Browsing by Author "Ngo, Khai D."
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- Analysis and Design of a DCM SEPIC PFC with Adjustable Output VoltageChen, Rui (Virginia Tech, 2015-03-31)Power Factor Correction rectifiers are widely adopted as the first stage in most grid-tied power conversion systems. Among all PFC converts for single phase system, Boost PFC is the most popular one due to simplicity of structure and high performance. Although the efficiency of Boost PFC keeps increasing with the evolution of semiconductor technology, the intrinsic feature of high output voltage may result cumbersome system structure with multiple power conversion stages and even diminished system efficiency. This disadvantage is aggravated especially in systems where resonant converters are selected as second stage. Especially for domestic induction cooker application, step-down PFC with wide range output regulation capability would be a reasonable solution, Conventional induction cooker is composed by input filter, diode-bridge rectifier, and full bridge or half bridge series resonant circuit (SRC). High frequency magnetic field is induced through the switching action to heat the pan. The power level is usually controlled through pulse frequency modulation (PFM). In such configuration, first, a bulky input differential filter is required to filter out the high frequency operating current in SRC. Second, as the output power decreases, the operating point of SRC is moved away from the optimum point, which would result large amount circulating energy. Third, when the pan is made of well conducting and non-ferromagnetic material such as aluminum, due to the heating resistance become much smaller and peak output voltage of the switching bridge equals to the peak voltage of the grid, operating the SRC at the series resonant frequency can result excessive current flowing through the switch and the heating coil. Thus for pan with smaller heating resistance, even at maximum power, the operating frequency is pushed further away from the series resonant point, which also results efficiency loss. To address these potential issues, a PFC circuit features continuous conducting input current, high power factor, step-down capability and wide range output regulation would be preferred. The Analysis and design work is present in this article for a non-isolated hard switching DCM SEPIC PFC. Due to DCM operation of SPEIC converter, wide adjustable step-down output voltage, continuous conduction of input current and elimination of reverse recovery loss can be achieved at same time. The thesis begins with circuit operation analysis for both DC-DC and PFC operation. Based on averaged switching model, small signal model and corresponding transfer functions are derived. Especially, the impact from small intermediate capacitor on steady state value are discussed. With the concept of ripple steering, theoretic analysis is applied to SEPIC converter with two coupled inductors. The results indicate if the coupling coefficient is well designed, the equivalent input inductance can be multiple times larger than the self-inductance. Because of this, while maintaining input current ripple same, the two inductors of SEPIC can be implemented with two smaller coupled inductors. Thus both the total volume of inductors and the total number of windings can be reduced, and the power density and efficiency can be improved. Based on magnetic reluctance model, a corresponding winding scheme to control the coupling coefficient between two coupled inductors is analyzed. Also the impact of coupled inductors on the small signal transfer function is discussed. For the voltage follower control scheme of DCM PFC, single loop controller and notch filter design are discussed. With properly designed notch filter or the PR controller in another word, the closed loop bandwidth can be increased; simple PI controller is sufficient to achieve high power factor; THD of the input current can be greatly reduced. Finally, to validate the analysis and design procedure, a 1 kW prototype is built. With 120 Vrms AC input, 60V to 100V output, experimental results demonstrate unity power factor, wide output voltage regulation can be achieved within a single stage, and the 1 kW efficiency is around 93%.
- Behavioral EMI-Models of Switched Power ConvertersBishnoi, Hemant (Virginia Tech, 2013-11-05)Measurement-based behavioral electromagnetic interference (EMI) models have been shown earlier to accurately capture the EMI behavior of switched power converters. These models are compact, linear, and run in frequency domain, enabling faster and more stable simulations compared to the detailed lumped circuit models. So far, the behavioral EMI modeling techniques are developed and applied to the converter's input side only. The resulting models are therefore referred to as "terminated EMI models". Under the condition that the output side of the converter remains fixed, these models can predict the input side EMI for any change in the impedance of the input side network. However, any change at the output side would require re-extraction of the behavioral model. Thus the terminated EMI models are incapable of predicting the change in the input side EMI due to changes at the output side of the converter or vice versa. The above mentioned limitation has been overcome by an "un-terminated EMI model" proposed in this dissertation. Un-terminated EMI models are developed here to predict both the common-mode (CM) and the differential (DM) noise currents at the input and the output sides of a motor-drive system. The modeling procedure itself has been simplified and now requires fewer measurements and results in less noise in the identified model parameters. Both CM and DM models are then combined to predict the total noise in the motor drive system. All models are validated by experiments and their limitations identified. A significant portion of this dissertation is then devoted to the application of behavioral EMI models in the design of EMI filters. Comprehensive design procedures are developed for both DM and CM filters in a motor-drive system. The filters designed using the proposed methods are experimentally shown to satisfy the DO-160 conducted emissions standards. The dissertation ends with a summary of contributions, limitations, and some future research directions.
