Very-high-frequency low-voltage power delivery

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2013.Cataloged from PDF version of thesis.Includes bibliographical references (p. 217-223).Power conversion for the myriad low-voltage electronic circuits in use today, including portable electronic devices, digital electronics, sensors and communication circuits, is becoming increasingly challenging due to the desire for lower voltages, higher conversion ratios and higher bandwidth. Future computation systems also pose a major challenge in energy delivery that is difficult to meet with existing devices and design strategies. To reduce interconnect bottlenecks and enable more flexible energy utilization, it is desired to deliver power across interconnects at high voltage and low current with on- or over-die transformation to low voltage and high current, while providing localized voltage regulation in numerous zones. This thesis introduces elements for hybrid GaN-Si dc-de power converters operating at very high frequencies (VHF, 30-300 MHz) for low-voltage applications. Contributions include development of a new VHF frequency multiplier inverter suitable for step-down power conversion, and a Si CMOS switched-capacitor step-down rectifier. These are applied to develop a prototype GaN-Si hybrid dc-dc converter operating at 50 MHz. Additionally, this thesis exploits these elements to propose an ac power delivery architecture for low-voltage electronics in which power is delivered across the interconnect to the load at VHF ac, with local on-die transformation and rectification to dc. With the proposed technologies and emerging passives, it is predicted that the ac power delivery system can achieve over 90 % efficiency with greater than 1 W/mm² power density and 5:1 voltage conversion ratio. A prototype system has been designed and fabricated using a TSMC 0.25 [mu]m CMOS process to validate the concept. It operates at 50 MHz with output power of 4 W. The prototype converter has 8:1 voltage conversion ratio with input voltage of 20 V and output voltage of 2.5 V. To the author's best knowledge, this is the first ac power delivery architecture for low-voltage electronics ever built and tested.by Wei Li.Ph.D

Power Converters in Power Electronics

In recent years, power converters have played an important role in power electronics technology for different applications, such as renewable energy systems, electric vehicles, pulsed power generation, and biomedical sciences. Power converters, in the realm of power electronics, are becoming essential for generating electrical power energy in various ways. This Special Issue focuses on the development of novel power converter topologies in power electronics. The topics of interest include, but are not limited to: Z-source converters; multilevel power converter topologies; switched-capacitor-based power converters; power converters for battery management systems; power converters in wireless power transfer techniques; the reliability of power conversion systems; and modulation techniques for advanced power converters

Identification of amino acid residues of the NR2A subunit that control glutamate potency in recombinant NR1/NR2A NMDA receptors.

The NMDA type of glutamate receptor requires both glycine and glutamate to activate it efficiently. The receptor is thought to be an oligomer of two types of subunit, NR1 and NR2. Site-directed mutagenesis has shown that glycine potency is controlled by residues located in two areas on the NR1 subunit, one N-terminal of Ml and the other C-terminal of M3 (termed the S1 and S2 domains respectively). To test the hypothesis that the glutamate site exists on the NR2 subunits, the role of amino acid residues in similar areas on the NR2A subunit were investigated. These areas show homology with the ligand binding domains of bacterial periplasmic amino acid binding proteins. Two mutations caused an increase of glutamate EC50 by at least two orders of magnitude, (H466A and G669A) and one (T671A) by three orders, compared with wild-type. In contrast, glycine EC50s did not differ by more than 2-fold compared with the wild-type. Schild analysis was used to measure the binding of the competitive NMDA receptor antagonist D-(-)-2-amino-5-phosphonopentanoic acid (APV) on the most 'shifted' mutant (T671 A). The slope of the Schild plot for the mutant receptor did not differ greatly from unity, 0.94 ± 0.16 (mean ± S.D.M), which is consistent with APV being a competitive antagonist on the mutant receptor. However the affinity of the receptor for APV (KB = 321 ±30 μM) was 255-fold less than wild-type (KB = 1-26 ± 0.07 μM) (means ± S.D.M). The large reduction of glutamate potency, together with unchanged Hill slope, and no gross reduction in maximum response, suggests that residues contributing to glutamate binding are located on the NR2A subunit. This is supported by the much- reduced affinity for APV in the T671A mutant. Such evidence suggests that this residue is important for the binding of glutamate to NR1/NR2A receptors. In a second project, transgenic founder mice were generated by pronuclear microinjection containing a 8 kb 5'UT fragment from the mouse NR1 gene driving the expression of the tetracycline-sensitive transactivator (tTA). This mouse may be useful for the study of neuronal-specific inducible gene expression in the future

