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In recent years, advancements in modern technologies have grown the demand for low-power wireless devices. Considering that enhancing the lifetime of the required battery to maintain the operation of these devices is still impractical, harvesting energy from ambient sources has become a promising solution to power portable low power electronic devices. Harvesting ambient energy from the electromagnetic wave (EM), which is referred to as radio frequency energy harvesting (RFEH), is one of the most popular power extracting methods. Scavenging energy can be used to fully supply the power required for wearable electronics devices, RFID, medical implantable devices, wireless sensors, internet of things (IoT) etc. RF energy is readily available in urban environments due to the abundant existence of HF and UHF technologies. Therefore, there is a great interest in studying systems working in UHF bands, including 300MHz to 3GHz frequencies. Radio frequency energy harvesting is a method which converts the received signals into electricity. This technique offers various environmentally friendly alternative energy sources. Specifically, RFEH has interesting attributes that make it very practical for low-power electronics and wireless sensor networks (WSNs). Ambient RF energy can be provided by commercial RF broadcasting stations such as Wi-Fi, GSM, radar or TV. In this study, particular attention is given to design efficient low power circuits suitable to be applied for RFEH as a green technology, which is very suitable for overcoming problems such as powering wireless sensors located in inaccessible places or harsh environments, the possibility to power directly electronic devices, recharge batteries and etc. In RFEH, it is very important to enhance the efficiency of the circuits and systems to maximize the amount of harvested energy. This thesis is mainly concerned with the design, simulation, and implementation of AC to DC circuits including phase shifter, rectifier, and DC to DC converter which is specifically designed for RFEH. It can be applied in various applications such as telecommunications, wireless sensors, medical devices, wireless charging, Internet of Things (IoT) and etc. In the designed system in this thesis, the signal must be passed through a phase shifter, rectifier, and voltage multiplier to reach the required level of output voltage. In another word, this system rectifies the sinusoidal AC waveform to DC and multiplies it to get higher voltages. In this thesis, we propose 1 and 7-stage rectifiers, phase shifters and isolated/non-isolated DC to DC converters will be investigated individually in a general manner and integrated together to have the desired range of outputs for considered applications. This research methodology has three major phases: Phase 1: Theoretical analyses, Phase 2: Simulation investigations and Phase 3: Practical verification. This thesis presents a review on the history of different circuits used to design a low power system for EH. Certain achievements in recent decades make power harvesting a reality, capable of providing alternative sources of energy for a wider range of applications. This review provides a summary of RFEH technologies to use as a guide for the design of RFEH units. Additionally, comprehensive analysis and discussions of various designs of rectifiers, isolated and non-isolated DC to DC converters and phase shifters in addition to their trade-offs for RF energy harvesting purposes are included. In this thesis, novel designs of Dickson rectifiers with high voltage gain and efficiency operating with an input frequency of 915MHz is presented. The proposed circuits introduce a new method of deriving output characteristics of rectification circuit in terms of voltage. The design consists of different stages of the Dickson voltage multiplier. The rectifiers benefit from two input AC sources with 180° phase shift. This Dickson circuit is further discussed in two levels; the first one is a 1-stage rectifier operating with Schottky diodes or diode-connected MOSFETs, and the second is a 7-stage rectifier discussed with both Schottky diodes and diode-connected MOSFETs producing higher output voltages. Furthermore, the prototype of 1-stage rectifier is presented where the input voltage is between -10dBm and 2dBm and the output voltage gained is from 318mV to 1700mV, respectively. Also, the prototype of 7-stage rectifier is presented where the input voltage is -10dBm, -8dBm and -6dBm and the output voltage is gained 1220mV, 1330mV and 1550mV, respectively. Additionally, a new non-isolated high voltage gain, high efficiency zero voltage switching (ZVS) resonant DC to DC converter working under ZVS condition is introduced, which can work in high frequencies with high power conversion rate as well as low losses. The proposed converter can provide 5V output from 350mV input voltage with efficiency of 72.8%. Furthermore, we proposed an isolated DC to DC converter which provides the output voltage of 6V with efficiency of 68%. Due to have an isolation transformer, this converter prevents electric shocks which makes it suitable for applications requiring more safety. All the theoretical analyses are verified by MATLAB and circuits are simulated in PSIM. In addition, two combinations of high voltage gain circuits are introduced for low power applications such as RFEH. The first combination consists of a phase shifter, 1-stage rectifier and resonant ZVS DC to DC converter which has an output voltage of 6V with an efficiency of 71%. The second consists of a phase shifter, 1-stage rectifier and isolated resonant ZVS DC to DC converter with output voltage and efficiency of 5V and 65%, respectively. In conclusion, this thesis is presented in 6 chapters discussing the designed high voltage gain high efficiency low power circuits to convert AC input with frequency of 915MHz to DC output. The circuits can be applied in different low power applications such as energy harvesting systems specifically RFEH.