- Circuits and Modulation Schemes to Achieve High Power-Density in SiC Grid-connected ConvertersOhn, Sungjae (Virginia Tech, 2019-05-16)The emergence of silicon-carbide (SiC) devices has been a 'game changer' in the field of power electronics. With desirable material properties such as low-loss characteristics, high blocking voltage, and high junction temperature operation, they are expected to drastically increase the power density of power electronics systems. Recent state-of-the-art designs show the power density over 17 ; however, certain factors limit the power density to increase beyond this limit. In this dissertation, three key factors are selected to increase the power density of SiC-based grid-connected three-phase converters. Throughout this dissertation, the techniques and strategies to increase the power density of SiC three-phase converters were investigated. Firstly, a magnetic integration method was introduced for the coupled inductors in the interleaved three-phase converters. Due to limited current-capacity compared to the silicon insulated-gate bipolar transistors (Si-IGBTs), discrete SiC devices or SiC modules, operate in parallel to handle a large current. When three-phase inverters are paralleled, interleaving can be used, and coupled inductors are employed to limit the circulating current. In Chapter 2, the conventional integration method was extended to integrate three coupled inductors into two; one for differential-mode circulating current and the other for common-mode circulating current. By comparing with prior research work, a 20% reduction in size and weight is demonstrated. From Chapter 3 to Chapter 5, a full-SiC uninterruptible power supply (UPS) was investigated. With the high switching frequency and fast switching dynamics of SiC devices, strategies on electromagnetic inference become more important, compared to Si-IGBT based inverters. Chapter 3 focuses on a common-mode equivalent circuit model for a topology and pulse width modulation (PWM) scheme selection, to set a noise mitigation strategy in the design phase. A three terminal common-mode electromagnetic interference (EMI) model is proposed, which predicts the impact of the dc-dc stage and a large battery-rack on the output CM noise. Based on the model, severe deterioration of noise by the dc-dc stage and battery-rack can be predicted. Special attention was paid on the selection of the dc-dc stage's topology and the PWM scheme to minimize the impact. With the mitigation strategy, a maximum 16 dB reduction on CM EMI can be achieved for a wide frequency range. In Chapter 4, an active PWM scheme for a full-SiC three-level back-to-back converter was proposed. The PWM scheme targets the size reduction of two key components: dc-link capacitors and a common-mode EMI filter. The increase in switching frequency calls for a large common-mode EMI filter, and dc-link capacitors in the three-level topology may take a considerable portion in the total volume. To reduce the common-mode noise emission, different combinations of the voltage vectors are investigated to generate center-aligned single pulse common-mode voltage. By such an alignment of common-mode voltage with different vector combinations, noise cancellation between the rectifier and the inverter can be maximally utilized, while the balancing of neutral point voltage can be achieved by the transition between the combinations. Also, to reduce the size of the dc-link capacitor for the three-level back-to-back converter, a compensation algorithm for neutral point voltage unbalance was developed for both differential-mode voltage and the common-mode voltage of the ac-ac stage. The experimental results show a 4 dB reduction on CM EMI, which leads to a 30% reduction on the required CM inductance value. When a 10% variation of neutral point voltage can be handled, the dc-link capacitance can be reduced by 56%. In Chapter 5, a 20 kW full-SiC UPS prototype was built to demonstrate a possible size-reduction with the proposed PWM scheme, as well as a selection of topologies and PWM schemes based on the model. The power density and efficiency are compared with the state-of-the-art Si-IGBT based UPSs. Chapter 6 seeks to improve power density by a change in a modulation method. Triangular conduction mode (TCM) operation of the three-level full-SiC inverter was investigated. The switching loss of SiC devices is reported to be concentrated on the turn-on instant. With zero-voltage turn-on of all switches, the switching frequency of a three-level three-phase SiC inverter can be drastically increased, compared to the hard-switching operation. This contributes to the size-reduction of the filter inductors and EMI filters. Based on the design to achieve a 99% peak efficiency, a comparison was made with a full-SiC three-level inverter, operating in continuous conduction mode (CCM), to verify the benefit of the soft switching scheme on the power density. A design procedure for an LCL filter of paralleled TCM inverters was developed. With 3.5 times high switching frequency, the total weight of the filter stage of the TCM inverter can be reduced by 15%, compared to that of the CCM inverter. Throughout this dissertation, techniques for size reduction of key components are introduced, including coupled inductors in parallel inverters, an EMI filter, dc-link capacitors, and the main boost inductor. From Chapter 2 to 5, the physical size or required value of these key components could be reduced by 20% to 56% by different schemes such as magnetic integration, EMI mitigation strategy through modeling, and an active PWM scheme. An optimization result for a full-SiC UPS showed a 40% decrease in the total volume, compared to the state-of-the-art Si-IGBT solution. Soft-switching modulation for SiC-based three-phase inverters can bring a significant increase in the switching frequency and has the potential to enhance power-density notably. A three-level three-phase full-SiC 40 kW PV inverter with TCM operation contributed to a 15% reduction on the filter weight.