Silicon carbide based DC-DC converters for deployment in hostile environments

PhD ThesisThe development of power modules for deployment in hostile environments, where the elevated ambient temperatures demand high temperature capability of the entire converter system, requires innovative power electronic circuits to meet stringent requirements in terms of efficiency, power-density and reliability. To simultaneously meet these conflicting requirements in extreme environment applications is quite challenging. To realise these power modules, the relevant control circuitry also needs to operate at elevated temperatures. The recent advances in silicon carbide devices has allowed the realisation of not just high frequency, high efficiency power converters, but also the power electronic converters that can operate at elevated temperatures, beyond those possible using conventional silicon-based technology. High power-density power converters are key components for power supply systems in applications where space and weight are critical parameters. The demand for higher power density requires the use of high-frequency DC-DC converters to overcome the increase in size and power losses due to the use of transformers. The increase in converter switching frequency reduces the size of passive components whilst increasing the electromagnetic interference (EMI) emissions. A performance comparison of SiC MOSFETs and JFETs in a high-power DC-DC converter to form part of a single phase PV inverter system is presented. The drive design requirements for optimum performance in the energy conversion system are also detailed. The converter was tested under continuous conduction mode at frequencies up to 250 kHz. The converter power efficiency, switch power loss and temperature measurements are then compared with the ultra-high speed CoolMOS switches and SiC diodes. The high voltage, high frequency and high temperature operation capability of the SiC DUTs are also demonstrated. The all SiC converters showed more stable efficiencies of 95.5% and 96% for the switching frequency range for the SiC MOSFET and JFET, respectively. A comparison of radiated noise showed the highest noise signature for the SiC JFET and lowest for the SiC MOSFET. The negative gate voltage requirement of the SiC MOSFET introduces up to 6 dBμV increase in radiated noise, due to the induced current in the high frequency resonant stray loop in the gate drive negative power plane. ii A gate driver is an essential part of any power electronic circuitry to control the switching of the power semiconductor devices. The desire to place the gate driver physically close to the power switches in the converter, leads to the necessity of a temperature resilient PWM generator to control the power electronics module. At elevated temperatures, the ability to control electrical systems will be a key enabler for future technology enhancements. Here an SiC/SOI-based PWM gate driver is proposed and designed using a current source technique to accomplish variable duty-cycle PWM generation. The ring oscillator and constant current source stages use low power normally-on, epitaxial SiC-JFETs fabricated at Newcastle University. The amplification and control stages use enhancement-mode signal SOI MOSFETs. Both SOI MOSFETs will be replaced by future high current SiC-JFETs with only minor modification to the clamp-stage circuit design. In the proposed design, the duty cycle can be varied from 10% to 90%. The PWM generator is then evaluated in a 200 kHz step-up converter which results in a 91% efficiency at 81% duty cycle. High temperature environments are incompatible with standard battery technologies, and so, energy harvesting is a suitable technology when remote monitoring of these extreme environments is performed through the use of wireless sensor nodes. Energy harvesting devices often produce voltages which are unusable directly by electronic loads and so require power management circuits to convert the electrical output to a level which is usable by monitoring electronics and sensors. Therefore a DC-DC step-up converter that can handle low input voltages is required. The first demonstration of a novel self-starting DC-DC converter is reported, to supply power to a wireless sensor node for deployment in high temperature environments. Utilising SiC devices a novel boost converter topology has been realised which is suitable for boosting a low voltage to a level sufficient to power a sensor node at temperatures up to 300 °C. The converter operates in the boundary between continuous and discontinuous mode of operation and has a VCR of 3 at 300 °C. This topology is able to self start and so requires no external control circuitry, making it ideal for energy harvesting applications, where the energy supply may be intermittent.EPSRC and BAE SYSTEMS through the Dorothy Hodgkin Postgraduate Awar

Communication satellite technology: State of the art and development opportunities

Opportunities in communication satellite technology are identified and defined. Factors that tend to limit the ready availability of satellite communication to an increasingly wide group of users are evaluated. Current primary limitations on this wide utilization are the availability of frequency and/or synchronous equatorial satellite positions and the cost of individual user Earth terminals. The former could be ameliorated through the reuse of frequencies, the use of higher frequency bands, and the reduction of antenna side lobes. The latter limitation requires innovative hardware, design, careful system design, and large scale production