Electronic devices capable of converting ambient vibration energy into useful electrical power are highly desirable for next generation of self-powered, wireless, and wearable devices. It enables them to work independently and with no need of replacement and management of batteries. Piezoelectric materials are of prime interest among researchers as successful candidate to act as the harvesting source for such devices. However, several challenges in the characterisation and energy extraction from ambient vibrations need to be addressed to enable successful commercial application. One of the key problems is a lack of dedicated measurement tools for complex characterisation of the electrical output when harvesting from structures that incorporate piezoelectric transducers. In addition, due to a complex mechanical-electrical form of operation of energy harvesters, their performance testing during operation is challenging as the functioning of the device depends on environmental parameters such as temperature, humidity, frequency, and excitation amplitude. To open up research on piezoelectric energy harvesters to a broader audience, the main issues and challenges are discussed, and potential solutions with respect to new measurement techniques, experimental setups and method for characterisation of harvesting devices are proposed in this thesis. This is realised via the development of a novel, adjustable measurement system capable of complex characterisation of energy harvesting devices at room temperature and at elevated temperatures as encountered in real life applications. Additionally, a new method for evaluation of piezoelectric energy harvester during its operation allowing for estimation of device core parameters and simple, yet effective performance characterisation is also proposed. In line with current world research in the field, significant improvements in the area of piezoelectric ceramics, piezoelectric composites and mechanical structures are also proposed. This is realised with a study on freeze-cast piezoelectric composites where complex porous structures were found to have enhanced energy harvesting figures of merit and increased piezoelectric coefficients, demonstrating improved effectiveness in energy harvesting system. Finally, a novel cantilever beam structure operating in bend-twisting mode for broadening of the available frequency bandwidth and increased power density of piezoelectric energy harvester is proposed. The work presented in this thesis further expands on the potential applications of freeze-cast porous piezoelectric materials, mechanical twist bending modes, and shows new opportunities for manufacturing of future electronic self-powered devices.

Graphene has the highest mobility of any material at room temperature; this property has attracted a great deal of interest for applications in high frequency electronics, specifically transistors and diodes. To date, there has been little success using graphene for these purposes because it lacks the bandgap necessary to create an efficient device. This work aims to approach this problem from a different angle; using device architecture that potentially does not need a bandgap. This could allow graphene's excellent electrical properties to be exploited fully. The first example of this is the ballistic rectifier; a device that exploits the long mean free path of two dimensional electron gasses so that carriers can be treated like "billiard balls". Here we demonstrate two different four-terminal ballistic rectifiers that redirect carriers from the two input leads to one of the two output leads; the effect of this is to rectify an AC signal into a DC signal. An extremely high voltage responsivity of 23,000 V/W and a very low noise equivalent power of 0.64 pW/Hz1/2 are achieved from a low-frequency AC signal at room temperature. This same device has been tested at 220GHz and showed no signs of a cut-off frequency. Another rectifier tested here is the self-switching diode, a device that uses two side gates attached to its own source to locally gate its own conducting channel. This architecture demonstrates a modest peak responsivity of 690 V/W, a result of graphene's missing bandgap. A side-gated transistor with a modest on/off ratio of ~2.33 is also fabricated in order to better understand the limited capabilities of the graphene self-switching diode. Part of the novelty of this work is the introduction of a modified stamp transfer technique that allowed more flexibility creating hetero-structures. A dry etching recipe for hetero-structures is introduced that does not damage soft masks allowing for a new type of ultra-clean 1D contact. This new contact demonstrates considerably better contact resistance and reliability than previous generations; important for any high frequency application.

With ample radio transmitters scattered throughout urban landscape, RF energy scavenging emerges as a promising approach to extract energy from propagating radio waves in the ambient environment to continuously charge low power electronics. With the ability of generating power from RF energy, the need for batteries could be eliminated. The effective distance of a RF energy scavenging system is highly dependent on its conversion efficiency. This results in significant limitations on the mobility and space requirement of conventional RF energy scavenging systems as they operate only in presence of physically large antennas and conversion circuits to achieve acceptable efficiency. This thesis presents a number of novel design strategies in the antenna and rectifier designs for miniaturized RF energy scavenging system. In the first stage, different energy scavenging systems including solar energy scavenging system, thermoelectric energy scavenging system, wind energy scavenging system, kinetic energy scavenging system, radio frequency energy scavenging system and hybrid energy scavenging system are investigated with regard to their principle and performance. Compared with the other systems, RF energy scavenging system has its advantages on system size and power density with relatively stable energy source. For a typical RF energy scavenging system, antenna and rectifier (AC-DC convertor) are the two essential components to extract RF energy and convert to usable electricity. As the antenna occupies most of the area in the RF energy scavenging system, reduction in antenna size is necessary in order to design a miniaturized system. Several antennas with different characteristics are proposed in the second stage. Firstly, ultra-wideband microstrip antennas printed on a thin substrate with a thickness of 0.2 mm are designed for both half-wave and full-wave wideband RF energy scavenging. Ambient RF power is distributed over a wide range of frequency bands. A wideband RF energy scavenging system can extract power from different frequencies to maximize the input power, hence, generating sufficient output power for charging devices. Wideband operation with 4 GHz bandwidth is obtained by the proposed microstrip antenna. Secondly, multi-band planar inverted-F antennas with low profile are proposed for frequency bands of GSM 900, DCS 1800 and Wi-Fi 2.4 GHz, which are the three most promising frequency bands for RF energy scavenging. Compared with previous designs, the triple band antenna has smaller dimensions with higher antenna gain. Thirdly, a novel miniature inverted-F antenna without empty space covering Wi-Fi 2.4 GHz frequency band is presented dedicated for indoor RF energy scavenging. The antenna has dimensions of only 10 × 5 × 3.5 mm3 with appreciable efficiency across the operating frequency range. In the final stage, a passive CMOS charge pump rectifier in 0.35 μm CMOS technology is proposed for AC to DC conversion. Bootstrapping capacitors are employed to reduce the effective threshold voltage drop of the selected MOS transistors. Transistor sizes are optimized to be 200/0.5 μm. The proposed rectifier achieves improvements in both power conversion efficiency and voltage conversion efficiency compared with conventional designs. The design strategies proposed in this thesis contribute towards the realization of miniaturized RF energy scavenging systems.