- Computer-Aided Formulation of Magnetic Pastes for Magnetic Components in Power ElectronicsDing, Chao (Virginia Tech, 2021-05-25)Magnetic components are necessary for switch-mode power electronics converters, but they are often the bulkiest and heaviest in the system. Novel magnetic designs with intricate structures lead to the size reduction of power electronics converters but pose challenges to the fabrication process and material availability. Because of their low-temperature and pressure-less process-ability, magnetic pastes would be the material of choice to make magnetic cores with complex geometries. However, most magnetic pastes reported in the literature suffer from low relative permeability (µr < 26) due to the low magnetic fraction limited by viscosity. The conventional approach of developing magnetic pastes involves experimental iterations with trial-and-error efforts to determine the optimal compositions. To shorten the development cycle and take advantage of the computational power in the current age, this work focuses on exploring, validating, and demonstrating a computer-aided methodology to correlate material's processing, microstructure, and property to guide the development of magnetic pastes. The discrete element method (DEM) simulation was explored to create materials' microstructure and the finite element method (FEM) simulation was utilized to study the magnetic permeability based on the microstructure created by DEM or taken from an actual material sample. The combination of DEM and FEM provided the linkage among processing-microstructure-property relations. Then, the methodology was verified and demonstrated by improving a starting formulation. The formulation was modeled with DEM based on multiple variables, e.g., particle shape, size, size distribution, mixing ratio, gap, gap distribution, magnetic volume fraction, etc. The optimal mixing ratio of different powders to achieve the maximum magnetic fraction was determined by DEM. Experimental results confirmed the predicted optimal mixing ratio. To further take advantage of the computational tools, the magnetic permeability of the magnetic pastes was computed by FEM based on the DEM-generated microstructures. The effects of powder mixing ratio and magnetic volume fraction on the magnetic permeability were studied, respectively. Compared with the experimental values, the microstructure-based FEM simulations could predict the magnetic permeability of the formulations with varied powder mixing ratios or magnetic volume fractions with an average error of only 10 %. Another critical aspect of employing magnetic pastes for magnetic components in power electronics is capable of tailoring their magnetic permeability to meet different design needs. The methodology was further verified and demonstrated by guiding the selection of composition parameters for tailorable magnetic permeability of a starting formulation with flaky particles. An FEM model was constructed from a microstructural image and varied parameters were explored (particle permeability, matrix permeability, particle volume fraction, etc.) to tailor the magnetic permeability. To verify the simulated results, a set of magnetic pastes with various volume fractions of flakes was prepared experimentally and characterized for their permeability. Comparing the simulated and measured permeability, the error was found to be less than 10 %. Last, the guideline was demonstrated to predict a material composition to achieve a target relative permeability of 30. From the predicted composition, the magnetic paste was prepared and characterized. The error between experimental permeability and the target was only 5 %. With the guideline, one can formulate magnetic pastes with tailorable permeability with minimal experimental effort and select the composition parameters to achieve a target permeability. After developing a series of magnetic pastes with tailorable permeability and a maximum value of 35, the feasibility of making magnetic components with magnetic pastes was demonstrated. The commonly used magnetic cores – C-core, E-core, toroid core, bar core, and plate core were fabricated by a low-temperature (< 200 °C) and pressure-less molding process. Several innovative magnetic components with intricate core structures were also fabricated to demonstrate the shape-forming flexibility. The magnetic paste can also be used as the feedstock for paste-extrusion-based additive manufacturing, which further enhances the shape-forming capability. For demonstration, a multi-permeability core was fabricated by 3D printing the magnetic pastes with tailored permeability. The feasibility of making high-performance magnetic components by additive manufacturing or low-temperature pressure-less molding of magnetic pastes opens the door to power electronics researchers to explore more innovative magnetic designs to further improve the efficiency and power density of the power electronics converters.
