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Browsing by Author "Skutt, Glenn R."

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    Evaluation and Design of a SiC-Based Bidirectional Isolated DC/DC Converter
    Chu, Alex (Virginia Tech, 2018-02-01)
    Galvanic isolation between the grid and energy storage unit is typically required for bidirectional power distribution systems. Due to the recent advancement in wide-bandgap semiconductor devices, it has become feasible to achieve the galvanic isolation using bidirectional isolated DC/DC converters instead of line-frequency transformers. A survey of the latest generation SiC MOSFET is performed. The devices were compared against each other based on their key parameters. It was determined that under the given specifications, the most suitable devices are X3M0016120K 1.2 kV 16 mohm and C3M0010090K 900 V 10 mohm SiC MOSFETs from Wolfspeed. Two of the most commonly utilized bidirectional isolated DC/DC converter topologies, dual active bridge and CLLC resonant converter are introduced. The operating principle of these converter topologies are explained. A comparative analysis between the two converter topologies, focusing on total device loss, has been performed. It was found that the CLLC converter has lower total device loss compared to the dual active bridge converter under the given specifications. Loss analysis for the isolation transformer in the CLLC resonant converter was also performed at different switching frequencies. It was determined that the total converter loss was lowest at a switching frequency of 250 kHz A prototype for the CLLC resonant converter switching at 250 kHz was then designed and built. Bidirectional power delivery for the converter was verified for power levels up to 25 kW. The converter waveforms and efficiency data were captured at different power levels. Under forward mode operation, a peak efficiency of 98.3% at 15 kW was recorded, along with a full load efficiency value of 98.1% at 25 kW. Under reverse mode operation, a peak efficiency of 98.8% was measured at 17.8 kW. The full load efficiency at 25 kW under reverse mode operation is 98.5%.
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    High-Frequency Dimensional Effects in Ferrite-Core Magnetic Devices
    Skutt, Glenn R. (Virginia Tech, 1996-10-04)
    MnZn ferrites are widely used in power electronics applications where the switching frequency is in the range of several tens of kilohertz to a megahertz. In this range of frequencies the combination of relatively high permeability and relatively low conductivity found in MnZn ferrite helps to minimize the size of magnetic devices while maintaining high efficiency. The continuing improvement in semiconductor switches and circuit topologies has led to use of high-frequency switching circuits at ever increasing power levels. The magnetic devices for these high-power, high-frequency circuits require magnetic CORES that are significantly larger than standard ferrite-core devices used at lower power levels. Often such large ferrite cores must be custom designed, and at present this custom design is based on available material information without regard for the physical size of the structure. This thesis examines the issues encountered in the use of larger MnZn ferrite cores for high-frequency, high-power applications. The two main issues of concern are the increased power dissipation due to induced currents in the structure and the change in inductance that results as the flux within the core is redistributed at higher frequencies. In order to model these problems using either numerical or analytical methods requires a reliable and complete set of material information. A significant portion of this work is devoted to methods for acquiring such material information since such information is not generally available from the manufacturers. Once the material constants required for the analysis are determined, they are used in both closed-form and numerical model to illustrate that large ferrite cores suffer significant increases in loss and significant decreases in inductance for frequencies as low as several hundred kilohertz. The separate impacts of the electrical and magnetic losses in the core are illustrated through the use of linear finite element analyses of several example core structures. The device impedances calculated using the FEA tools show fair agreement with measurement. An analysis of gapped structures and segmented cross-sections shows that these design options can reduce the severity of the dimensional problems for some designs.