The increased power consumption and power density demands of modern technologies combined with the focus on global energy savings have increased the demands on DC/DC power supplies. DC/DC converters are ubiquitous in everyday life, found in products ranging from small handheld electronics requiring a few watts to warehouse sized server farms demanding over 50 megawatts. To improve efficiency and power density while reducing complexity and cost the modular building block approach is gaining popularity. These modular building blocks replace individually designed specialty power supplies, providing instead an optimized complete solution. To meet the demands for lower loss and higher power density, higher efficiency and higher frequency must be targeted in future designs. The objective of this dissertation is to explore and propose methods to improve the power density and performance of point of load modules ranging from 10 to 600W. For non-isolated, low current point of load applications targeting outputs ranging from one to ten ampere, the use of a three level converter is proposed to improve efficiency and power density. The three level converter can reduce the voltage stress across the devices by a factor of two compared to the traditional buck; reducing switching losses, and allowing for the use of improved low voltage lateral and lateral trench devices. The three level can also significantly reduce the size of the inductor, facilitating 3D converter integration with a low profile magnetic by doubling the effective switching frequency and reducing the volt-second across the inductor. This work also proposes solutions for the drive circuit, startup, and flying capacitor balancing issues introduced by moving to the three level topology. The emerging technology of gallium nitride can offer the ability to push the frequency of traditional buck converters to new levels. Silicon based semiconductors are a mature technology and the potential to further push frequency for improved power density is limited. GaN transistors are high electron mobility transistors offering a higher band gap, electron mobility, and electron velocity than Si devices. These material characteristics make the GaN device more suitable for higher frequency and voltage operation. This work will discuss the fundamentals of utilizing the GaN transistor in high frequency buck converter design; addressing the packaging of the GaN transistor, fundamental operating differences between GaN and Si devices, driving of GaN devices, and the impact of dead time on loss in the GaN buck converter. An analytical loss model for the GaN buck converter is also introduced. With significant improvements in device technology and packaging, the circuit layout parasitics begins to limit the switching frequency and performance. This work will explore the design of a high frequency, high density 12V integrated buck converter, identifying the impact of parasitics on converter performance, propose design improvements to reduce critical parasitics, and assess the impact of frequency on passive integration. The final part of this research considers the thermal design of a high density 3D integrated module; this addresses the thermal limitations of standard PCB substrates for high power density designs and proposes the use of a direct bond copper (DBC) substrate to improve thermal performance in the module. For 48V isolated applications, the current solutions are limited in frequency by high loss generated from the use of traditional topologies, devices, packaging, and transformer design. This dissertation considers the high frequency design of a highly efficient unregulated bus converter targeting intermediate bus architectures for use in telecom, networking, and high end computing applications. This work will explore the impact of switching frequency on transformer core volume, leakage inductance, and winding resistance. The use of distributed matrix transformers to reduce leakage inductance and winding resistance, improving high frequency transformer performance will be considered. A novel integrated matrix transformer structure is proposed to reduce core loss and core volume while maintaining low leakage inductance and winding resistance. Lastly, this work will push for higher frequency, higher efficiency, and higher power density with the use of low loss GaN devices. / Ph. D.

Moore's Law has been used to describe and predict the blossom of IC industries, so increasing the data density is clearly the ultimate goal of all technological development. If the power density of power electronics converters can be analogized to the data density of IC's, then power density is a critical indicator and inherent driving force to the development of power electronics. Increasing the power density while reducing or keeping the cost would allow power electronics to be used in more applications. One of the design challenges of the high-density power converter design is to have high-density magnetic components which are usually the most bulky parts in a converter. Increasing the switching frequency to shrink the passive component size is the biggest contribution towards increasing power density. However, two factors, losses and parasitics, loom and compromise the effect. Losses of high-frequency magnetic components are complicated due to the eddy current effect in magnetic cores and copper windings. Parasitics of magnetic components, including leakage inductances and winding capacitances, can significantly change converter behavior. Therefore, modeling loss and parasitic mechanism and control them for certain design are major challenges and need to be explored extensively. In this dissertation, the abovementioned issues of high-frequency transformers are explored, particularly in regards to high-power converter applications. Loss calculations accommodating resonant operating waveform and Litz wire windings are explored. Leakage inductance modeling for large-number-of-stand Litz wire windings is proposed. The optimal design procedure based on the models is developed. / Ph. D.