In industry, adjustable speed drives (ASDs) are widely employed in driving AC motors for variable speed applications due to the high performance and high energy efficiency obtained in such systems. However, ASDs have an impact on the power quality and utilisation of AC power feeds by injecting current harmonics and causing resonances, additional losses, and voltage distortion at the point of common coupling. Due to these problems, electric power utilities have established stringent rules and regulations to limit the effects of this distortion. As a result, efficient, reliable, and economical harmonic mitigation techniques must now be implemented in practical systems to achieve compliance at reasonable cost. A variety of techniques exist to control the harmonic current injected by ASDs, and allow three-phase AC-line-connected medium-power systems to meet stringent power quality standards. Of these, the broadband harmonic passive filter deserves special attention because of its good harmonic mitigation and reactive power compensation abilities, and low cost. It is also relatively free from harmonic resonance problems, has relatively simple structural complexity and involves considerably less engineering effort when compared to systems of single tuned shunt passive filters or active filters and active rectifier solutions. In this thesis, passive broadband harmonic filters are investigated. In particular, the improved broadband filter (IBF) which has superior overall performance and examples of its application are increasing rapidly. During this research project, the IBF operating principle is reviewed and its design principles are established. As the main disadvantage of most passive harmonic filters is the large-sized components, the first proposed design attempts to optimize the size of the filter components (L and C) utilized in the existing IBF topology. The second proposed design attempts to optimize the number and then the size of filter components resulting in an Advanced Broadband passive Filter (ABF) novel structure. The proposed design methods are based on frequency domain modelling of the system and then using a genetic algorithm optimization technique to search for optimal filter component values. The results obtained are compared with the results of a linear searching approach. The measured performance of the optimal filter designs (IBF and ABF) is evaluated under different loading conditions with typical levels of background voltage distortion. This involves assessing input current total harmonic distortion, input power factor, rectifier voltage regulation, efficiency, size and cost. The potential resonance problem is addressed and the influence of voltage imbalance on performance is investigated. The assessment is based on analysis, computer simulations and experimental results. The measured performance is compared to various typical passive harmonic filters for three-phase diode rectifier front-end type adjustable speed drives. Finally, the broadband filter design’s effectiveness and performance are evaluated by involving them in a standard IEEE distribution network operating under different penetration levels of connected nonlinear total loads (ASD system). The study is conducted via detailed modelling of the distribution network and the linked nonlinear loads using computer simulations.

More and more attention has been paid to the energy crisis due to the increasing energy demand from industrial and commercial applications. The utilisation of wind power, which is considered as one of the most promising renewable energy sources, has grown rapidly in the last three decades. In recent years, many power converter techniques have been developed to integrate wind power with the electrical grid. The use of power electronic converters allows for variable speed operation of wind turbines, and enhanced power extraction. This work, which is supported by EPSRC and Nheolis under the DHPA scheme, focuses on the design and analysis of control systems for wind power. In this work, two of the most popular AC-DC-AC topologies with permanent magnet synchronous generators (PMSG) have been developed. One consists of an uncontrollable rectifier, a boost converter and an inverter and a current control scheme is proposed to achieve the maximum power point tracking (MPPT). In the control strategy, the output current of the uncontrollable rectifier is controlled by a boost converter according to the current reference, which is determined by a climbing algorithm, to achieve MPPT. The synchronverter technology has been applied to control the inverter for the grid-connection. An experimental setup based on DSP has been designed to implement all the above mentioned experiments. In addition, a synchronverter-based parallel control strategy, which consists of a frequency droop loop and a voltage droop loop to achieve accurate sharing of real power and reactive power respectively, has been further studied. Moreover, a control strategy based on the synchronverter has been presented to force the inverter to have capacitive output impedance, so that the quality of the output voltage is improved. Abstract The other topology consists of a full-scale back-to-back converter, of which the rectifier is controllable. Two control strategies have been proposed to operate a three-phase rectifier to mimic a synchronous motor, following the idea of synchronverters to operate inverters to mimic synchronous generators. In the proposed schemes, the real power extracted from the source and the output voltage are the control variables, respectively, hence they can be employed in different applications. Furthermore, improved control strategies are proposed to self-synchronise with the grid. This does not only improve the performance of the system but also considerably reduces the complexity of the overall controller. All experiments have been implemented on a test rig based on dSPACE to demonstrate the excellent performance of the proposed control strategies with unity power factor, sinusoidal currents and good dynamics. Finally, an original control strategy based on the synchronverter technology has been proposed for back-to-back converters in wind power applications to make the whole system behave as a generator-motor-generator system.

Wireless power transfer has significantly improved the user experience in device charging and is currently an area of active research. Some major limitations of wireless charging systems include size constraints due to low frequency operation, the need for close proximity and tight alignment for efficient power transfer, and limited use of the device while charging. This work proposes several techniques to passively boost the voltage harvested and decrease the size of the power module. The rectifier and DC-DC converter explored in this dissertation extend technology proposed in existing literature that uses a resonator as the high quality factor component in the antenna matching network and energy storage element, respectively. This project proposes further development of these technologies by multiplexing the resonator between the rectifier and converter to reduce power module size and decrease generated electromagnetic interference by eliminating the need for magnetics. Preliminary testing has shown a resonator is more effective than an inductor (by a maximum of 30 percent) in boosting the rectifier output voltage and can boost the rectifier output voltage by a maximum of 8 times compared to an unmatched rectifier at 18 MHz on a commercial-grade printed circuit board. Initial DC-DC converter simulations with NMOS switches and realistic diodes show a 100 kHz resonator can store energy in a boost DC-DC converter on the order of nanojoules for a 10 V to 14 V step up conversion with 68 percent efficiency. This is similar in performance to a simulated inductor-based converter, which has a 78 percent efficiency when operating with the same step up ratio. The resonator-based DC-DC converter was built on a printed circuit board (PCB) using discrete transistors and a 185 kHz ceramic resonator. Simulations and measurements both showed the proposed discrete transistors did not effectively boost a 5 V input voltage by 1.4 times as was shown previously. Different methods to address this issue such as a floating driver and PMOS switches were proposed. An integrated design was also implemented using the open-source SkyWater 130 nm CMOS process design kit (PDK). A magnetoelectric (ME) antenna is proposed to decrease the size of the antenna by operating at acoustic resonance. Another benefit is more lenient alignment rules, which provide more device usage flexibility while charging. The maximum voltage output from the fabricated ME antenna is 0.3 V with a Qi transmitter producing 0.8 W as the source. The size of the antenna is 2 cm by 1.5 cm, which is smaller than the commercial Qi receiving antennas of 4 cm by 4 cm. When used to harvest vibrational energy, a slightly larger antenna (3 cm by 1.5 cm) could provide a maximum output power of 0.3 uW for an input vibration acceleration of approximately +/- 8 g.