Contextual modulations of visual perception and visual cortex activity in humans

Visual perception and neural processing depend on more than retinal stimulation alone. They are modulated by contextual factors like cross-modal input, the current focus of attention or previous experience. In this thesis I investigate ways in which these factors affect vision. A first series of experiments investigates how co-occurring sounds modulate vision, with an emphasis on temporal aspects of visual processing. In three behavioral experiments I find that participants are unable to ignore the duration of co-occurring sounds when giving visual duration judgments. Furthermore, prolonged sound duration goes along with improved detection sensitivity for visual stimuli and thus extends beyond duration judgments per se. I go on to test a cross-modal illusion in which the perceived number of flashes in a rapid series is affected by the number of co-occurring beeps (the sound-Induced flash illusion). Combining data from structural magnetic resonance imaging (MRI) and a behavioral experiment I find that individual proneness to this illusion is linked with less grey matter volume in early visual cortex. Finally, I test how co-occurring sounds affect the cortical representation of more natural visual stimuli. A functional MRI (fMRI) experiment investigates patterns of activation evoked by short video clips in visual areas V1-3. The trial-by-trial reliability of such patterns is reduced for videos accompanied by mismatching sounds. Turning from cross-modal effects to more intrinsic sources of contextual modulation I test how attention affects visual representations in V1-3. Using fMRI and population receptive field (pRF) mapping I find that high perceptual load at fixation renders spatial tuning for the surrounding visual field coarser and goes along with pRFs being radially repelled. In a final behavioral and fMRI experiment I find that the perception of face features is modulated by retinal stimulus location. Eye and mouth stimuli are recognized better, and evoke more discriminable patterns of activation in face sensitive patches of cortex, when they are presented at canonical locations. Taken together, these experiments underscore the importance of contextual modulation for vision, reveal some previously unknown such factors and point to possible neural mechanisms underlying them. Finally, they argue for an understanding of vision as a process using all available cues to arrive at optimal estimates for the causes of sensory events

Fault management via dynamic reconfiguration for integrated modular avionics

The purpose of this research is to investigate fault management methodologies within Integrated Modular Avionics (IMA) systems, and develop techniques by which the use of dynamic reconfiguration can be implemented to restore higher levels of systems redundancy in the event of a systems fault. A proposed concept of dynamic configuration has been implemented on a test facility that allows controlled injection of common faults to a representative IMA system. This facility allows not only the observation of the response of the system management activities to manage the fault, but also analysis of real time data across the network to ensure distributed control activities are maintained. IMS technologies have evolved as a feasible direction for the next generation of avionic systems. Although federated systems are logical to design, certify and implement, they have some inherent limitations that are not cost beneficial to the customer over long life-cycles of complex systems, and hence the fundamental modular design, i.e. common processors running modular software functions, provides a flexibility in terms of configuration, implementation and upgradability that cannot be matched by well-established federated avionic system architectures. For example, rapid advances of computing technology means that dedicated hardware can become outmoded by component obsolescence which almost inevitably makes replacements unavailable during normal life-cycles of most avionic systems. To replace the obsolete part with a newer design involves a costly re-design and re-certification of any relevant or interacting functions with this unit. As such, aircraft are often known to go through expensive mid-life updates to upgrade all avionics systems. In contrast, a higher frequency of small capability upgrades would maximise the product performance, including cost of development and procurement, in constantly changing platform deployment environments. IMA is by no means a new concept and work has been carried out globally in order to mature the capability. There are even examples where this technology has been implemented as subsystems on service aircraft. However, IMA flexible configuration properties are yet to be exploited to their full extent; it is feasible that identification of faults or failures within the system would lead to the exploitation of these properties in order to dynamically reconfigure and maintain high levels of redundancy in the event of component failure. It is also conceivable to install redundant components such that an IMS can go through a process of graceful degradation, whereby the system accommodates a number of active failures, but can still maintain appropriate levels of reliability and service. This property extends the average maintenance-free operating period, ensuring that the platform has considerably less unscheduled down time and therefore increased availability. The content of this research work involved a number of key activities in order to investigate the feasibility of the issues outlined above. The first was the creation of a representative IMA system and the development of a systems management capability that performs the required configuration controls. The second aspect was the development of hardware test rig in order to facilitate a tangible demonstration of the IMA capability. A representative IMA was created using LabVIEW Embedded Tool Suit (ETS) real time operating system for minimal PC systems. Although this required further code written to perform IMS middleware functions and does not match up to the stringent air safety requirements, it provided a suitable test bed to demonstrate systems management capabilities. The overall IMA was demonstrated with a 100kg scale Maglev vehicle as a test subject. This platform provides a challenging real-time control problem, analogous to an aircraft flight control system, requiring the calculation of parallel control loops at a high sampling rate in order to maintain magnetic suspension. Although the dynamic properties of the test rig are not as complex as a modern aircraft, it has much less stringent operating requirements and therefore substantially less risk associated with failure to provide service. The main research contributions for the PhD are: 1.A solution for the dynamic reconfiguration problem for assigning required systems functions (namely a distributed, real-time control function with redundant processing channels) to available computing resources whilst protecting the functional concurrency and time critical needs of the control actions. 2.A systems management strategy that utilises the dynamic reconfiguration properties of an IMA System to restore high levels of redundancy in the presence of failures. The conclusion summarises the level of success of the implemented system in terms of an appropriate dynamic reconfiguration to the response of a fault signal. In addition, it highlights the issues with using an IMA to as a solution to operational goals of the target hardware, in terms of design and build complexity, overhead and resources