The ionosphere is the conducting part of the upper atmosphere that plays a significant role in trans-ionospheric high frequency (HF, 3-30 MHz) radiowave propagation. Solar activities, such as solar flares, radiation storms, coronal mass ejections (CMEs), alter the state of the ionosphere, a phenomenon known as Sudden Ionospheric Disturbance (SID), that can severely disrupt HF radio communication links by enhancing radiowave absorption and altering signal frequency and phase. The Super Dual Auroral Radar Network (SuperDARN) is an international network of low-power HF coherent scatter radars distributed across the globe to probe the ionosphere and its relation to solar activities. In this study, we used SuperDARN HF radar measurements with coordinated spacecraft and riometer observations to investigate statistical characteristics and the driving mechanisms of various manifestations of solar flare-driven SIDs in HF observations. We begin in Chapter 2 with a statistical characterization of various effects of solar flares on SuperDARN observations. Simultaneous observations from GOES spacecraft and SuperDARN radars confirmed flare-driven HF absorption depends on solar zenith angle, operating frequency, and intensity of the solar flare. The study found flare-driven SID also affects the SuperDARN backscatter signal frequency, which produces a sudden rise in Doppler velocity observation, referred to as the ``Doppler flash'', which occurs before the HF absorption effect. In Chapter 3, we further investigate the HF absorption effect during successive solar flares and those co-occurring with other geomagnetic disturbances during the 2017 solar storm. We found successive solar flares can extend the ionospheric relaxation time and the variation of HF absorption with latitude is different depending on the type of disturbance. In Chapter 4, we looked into an inertial property of the ionosphere, sluggishness, its variations with solar flare intensity, and made some inferences about D-region ion-chemistry using a simulation study. Specifically, we found solar flares alter the D-region chemistry by enhancing the electron detachment rate due to a sudden rise in molecular vibrational and rotational energy under the influence of enhanced solar radiation. In Chapter 5, we describe a model framework that reproduces HF absorption observed by riometers. This chapter compares different model formulations for estimating HF absorption and discusses different driving influences of HF absorption. In Chapter 6, we have investigated different driving mechanisms of the Doppler flash observed by SuperDARN radars. We note two particular findings: (i) the Doppler flash is predominantly driven by a change in the F-region refractive index and (ii) a combination of solar flare-driven enhancement in photoionization, and changes in the zonal electric field and(or) ionospheric conductivity reduces upward ion-drift, which produces a lowering effect in the F-region HF radiowave reflection height. Collectively, these research findings provide a statistical characterization of various solar flare effects on the ionosphere seen in the HF observations, and insights into their driving mechanisms and impacts on ionospheric dynamics. / Doctor of Philosophy / The Earth's ionosphere, extending from about 60 km to 1000 km in altitude, is an electrically charged region of the upper atmosphere that exists primarily due to ionization by solar X-ray and extreme ultraviolet radiation. The ionosphere is an effective barrier to energetic electromagnetic (EM) radiation and charged particles originating from the Sun or any other extraterrestrial sources and protect us against harmful space radiation. High frequency (HF, 3-30 MHz) radio communication, broadly used for real-time medium and long-range communication, is strongly dependent on the state of the ionosphere, which is susceptible to solar activities, such as solar flares, solar energetic particles (SEPs), and coronal mass ejections (CMEs). Specifically, we are interested in the impacts of solar flares. In this study, we use Super Dual Auroral Radar Network (SuperDARN) HF radars, ground-based riometers, and coordinated spacecraft observations to investigate the driving mechanisms of various space weather impacts on the ionosphere and radiowave propagation following solar flares. We begin in Chapter 2 with a characterization of various kinds of ionospheric disturbances manifested in SuperDARN backscattered signal following solar flares. Specifically, we characterized HF absorption effects and frequency anomalies experienced by traveling radiowaves, also known as Shortwave Fadeout (SWF) and Sudden Frequency Deviations (SFDs), respectively. In SuperDARN HF radar