Data centers are indispensable for today's computing and networking society, which has a considerable power consumption and significant impact on power system. Meanwhile, the average energy usage efficiency of data centers is still not high, leading to significant power loss and system cost. In this dissertation, effective methods are proposed to investigate the data center load characteristics, improve data center power usage efficiency, and reduce the system cost. First, a dynamic power model of a typical data center ac power system is proposed, which is complete and able to predict the data center's dynamic performance. Also, a converter-based data center power emulator serving as an all-in-one load is developed. The power emulator has been verified experimentally in a regional network in the HTB. Dynamic performances during voltage sag events and server load variations are emulated and discussed. Then, a gallium nitride (GaN) based critical conduction mode (CRM) totem-pole power factor correction (PFC) rectifier is designed as the single-phase front-end rectifier to improve the data center power distribution efficiency. Zero voltage switching (ZVS) modulation with ZVS time margin is developed, and a digital variable ON-time control is employed. A hardware prototype of the PFC rectifier is built and demonstrated with high efficiency. To achieve low input current total harmonic distortion (iTHD), current distortion mechanisms are analyzed, and effective solutions for mitigating current distortion are proposed and validated with experiments. The idea of providing reactive power compensation with the rack-level GaN-based front-end rectifiers is proposed for data centers to reduce data center's power loss and system cost. Full-range ZVS modulation is extended into non-unity PF condition and a GaN-based T-type totem-pole rectifier with reactive power control is proposed. A hardware prototype of the proposed rectifier is built and demonstrated experimentally with high power efficiency and flexible reactive power regulation. Experimental emulation of the whole data center system also validates the capability of reactive power compensation by the front-end rectifiers, which can also generate or consume more reactive power to achieve flexible PF regulation and help support the power system.

Among various energy harvesting technologies reverse electrowetting-on-dielectric energy harvesting (REWOD) has been proved to harvest energy from low frequency motion such as many human motion activities (e.g. walking, running, jogging etc.). Voltage rectification and DC-DC boosting of low magnitude AC voltage from REWOD can be used to reliably self-power the wearable sensors. In this work, a commercial component-based rectifier and DC-DC converter is designed and experimentally verified, for further miniaturization standard 180 nm CMOS process is used to design the rectifier and the DC-DC boost converter.This work also includes the MATLAB based model for REWOD energy harvester for various REWOD models. In REWOD energy harvesting, a mechanical input during the motion causes the electrolyte placed in between two dissimilar electrodes to squeeze back and forth thereby periodically changing the effective interfacial area, hence generating alternating current. The alternating current is given to the rectifier design. There is no realistic model that has been developed yet for this technique. Thereby, a MATLAB based REWOD model is developed for the realistic simulation of the REWOD phenomenon. In the work, a comparison of different REWOD models such as planar surface, rough surface and porous models are performed demonstrating the variations in capacitance, current and voltage.

Molybdenum (Mo) is an essential element that plays an important role in global nitrogen, carbon, and sulfur cycles with a critical role in human metabolism and ecological balance. It becomes catalytically active when complexed with the pyranopterin dithiolene ligand (PDT), forming the nearly ubiquitous molybdenum cofactor (Moco). The complex biosynthetic pathway of Moco, its presence in all three domains of life, and its role as a constituent cofactor in the last universal common ancestor (LUCA) all point toward the importance of the PDT in the development of life on Earth. Molybdoenzymes catalyze the two-electron oxidation or reduction of substrates that is usually coupled to an oxygen atom transfer. The PDT adopts several tautomeric and oxidation states in proteins, and this has been suggested to contribute to enzyme function through its ability to facilitate redox reactions by acting as an electron transfer conduit or a mediator of the enzyme redox potential. In the following studies, the role of the PDT in electron transfer and catalysis is investigated. In the first of these studies, the potential role of the PDT as a directional electron transfer conduit during catalysis is assessed; in this, we have provided the first evidence that the PDT can function as biological unimolecular rectifier. Unimolecular rectifiers were first proposed by Aviram and Ratner over 40 years ago, but true rectification has been difficult to achieve, with RRs rarely exceeding 10. Our results indicate that the PDT can achieve RRs ~2-10, depending upon the tautomeric/oxidation state of the ligand and its relative connectivity to the electrodes. These results may elucidate how directional electron transfer is achieved in a biological system. In the second of these studies, we sought an explanation for the conflicting crystallographic, kinetic, and spectroscopic data that has been published on the E. coli molybdoenzyme, MsrP. This enzyme functions as a periplasmic methionine sulfoxide reductase to “rescue” proteins damaged by oxidative stress from reactive chlorine species (RCS). The originally proposed active site of as-isolated MsrP, assigned by X-ray crystallography, is at odds with published spectroscopic data, and has led to the formulation of an unusual catalytic mechanism, in which the reducing equivalents necessary for catalysis are provided by the PDT ligand and not the metal. This mechanism, if correct, would represent a major paradigm shift from the accepted catalytic mechanisms of all other molybdoenzymes, in which the metal undergoes redox changes to provide the reducing equivalents necessary for catalysis. In this study, we provide compelling evidence that this mechanism and the originally proposed active site structure of as-isolated MsrP is incorrect. We provide evidence that as-isolated MsrP represents a thiol-inhibited species and reconciles the conflicting spectroscopic data. Lastly, we propose a new mechanism for the catalytic cycle of MsrP, in which the PDT does not undergo redox changes and the reducing equivalents are provided by Mo.