- Design and Processing of Ferrite Paste Feedstock for Additive Manufacturing of Power Magnetic ComponentsLiu, Lanbing (Virginia Tech, 2020-06-19)Reducing the size of bulky magnetic components (inductors and transformers) in power converters can be achieved by increasing switching frequency and applying innovative designs of magnetic components. Ferrite is the most suitable bulk magnetic material for working at high frequencies but it is difficult to fabricate novel designs of ferrite magnetic components because of the limitations of conventional fabrication methods. Additive manufacturing (AM) has the potential to make customize ferrite magnetic components. One big challenge in 3D printing ferrite magnetic components is the lack of compatible and functional ferrite materials as printers' feedstock. This work focuses on developing ferrite feedstock for 3D printing ferrite magnetic components and providing a guideline for formulating ferrite feedstock by studying the effects of materials and processing parameters on major properties of the ferrite feedstock. The ferrite feedstock should not only be processable by a 3D printer but also make functional ferrite material that can work in power converters. To meet the requirements, the following four aspects of the feedstock are considered in this study: 1. the feedstock should be sinterable to achieve high enough magnetic permeability; 2. magnetic permeability of the feedstock can be easily tailored; 3. rheological properties of the feedstock should ensure reasonable printing resolution; 4. the feedstock can print high aspect ratio structures without slumping. Based on the four major considerations and the desired properties, materials were selected for formulating the ferrite feedstock. The effects of materials and processing variables on the major properties of the ferrite feedstock need to be studied to develop a formulation guidance of the feedstock. The effects of materials fractions and the post-printing peak sintering temperature of the feedstock on maximizing magnetic permeability were studied. The peak sintering temperature had a significant impact on permeability and solid loading (SL) and solid loading excluding diluent (SLED) had smaller impacts. Densities and microstructures of the sintered ferrite cores were characterized to illustrate how the variables affect magnetic permeability. Adding sintering additives to the feedstock was selected as an easy and effective way to tailor the permeability of the ferrite feedstock. The effect of the fractions of two types of additives, SiO2 and Co3O4, on permeability of ferrite were studied. Both SiO2 and Co3O4 can effectively reduce the permeability of the ferrite. A novel multi-permeability toroid core design was 3D-printed with ferrite feedstocks having different fractions of SiO2 to demonstrate the feasibility of fabricating special designs of ferrite magnetics using feedstocks with additives. Core-loss densities of ferrite cores fabricated with feedstocks having different fractions of the two additives were also characterized since it is another important property of ferrite cores in high-frequency converters. Adding SiO2 significantly increases the core-loss density of ferrite cores while adding proper fractions of Co3O4 decreased core-loss density at low magnetic flux densities. The mechanisms of how Co3O4 affect permeability and core-loss density were discussed. The effect of the solid loading (SL) on print-line width resolution was studied by conducting line printing tests. The experiment results showed the best print-line width resolution was achieved using the feedstock with an intermediate SL. The is, which considered both viscosity of the feedstock and coagulation in the feedstock suspension, were discussed. The effect of solid loading excluding diluent (SLED) and UV illumination time on the achievable aspect ratio of printed feedstock was studied. Yield shear strength (y) of feedstocks composition versus UV-curing time were characterized. We evaluated various phenomenological models reported in the literature for predicting the critical yield shear strength (y*) required to obtain a paste structure for a certain aspect ratio. Knowing y* would help to determine the shortest time needed for UV illumination. Applying the model that best fitted to our experimental results, we developed a processing guideline that from specified magnetic permeability and dimensions of a ferrite core, would prescribe the needed SLED and the minimal UV curing time for printing. The guideline was demonstrated by the successful fabrication of tall ferrite inductor cores commonly found in power converters. The main contributions of this study are listed below: 1. Designed, formulated, and characterized ferrite feedstock that not only has functionality for power electronics applications but is also compatible with a direct extrusion type 3D printer. The feedstock can be made into ferrite cores with relative permeability ranging from 10 to 500 which are much higher than those of soft ferrite feedstocks currently reported elsewhere. The packing densities of 950℃ sintered ferrite cores made from the feedstock can be as high as 95%. With the Hyrel 30M 3D-printer, the smallest nozzle orifice diameter that the feedstock can be extruded from is 0.42 mm. We demonstrated printing of the feedstock into a cylinders with a height of 18 mm and an aspect ratio of 3 without slumping issue. 2. Identified the effects of materials and processing variales on 4 major considerations of the ferrite feedstock including maximizing sintered packing density, tailoring permeability, print-line resolution, and achievable dimensions of the printed feedstock without slumping. A deeper understanding of the mechanisms of how the variables affect main properties of the feedstock was provided. 3. Provided a preparation guideline of the ferrite feedstock that prescribe feedstock formulation and UV illumination time per print-layer from the target relative permeability and dimension of a ferrite core.