observations, SFDs are recorded as a sudden enhancement in Doppler velocity, which is referred to as the ``Doppler flash''. In Chapter 3, we investigate a special event study that elucidates the nonlinear physics behind HF absorption caused by multiple simultaneous solar flares and flares co-occurring with SEPs and CMEs. In Chapter 4, we explore an inertial property of the ionosphere, known as sluggishness, and its dependence on solar flares can provide important information about the chemical proprieties of the ionosphere. We found that the enhanced solar radiation during a solar flare increases the molecular vibrational and rotational energy that in turn enhances the electron detachment rate and reduces ionospheric sluggishness. In Chapter 5, we describe a framework to estimate HF absorption observed by riometers following solar flares. We analyze the influence of different physical parameters, such as collision frequency and electron temperature, on HF absorption. In Chapter 6, we delved into the physical processes that drive the Doppler flash in SuperDARN observations following solar flares. We find, (i) the Doppler flash is predominately driven by change in the F-region refractive index and (ii) a combination of solar flare-driven enhancement in photoionization, and change in zonal electric field and(or) ionospheric conductivity reduces upward ion-drift, which produces a lowering effect in the F-region HF radiowave reflection height. Taken together, these research findings provide new insights into solar flare impacts on the ionosphere and could be used to improve forecasting of ionospheric space weather disturbances following solar flares.

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.

With the rapid increase of cloud computing and the high demand for digital content, it is estimated that the power consumption of the IT industry will reach 10 % of the total electric power in the USA by 2020. Multi-core processors (CPUs) and graphics processing units (GPUs) are the key elements in fulfilling all of the digital content requirements, but come with a price of more power-hungry processors, driving the power per server rack to 20 KW levels. The need for more efficient power management solutions on the architecture level, down to the converter level, is inevitable. Recently, data centers have replaced the 12V DC server rack distribution with a 48V DC distribution, producing a significant overall system efficiency improvement. However, 48V rack architecture raises significant challenges for the voltage regulator modules (VRMs) required for powering the processor. The 48V VRM in the vicinity of the CPU needs to be designed with very high efficiency, high power density, high light-load efficiency, as well as meet all transient requirements by the CPU and GPU. Transferring the well-developed multi-phase buck converter used in the 12V VRM to the 48V distribution platform is not that simple. The buck converter operating with 48V, stepping down to sub 2V, will be subjected to significant switching related loss, resulting in lower overall system efficiency. These challenges drive the need to look for more efficient architectures for 48V VRM solutions. Two-stage conversions can help solve the design challenges for 48V VRMs. A first-stage unregulated converter is used to step-down the 48V to a specific intermediate bus voltage. This voltage will feed a multi-phase buck converter that powers the CPU. An unregulated LLC converter is used for the first-stage converter, with zero voltage switching (ZVS) operation for the primary side switches, and zero current switching (ZCS) along with ZVS operation, for the secondary side synchronous rectifiers (SRs). The LLC converter can operate at high frequency, in order to reduce the magnetic components size, while achieving high-efficiency. The high-efficiency first-stage, along with the scalability and high bandwidth control of the second-stage, allows this architecture to achieve high-efficiency and power density. This architecture is simpler to adopt by industry, by plugging the unregulated converter before the existing multi-phase buck converters on today's platforms. The first challenge for this architecture is the transformer design of the first-stage LLC converter. It must avoid all of the loss associated with high frequency operations, and still achieve high power density without scarifying efficiency. In this thesis, the integrated matrix transformer structure is optimized by SR integration with windings, interleaved primary side termination, and a better PCB winding arrangement to achieve high-efficiency and power density, and minimize the losses associated with high-frequency operations. The second challenge is the light