Residual stress measurement methods have commonly been used to characterize states of stress in various elastic isotropic materials. In order to investigate the effects of elastic anisotropy on residual stress measurements, commercially pure grade 2 titanium (CP Ti Gr 2) was selected to study a strong texture, or preferred grain orientation. Warm rolled and air-cooled CP titanium is well known to have a texture from the factory. This texture and resulting elastic anisotropy were confirmed using two material characterization methods, resonant ultrasound spectroscopy (RUS) and electron backscatter diffraction (EBSD). The texture was further developed using a rolling mill to cold roll the titanium. A vacuum furnace set to a temperature of 550 C for one hour was used to stress relieve the titanium without reducing the texture. RUS and EBSD methods were used again to confirm the texture achieved by cold rolling. Well-characterized residual stresses were introduced with a shrink-fit ring and plug. The residual stress hole drilling method was used to characterize stresses in the rolling and transverse directions of the ring and plug assemblies. Stress profiles from hole drilling indicated some possible elastic anisotropic effects in two assemblies and are presented. However, more assemblies are needed to confirm the results. A stress determination technique with higher sensitivity may be necessary to substantiate assembly stress profile results.

Yes / In this paper, we present the design of a compact and highly sensitive microwave sensor based on a metamaterial complementary split-ring resonator (CSRR), for liquid characterization at microwave frequencies. The design consists of a two-port microstrip-fed rectangular patch resonating structure printed on a 20 × 28 mm2 Roger RO3035 substrate with a thickness of 0.75 mm, a relative permittivity of 3.5, and a loss tangent of 0.0015. A CSRR is etched on the ground plane for the purpose of sensor miniaturization. The investigated liquid sample is put in a capillary glass tube lying parallel to the surface of the sensor. The parallel placement of the liquid test tube makes the design twice as efficient as a normal one in terms of sensitivity and Q factor. By bending the proposed structure, further enhancements of the sensor design can be obtained. These changes result in a shift in the resonant frequency and Q factor of the sensor. Hence, we could improve the sensitivity 10-fold compared to the flat structure. Subsequently, two configurations of sensors were designed and tested using CST simulation software, validated using HFSS simulation software, and compared to structures available in the literature, obtaining good agreement. A prototype of the flat configuration was fabricated and experimentally tested. Simulation results were found to be in good agreement with the experiments. The proposed devices exhibit the advantage of exploring multiple rapid and easy measurements using different test tubes, making the measurement faster, easier, and more cost-effective; therefore, the proposed high-sensitivity sensors are ideal candidates for various sensing applications. / This work was supported by the Moore4Medical project, funded within ECSEL JU in collaboration with the EU H2020 Framework Programme (H2020/2014–2020) under grant agreement H2020-ECSEL-2019-IA-876190, and the Fundação para a Ciência e Tecnologia (ECSEL/0006/2019). This project received funding in part from the DGRSDT (Direction Générale de la Recherche Scientifique et du Développement Technologique), MESRS (Ministry of Higher Education and Scientific Research), Algeria. This work was also supported by the General Directorate of Scientific Research and Technological Development (DGRSDT)–Ministry of Higher Education and Scientific Research (MESRS), Algeria, and funded by the FCT/MEC through national funds and, when applicable, co-financed by the ERDF, under the PT2020 Partnership Agreement under the UID/EEA/50008/2020 project.