- Design Methodology and Materials for Additive Manufacturing of Magnetic ComponentsYan, Yi (Virginia Tech, 2017-04-11)Magnetic components such as inductors and transformers are generally the largest circuit elements in switch-mode power systems for controlling and processing electrical energy. To meet the demands of higher conversion efficiency and power density, there is a growing need to simplify the process of fabricating magnetics for better integration with other power electronics components. The potential benefits of additive manufacturing (AM), or more commonly known as three-dimensional (3D) printing technologies, include shorter lead times, mass customization, reduced parts count, more complex shapes, less material waste, and lower life-cycle energy usage—all of which are needed for manufacturing power magnetics. In this work, an AM technology for fabricating and integrating magnetic components, including the design of manufacturing methodology and the development of the feedstock material, was investigated. A process flow chart of additive manufacturing functional multi-material parts was developed and applied for the fabrication of magnetic components. One of the barriers preventing the application of 3D-printing in power magnetics manufacturing is the lack of compatible and efficient magnetic materials for the printer's feedstock. In this work, several magnetic-filled-benzocyclobutene (BCB) pastes curable below 250 degree C were formulated for a commercial multi-material extrusion-based 3D-printer to form the core part. Two magnetic fillers were used: round-shaped particles of permalloy, and flake-shaped particles of Metglas 2750M. To guide the formulation, 3D finite-element models of the composite, consisting of periodic unit cells of magnetic particles and flakes in the polymer-matrix, was constructed. Ansoft Maxwell was used to simulate magnetic properties of the composite. Based on the simulation results, the pastes consisted of 10 wt% of BCB and 90 wt% of magnetic fillers—the latter containing varying amounts of Metglas from 0 to 12.5 wt%. All the pastes displayed shear thinning behavior and were shown to be compatible with the AM platform. However, the viscoelastic behavior of the pastes did not exhibit solid-like behavior, instead requiring layer-by-layer drying to form a thick structure during printing. The key properties of the cured magnetic pastes were characterized. For example, bulk DC electrical resistivity approached 107 Ω⋅cm, and the relative permeability increased with Metglas addition, reaching a value of 26 at 12.5 wt%. However, the core loss data at 1 MHz and 5 MHz showed that the addition of Metglas flakes also increased core loss density. To demonstrate the feasibility of fabricating magnetic components via 3D-printing, several inductors of differing structural complexities (planar, toroid, and constant-flux inductors) were designed. An AM process for fabricating magnetic components by using as-prepared magnetic paste and a commercial nanosilver paste was developed and optimized. The properties of as-fabricated magnetic components, including inductance and DC winding resistance, were characterized to prove the feasibility of fabricating magnetic components via 3D-printing. The microstructures of the 3D-printed magnetic components were characterized by Scanning-electron-microscope (SEM). Results indicate that both the winding and core magnetic properties could be improved by adjusting the formulation and flow characteristics of the feed paste, by fine-tuning printer parameters (e.g., motor speed, extrusion rate, and nozzle size), and by updating the curing profile in the post-process. The main contributions of this study are listed below: 1. Developed a process flow chart for additive manufacturing of functional multi-material components. This methodology can be used as a general reference in any other research area targeting the utilization of AM technology. 2. Designed, formulated and characterized low-temperature curable magnetic pastes. The pastes are physically compatible with the additive manufacturing platform and have applications in the area of power electronics integration. 3. Provided an enhanced understanding of the core-loss mechanisms of soft magnetic materials and soft magnetic composites at high frequency applications.
- Design of DC-DC converters using Tunable Piezoelectric TransformersKhanna, Mudit (Virginia Tech, 2017-06-26)This thesis introduces the ‘tunable’ piezoelectric transformers (TPT) which provide an extra control terminal, used in this case, to regulate the output voltage. A detailed mathematical analysis is done on the electrical equivalent circuit of the TPT to understand the effect of control terminal loading on the circuit performance. Based on this analysis, a variable capacitor connected across the control terminal is proposed to regulate the output voltage for line and load variations is suggested. The concept of ‘tunability’ in a TPT is introduced and mathematical conditions are derived to achieve the required ‘tunability’. This analysis can help a TPT designer to design the TPT for a specific application and predict the load and line regulations limits for a given design. A circuit implementation of the variable capacitor, intended for control, is presented. With the proposed control circuit design, the effective value of a fixe capacitor can be controlled by controlling the duty cycle of a switch. Hence, this enables pulse width modulated (PWM) control for the TPT based converter operating at a constant frequency. Fixed frequency operation enables a high efficiency operation of TPT near its resonant frequency and the complete secondary control requires no isolation in the voltage feedback and control circuit. This prevents any ‘cross-talk’ between primary and secondary terminals and reduces the component count. The design of series input inductor for achieving zero voltage switching (ZVS) in the inverter switches for the new control is also discussed. Experimental results for two different TPT designs are presented. Their differences in structure and its effect on the circuit performance has been discussed to support the mathematical analysis.
- Design of Power Converter and Wireless Data Acquisition System for TEG Energy HarvesterXing, Shaoxu (Virginia Tech, 2016-11-01)In order to avoid the accidents like Fukushima Disaster and monitor the operation status of nuclear power plant, a wireless sensor system which is powered by the Thermoelectric Generator (TEG) Energy Harvester is designed and built. Meanwhile, a power converter circuit has also been designed to converter the output voltage of TEG Energy Harvester to a DC voltage to charge the battery or power the application systems. Several prototypes based on this power converter circuit have been built for Thermoelectric Generator (TEG) energy harvester and tested in both working and laboratory conditions. The reliability of the TEG Energy Harvester system in the gamma radiation environment has been examined in the experiments. Based on the experiments results, the design was optimized. And an optimized Maximum Power Point Tracking algorithm has also been applied in the prototype to extract the maximum power from the TEG Energy Harvester in all conditions. The TEG Energy Harvester system would be greatly simplified as a new type of sensor will be applied. The design of the signal conditioning circuit for this sensor has also been presented.