Abstract: The “Internet of Things” (IoT) promises to extend internet connectivity to billions of devices. A large number of these devices, such as wireless sensors, that will be wirelessly connected to the internet will not have a wired connection to the electricity grid relying on the energy stored in the batteries to power themselves. The battery power source is a bottleneck for the scaling of a wireless sensor network to thousands or millions of nodes due to its lifetime and capacity limitations. Energy harvesting presents a viable solution for powering at least some wireless sensors and devices. Ambient sources of energy such as solar, vibrational, thermal and radio frequency (RF) can be harvested and utilized. RF energy harvesting is one of the most popular energy scavenging methods because of its ever-expanding availability, integration capability, and compatibility with wireless networks. The major limitation of harvesting RF energy is the limited amount of the energy that can be scavenged due to the path loss, rapid attenuation of signals over distance, the low power efficiency of RF-DC converters, and the limited maximum allowed transmitted signal strength as restricted the regulations. To build a wireless sensor entirely powered by harvested RF energy, it is crucial to enhance the efficiency of RF rectifiers and to minimize the power consumption of the sensor circuitry and wireless transmitter that are required to transmit the sensed data to a reader. In this Ph.D. work, we addressed sensor challenges by designing a highly efficient RF energy harvester and an ultra-low-power wireless temperature solely powered by RF energy harvesting. In this dissertation, we first describe a wide input range, 4-stage threshold voltage compensated RF-to-DC power converter, designed to efficiently convert RF signals to dc voltages by applying an optimum compensation voltage produced by subthreshold auxiliary transistors. The proposed optimally compensated rectifiers achieve higher efficiency over a wider input power range compared to other threshold voltage compensation circuits, where the level of the compensation is limited by the circuit structure and varies with input power. This proposed compensation technique can be applied to a rectifier chain with a relatively small number of stages. Designed and implemented in 130-nm CMOS technology, the proposed rectifier exhibits a measured PCE of above 20% over the 8.5-dB input power range while driving a 1-MΩ load resistor at 896-MHz. To prove the feasibility that a wireless sensor can be entirely powered by RF energy harvesting, a fully integrated RF-powered temperature sensor with non-intermittent operation is presented. The proposed sensor is powered up wirelessly from a 915-MHz incident signal using a power-efficient RF energy harvester, uses a subthreshold ring oscillator that produces a highly temperature-dependent oscillation frequency acting as a temperature-to-frequency converter, and finally transfers the frequency-modulated signal to an external reader using back scattering. The power management circuits are eliminated in the designed sensor to arrive at a minimalistic design. For proper operation, a novel voltage regulator is developed that produces a relatively constant output voltage as the supply voltage of the ring oscillator for a large range of harvested input energy but allows the output voltage to change as a function of the temperature for the added temperature sensitivity of the overall sensor. The power consumption of the proposed sensor is only 1.05 µW at room temperature, which enables continuous operation of the sensor from an incident energy of −16 dBm. Fabricated in IBM’s 130-nm CMOS technology, the proposed sensor occupies a die area of 0.23 mm2.

This thesis presents a multi-stage rectifier for wireless power transfer in biomedical implant systems. The rectifier is built using Schottky diodes. The design has been simulated in 0.5µm and 130nm CMOS processes. The challenges for a rectifier in a wireless power transfer systems are observed to be the efficiency, output voltage yield, operating frequency range and the minimum input voltage the rectifier can convert. The rectifier outperformed the contemporary works in the mentioned criteria.

The call for a larger degree of engineering innovation grows as wireless power transfer increases in popularity. In this thesis, 6.78 MHz resonant wireless power transfer is explained. Challenges in WPT such as dynamic load variation and electromagnetic interference due to harmonic distortion are discussed, and a literature review is conducted to convey how the current state of the art is addressing these challenges.A GaN-based synchronous rectifier is proposed as a viable solution, and a model of the circuit is constructed. The precisely derived model is compared to a linearized model to illustrate the importance of exactness within the model derivation. The model is then used to quantify the design space of circuit parameters Lr and Cr with regard to harmonic distortion, input phase control, and efficiency. Practical design decisions concerning the 6.78 MHz system are explained. These include gate driver choice and mitigation of PCB parasitics. The model is verified with open loop experimentation using a linear power amplifier, FPGA, electronic load, and two function generators. Current zero-crossing sensing is then introduced in order to achieve self-regulation of both the switching frequency and input phase. The details of the FPGA code and sensing scheme used to obtain this closed loop functionality are described in detail. Finally, conclusions are drawn, and future work is identified.