load efficiency improvement. In this thesis a light load efficiency improvement is proposed by a dynamic change of the intermediate bus voltage, resulting in more than 8 % light load efficiency improvements. The third challenge is the selection of the optimal bus voltage for the two-stage architecture. The impact of different bus voltages was analyzed in order to maximize the overall conversion efficiency. Multiple 48V unregulated converters were designed with maximum efficiency >98 %, and power densities >1000 W/in3, with different output voltages, to select the optimal bus voltage for the two-stage VRM. Although the two-stage VRM is more scalable and simpler to design and adopt by current industry, the efficiency will reduce as full power flows in two cascaded DC/DC converters. Single-stage conversion can achieve higher-efficiency and power-density. In this thesis, a quasi-parallel Sigma converter is proposed for the 48V VRM application. In this structure, the power is shared between two converters, resulting in higher conversion efficiency. With the aid of an optimized integrated magnetic design, a Sigma converter suitable for narrow voltage range applications was designed with 420 W/in3 and a maximum efficiency of 94 %. Later, another Sigma converter suitable for wide voltage range applications was designed with 700W/in3 and a maximum efficiency of 95 %. Both designs can achieve higher efficiency than the two-stage VRM and all other state-of-art solutions. The challenges associated with the Sigma converter, such as startup and closed loop control were addressed, in order to make it a viable solution for the VRM application. The 48V rack architecture requires regulated 12V output converters for various loads. In this thesis, a regulated LLC is used to design a high-efficiency and power-density 48V bus converter. A novel integration method of the inductor and transformer helps the LLC achieve the required regulation capability with minimum losses, resulting in a converter that can provide 1KW of continuous power with efficiency of 97.8 % and 700 W/in3 power density. This dissertation discusses new power architectures with an optimized design for the 48V rack architectures. With the academic contributions in this dissertation, different conversion architectures can be utilized for 48V VRM solutions that solve all of the challenges associated with it, such as scalability, high-efficiency, high density, and high BW control. / Doctor of Philosophy / With the rapid increase of cloud computing and the high demand for digital content, it is estimated that the power consumption of the IT industry will reach 10 % of the total electric power in the USA by 2020. Multi-core processors (CPUs) and graphics processing units (GPUs) are the key elements in fulfilling all of the digital content requirements but come with a price of more power-hungry processors, driving the power per server rack to 20 KW levels. The need for more efficient power management solutions on the architecture level, down to the converter level, is inevitable. The data center manufacturers have recently adopted a more efficient architecture that supplies a 48V DC server rack distribution instead of a 12V DC distribution to the server motherboard. This helped reduce costs and losses, but as a consequence, raised a challenge in the design of the DC/DC voltage regulator modules (VRM) supplied by the 48V, in order to power the CPU and GPU. In this work, different architectures will be explored for the 48V VRM, and the trade-off between them will be evaluated. The main target is to design the VRM with very high-efficiency and high-power density to reduce the cost and size of the CPU/GPU motherboards. First, a two-stage power conversion structure will be used. The benefit of this structure is that it relies on existing technology using the 12V VRM for powering the CPU. The only modification required is the addition of another converter to step the 48V to the 12V level. This architecture can be easily adopted by industry, with only small modifications required on the system design level. Secondly, a single-stage power conversion structure is proposed that achieves higher efficiency and power density compared to the two-stage approach; however, the structure is very challenging to design and to meet all requirements by the CPU/GPU applications. All of these challenges will be addressed and solved in this work. The proposed architectures will be designed using an optimized magnetic structure. These structures achieve very high efficiency and power density in their designed architectures, compared to state-of-art solutions. In addition, they can be easily manufactured using automated manufacturing processes.