ith the rapid advancements in modern technology and the increasing demand for efficient energy conversion, the field of medium voltage power conversion has experienced significant progress in recent years. This progress is driven by its high efficiency and improved scalability. Medium voltage power conversion finds applications in various areas such as data centers, electric vehicle fast charging, and smart grids. It enables the reduction of power delivery stages and minimizes the required physical space. The scalability and modularity of this technology offer the flexibility to expand the power level as needed. According to the International Energy Agency, data centers and electric vehicle charging are projected to consume over 10% of the world's total electricity consumption by 2040. To power this amount, approximately 800 nuclear power reactors with a capacity of 1 GW each would be required. Therefore, even small savings in power consumption can have a substantial impact. The solid-state transformer (SST) is a promising technique for medium voltage conversion that offers high-frequency operation, resulting in reduced volume and excellent insulation capabilities. Currently, the medium voltage transformer poses a challenge for SST systems due to the requirements for high insulation levels, efficient thermal management, improved efficiency, and higher power density. Unlike conventional line-frequency transformers, the solid-state transformer operates at relatively high frequencies, typically in the range of tens of kilohertz. This higher frequency enables a reduction in the cross-sectional area of the magnetic components, leading to a smaller and lighter design. However, the high-frequency transformer used in the solid-state transformer does face certain limitations. Balancing insulation capability with the goal of achieving high power density presents a dilemma. To ensure medium voltage insulation, a thick insulation layer is required for the transformer. However, the high-frequency Litz wire and compact size of the transformer make it challenging to achieve partial discharge-free operation, unlike traditional line-frequency transformers. To address these challenges and achieve both medium voltage insulation capability and high power density, improvements in the insulation structure have been made. The dissertation firstly proposes the application of a shielding layer and related stress grading layer in the insulation structure. This helps confine the electric field within the primary side winding encapsulation rather than in the air. As a result, there is minimal electric field present in the air, allowing for further reduction in the transformer volume as there is no longer a need for insulation margin. With the enhanced insulation structure, the transformer can operate at even higher frequencies. However, it is important to note that the reduction in size is not directly proportional to the increase in frequency due to the impact of the insulation layer. To address this, a straightforward and comprehensive optimization method is proposed for the first time. This method considers the trade-off between loss and volume, taking into account multiple design objectives and parameters. An optimized 800/400 V, 200 kHz, 15 kW CLLC converter is demonstrated. The peak efficiency of this optimized converter reaches 98.8%, and the power density is 3.7 kW/L. The transformer also exhibits good insulation capability, with a partial discharge-free level reaching 7.7 kV. Additionally, achieving a suitable insulation level for the DC-DC module poses challenges due to thermal limitations. Insulation materials are not efficient thermal conductors, and as insulation levels increase, the thickness of the insulation layer must also increase, resulting in a significant rise in thermal resistance. To address this issue for applications requiring a 13.2 kV grid, an alternative insulation material called FR4 is considered in this dissertation. FR4, which can be implemented as the insulation layer for a PCB winding, offers the advantage of being fabricated together with the winding during the PCB manufacturing process. This process takes place in a vacuum environment, reducing the presence of air cavities that could lead to partial discharge within the insulation structure. Thus, the entire insulation fabrication process can be simplified. To enhance the insulation capability further, the dissertation proposes the incorporation of an arc section within the PCB winding. This design reduces the electric field crowding in the corner area. However, winding losses in the PCB winding remain a concern. To mitigate these losses, an ER core structure is introduced to balance the magnetic flux within the transformer core. This balanced distribution of the magnetic field helps reduce leakage flux into the air, subsequently reducing winding losses. The dissertation also suggests a sandwich winding structure to decrease the magnetomotive force in the winding, in comparison to a completely separate winding structure. Another optimization process for the PCB winding is performed to strike a better balance between size and loss in the transformer. In line with these improvements, another 800/400 V, 200 kHz CLLC transformer is designed utilizing the PCB winding approach. Compared to the Litz wire-based transformer, the efficiency performance is similar, but the power density is doubled due to the low-profile design enabled by the PCB winding. In terms of insulation capability, the FR4 insulation, with its high dielectric strength, allows the transformer to be partial discharge-free even with the same insulation thickness as the epoxy used in the Litz wire transformer for the 13.2 kV applications. Thirdly, considering the power limitation mainly because of the thermal issue in the primary side PCB winding, the PCB Litz wire concept is proposed to further improve the winding loss. To further improve the power level of the PCB winding transformer, the winding should be designed wider to reduce the DC winding resistance. However, the current distributes in a bad manner due to the proximity effect in the winding. That makes winding width increment insignificant to the loss reduce. The Litz wire is widely used in the high-frequency power conversion applications. A similar concept has been proposed in this dissertation in the PCB winding. Using two layers constructing one turns, the interwoven strategy can be implemented in the PCB winding to achieve the flux cancellation effect. That helps to make the current distribute uniformly inside the PCB winding. The PCB Litz construction method and connection method is introduced in this