Wireless power transfer (WPT) technology is becoming attractive in a wide variety of applications such as electric-vehicle charging, induction heating, charging portable applications, industrial robots, and biomedical implants. Recent studies have shown various techniques to implement wireless power transfer and these techniques differ based on the type of applications. For example, for electric vehicle charging, the power levels are in the range 5 kW to 25 kW and the operating frequency is in the range 70 kHz to 110 kHz. On the other hand, for consumer applications, the power levels vary from a few watts to hundreds of watts and operates at frequencies of the order of 5 MHz to 10 MHz. This thesis addresses the analysis, design, implementation, and simulation of a wireless charging system targeted towards a high-frequency, low-power portable application with wide separation between transmitter and receiver.The WPT system is composed of three important blocks: inverter (or transmitter), transformer (or coil), and rectifier (or receiver). Hard-switching inverters and rectifiers have major drawbacks at high frequencies due to large switching power loss. Therefore, soft-switching Class-E topology is chosen. The Class-E dc-ac inverter with CLL resonant tank, also referred to as pi2a impedance matching network is analyzed, designed, and simulated to observe its superior performance over other topologies at varying coupling coefficients and loads. Four soft-switching rectifier topologies are analyzed, designed, and simulated to evaluate their behavior at high frequencies. Their compatibility with Class-E inverters in the presence of loosely-coupled transformers is discussed. The physical and commercial limitations of using transformers with magnetic core is presented. Therefore, the preferred solution, an air-core transformer is designed and integrated with the rectifier to evaluate their characteristics at selected coupling coefficient. The overall system including the inverter, loosely-coupled air-core transformer, and rectifier was designed for the following specifications: operating frequency 6.78 MHz, output power across a single-load 40 W, output voltage 25 V, and target coupling coefficient of 0.5. Simulation results have been provided to validate the theoretical predictions. The major challenges faced during the integration of these building blocks are addressed. Finally, conclusions, contributions, and scope for future work are provided.

Despite tremendous progress over the years, current brain-machine interface (BMI) systems are relatively bulky, highly invasive, and limited in their effectiveness except for highly constrained tasks such as moving a cursor on a computer screen. To improve performance of current BMI systems, it is necessary to dramatically increase spatial resolution and coverage across the brain without constraining the mobility of the subject. This calls for innovative approaches to high-density integrated neural recording and stimulation using non-invasive or minimally invasive microelectrode and custom silicon integrated circuits at extreme energy and area efficiency. In this thesis, I present energy-efficient fully integrated miniaturized implants for electrocortical recording and stimulation, and unobtrusive body-area networks systems for subcutaneous power delivery and data communication, as fundamental building blocks to next generation BMI. First I describe a fully wireless, encapsulated neural interface and acquisition chip (ENIAC) in 180nm silicon-on-insulator (SOI) complementary metal-oxide-semiconductor (CMOS) technology for 16-channel neural recording and stimulation including integrated 4x4 electrode array, coil antenna, and wireless power transfer and data telemetry without any external components, completely contained in less than 3mm3 volume suitable for minimally invasive surgical insertion on the cortical surface. A novel fully integrated wireless power receiver design with an RF-decoupled H-tree signal distribution network delivers 1mW power over 1 cm distance while mitigating RF interference in the sensitive analog front-end and acquisition circuits for recording of electrocorticography (ECoG) signals transmitted through the skull. Second I highlight a 1mm2 16-channel neural recording and acquisition system-on-chip in 65nm CMOS offering 92 dB input dynamic range and

Abstract: The vision of realizing Internet of Things (IoT) pervasively connecting large number of sensors and devices that can sense and communicate is particularly attractive in today's world offering a wide range of applications. The scaling of wireless sensor network to thousands or millions of nodes is currently impractical if the energy required for the operation of these sensors is supplied by the batteries. The batteries have to be periodically recharged and replaced because of their limited storage capacities and lifetimes. Energy harvesting presents a viable solution for easily powering these sensor devices. The most common sources that can be harvested are light, vibration, thermoelectric, and radio frequency (RF) energy. Scavenging ambient energy from the available radio waves commonly referred as RF energy harvesting is one of the most popular energy excavating method. RF-powered devices have to extract energy from the received RF signals with typically low power densities as the signal is attenuated because of the multipath losses in the air owing to the large operating distance between the radiating source and the sensor nodes. Although RF-to-DC conversion can be done with high efficiency at large power levels, the conversion of low power RF signals to DC power remains very inefficient due to the threshold voltage requirement of the rectifying devices, and the losses due to the leakage of the stored energy making the amount of energy harvested from RF sources often insufficient for powering wireless sensors. This research is focused on the design of an integrated energy harvesting system in CMOS technology. The main objective of the research is to improve the sensitivity and RF-to-DC power conversion efficiency (PCE) of the RF rectifiers while providing a large output voltage from low received power levels. A hybrid forward and backward threshold-voltage compensated rectifier circuit is proposed employing PMOS transistors and “n” number of NMOS transistors for an n-level compensated multi-stage rectifier eliminating the need for triple-well technology while compensating the threshold voltage of all the rectifying transistors. The proposed hybrid rectifier circuit passively lowers the threshold voltage of the rectifying devices and largely improves the sensitivity of the rectifier circuit by increasing the level of compensation which improves the forward conduction. However, reduction in the threshold voltage of the reverse biased transistors leads to increased reverse leakage current degrading the rectifier's PCE. An adaptive rectifier scheme is proposed to efficiently convert RF signals to DC voltages utilizing minimum auxiliary PMOS transistors to control the threshold voltage of the transistors in the main rectifier chain dynamically. The proposed circuit adaptively reduces the threshold voltage of the forward biased transistors to increase the harvested power and the output voltage, and increases the threshold voltage of the reverse biased transistors to reduce the leakage current to prevent the loss of previously stored energy. Design strategy to optimize the impedance matching circuit of an RF energy harvester to maximize the harvested power for a range of received power levels with known probability density distributions is presented. Optimization of the RF energy harvesting circuit results in an increased harvested power and a large PCE for a defined range of received power levels. Prototypes of the conventional, hybrid, and adaptive rectifiers are designed and fabricated in IBM's 0.13 µm CMOS technology. Off-chip impedance matching circuits are implemented on a customized printed circuit board (PCB). The proposed adaptive threshold voltage compensated rectifier circuit achieves a maximum PCE of 32% at an input power of -15 dBm (32 µW) with an output DC voltage of 3.2 V for a 1 MΩ load. At a remarkably low input power of -21.6 dBm (6.9 µW) for a 1 MΩ load, the proposed hybrid rectifier circuit produces an output voltage of 1 V and achieves a maximum PCE of 22.6% at -16.8 dBm (20.9 µW) while delivering 2.2 V to the load.