Three-phase rectifiers/inverters are widely used in grid-tied applications. Take the electric vehicle (EV) charging systems as an example. Within a certain space designated for the chargers, quick charging yet high efficiency are demanded. According to the current industry practice, with a power rating between 10 and 30 kW, the power density are limited by silicon (Si) power semiconductor devices, which make the systems operate at only up to around 30 kHz. The emerging wide bandgap (WBG) power semiconductor devices are considered as game changing devices to exceed the limits brought by their Si counterparts. Much higher switching frequency, higher power density and higher system efficiency are expected to be achieved with WBG power semiconductor devices. Among different types of WBG power semiconductor devices, Silicon Carbide Metal-Oxide-Semiconductor Field-Effect Transistors (SiC MOSFETs) are more popular in current research conducted for tens of kW power converter applications. However, the commonly adopted hard switching operation in this application still leads to significant switching loss at high frequency operation even for SiC-based systems. With the unique feature that the turn-off energy is almost negligible compared with the turn-on energy, critical conduction mode (CRM) based zero voltage soft switching turn-on operation is preferred for the SiC MOSFETs to eliminate the turn-on loss with small penalty on the conduction loss and on the turn-off loss. With this soft switching operation, switching frequency of SiC-based systems is able to be pushed to more than ten times higher than Si-based systems, and therefore higher power density yet even higher system efficiency can be achieved. The CRM-based soft switching is applied to three-phase rectifiers/inverters under the unity power factor operating condition first. Decoupled CRM-based control is enabled, and the inherent drawback of wide switching frequency variation range at CRM-based operation is overcome by the proposed novel modulation technique. It is the first time that CRM-based soft switching modulation is demonstrated in the most conventional three-phase H-bridge ac–dc converter, and more than three-time size reduction compared with current industry practice yet 99.0% peak efficiency are achieved at above 300 kHz switching frequency operation. Then this proposed soft switching modulation technique is extended to non-unity power factor operating conditions especially for grid-tied inverter system applications. With several improvements on the modulation, a generalized CRM-based soft switching modulation technique is proposed, which is applicable to both the unity and non-unity power factor conditions. With the power factor down to 0.8 lagging or leading according to commercial products, above 98.0% peak efficiency is achieved with the generalized soft switching modulation technique at above 300 kHz switching frequency operation. Furthermore from the aspect of electromagnetic interference (EMI), compared with the traditional Si-based design, CRM operation brings higher differential-mode (DM) EMI noise, and higher dv/dt with SiC MOSFETs brings higher common-mode (CM) EMI noise. What's more, hundreds of kHz switching frequency operation makes the main components of the system EMI spectrum located within the frequency range related to the EMI standard (150 kHz – 30 MHz). Therefore, several methods are adopted for the reduction of EMI noise. The total inductor current ripple is reduced with multi-channel interleaving control in order to reduce DM EMI noise. The balance technique is applied in order to reduce CM EMI noise. With PCB winding coupled inductors, the well-controlled parasitic parameters make the balance technique able to be effective for a uniform reduction of CM EMI noise from 150 kHz to above 20 MHz. In addition, PCB winding based magnetic designs are beneficial to achieving manufacture automation and reducing the labor cost. / Doctor of Philosophy / Power electronics and power conversion are crucial to many applications related to electricity, such as consumer electronics, domestic and commercial appliances, automobiles, data centers, utilities and infrastructure. In today's market, quality and reliability are usually considered as a given; high efficiency (low loss), high power density (small size and weight) and low cost are the main focuses in the design of power electronics products. In the past several decades, significant achievements in power electronics have been witnessed thanks to the silicon (Si) semiconductor technology, especially the Si power semiconductor devices. Nowadays, the development of Si power semiconductor devices is already close to the theoretical limits of the material itself. Therefore, in order to meet the increasing demands from customers in different applications, wide bandgap (WBG) based power semiconductor devices, namely Gallium Nitride (GaN) and Silicon Carbide (SiC), are becoming attractive because of its great potential compared with their Si counterparts. In literature, great contributions have already been made to understanding the WBG based power semiconductor devices. It is exciting and encouraging that some of the GaN-based power electronics products featuring high efficiency, high power density and low cost have been commercialized in consumer electronics applications. However, when pursuing these objectives, previous literature has not shown any applications of high frequency soft switching technology into the high power ac–dc conversion (usually three-phase ac–dc) in a simple way as the low power ac–dc conversion (usually single-phase ac–dc) in consumer electronics products. The key to achieving high efficiency, high power density and low cost is the high frequency soft switching operation. For single-phase ac–dc systems, the research on the realization of soft switching by control strategies instead of additional physical complexity has been intensively conducted, and this technology has also been adopted in the current industry practice. Therefore, the major achievement of this work is the development of a generalized soft switching control strategy for three-phase ac–dc systems, without adding any physical complexity, which is applicable to the simplest and most conventional three-phase ac-dc circuit topology. The proposed soft switching control strategy features bidirectional (rectifiers/inverters) power conversion, active/reactive power transfer, grid-tied/stand-alone modes, and scalability to multi-channel interleaved operation. Furthermore, with high frequency, the integration of magnetic components with embedded windings in the printed circuit board (PCB) becomes feasible, which is also beneficial to achieving electromagnetic compatibility (EMC) and manufacture automation. Based on the proposed control strategy and design methodology, a SiC-based 25-kW three-phase high frequency soft switching rectifier/inverter is developed for various applications such as electric vehicle (EV) charging stations, uninterruptible power supplies (UPS) and renewable energy based utilities.

