage-fed trans-Z-source inverters, and the expanded motoring operation range in the current-fed trans-Z-source inverters. Two 3-kW prototypes prove the analysis of the voltage-fed and current-fed trans-Z-source inverters respectively.To reduce the number of the serially connected battery cells/modules that are used in the energy systems, an improvement of a battery balance circuit is presented via phase-shift control. Each pair of battery cells are paralleled with a magnetically coupled half-bridge balance circuit. This cost-effective solution is able to achieve cell balancing and defective cell tolerance with more evenly distributed current stress, less component count and lower device ratings than its counterparts. Experimental results on Li-ion batteries verify the concept.
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