chapter to reduce the design burden with such a complicated winding pattern. Some design considerations are also proposed to optimize the PCB Litz concept. This dissertation solves the challenges in magnetic design in high-frequency DC/DC converters in the solid-state transformer with medium voltage insulation. This includes the Litz wire transformer and the PCB winding based transformer. With the academic contribution in this dissertation, the insulation performance is better for both Litz wire transformer and PCB winding based transformer. The straightforward and comprehensive optimization method is benefit for both academic and industry for transformer design in this application. The proposed PCB winding transformer makes the insulation fabrication much easier compared to the conventional fabrication method. And the PCB Litz concept helps to further reduce the winding loss, which makes it possible to further lift the power level in the PCB winding based transformer. / Doctor of Philosophy / With the rapid advancements in modern technology and the increasing demand for efficient energy conversion, the field of medium voltage power conversion has experienced significant progress in recent years. This progress is driven by its high efficiency and improved scalability. Medium voltage power conversion finds applications in various areas such as data centers, electric vehicle fast charging, and smart grids. It enables the reduction of power delivery stages and minimizes the required physical space. The scalability and modularity of this technology offer the flexibility to expand the power level as needed. According to the International Energy Agency, data centers and electric vehicle charging are projected to consume over 10% of the world's total electricity consumption by 2040. To power this amount, approximately 800 nuclear power reactors with a capacity of 1 GW each would be required. Therefore, even small savings in power consumption can have a substantial impact. The solid-state transformer (SST) is a promising technique for medium voltage conversion that offers high-frequency operation, resulting in reduced volume and excellent insulation capabilities. Currently, the medium voltage transformer poses a challenge for SST systems due to the requirements for high insulation levels, efficient thermal management, improved efficiency, and higher power density. Unlike conventional line-frequency transformers, the solid-state transformer operates at the range of tens of kilohertz. This higher frequency enables a reduction in the cross-sectional area of the magnetic components, leading to a smaller and lighter design. Balancing insulation capability with the goal of achieving high power density presents a dilemma. To ensure medium voltage insulation, a thick insulation layer is required for the transformer. However, the high-frequency Litz wire and compact size of the transformer make it challenging to achieve partial discharge-free, unlike traditional line-frequency transformers. To address these challenges and achieve both medium voltage insulation capability and high power density, improvements in the insulation structure have been made. A straightforward and comprehensive optimization method is proposed for the first time. This method considers the trade-off between loss and volume, taking into account multiple design objectives and parameters. An optimized 800/400 V, 200 kHz, 15 kW CLLC converter is demonstrated. The peak efficiency of this optimized converter reaches 98.8%, and the power density is 3.7 kW/L. The transformer also exhibits good insulation capability, with a partial discharge-free level reaching 7.7 kV. Additionally, insulation materials are not efficient thermal conductors, and as insulation levels increase, the thickness of the insulation layer must also increase, resulting in a significant rise in thermal resistance. An alternative insulation material called FR4 is considered in this dissertation. FR4, which can be implemented as the insulation layer for a PCB winding, offers the advantage of being fabricated with the winding during the PCB manufacturing process. To enhance the insulation capability further, the dissertation proposes an arc section within the PCB winding. This design reduces the electric field crowding in the corner area. The dissertation also suggests a sandwich winding structure to decrease the magnetomotive force in the winding, in comparison to a completely separate winding structure. Another optimization process for the PCB winding is performed to strike a better balance between size and loss in the transformer. In line with these improvements, another 200 kHz CLLC transformer is designed utilizing the PCB winding approach with doubled converter power density. In terms of insulation capability, the FR4 insulation, allows the transformer to be partial discharge-free for the 13.2 kV applications. Thirdly, considering the power limitation mainly because of the thermal issue in the primary side PCB winding, the PCB Litz wire concept is proposed to further improve the winding loss. The current distributes in a bad manner due to the proximity effect in the PCB winding. That makes winding width increment insignificant to the loss reduce. The Litz wire is widely used in the high-frequency power conversion applications. A similar concept has been proposed in this dissertation in the PCB winding. Using two layers constructing one turns, the interwoven strategy can be implemented in the PCB winding to achieve the flux cancellation effect. That helps to make the current distribute uniformly inside the PCB winding. The PCB Litz construction method and connection method is introduced in this chapter to reduce the design burden with such a complicated winding pattern. Some design considerations are also proposed to optimize the PCB Litz concept. This dissertation solves the challenges in magnetic design in high-frequency DC/DC converters in the solid-state transformer with medium voltage insulation. This includes the Litz wire transformer and the PCB winding based transformer. With the academic contribution in this dissertation, the insulation performance is better for both Litz wire transformer and PCB winding based transformer. The straightforward and comprehensive optimization method is benefit for both academic and industry for transformer design in this application. The proposed PCB winding transformer makes the insulation fabrication much easier compared to the conventional fabrication method. And the PCB Litz concept helps to further reduce the winding loss, which makes it possible to further lift the power level in the PCB winding based transformer.