Insulation system is an inseparable part of electrical equipment. In this study, one of the most important aging factors in insulation systems known as partial discharge (PD) is targeted. PD phenomenon has been studied for more than a century and yet new technologies still demand the investigation of PD impact. Nowadays, electrification is penetrating into various fossil-fuel-based industries such as transportation system that demands the reliability of electrical equipment under various harsh environmental conditions. Due to the lack of knowledge on the behavior of insulation systems, research in this area is intensively needed. The current study probes into the partial discharge phenomenon from two aspects and the groundwork for both aspects are provided by experimentation of multiple PD types. In the first goal, a finite-element analysis (FEA) approach is developed based on measurement data to estimate electric field distribution. The FEA model is coupled with a programming scheme to evaluate PD conditions, calculate PD metrics, and perform statistical analysis of the results. For the second target, it is aimed to use deep neural networks to identify and discriminate different sources of PD. The measurement data are used to generate thousands of phase-resolved PD (PRPD) images that will be used for training deep learning models. To meet the characteristics of the dataset, a deep residual neural network is designed and optimized to discriminate PD sources in an accurate, stable, and time-efficient way. The outcome of this research enhances the reliability of electrical apparatus through a better understanding of the PD behavior and lays a foundation for automatic monitoring of PD sources. / Doctor of Philosophy / Electrical equipment functions properly when its conductive elements are electrically insulated. The science of dealing with insulation systems has become more prominent in recent years due to the novel challenges and circumstances introduced by the rapid electrification trend. As an instance, the electrification trend in transportation systems can impose a multitude of environmental, thermal, and mechanical constraints which were not traditionally considered. These new challenges have led to an accelerated deterioration rate of insulation materials. To address this concern, this study targets the experimentation and modeling of the main aging mechanism in electrical equipment known as partial discharge (PD). A numerical model based on finite-element analysis (FEA) is developed that agrees with the test results and can accurately predict the aging of insulating materials due to the PD phenomenon. Moreover, the growing interest toward electrification of the aviation industry (as a response to the climate change crisis) requires the study of insulating materials under low-pressure (high-altitude) conditions. Theoretical and experimental data confirm the more frequent occurrence of PDs and their higher intensity under low-pressure conditions. Safety of operation is the highest priority in airborne transportation, yet no study has addressed the condition monitoring system as a necessary asset of the electric aircraft. To address this research gap, this work develops a dielectric online condition monitoring system (DOCMS) that actively monitors the deterioration level of insulation using deep learning methods. Based on standardized measurements under low-pressure conditions, the data are preprocessed to train the deep neural network with the pattern of PD activities. The proposed scheme can achieve >82% with short-term signals emitted measured from the system.