- Design Optimization of Hybrid Switch Soft-Switching Inverters using Multi-Scale Electro-Thermal SimulationReichl, John Vincent (Virginia Tech, 2015-11-17)The development of a fully automated tool that is used to optimize the design of a hybrid switch soft-switching inverter using a library of dynamic electro-thermal component models parameterized in terms of electrical, structural and material properties is presented. A multi-scale electro-thermal simulation approach is developed allowing for a large number of parametric studies involving multiple design variables to be considered, drastically reducing simulation time. Traditionally, electro-thermal simulation and analysis has been used to predict the behavior of pre-existing designs. While the traditional approach to electro-thermal analysis can help shape cooling requirements and heat sink designs to maintain certain junction temperatures, there is no guarantee that the design under study is the most optimal. This dissertation uses electro-thermal simulation to guarantee an optimal design and thus truly minimizing cooling requirements and improving device reliability. The proposed optimization tool is used to provide a step-by-step design optimization of a two-coupled magnetic hybrid soft-switching inverter. The soft-switching inverter uses a two-coupled magnetic approach for transformer reset condition [1], a variable timing control for achieving ZVS over the entire load range [2], and utilizes a hybrid switch approach for the main device [3]. Design parameters such as device chip area, gate drive timing control and external resonant capacitor and inductor are used to minimize device loss subject to design constraints such as converter minimum on-time, maximum device chip area, and transformer reset condition. Since the amount of heat that is dissipated has been minimized, the optimal cooling requirements can be determined by reducing the cooling convection coefficients until desired junction temperatures are achieved. The optimized design is then compared and contrasted with an already existing design from the Virginia Tech freedom car project using the generation II module. It will be shown that the proposed tool improves the baseline design by 16% in loss and reduces the cooling requirements by 42%. Validation of the device model against measured data along with the procedures for device parameter extraction is also provided. Validation of the thermal model against measured data is also provided.
- Design, Analysis and Experimental Verification of a Mechanically Compliant Interface for Fabricating Reliable, Double-Side Cooled, High Temperature, Sintered Silver Interconnected Power ModulesBerry, David W. (Virginia Tech, 2014-09-08)This research developed a double-side power electronics packaging scheme for high temperature applications exemplified by 1200 V, 150 A silicon devices. The power modules, based on both quarter and half-bridge topologies, were assembled using sintered silver device attachment rather than conventional solder alloys. Thermomechanical stresses in the double-side architecture were mitigated with a compliant layer fabricated from elliptical silver tubes. This research presents an introduction to conventional packaging techniques and their weaknesses. These shortcomings provide the basis for a module design which improves upon module thermal management while also addressing electrical and reliability requirements. The optimum package design enhances heat dissipation with the addition of a substrate bonded to the top electrical pads of the semiconductor devices. The use of sintered silver also increases the useful application temperature by avoiding the creep failure mechanisms of solder alloys. The modules were characterized extensively to quantify thermal and electrical performance. In the case of thermal characterization, the double-side architecture required multiple testing configurations to fully understand the parallel heat flow paths. These results were compared to models constructed using finite element analysis (FEA). The FEA models were also utilized for measurement of strains in multiple package designs to better determine the effects of increased compliance on the relative package cycling lifetime. These lifetimes were then assessed, in part, using experimental passive and cycling tests on functional double-side packages. The resulting power modules exhibited significant decreases in thermal resistance when they are cooled, as designed, from both sides of the module. Even single sided cooling options reveal significant advantages and transient thermal impedance was found to be significantly lower. Power module models revealed the compliant layer was successful in reducing the device shear stresses which was experimentally validated through the use of DC power stage testing. It was found, through double pulse testing and electrical modeling, that parasitic inductances were reduced by utilizing planar bonding and planar symmetrical traces. Finally, modeling of the double-side package with added tube compliance revealed a decrease in plastic and shear strains when compared to other single and double-side package designs. This reduction directly translates to increased cycling lifetime using well known strain based fatigue models.