Development of an active power filter based on wide-bandgap semiconductors

Pla de Doctorat Industrial, Generalitat de CatalynuaElectrical and electronic equipment needs sinusoidal currents and voltages to function properly. Equipment such as computers, household appliances, electric vehicle chargers, and LED lights can distort the grid and worsen grid quality. Distorted electrical grids can cause malfunctions, reduce service life, and decrease the performance of connected equipment. Industry commonly solves these problems using active power filters, which can minimise the harmonics of the grid, eliminate undesirable reactive power, and restore balance to unbalanced power grids. This thesis deals with the design and implementation of an active power filter based on wide-bandgap semiconductors, which have properties that are superior to classical silicon devices. An active power filter’s design must take advantage of these benefits to build converters that are smaller, more efficient, and consume fewer resources. However, wide-bandgap semiconductors also present design challenges. Because the most commonly used active power filters in the industry are based on two-level voltage source converters, the research for this doctoral thesis focuses on this converter topology. Moreover, its main objective is to contribute new modulation techniques that are specially designed to work with wide-bandgap semiconductors. The proposed modulations consider different aspects, such as the computational cost of the algorithms, converter losses, and the electromagnetic distortion generated. First, this thesis presents a hexagonal sigma-delta (H-S¿) modulation based on sigma-delta (S¿) modulation. The properties of this modulation are studied, and the technique is compared with other widely used modulations. The comparison considers efficiency, harmonic distortion, the electromagnetic compatibility of the converter, and the type of wideband semiconductor used. In addition, a fast algorithm is mathematically developed to simplify the presented modulation and reduce its computational cost. Secondly, this thesis presents a family of sigma-delta modulations specially designed to improve electromagnetic compatibility: the reduced common-mode voltage sigma-delta (RCMV-S¿) modulations. These modulations avoid using the vectors that generate the maximum common-mode voltage, which significantly reduces the generated electromagnetic distortion without affecting the performance of the converter and its harmonic distortion. Finally, the proposed modulations are applied in a wide-bandgap power converter working as an active filter. Thus, it is verified that the techniques presented in this thesis will obtain satisfactory results when implemented in commercial active power filters.Els equips elèctrics i electrònics necessiten corrents i tensions sinusoïdals per funcionar correctament. Existeixen equips com els ordinadors, els electrodomèstics, els carregadors de vehicle elèctric o les llums LED, que poden distorsionar la xarxa i empitjorar la qualitat d'aquesta. Les xarxes elèctriques distorsionades poden causar el mal funcionament dels equips que s'hi connecten, reduir la seva vida útil i també empitjorar la seva eficiència. A la industria és habitual utilitzar filtres actius per a solucionar aquests problemes. Els filtres actius permeten minimitzar els harmònics presents a la Δxarxa, eliminar la potència reactiva no desitjada i equilibrar xarxes elèctriques desequilibrades. Aquesta tesi tracta sobre el disseny i la implementació d'un filtre actiu basat en semiconductors de banda ampla. Aquests semiconductors presenten propietats superiors als clàssics dispositius de silici. El disseny d'un filtre actiu ha d'aprofitar aquests avantatges per a construir convertidors més petits, eficients i que consumeixin menys recursos. Tanmateix, els semiconductors de banda ampla també presenten problemes que el disseny ha de solucionar. Els filtres actius més utilitzats en la indústria són els basats en convertidors de font de tensió (voltatge source converters) amb dos nivells. La recerca d'aquesta tesi doctoral està focalitzada en aquesta topologia de convertidor, i el seu principal objectiu és l’aportació de noves tècniques de modulació especialment dissenyades per treballar amb semiconductors de banda ampla. Les modulacions proposades tenen en compte diferents aspectes: el cost computacional dels algoritmes, les pèrdues del convertidor i la distorsió electromagnètica generada. En primer lloc, es presenta una modulació sigma-delta hexagonal (H-__) que es basa en la modulació sigma-delta (ΣΔ). S'estudien les propietats d'aquesta modulació i la tècnica es compara amb altres modulacions àmpliament usades. La comparativa realitzada considera l’eficiència, la distorsió harmònica, la compatibilitat electromagnètica del convertidor i el tipus de semiconductor de banda ampla emprat. Addicionalment, es desenvolupa matemàticament un algoritme ràpid per simplificar la modulació presentada i reduir el seu cost computacional. En segon lloc, es presenta una família de modulacions sigma-delta especialment dissenyades per millorar la compatibilitat electromagnètica: les modulacions sigmadelta amb tensió en mode comú reduïda (RCMV-ΣΔ ). Aquestes modulacions eviten fer servir els vectors que generen la màxima tensió en mode comú. D'aquesta manera es redueix significativament la distorsió electromagnètica generada sense afectar de forma notable al rendiment del convertidor ni a la seva distorsió harmònica. Finalment, les modulacions proposades s'apliquen en un convertidor de potència, basat en semiconductors de banda ampla, que treballa com a filtre actiu. Això es verifica que les tècniques presentades en aquesta tesi poden ser implementades en filtres actius comercials obtenint resultats satisfactoris.Postprint (published version