This dissertation proposes a high-power, high-frequency and high-density three-level parallel resonant converter for capacitor charging. DC-DC pulsed power converters are widely used in military and medical systems, where the power density requirement is often stringent. The primary means for reducing the power converter size has been to reduce loss for reduced cooling systems and to increase the frequency for reduced passive components. Three-level resonant converters, which combine the merits of the three-level structure and resonant converters, are an attractive topology for these applications. The three-level configuration allows for the use of lower-voltage-rating and faster devices, while the resonant converter reduces switching loss and enhances switching capability. This dissertation begins with an analysis of the influence of variations in the structure of the resonant tank on the transformer volume, with the aim of achieving a high power density three-level DC-DC converter. As one of the most bulky and expensive components in the power converter, the different positions of the transformer within the resonant tank cause significant differences in the transformer's volume and the voltage and current stress on the resonant elements. While it does not change the resonant converter design or performance, the improper selection of the resonant tank structure in regard to the transformer will offset the benefits gained by increasing the switching frequency, sometimes even making the power density even worse than the power density when using a low switching frequency. A methodology based on different structural variations is proposed for a high-density design, as well as an optimized charging profile for transformer volume reduction. The optimal charging profile cannot be perfectly achieved by a traditional output-voltage based variable switching frequency control, which either needs excess margin to guarantee ZVS, or delivers maximum power with the danger of losing ZVS. Moreover, it cannot work for widely varied input voltages. The PLL is introduced to overcome these issues. With PLL charging control, the power can be improved by 10% with a narrow frequency range. The three-level structure in particular suffers unbalanced voltage stress in some abnormal conditions, and a fault could easily destroy the system due to minimized margin. Based on thoroughly analysis on the three-level behaviors for unbalanced voltage stress phenomena and fault conditions, a novel protection scheme based on monitoring the flying capacitor voltage is proposed for the three-level structure, as well as solutions to some abnormal conditions for unbalanced voltage stresses. A protection circuit is designed to achieve the protection scheme. A final prototype, built with a custom-packed MOSFET module, a SiC Schottky diode, a nanocrystalline core transformer with an integrated resonant inductor, and a custom-designed oil-cooled mica capacitor, achieves a breakthrough power density of 140W/in3 far beyond the highest-end power density reported (<100 W/in3) in power converter applications. / Ph. D.

The future power conversion system not only must meet the characteristics demanded by the load, but also have to achieve high power density with high efficiency, high ambient temperature, and high reliability. Density and efficiency are two key drivers and metrics for the advancement of power conversion technologies. Generally speaking, a high performance active device is the first force to push power density to meet the requirement of modern systems. Silicon has been a dominant material in power management since the late 1950s. However, due to continuous device optimizations and improvements in the production process, the material properties of silicon have increasingly become the limiting factor. Workarounds like the super junction stretch the limits but usually at substantial cost. The use of gallium nitride devices is gathering momentum, with a number of recent market introductions for a wide range of applications such as point-of-load (POL) converters, off-line switching power supplies, battery chargers and motor drives. GaN devices have a much lower gate charge and lower output capacitance than silicon MOSFETs and, therefore, are capable of operating at a switching frequency 10 times greater. This can significantly impact the power density of power converters, their form factor, and even current design and manufacturing practices. To realize the benefits of GaN devices resulting from significantly higher operating frequencies, a number of issues have to be addressed, such as converter topology, soft-switching technique, high frequency gate driver, high frequency magnetics, packaging, control, and thermal management. This work studies the insight switching characteristics of high-voltage GaN devices including some specific issues related to the cascode GaN. The package impact on the switching performance and device reliability will be illustrated in details. A stack-die package is proposed for cascode GaN devices to minimize the impact of package parasitic inductance on switching transition. Comparison of hard-switching and soft-switching operation is carried based on device model and experiments, which shows the necessity of soft-switching for GaN devices at high frequency. This work also addresses high dv/dt and di/dt related gate drive issues associated with the higher switching speed of GaN devices. Particularly, the conventional driving solution could fail on the high side switch in a half-bridge configuration due to relative large common-mode noise current. Two simple and effective driving methods are proposed to improve noise immunity and maintain high driving speed. Finally, this work illustrates the utilization of GaN in an emerging application, high density AC-DC adapter. Many design considerations are presented in detail. The GaN-based adapter is capable of operating at 1-2 MHz frequency with an improved efficiency up to 94%. Several design examples at different power levels, with a power density in the range of 20~35W/in3, which is a three-fold improvement over the state-of-the-art product, are successfully demonstrated. In conclusion, this work is focus on the characterization, and evaluation of GaN devices. Packaging, high frequency driving and soft-switching technique are addressed to fully explore the potential of GaN devices. High density adapters are demonstrated to show the advance of GaN device and its impact on system design. / 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