In his thesis, CONDUCTED EMC MODELLING IN MODERN DC-DC POWER CONVERTERS, the conducted electromagnetic effects of high-density high-switching frequency modern power converters are studied. The noise source and noise path were modelled and the results calibrated to accredited noise levels. A VHF frequency model was developed and verified using different analysis packages and compared for accuracy, affordability and ease of use. Noise modes were separated and verified for accuracy. The mechanisms of the noise modes were studied and noise mitigation techniques presented. / Thesis (PhD)--University of Pretoria, 2017. / Electrical, Electronic and Computer Engineering / PhD / Unrestricted

This thesis proposes a control system to widen the bandwidth of piezoelectric transducers (PZTs) for vibration energy harvesting while extracting maximum power. A straightforward complex conjugate match achieves maximum power transfer only at a single frequency while requiring an impractically large inductance. The proposed system intends to address these problems. It incorporates a bi-directional DC/DC converter with feed-forward control to achieve a complex conjugate match over a wide range of frequencies.  Analysis of the proposed system and simulation results are presented to verify validity of the proposed method. / Master of Science

Battery-powered trends in consumer electronics, transportation, and renewable energy sectors increase demands on DC/DC converter technology. Higher switching frequency and efficiency reduces solution size and cost, while increasing power capabilities. Still, switching noise remains the primary drawback associated with any DC/DC converter. Reducing a converter’s input ripple helps prevent switching noise from spreading to other systems on a shared DC power bus. This thesis covers the analysis, simulation, and implementation of a recently-proposed boost converter topology, alongside an equivalent standard boost converter, operating in steady-state, continuous conduction mode. A Matlab-based simulation predicts each converter’s input ripple performance using a state-space model. The converters’ hardware implementation minimizes component and layout differences to create an equivalent comparison. The simulation and hardware measurements demonstrate a 40% input current ripple reduction using the modified topology. Replacing standard boost converters with the modified topology minimizes the switching noise conducted through a system’s DC power network.

The inherent difficulty in designing high voltage power supplies is often compounded by demands of high reliability, high performance, and safe functionality. A proposed high step-up ratio DC-DC converter meets the exacting requirements of applications such as uninterruptible power systems, radar, and pulsed power systems. The proposed DC-DC converter topology combines a multi-phase buck input stage with a novel self-tracking zero-voltage-switching (ZVS) resonant output stage. Traditionally, the inclusion of multiple power processing stages within a power supply topology severely degrades the overall converter efficiency. Due to the inherent high efficiency per stage, however, this effect is minimized. The self-tracking switching scheme ensures that ZVS occurs across the full range of load variation. Furthermore, the switching scheme allows significantly increased flexibility in component tolerances compared to traditional resonant converter designs. The converter also demonstrates indefinite short-circuit protection and true ZVS during transient processes. Computer simulation and hardware analysis verify the efficacy of the topology as a rugged and efficient converter.

Electric vehicles (EVs) offer significant advantages over conventional internal combustion engine vehicles, including zero emissions and convenient overnight charging. However, there are still several challenges that need to be addressed. These challenges include limited driving ranges, slow refueling options while on-the-go, concerns related to the supply of lithium for batteries, and emissions associated with certain sources of electricity generation, such as coal. Adding on-board solar generation and/or fuel cell range extenders to EVs can help to mitigate some of these challenges, but also adds the need for optimal power electronic converters to manage the power flow of these multiple on-board energy sources, which is the focus of this thesis. This thesis first performs a comprehensive review of EV onboard chargers (OBCs) including charger system requirements by different standards and codes and different DC/DC power converters in the current infrastructures. Various power levels are compared and evaluated based on their component ratings, efficiency, cost, and power density. Secondly, there has been recent interest in harnessing solar power within electric vehicles, leading to the emergence of solar-charged electric vehicles (SEVs), which can offer extended driving ranges and less need for grid charging. These vehicles also offer a new opportunity for distributed generation when their traction batteries are fully charged, and the plugged-in vehicle is still generating solar energy. However, this also presents a unique power electronic dilemma. The OBC must exhibit high efficiency in two scenarios: firstly, during normal charging from the grid at power levels around 6.6 kW, and secondly, during vehicle-to-grid operation at significantly lower solar power levels, typically below 800 W. Unfortunately, conventional OBC designs tend to have low efficiency when operating at light loads. To tackle this challenge, this thesis proposes a novel bidirectional LLC-based converter, for use within the OBC, that achieves higher vehicle-to-grid efficiency at light loads than a traditional dual bridge converter. Detailed PLECS simulation results and experimental results are presented to verify the circuit. Thirdly, the presence of manufacturing variations can introduce parameter mismatches, resulting in voltage imbalances across capacitors in the proposed converter, or in other resonant converters with multiple transformer windings and two series-connected capacitors with a center connection. Such voltage imbalances pose significant concerns regarding safety and reliability. However, the existing capacitor balancing strategies developed for other converter topologies are not directly applicable to these new resonant multi-winding topologies. To address this issue, this thesis presents a novel method for achieving capacitor voltage balancing in a resonant multi-winding converter. The proposed method employs a straightforward approach to determine the appropriate balancing switching states. Time domain analysis is conducted to quantify the number of control cycles required, and an adaptive control strategy is introduced to enhance the balancing performance. The effectiveness of the proposed method and the beneficial effects on the converter's efficiency and bus capacitor sizes are validated through experimental investigations involving multiple bus capacitor sizes. Finally, though SEVs offer advantages over non-solar EVs, some challenges remain such as lithium supply concerns for large batteries, slow recharging, and driving range that is still limited compared to conventional vehicles. Fuel cell range-extended vehicles (FCREVs) can add a small fuel cell and hydrogen tank to allow quick refueling for long trips, and still use a reduced-size plug-in battery for the majority of short trips. This allows the driver to use efficient and convenient overnight charging for most daily commutes, and refuel with hydrogen on long-distance driving days if hydrogen stations are available. The smaller battery means that lithium requirements are reduced. Further, by adding on-board solar generation to a FCREV (S-FCREV), range can be further extended and grid charging requirements can be reduced. However, using conventional separate converters for a S-FCREV would be complex and costly, having a high number of semiconductor devices. To overcome this, the thesis proposes a practical multi-port converter that fulfills S-FCREV requirements with reduced components. A novel triple PWM and triple phase shift (TPTPS) control is proposed. Simulation and experimental results validate the proposed topology's operation and efficiency, offering a promising solution for integrating power electronics in S-FCREV applications. / Thesis / Doctor of Philosophy (PhD) / In the pursuit of sustainable transportation, recent scholarly investigations have placed significant emphasis on the advancement of electric vehicles (EVs) with a particular focus on solar-charged EVs and fuel cell range-extended vehicles (FCREVs) in order to help mitigate some of the drawbacks of battery EVs such as limited driving range, long refueling times, and charging impacts on the grid. Power electronic converters play a crucial role in managing the transfer of power between multiple energy sources, such as on-board solar panels, fuel cells, batteries, and connection to the grid. The objective of this thesis is to propose novel topologies and control for power electronic converters in solar-charged EVs and solar-charged FCREVs. Firstly, a novel bidirectional DC/DC topology is proposed for solar-charged EVs that allows a high-efficiency transfer of excess solar energy to the grid when the EV battery is full. Additionally, a novel control methodology for balancing the DC bus capacitors is introduced, aiming to reduce capacitor size and mitigate circulating unbalanced currents. Lastly, this thesis presents the pioneering practical implementation of a multi-port converter for a solar-charged FCREV, along with its adaptable control approach, enabling efficient power flow management among the grid, on-board battery, solar panels, and fuel cell.