With the fast advance in VLSI technology, smaller, more powerful digital system is available. It requires power supply with higher power density, lower profile and higher efficiency. PWM topologies have been widely used for this application. Unfortunately, hold up time requirement put huge penalties on the performance of these topologies. Also, high switching loss limited the power density achievable for these topologies. Two techniques to deal with hold up time issue are discussed in this dissertation: range winding solution and asymmetric winding solution, the efficiency at normal operation point could be improved with these methods. To reduce secondary rectifier conduction loss, QSW synchronous rectifier is developed, which also helps to achieve ZVS for symmetrical half bridge converter. Although with these methods, the efficiency of front end DC/DC converter could be improved, the excessive switching loss prohibited higher switching frequency. To achieve the targets, topologies with high switching frequency and high efficiency must be developed. Three resonant topologies: SRC, PRC and SPRC, are been investigated for this application because of their fame of low switching loss. Unfortunately, to design with hold up requirement, none of them could provide significant improvements over PWM converter. Although the negative outcome, the desired characteristic for front end application could be derived. Base on the desired characteristic, a thorough search is performed for three elements resonant tanks. LLC resonant topology is found to posses the desired characteristic. From comparison, LLC resonant converter could reduce the total loss by 40% at same switching frequency. With doubled switching frequency, efficiency of LLC resonant converter is still far better than PWM converters. To design the power stage of LLC resonant converter, DC analysis is performed with two methods: simulation and fundamental component simplification. Magnetic design is also discussed. The proposed integrated magnetic structure could achieve smaller volume, higher efficiency and easy manufacture. To make practical use of the topology, over load protection is a critical issue. Three methods to limit the stress under over load situation are discussed. With these methods, the converter could not only survive the over load condition, but also operate for long time under over load condition. Next small signal characteristic of the converter is investigated in order to design the feedback control. For resonant converter, state space average method is no longer valid. Two methods are used to investigate the small signal characteristic of LLC resonant converter: simulation and extended describing function method. Compare with test results, both methods could provide satisfactory results. To achieve both breadth and depth, two methods are both used to reveal the myth. With this information, compensator for feedback control could be designed. Test circuit of LLC resonant converter was developed for front end DC/DC application. With LLC topology, power density of 48W/in3 could be achieved compare with 13W/in3 for PWM converter. / Ph. D.

Wireless power transfer (WPT) with loosely coupled coils is a promising solution to deliver power to a battery in a variety of applications. Due to its convenience, wireless power transfer technology has become popular in consumer electronics. Thus far, the majority of the coupled coils in these systems are planar structure, and the magnetic field induced by the transmitter coil is in one direction, meaning that the energy power transfer capability degrades greatly when there is some angle misalignment between the coupled coils. To improve the charging flexibility, a three–dimensional (3D) coils structure is proposed to transfer energy in different directions. With appropriate modulation current flowing through each transmitter coil, the magnetic field rotates in different directions and covers all the directions in 3D space. With omnidirectional magnetic field, the charging platform can provide energy transfer in any direction; therefore, the angle alignment between the transmitter coil and receiver coil is no longer needed. Compensation networks are normally used to improve the power transfer capability of a WPT system with loosely coupled coils. The resonant circuits, formed by the loosely coupled coils and external compensation inductors or capacitors, are crucial in the converter design. In WPT system, the coupling coefficient between the transmitting coil and the receiving coil is subject to the receiver's positioning. The variable coupling condition is a big challenge to the resonant topology selection. The detailed requirements of the resonant converter in an omnidirectional WPT system are identified as follows: 1). coupling independent resonant frequency; 2). load independent output voltage; 3). load independent transmitter coil current; 4). maximum efficiency power transfer; 5). soft switching of active devices. A LCCL-LC resonant converter is derived to satisfy all of the five requirements. In consumer electronics applications, Megahertz (MHz) WPT systems are used to improve the charging spatial freedom. 