A High Power Density Three-level Parallel Resonant Converter for Capacitor Charging
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This dissertation proposes a high-power, high-frequency and high-density three-level parallel resonant converter for capacitor charging. DC-DC pulsed power converters are widely used in military and medical systems, where the power density requirement is often stringent. The primary means for reducing the power converter size has been to reduce loss for reduced cooling systems and to increase the frequency for reduced passive components. Three-level resonant converters, which combine the merits of the three-level structure and resonant converters, are an attractive topology for these applications. The three-level configuration allows for the use of lower-voltage-rating and faster devices, while the resonant converter reduces switching loss and enhances switching capability. This dissertation begins with an analysis of the influence of variations in the structure of the resonant tank on the transformer volume, with the aim of achieving a high power density three-level DC-DC converter. As one of the most bulky and expensive components in the power converter, the different positions of the transformer within the resonant tank cause significant differences in the transformerâ s volume and the voltage and current stress on the resonant elements. While it does not change the resonant converter design or performance, the improper selection of the resonant tank structure in regard to the transformer will offset the benefits gained by increasing the switching frequency, sometimes even making the power density even worse than the power density when using a low switching frequency. A methodology based on different structural variations is proposed for a high-density design, as well as an optimized charging profile for transformer volume reduction. The optimal charging profile cannot be perfectly achieved by a traditional output-voltage based variable switching frequency control, which either needs excess margin to guarantee ZVS, or delivers maximum power with the danger of losing ZVS. Moreover, it cannot work for widely varied input voltages. The PLL is introduced to overcome these issues. With PLL charging control, the power can be improved by 10% with a narrow frequency range. The three-level structure in particular suffers unbalanced voltage stress in some abnormal conditions, and a fault could easily destroy the system due to minimized margin. Based on thoroughly analysis on the three-level behaviors for unbalanced voltage stress phenomena and fault conditions, a novel protection scheme based on monitoring the flying capacitor voltage is proposed for the three-level structure, as well as solutions to some abnormal conditions for unbalanced voltage stresses. A protection circuit is designed to achieve the protection scheme. A final prototype, built with a custom-packed MOSFET module, a SiC Schottky diode, a nanocrystalline core transformer with an integrated resonant inductor, and a custom-designed oil-cooled mica capacitor, achieves a breakthrough power density of 140W/in3 far beyond the highest-end power density reported (<100 W/in3) in power converter applications.
- Doctoral Dissertations