Abstract: Over the past decade improvements in semiconductor technology have led to faster power electronic switches with higher ratings. These devices have been used to improve the performance of various power converter systems. In particular, the Voltage Source Converter (VSC) has been widely used in power systems to facilitate power flow control and improve power quality. The research described in this dissertation mainly relates to two important power quality applications of the VSC: the Static Synchronous Compensator (STATCOM) which performs power factor compensation (PFC), and the Active Power Filter (APF) which attenuates system harmonics. Both applications can be formulated as trajectory tracking problems for a nonlinear dynamic system, and the approaches taken here involve a direct model-based compensation for the nonlinearity in order to establish a formal statement regarding the stability of the closed-loop system. An emphasis of these designs is placed on adaptive control to directly account for uncertainties in the system parameter. First, the thesis focuses on the STATCOM application of the VSC. We present two different third order models based on different power balances. Using these models we propose a nonlinear flatness-based control which guarantees tracking of constant dc voltages and q-axis current. The particular novelty of the approach lies in the use of steady state relations to improve existing work. Next we present the state-of-the art in nonlinear adaptive control theory and apply an existing approach to PFC while dealing with uncertainty in two system parameters. We show that it is not possible to apply existing theory to an important choice of uncertainty in parameters. Hence, we propose two alternate adaptive designs which directly account for uncertainty in three parameters. The first design is a so-called adaptive PI vector control which is reminiscent of the classical vector control method. The other approach is the so-called full adaptive control. A Lyapunov function is derived to demonstrate these designs provide asymptotic convergence of the tracking error. Two designs are presented for the APF application: a traditional PI control and an adaptive control. The adaptive control shows improved tracking of power factor, harmonic compensation, and robustness to model error. In parallel to the control theory derived and simulation work, experimental validation was performed on a VSC test stand.

This thesis looks at a new design for a dual input bidirectional power converter (DIBPC) for charging and discharging a PHEV battery. The design incorporates a power factor correcting rectifier aimed at optimizing the battery charging efficiency from either a 120 VAC or 240 VAC source or discharging the battery to a usable AC voltage at 120 VAC. For simplicity and cost-effectiveness, the DIBPC is constructed using a standard IGBT 6-pack intended for motor control. The DIBPC is designed specifically to provide efficient operation with 120 VAC and 240 VAC inputs while achieving a very low THDI. The DIBPC also needs to be able to provide AC output power at 120 VAC with the flexibility to output at 240 VAC in the future. The DIBPC was tested first in simulation, and then in experimentation. The DIBPC consists of two portions, an AC/DC converter and a DC/DC converter. Although both were simulated, only the AC/DC converter was constructed. Testing under various load values and in each mode of operation provided ample data to show the DIBPC can meet all design goals. When operating as a rectifier, the DIBPC produces between 7.4% and 13.35% THDI and a DC voltage ripple of 8 VP-P or less at 400 VDC. At 120 VAC and 240 VAC an efficiency of 84.5% and 94.6% was achieved, respectively. When operating as an inverter, the DIBPC produces less than 6% THDV and 7% THDI, while outputting a voltage between 114 and 128 VRMS. Overall, the THDI in the charging mode easily meets and exceeds all standards and design constraints set forth, including IEC 61000-3-4. The efficiency with a 120 VAC input, however, is less than expected - about 84%.

Gaurav, Bawa. A Switched Capacitor based micro-stimulator for Deep Brain Stimulation. (under the direction of Dr. Maysam Ghovanloo and Dr. Leda Lunardi) This thesis presents a novel technique for the development of an implantable stimulator for Deep Brain Stimulation. The key idea is to harvest energy from an inductive link using an efficient switching mechanism at the secondary side. To validate the concept, extensive characterization was performed by simulating explicit differential equations in MATLAB in addition to a prototype implementation in standard CMOS technology. In principle, a zero current switching methodology at the secondary coil has proved to be extremely energy efficient due to no extra wastage of power in the parallel resonant circuits. A high efficiency full-wave rectifier has been implemented in standard CMOS technology, which ensures a measured efficiency of ~ 85 % while delivering ~ 19 mW of power to the load at 500 KHz input frequency. The underlying principle is that of synchronous rectification, minimization of MOS switch resistance through biasing in deep triode and blockage of reverse currents from the load to source using a lossless capacitive voltage divider technique. Dual-mode backtelemetry has been incorporated in a full-wave active rectifier with minimal area overhead and an increased data rate and reading range in measurement results, when the rectifier loading varies over time.

A rectifier is an electrical device, comprising one or more semiconductor devices arranged for converting alternating current to direct current by blocking the negative or positive portion of the waveform. The purpose of this study would be to evaluate dynamic and static electrical characteristics of rectifier chips fabricated with (a) DY8 process and (b) YI8 process and compare them with the existing DI8 process rectifiers. These new rectifiers were tested to compare their performance to meet or exceed requirements of lower forward voltages, leakage currents, reverse recovery time, and greater sustainability at higher temperatures compared to diodes manufactured using boron as base (DI8 process diodes) for similar input variables.

