Publications

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This work investigates the use of hybrid switched capacitor converter (HSCC) topologies with wide bandgap devices to achieve high efficiency DC-DC power conversion with high gain, high voltage outputs. This class of converter may be useful for several applications that include a medium voltage bus, such as solar PV, electric aircraft, or even all-electric ship architectures. Three converter prototypes are considered and evaluated in hardware, including a basic (unipolar) HSCC and two bipolar HSCC variants. The converter operation is discussed, and the bipolar prototypes are demonstrated to achieve high-gain, high-voltage output. Finally, the latest bipolar switched capacitor prototype is demonstrated to boost 480 V to 10 kV (Gain > 20) with 97.9% efficiency at 4.96 kW output power.

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Gallium Nitride (GaN) semiconductors have extremely low switching loss, high breakdown voltage, and high junction temperature rating. These characteristics enable improved device performance and thus improved switch mode power converter designs. This paper evaluates the Pareto-optimal performance improvements for a DC generation system with predicted GaN loss characteristics and a rigorous multi-objective optimization based design paradigm. The optimization results show that the application of GaN can achieve a 6.4% mass savings relative to Silicon Carbide (SiC) and 40% mass savings relative to Silicon (Si) at the same loss level for a 10 kW application.

In this work, a novel DC-DC converter topology, an adaptation of the Hybrid Switched Capacitor Circuit (HSCC), is considered for use in high-gain, high voltage applications that also require high efficiency and superior power density. In particular, a bipolar HSCC design is described, and a candidate control methodology is set forth and developed analytically. The converter performance is demonstrated to be consistent with analysis. In addition, the converter is demonstrated to step 460V up to 8.63 kV (gain of 19) at 3.63 kW and nearly 97.0% efficiency.

A system is presented that is capable of measuring subnanosecond reverse recovery times of diodes in wide-bandgap materials over a wide range of forward biases (0 - 1 A) and reverse voltages (0 - 10 kV). The system utilizes the step recovery technique and comprises a cable pulser based on a silicon (Si) Photoconductive Semiconductor Switch (PCSS) triggered with an Ultrashort Pulse Laser, a pulse charging circuit, a diode biasing circuit, and resistive and capacitive voltage monitors. The PCSS-based cable pulser transmits a 130 ps rise time pulse down a transmission line to a capacitively coupled diode, which acts as the terminating element of the transmission line. The temporal nature of the pulse reflected by the diode provides the reverse recovery characteristics of the diode, measured with a high bandwidth capacitive probe integrated into the cable pulser. This system was used to measure the reverse recovery times (including the creation and charging of the depletion region) for two Avogy gallium nitride diodes; the initial reverse recovery time was found to be 4 ns and varied minimally over reverse biases of 50-100 V and forward current of 1-100 mA.

In most distributed power electronic systems, the transmission line effects associated with cabling are neglected due to the expectation that cables are sufficiently short to be modeled as a lumped parameter model. However, as converter switching speeds and control bandwidth increase, especially in large distributed power electronic based systems, the transmission line effects may become an important consideration when establishing margins of stability. In this work, immittance based stability analysis is applied to power electronic systems with long cables between source and load converter. In particular, the Energy Systems Analysis Consortium (ESAC) method is utilized to compute limits on cable length so as to maintain prescribed stability margins. Simulation results are presented in support of the approach.

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With the next generation of semiconductor materials in development, significant strides in the Size, Weight, and Power (SWaP) characteristics of power conversion systems are presently underway. In particular, much of the improvements in system-level efficiencies and power densities due to wide-bandgap (WBG) and ultra-wide-bandgap (UWBG) device incorporation are realized through higher voltage, higher frequency, and higher temperature operation. Concomitantly, there is a demand for ever smaller device footprints with high voltage, high power handling ability while maintaining ultra-low inductive/capacitive parasitics for high frequency operation. For our work, we are developing small size vertical gallium nitride (GaN) and aluminum gallium nitride (AlGaN) power diodes and transistors with breakdown and hold-off voltages as high as 15kV. The small size and high power densities of these devices create stringent requirements on both the size (balanced between larger sizing for increased voltage hold-off with smaller sizing for reduced parasitics) and heat dissipation capabilities of the associated packaging. To accommodate these requirements and to be able to characterize these novel device designs, we have developed specialized packages as well as test hardware and capabilities. This work describes the requirements of these new devices, the development of the high voltage, high power packages, and the high-voltage, high-Temperature test capabilities needed to characterize and use the completed components. In the course of this work, we have settled on a multi-step methodology for assessing the performance of these new power devices, which we also present.

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The effects of paralleling low-current vertical Gallium Nitride (v-GaN) diodes in a custom power module are reported. Four paralleled v-GaN diodes were demonstrated to operate in a buck converter at 1.3 Apeak (792 mArms) at 240 V and 15 kHz switching frequency. Additionally, high-fidelity SPICE simulations demonstrate the effects of device parameter variation on power sharing in a power module. The device parameters studied were found to have a sub-linear relationship with power sharing, indicating a relaxed need to bin parts for paralleling. This result is very encouraging for power electronics based on low-current v-GaN and demonstrates its potential for use in high-power systems.

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Increasing the penetration of distributed renewable sources, including photovoltaic (PV) sources, poses technical challenges for grid management. The grid has been optimized over decades to rely upon large centralized power plants with well-established feedback controls, but now non-dispatchable, renewable sources are displacing these controllable generators. By programming autonomous functionality into distributed energy resources-in particular, PV inverters-the aggregated PV resources can act collectively to mitigate grid disturbances. This paper focuses on the problem of frequency regulation. Specifically, the use of existing IEC 61850-90-7 grid support functions to improve grid frequency response using a frequency-watt function was investigated. The proposed approach dampens frequency disturbances associated with variable irradiance conditions as well as contingency events without incorporating expensive energy storage systems or supplemental generation, but it does require some curtailment of power to enable headroom for control action. Thus, this study includes a determination of the trade-offs between reduced energy delivery and dynamic performance. This paper includes simulation results for an island grid and hardware results for a testbed that includes a load, a 225 kW diesel generator, and a 24 kW inverter.

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Increasing the penetration of distributed renewable sources, including photovoltaic (PV) sources, poses technical challenges for grid management. The grid has been optimized over decades to rely upon large centralized power plants with well-established feedback controls, but now non-dispatchable, renewable sources are displacing these controllable generators. This one-year study was funded by the Department of Energy (DOE) SunShot program and is intended to better utilize those variable resources by providing electric utilities with the tools to implement frequency regulation and primary frequency reserves using aggregated renewable resources, known as a virtual power plant. The goal is to eventually enable the integration of 100s of Gigawatts into US power systems.