- Design, Analysis and Testing of a Self-reactive Wave Energy Point Absorber with Mechanical Power Take-offLi, Xiaofan (Virginia Tech, 2020-11-06)Ocean wave as a renewable energy source possesses great potential for solving the world energy crisis and benefit human beings. The total theoretical potential wave power on the ocean-facing coastlines of the world is around 30,000 TWh, although cannot all be adopted for generating electricity, the amount of the power can be absorbed still can occupy a large portion of the world's total energy consumption. However, multiple reasons have stopped the ocean wave energy from being widely adopted, and among those reasons, the most important one is immature of the Power Take-off (PTO) technology. In this dissertation, a self-reactive two-body wave energy point absorber that is embedded with a novel PTO using the unique mechanism of Mechanical Motion Rectifier (MMR) is investigated through design, analysis and testing to improve the energy harvesting efficiency and the reliability of the PTO. The MMR mechanism can transfer the reciprocated bi-directional movement of the ocean wave into unidirectional rotation of the generator. As a result, this mechanism brings in two advantages towards the PTO. The first advantage it possess is that the alternating stress of the PTO is changed into normal stress, hence the reliability of the components are expected to be improved significantly. The other advantage it brings in is a unique phenomenon of engagement and disengagement during the operation, which lead to a piecewise nonlinear dynamic property of the PTO. This nonlinearity of the PTO can contribute to an expanded frequency domain bandwidth and better efficiency, which are verified through both numerical simulation and in-lab experiment. During the in-lab test, the prototyped PTO achieved energy transfer efficiency as high as 81.2%, and over 40% of efficiency improvement compared with the traditional non-MMR PTO under low-speed condition, proving the previously proposed advantage. Through a more comprehensive study, the MMR PTO is further characterized and a refined dynamic model. The refined model can accurately predict the dynamic response of the PTO. The major factors that can influence the performance of the MMR PTO, which are the inertia of the PTO, the damping coefficient, and the excitation frequency, are explored through analysis and experiment comprehensively. The results show that the increase on the inertia of the PTO and excitation frequency, and decrease on the damping coefficient can lead to a longer disengagement of the PTO and can be expressed analytically. Besides the research on the PTO, the body structure of the point absorber is analyzed. Due to the low-frequency of the ocean wave excitation, usually a very large body dimension for the floating buoy of the point absorber is desired to match with that frequency. To solve this issue, a self-reactive two-body structure is designed where an additional frequency between the two interactive bodies are added to match the ocean wave frequency by adopting an additional reactive submerged body. The self-reactive two-body structure is tested in a wave to compare with the single body design. The results show that the two-body structure can successfully achieve the frequency matching function, and it can improve more than 50% of total power absorption compared with the single body design.
- Design, Fabrication and Characterization of a GaAs/InxGa1-xAs/GaAs Heterojunction Bipolar TransistorLidsky, David (Virginia Tech, 2014-10-16)Designs for PnP GaAs/InxGa1-xAs/GaAs heterojunction bipolar transistors (HBTs) are proposed and simulated with the aid of commercial software. Band diagrams, Gummel plots and common emitter characteristics are shown for the specific case of x=1, x=0.7, and x linearly graded from 0.75 to 0.7. Of the three designs, it is found that the linearly graded case has the lowest leakage current and the highest current gain. IV curves for all four possible classes of InAs/GaAs heterojunction (nN, nP, pN, pP) are calculated. A pN heterojunction is fabricated and characterized. In spite of the 7% lattice mismatch between InAs and GaAs, the diode has an ideality factor of 1.26 over three decades in the forward direction. In the reverse direction, the leakage current grows exponentially with the magnitude of the bias, and shows an effective ideality factor of 3.17, in stark disagreement with simulation. IV curves are taken over a temperature range of 105 K to 405 and activation energies are extracted. For benchmarking the device processing and the characterization apparatus, a conventional GaAs homojunction diode was fabricated and characterized, showing current rectification ratio of 109 between plus one volt and minus one volt. Because the PnP material for the optimal HBT design was not available, an Npn GaAs/InAs/InAs HBT structure was processed, characterized, and analyzed. The Npn device fails in both theory and in practice; however, by making a real structure, valuable lessons were learned for crystal growth, mask design, processing, and metal contacts.
- Design, Modeling and Control of Vibration Systems with Electromagnetic Energy Harvesters and their Application to Vehicle SuspensionsLiu, Yilun (Virginia Tech, 2016-11-07)Instead of dissipating vibration energy into heat waste via viscous damping elements, this dissertation proposes an innovative vibration control method which can simultaneously mitigate vibration and harvest the associated vibration energy using electromagnetic energy harvesters. This dissertation shows that the electromagnetic energy harvester can work as a controllable damper as well as an energy harvester. The semi-active control of a linear electromagnetic energy harvester, for improvement of suspension performance, has been experimentally implemented in a scaled-down quarter-car suspension system. While improving performance, power produced by the harvester can be harvested through energy harvesting circuits. This dissertation also proposes a mechanical-motion-rectifier(MMR)-based electromagnetic energy harvester using a ball-screw mechanism and two one-way clutches for the application of replacing the viscous damper in vehicle suspensions. Compared to commercial linear harvesters, the proposed design is able to provide large damping forces and increase power-dissipation density, making it suitable to vehicle suspensions. In addition, the proposed MMR-based harvester can convert reciprocating vibration into unidirectional rotation of the generator. This feature significantly increases energy-harvesting efficiency by enabling the generator to rotate at a relatively steady speed during irregular vibrations and improves the system reliability by reducing impact forces among transmission gears. Extensive theoretical and experimental analysis have been conducted to characterize the proposed MMR-based energy harvester. The coupled dynamics of the suspension system with the MMR-based energy harvester are also explored and optimized. Furthermore, a new control algorithm is proposed to control the MMR-based energy harvester considering its unique dynamics induced by the one-way clutches. The results show that the controlled proposed electromagnetic energy harvester can possibly improve ride comfort of vehicles over conventional oil dampers and simultaneously harvest the associated vibration energy.