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DC/DC Power converter design has been following a trend of reducing size while also increasing performance for the last several years. This push for higher power output and smaller footprint and profile requires integration and higher switching frequencies in order to continue. Higher frequencies require physical integration to eliminate problems induced by parasitics, which increase losses. GE's Power Overlay and Philip's PCB integration schemes have been clear steps in the quest to reduce size with new system design techniques. However, both have downsides. GE Power Overlay embeds the devices inside a milled AlN ceramic cavity and then layers interconnections on top using polyimide dielectric interlayers. The milling of AlN ceramic is a very costly and time consuming task due to the brittleness of the material, and the interlayers add additional complexity to the fabrication process. Philip's PCB integration was primarily aimed at integrating passives along with the PCB process for reduction of size. Inductor windings and capacitive layers were built up along with FR4 epoxy layers using typical PCB fabrication methods. However, unlike GE's Power Overlay, the substrate material was several times lower in thermal conductivity which invariably has corresponding thermal penalties. The work presented here reconciles the good of both integration techniques. Initially called Embedded Power, alumina ceramic was used as the substrate and rather than milling holes for the devices, holes were laser cut all the way through and interconnections were made by using interlayers and sputtered copper deposition, similar to GE's method. Integration of passives was done using LTCC ferrite to make an inductor of thin profile, rather than embedding cores and windings inside PCB. However, fabrication remained time consuming due to numerous solder masking and sputtering steps and thermal performance was not optimized due to the use of alumina ceramic. A revised design method called Stacked Power is presented in this dissertation that follows on the work of Embedded Power, but improves on it by simplifying fabrication through the elimination of thermally-restrictive interlayers, as well as time consuming sputtering and electroplating of copper interconnections. Instead, AlN Direct Bonded Copper is used as a multifunctional material thanks to its many-times-greater thermal conductivity than PCB or alumina, solderable device dies are implemented in a vertical fashion, and interconnections are simply made using copper straps soldered into place. For applications where moisture contamination and breakdown isolation are potential problems, dip conformal coating can easily be applied, replacing laborious solder masking. The work in this dissertation describes the fabrication methodology for Stacked Power, demonstrates the thermal advantages, and shows examples of high-frequency buck converters that achieve super-high levels of power density in the smallest of volumes and require no more thermal management than modest airflow. The added cost incurred with aluminum nitride is traded for distinct advantages in terms of low-profile, low airflow requirements for the power output, capability of natural convection for use in locations where fans are prohibitive and compact size for ease of implementation. / Ph. D.

As the world's energy use continues to grow, the development of clean distributed generation becomes increasingly important. Fuel cells are an environmentally friendly renewable energy source that can be used in a wide range of applications and are ideal for distributed power applications. In this study, the power conversion element of a dual single-phase, 10 kW stand-alone fuel cell system is analyzed. The modular converter consists of a DC-DC front-end cascaded with a half-bridge inverter. The entire system is accurately modeled, to help determine any interactions that may arise. Control strategies based on simplicity, performance, and cost are evaluated. A simple voltage loop, with careful consideration to avoid transformer saturation, is employed for the phase-shifted DC-DC converter. Several experimental transfer functions were measured to confirm the modeling assumptions and verify the control design of the DC-DC converter. Two control options for the inverter are explored in detail, and experimental results confirm that the modulation index must be controlled to regulate the output voltage during various load conditions. The final system is implemented without the use of current sensors, thus keeping the inverter cost down. Experimental results using a power supply are given for resistive, inductive, and nonlinear loads and the performance is acceptable. Fuel cell test results, including transient response, are also displayed and analyzed. / Master of Science

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%. / Master of Science