6.78 MHz is selected as the system operation in AirFuel standard, a wireless charging standard for commercial electronics. The zero voltage switching (ZVS) operation of the switching devices is essential in reducing the switching loss and the switching related electromagnetic interference (EMI) issue in a MHz system; therefore, a comprehensive evaluation of ZVS condition in an omnidirectional WPT system is performed. And a design methodology of the LCCL-LC converter to achieve ZVS operation is proposed. The big hurdle of the WPT technology is the safety issue related to human exposure of electromagnetic fields (EMF). A double layer shield structure, including a magnetic layer and a conductive layer, is proposed in a three dimensional charging setup to reduce the stray magnetic field level. A parametric analysis of the double shield structure is conducted to improve the attenuation capability of the shielding structure. In an omnidirectional WPT system, the energy can be transferred in any direction; however the receiving devices has its preferred field direction based on its positioning and orientation. To focus power transfer towards targeted loads, a smart detection algorithm for identifying the positioning and orientation of receiver devices based on the input power information is presented. The system efficiency is further improved by a maximum efficiency point tracking function. A novel power flow control with a load combination strategy to charge multiple loads simultaneously is explained. The charging speed of the omnidirectional WPT system is greatly improved with proposed power flow control. / Doctor of Philosophy / Wireless power transfer (WPT) is a promising solution to deliver power to a battery in a variety of applications. Due to its convenience, wireless power transfer technology with loosely coupled coils has become popular in consumer electronics. In such system, the receiving coil embedded in the receiving device picks up magnetic field induced by the transmitter coil; therefore, energy is transferred through the magnetic field and contactless charging is achieved. Thus far, the majority of the coupled coils in these systems are planar structure, and the magnetic field induced by the transmitter coil is in one direction, meaning that the energy power transfer capability degrades greatly when there is some angle misalignment between the coupled coils. To improve the charging flexibility, a three–dimensional (3D) coils structure is proposed to transfer energy in different directions, also known as in omnidirectional manner. With omnidirectional magnetic field, the charging platform can provide energy transfer in any direction; therefore, the angle alignment between the transmitter coil and receiver coil is no longer needed. In a WPT system with loosely coupled coils, the energy transfer capability suffers from weak coupling condition. To improve the power transfer capability, the electrical resonance concept between the inductor and capacitor at the power transfer frequency is adopted. A novel compensation network is proposed to form a resonant tank with the loosely coupled coils and maximize the power transfer at the operating frequency. As for the WPT system with loosely coupled coils, the energy transfer capability is also proportional to the operating frequency. Therefore, Megahertz (MHz) WPT systems are used to improve the charging spatial freedom. 6.78 MHz is selected as the system operation in AirFuel standard, a wireless charging standard for commercial electronics. The zero voltage switching (ZVS) operation of the switching devices is essential in reducing the switching loss and the switching related electromagnetic interference (EMI) issue in a MHz system; therefore, a comprehensive evaluation of ZVS condition in an omnidirectional WPT system is performed. The big hurdle of the WPT technology is the safety concern related to human exposure of electromagnetic fields (EMF). Therefore, a double layer shield structure is first applied in a three dimensional charging setup to confine the electromagnetic fields effectively. The stray field level in our charging platform is well below the safety level required by the regulation agent. Although the energy can be transferred in an omnidirectional manner in the proposed charging platform, the energy should be directed to the target loads to avoid unnecessary energy waste. Therefore, a smart detection method is proposed to detect the receiver coil's orientation and focus the energy transfer to certain direction preferred by the receiver in the setup. The energy beaming strategy greatly improves the charging speed of the charging setup.

The steady state model and analysis of a phase-shift controlled series resonant inverter (PSC-SRl) is presented. This steady state model includes the evaluation of the zero-voltage switching (ZVS) condition and the determination of the ZVS operating region. Based upon this analysis a frequency control strategy that minimizes circulating energies is proposed. Also, a methodology to design the power stage components, and to predict the duty ratio and the operating frequency range is presented using a PSC-SRl design example operating at 500 kHz and 10 kW. In addition, a novel and simple frequency control circuit that implements the proposed frequency control strategy is provided. Besides, the analysis of the PSC-SRl complete power stage and two control-loop system (frequency control and duty ratio control) is given. Furthermore, the small-signal model and the compensation schemes for each of the control loops is presented. The analytical predictions are compared with experimental data measured from a 500 kHz, 10 kW laboratory prototype and conclusions are drawn. / Ph. D.