Devonian origin and Cenozoic radiation in the clubmosses (Lycopodiaceae)

Together with the heterosporous lycophytes, the clubmoss family (Lycopodiaceae) is the sister lineage to all other vascular land plants. Given the family’s important position in the land-plant phylogeny, studying the evolutionary history of this group is an important step towards a better understanding of plant evolution. Despite this, little is known about the Lycopodiaceae, and a well-sampled, robust phylogeny of the group is lacking. The goal of this dissertation is to resolve the relationships among evolutionary lineages in the Lycopodiaceae and provide insight into the timing and drivers of diversification in the family. First, to place the evolution of the family within a global and historical context, I generated a densely sampled, time-calibrated phylogeny of the family. I sampled 50% of the estimated 400 extant species in the Lycopodiaceae and used eight fossils to calibrate the age of major divergence events in the family and across the land-plant phylogeny. Further, we used a probabilistic biogeographic model to infer the historical biogeography of the family. Together, these analyses indicate that the Lycopodiaceae originated in the late Devonian, began its early diversification in the Carboniferous, and accumulated much of its extant diversity during the Cenozoic. From a geographical perspective, major cladogenesis events in the family’s history appear to be linked to the breakup of the Pangaean and Gondwanan supercontinents, with long-distance dispersal playing a role in the establishment of younger evolutionary lineages. Second, I examined the drivers of diversification in the species-rich genus Phlegmariurus in the Neotropics. This clade includes an estimated 150 species and is most diverse in high-elevation habitats in the tropical Andes of South America. Using a time-calibrated phylogeny of the group and species distribution and niche data, I demonstrate a strong positive association between lineage diversification rates and the mean elevation of species’ distributions as well as a strong negative correlation between diversification rates and the size of species’ ranges. Further, we employ a paleoelevation-dependent diversification model to test for an association between the uplift of the Andes and diversification in the clade and demonstrate that speciation rates in Neotropical Phlegmariurus are positively associated with increasing elevations in the Andes. Third, I use a phylogenetic framework to test the monophyly of morphology-based species groups in Neotropical Phlegmariurus. I demonstrate that most groups are not monophyletic, and that convergent evolution is widespread in the genus. We use ancestral character-state reconstruction methods for six morphological traits to elucidate patterns of trait evolution and to circumscribe new species groups. A total of eleven new monophyletic species groups are proposed and defined