The purpose of this research has been to develop an agent based protection and reconfiguration scheme for power electronic building block based (PEBB) DC distribution systems. One of the foremost applications would be in the new zonal DC distribution on naval ships. The research involves the design of an agent based protection scheme which uses the PEBBs for current limiting and circuit breaking purposes. Considerations are given to reduce the system downtime under fault conditions, allow proper coordination and provide backup protection. The research also involves the design of a reconfiguration management scheme based on collaborative agents. The collaboration ensures that the reconfiguration is achieved at a global level, enhancing the system survivability under the conditions of multiple faults and damages. The coordination ensures that only the faulted part of the system is isolated and the reconfiguration makes sure that the power to the healthy part of the system is supplied continuously. The reconfiguration management also performs load shedding if the generation does not meet the load demand of the reconfigured system due to a fault or damage in the generator.

The development of a distribution system simulator that can model new power electronic devices as well as new novel distribution schemes, such as DC distribution has been donehere. The simulator adopts the Electro-Magnetic Transient Programs (EMTP) platform to facilitate the simulation. Basic power electronic building blocks have been developed to extend the capabilities of the EMTP. These blocks include a rectifier module, a DC buck converter module, a 3-phase inverter module and a single-phase inverter module. The paper shows simulation of a new distribution scheme for naval ships to illustrate that such a simulator facilitates the study of new distribution system designs, especially theprotection and control issues associated with new designs.

Power electronics building blocks (PEBBs) are standardized building blocks used to integrate power electronics systems. The PEBB approach can achieve low cost, high redundancy, high reliability, high flexibility and easy maintenance for large-scale power electronics systems. This thesis presents the implementation of a 100kW PEBB-based soft-switched bus regulator for an 800V DC distributed power system. The zero current transition (ZCT) soft-switching technique is used to improve the performance of the bus regulator by minimizing switching loss and improving overall efficiency. PEBB modules and a digital control building block are the subsystems of the DC bus regulator. This thesis addresses the design issues at subsystem and system levels. These include: operational principles and design of ZCT PEBB modules; design and implementation of the digital control block, based on DSP and EPLD; and modeling and control design of the DC bus regulator. There are several considerations when using the ZCT soft-switching technique in three-phase applications: the timing of the auxiliary switch gate signals must be arranged differently; there are low-frequency harmonics caused by the pulse width limits; and there is high thermal stress on the resonant capacitors. These issues are resolved by utilizing the sensed phase current information and the design freedom in the PWM modulator. A PWM modulation technique is proposed that can considerably reduce the switching events and further remove the associated loss while keeping THD low. Reduced switching events alleviate the thermal issue of the resonant capacitors. The same modulation technique can avoid the low-frequency harmonics caused by the pulse width limits and double the sampling frequency. The phase current information is used to deal with the control timing issue of the auxiliary switches and to control the three-phase soft-switching operation in order to achieve better efficiency. Additionally, the phase current information is used to implement dead time compensation to reduce THD. The soft-switched DC bus regulator has been tested up to a 100kW power level with 20kHz switching frequency. Experimental results demonstrate that high performance of the DC bus regulator is accomplished in terms of wide control bandwidth, low THD, unity power factor, high efficiency and high power density.

This dissertation covers three levels of issues and solutions dealing with unbalanced and/or nonlinear situations in power electronic systems, namely power converter level, power converter system level, and large-scale power electronics system level. At power converter level, after review of traditional PWM methods, especially two-dimensional space vector modulation schemes, three-dimensional space vector modulation schemes are proposed for four-legged voltage source converters, including inverters and rectifiers. The four-legged power converters with three-dimensional space vector modulation schemes have a better DC link voltage utilization and result in a low distortion. It is an effective solution to provide the neutral point for a three-phase four-wire system and to handle the neutral current due to unbalanced load or source and nonlinear loads. Comprehensive design guidelines for a four-legged inverter are presented. The four-legged rectifier is also presented which allows not only fault tolerant operation, but also provides the flexibility of equal resistance, equal current, or equal power operation under unbalanced source. Average large-signal models of four-legged power converters in both the a-b-c and d-q-o coordinates are derived. Small signal models are obtained in the d-q-o rotating coordinates. Voltage control loops are designed in the d-q-o coordinates for a high power utility power supply. Performance is studied under various load conditions. At the power converter system level, the load conditioner concept is proposed for high power applications. A power converter system structure is proposed which consists of a high-power low-switching frequency main inverter and a low-power high-switching frequency load conditioner. The load conditioner performs multiple functions, such as active filtering, active damping, and active decoupling with a high current control bandwidth. This hybrid approach allows the overall system to achieve high performance with high power and highly nonlinear loads. At the large-scale power electronics system level, the nonlinear loading effect of load converters is analyzed for a DC distribution system. Two solutions to the nonlinear loading effect are presented. One is to confine the nonlinear load effect with the sub-converter system, the other is to use a DC bus conditioner. The DC bus conditioner is the extension of the load conditioner concept.

A modified space vector modulation algorithm for a zero-voltage transition three-phase voltage source inverter/boost rectifier is presented. The converter is intended for high performance medium power applications requiring bi-directional power flow. The proposed modified space vector modulation allows the main switches to be operated with constant frequency and soft switching for any phase shift between the three-phase currents and voltages. The modulation algorithm also eliminates any low frequency distortion caused by the zero-voltage transition and can be extended to other soft-switching PWM three-phase converters. Experimental verification of the proposed algorithm is also presented as well as a comparison to the hard switched PWM converter.