Electric vehicles (EVs) and plug-in hybrid electric vehicles (PHEVs) are gaining popularity because they are more environmentally friendly, less noisy and more efficient. These vehicles have batteries can be charged by on-board battery chargers that can be conductive or inductive. In conductive chargers, the charger is physically connected to the grid by a connector. With the inductive chargers, energy can be transferred wirelessly over a large air-gap through inductive coupling, eliminating the physical connection between the charger and the grid. A typical on-board battery charger consists of a boost power factor correction (PFC) converter followed by a dc-dc converter. This dissertation focuses on the design, modeling and control of a bidirectional dc-dc converter for conductive battery charging application. In this dissertation, a detailed design procedure is presented for a bidirectional CLLLC-type resonant converter for a battery charging application. This converter is similar to an LLC-type resonant converter with an extra inductor and capacitor in the secondary side. Soft-switching can be ensured in all switches without additional snubber or clamp circuitry. Because of soft-switching in all switches, very high-frequency operation is possible, thus the size of the magnetics and the filter capacitors can be made small. To further reduce the size and cost of the converter, a CLLC-type resonant network with fewer magnetics is derived from the original CLLLC-type resonant network. First, an equivalent model for the bidirectional converter is derived for the steady-state analysis. Then, the design methodology is presented for the CLLLC-type resonant converter. Design of this converter includes determining the transformer turns ratio, design of the magnetizing inductance based on ZVS condition, design of the resonant inductances and capacitances. Then, the CLLC-type resonant network is derived from the CLLLC-type resonant network. To validate the proposed design procedure, a 3.5 kW converter was designed following the guidelines in the proposed methodology. A prototype was built and tested in the lab. Experimental results verified the design procedure presented. The dynamics analysis of any converter is necessary to design the control loop. The bandwidth, phase margin and gain margin of the control loops should be properly designed to guarantee a robust system. The dynamic analysis of the resonant converters have not been extensively studied, with the previous work mainly concentrated on the steady-state models. In this dissertation, the continuous-time large-signal model, the steady-state operating point, and the small-signal model are derived in an analytical closed-form. This model includes both the frequency and the phase-shift control. Simulation and experimental verification of the derived models are presented to validate the presented analysis. A detailed controller design methodology is proposed in this dissertation for the bidirectional CLLLC-type resonant converter for battery charging application. The dynamic characteristics of this converter change significantly as the battery charges or discharges. And, at some operating points, there is a high-Q resonant peaking in the open-loop bode-plot for any transfer functions in this converter. So, if the controller is not properly designed, the closed-loop system might become unstable at some operating points. In this paper, a controller design methodology is proposed that will guarantee a stable operation during the entire operating frequency range in both battery charging mode (BCM) and regeneration mode (RM). To validate the proposed controller design methodology, the output current and voltage loop controllers are designed for a 3.5 kW converter. The step response showed a stable system with good transient performance thus validating the proposed controller design methodology. / Ph. D.

Modern data center power architecture developing trend is analyzed, efficiency improvement method is also discussed. Literature survey of high frequency isolated power conversion system which is also called solid state transformer is given including application, topology, device and magnetic transformer. Then developing trend of this research area is clearly shown following by research target. State of art wide band gap device including silicon carbide (SiC) and gallium nitride (GaN) devices are characterized and compared, final selection is made based on comparison result. Mostly used high frequency high power DC/DC converter topology dual active bridge (DAB) is introduced and compared with novel CLLC resonant converter in terms of switching loss and conduction loss point of view. CLLC holds ZVS capability over all load range and smaller turn off current value. This is beneficial for high frequency operation and taken as our candidate. Device loss breakdown of CLLC converter is also given in the end. Medium voltage high frequency transformer is the key element in terms of insulation safety, power density and efficiency. Firstly, two mostly used transformer structures are compared. Then transformer insulation requirement is referred for 4160 V application according to IEEE standard. Solid insulation material are also compared and selected. Material thickness and insulation distance are also determined. Insulation capability is preliminary verified in FEA electric field simulation. Thirdly two transformer magnetic loss model are introduced including core loss model and litz wire winding loss model. Transformer turn