Date of Award

Summer 2025

Document Type

Open Access Dissertation

Department

Electrical Engineering

First Advisor

Kristen Booth

Abstract

Power electronics are essential in applications such as aerospace, electric vehicles, and electric ships where compact, efficient, and high power-density converters are increasingly demanded. This push toward miniaturization has led to converter designs operating at switching frequencies in the megahertz (MHz) range. Higher-frequency operation enables significant reductions in passive component sizes—particularly transformers and inductors—thereby facilitating dense packaging. However, these benefits come with new challenges, including increased thermal stress, complex semiconductor behavior, higher-frequency magnetic design constraints, and elevated electromagnetic interference (EMI).

This research addresses several key challenges in MHz converter design. The first part involves the development of a 1-MHz, 1-kW buck converter. Critical aspects such as printed circuit board (PCB) layout for minimizing parasitic inductance and EMI, PCB-based inductor design with emphasis on parasitic capacitance, and thermal behavior are considered. Additionally, the temperature-dependent characteristics of Gallium Nitride (GaN) High Electron Mobility Transistors (HEMTs) are investigated. A linear relationship is established between RDS,ON, power loss, and junction temperature, validated through experimental testing under elevated thermal stress.

Building on this foundation, the second part of the work focuses on a 5 kW LLC resonant converter operated at both 300 kHz and 1 MHz. A search-based optimization method for miniaturized tank design is proposed which is coordinated with minimizing losses in the primary bridge and reduce the magnetic footprint. Various combinations of magnetizing and resonant components are explored to identify configurations that minimize circulating current while maintaining soft switching. A PCB-based transformer is implemented to improve power density, replacing conventional copper or Litz wire windings, with resonant inductance integrated into the structure to reduce part count and enhance compactness.

A time-domain model is developed to analyze dead time (tdt) by characterizing the GaN device output charge behavior via the derived time-dependent output capacitance, CO(tr) = Qoss/Vds. This model links tdt with resonant current and CO(tr), providing insight into achieving reliable zero-voltage switching. The analysis captures the trade-off between full capacitor discharge and minimizing conduction losses, forming a critical design consideration for MHz LLC operation.

To address thermal challenges in the integrated transformer, a thermal network impedance model is developed for the planar magnetic structure under forced cooling. This model accurately predicts hotspot locations and informs effective thermal mitigation strategies. The model is validated through analytical calculations, finite element analysis, and experimental testing; all three show strong agreement. This integrated thermal modeling approach supports reliable operation and robust thermal design of MHz PCB-based transformers.

Prototypes of both converter designs are built and experimentally validated at 5 kW. The 1 MHz LLC converter achieves a peak efficiency of 97.2 % and a power density of 40 W/cm3, while the 300 kHz counterpart achieves a peak efficiency of 97.5 % and a power density of 17 W/cm3. Additionally, radiated EMI is measured across power levels for both converters. Overall, this work delivers practical insights into MHz converter design, highlighting the critical trade-offs among power density, efficiency, EMI, and thermal management.

Rights

© 2025, Aqarib Hussain

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