number is determined based on core loss and winding loss trade-off. Different core loss density and working frequency impact is carefully analyzed. Different materials show their best performance among different frequency range. Transformer prototype is developed following designed parameter. We test the developed 15 kW 500 kHz transformer under 4160 V dry type transformer IEEE Std. C57.12.01 standard, including basic lightning test, applied voltage test, partial discharge test. 500 kHz 15 kW CLLC converter gate drive is our design challenge in terms of symmetry propagation delay, cross talk phenomenon elimination and shoot through protection. Gate drive IC is carefully selected to achieve symmetrical propagation delay and high common mode dv/dt immunity. Zero turn off resistor is achieved with minimized gate loop inductance to prevent cross talk phenomenon. Desaturation protection is also employed to provide shoot through protection. Finally 15 kW 500 kHz CLLC resonant converter is developed based on 4160V 500 kHz transformer and tested up to full power level with 98% peak efficiency. / Master of Science

Solar energy as one type of the renewable energy becomes more and more popular which has led to increase the photovoltaic (PV) installations recently. One of the PV installations is the power conditioning system which is to convert the maximum available power output of the PV modules to the utility grid. Single-phase microinverters are commonly used to integrate the power to utility grid in modular power conditioning system. In the two-stage microinverter, each PV module is connected with a power converter which can transfer higher output power due to the tracking maximum power point (MPP) capability. However, it also has the disadvantages of lower power conversion efficiency due to the increased number of power electronics converters. The primary objective of this thesis is to develop a high-efficiency microconverter to increase the output power capability of the modular power conditioning systems. A topology with hybrid modes of operation are proposed to achieve wide-input regulation while achieving high efficiency. Two operating modes are introduced in details. Under high-input conditions, the converter acts like a buck converter, whereas the converter behaves as a boost converter under low-input conditions. The converter operates as the series resonant converter with normal-input voltage to achieve the highest efficiency. With this topology, the converter can achieve zero-voltage switching (ZVS) and/or zero-current switching (ZCS) of the primary side MOSFETs, ZCS and/or ZVS of the secondary side MOSFETs and ZCS of output diodes under all operational conditions. The experimental results based on a 300 W prototype are given with 98.1% of peak power stage efficiency and 97.6% of weighted California Energy Commission (CEC) efficiency including all auxiliary and control power under the normal-input voltage condition. / Master of Science

The objective of this thesis is to develop a dual-functional approach to effectively mitigate the earthquake induced vibrations of low- or mid-rise buildings, and at the same time to efficiently harvest utility-scale energy by using an optimally configured multi-resonant electromagnetic shunt in base isolation. In this research, two multi-resonant shunt configurations, parallel and series, were proposed and optimized based on the H2 criteria when the base isolation system is subjected to ground acceleration excitations. The performance of the proposed multi-resonant electromagnetic shunt was compared with traditional multiple tuned mass dampers (TMDs) applied to the base isolation system. It shows that, for multiple TMDs and multi-resonant electromagnetic shunt dampers with 5% total stiffness ratio, the parallel shunt electromagnetic shunt can achieve the best vibration mitigation performance among other types of multi-resonant dampers, including parallel TMDs, series TMDs and the series electromagnetic shunt damper. Case study of a base-isolated structure was analyzed to investigate the effectiveness of the proposed multi-resonant electromagnetic shunt. It shows that both multi-mode shunt circuits outperform single mode shunt circuit by suppressing the primary and the second vibration modes simultaneously. Comparatively, the parallel shunt circuit is more effective in vibration mitigation and energy harvesting, and is also more robust in parameter mistuning than the series shunt circuit. The time history response analysis shows that, under the recorded Northridge earthquake, the instant peak power and total average power capable to be harvested by the multi-resonant shunt can reach up to 1.18 MW and 203.37KW, respectively. This thesis further experimentally validated the effectiveness of the multi-resonant electromagnetic shunt on a scaled-down base-isolated building. The impact hammer test shows that the multi-resonant electromagnetic shunt can achieve enhanced vibration suppression by reducing the first resonant peak by 27.50dB and the second resonant peak by 22.57dB regarding the primary structure acceleration. The shake table test shows that under scaled Kobe and Northridge earthquake signals, the electromagnetic shunt can effectively reduce the vibration resonant peak value by 38.92% and 66.61%, respectively. The voltage simultaneously generated in the multi-mode shunt circuit was also obtained, which demonstrated the dual functions of the multi-resonant electromagnetic shunt in base isolation. / Master of Science