- Development of a Novel, Manufacturing Method of Producing Cost-Effective Thin-Film Heat Flux SensorsCherry, Rande James (Virginia Tech, 2015-11-13)A new method of manufacturing heat flux sensors was developed using a combination of copper etching and stencil printing nickel/silver conductive ink thermocouple materials onto a thin-film polyimide Kapton® substrate. The semi-automated production capabilities of this manufacturing process significantly decrease the cost of producing thin-film heat flux sensors while still maintaining acceptable performance characteristics. Material testing was performed to first determine the most appropriate materials as well as the theoretical sensitivity and time response of the final sensor. Seebeck coefficient of a thermocouple formed using the combination of EMS CI-1001 silver and EMS CI-5001 nickel ink was measured to be 18.3 ± 0.9 uV/ deg C. Calibrations were then performed on a sample of sensors produced using the novel manufacturing process to verify theoretical values for both sensitivity and time response. The printed heat flux sensor (PHFS) made using this process has a nominal voltage output sensitivity of 4.10 ± 0.23 mV/(W/cm2) and first order time constant response time of 0.592 ± 0.026 seconds. Lastly, a cost analysis was performed to estimate that the final cost to produce the PHFS is approximately $7.73 per sensor. This cost is significantly lower than commercially available sensors which range from $210 upwards to $3000.
- Die-Attachment on Copper by Nanosilver Sintering: Processing, Characterization and ReliabilityZheng, Hanguang (Virginia Tech, 2015-04-29)Die-attachment, as the first level of electronics packaging, plays a key role for the overall performance of the power electronics packages. Nanosilver sintering has becoming an emerging solder-free, environmental friendly die-attach technology. Researchers have demonstrated the feasibility of die-attachment on silver (Ag) or gold (Au) surfaces by pressure-less or low-pressure (< 5 MPa) nanosilver sintering. This study extended the application of nanosilver sintering die-attach technique to copper (Cu) surface. The main challenge of nanosilver sintering on Cu is the formation of thick Cu oxide during processing, which may lead to weak joints. In this study, different processes were developed based on the die size: for small-area dice (< 5 * 5 mm2), different sintering atmospheres (e.g. forming gas) were applied to protect Cu surface from oxidation; for large-area dice (> 5 * 5 mm2), a double-print, low-pressure (< 5 MPa) assisted sintering process was developed. For both processes, die-shear tests demonstrated die-shear strength can reach 40 MPa. The effects of different sintering parameters of the processing were analyzed by different material characterization techniques. With forming gas as sintering atmosphere, not only Cu surface was protected from oxidation, but also the organics in the paste were degraded with nanosilver particles as catalyst. External pressure applied in the processing not only increased the density of sintered Ag, but also enhanced the contact area of sintered-Ag/Cu interface. Microstructure of Ag/Cu interface were characterized by transmission electron microscopy (TEM). Characterization results indicate that Ag/Cu metallic bonds formed at the interface, which verified the high die-shear strength of the die-attachment. Thermal performance of nanosilver sintered die-attachment on Cu was evaluated. A system was designed and constructed for measuring both transient thermal impedance (Zth) and steady-state thermal resistance (Rth) of insulated gate bipolar transistor (IGBT) packages. The coefficient of variation (CV) of Zth measurement by the system was lower than 0.5%. Lead-free solder (SAC305) was applied in comparison of thermal performance with nanosilver paste. With same sample geometry and heating power level, nanosilver sintered joints on Cu showed in average 12.6% lower Zth and 20.1% lower Rth than SAC305 soldered joints. Great thermal performances of nanosilver sintering die-attachment on Cu were mainly due to the low thermal resistivity of sintered-Ag and the good bonding quality. Both passive temperature cycling and active power cycling tests were conducted to evaluate the reliability of nanosilver sintered joints on Cu. For passive temperature cycling tests (-40 - 125 C), the die-shear strengths of mechanical samples had no significant drop over 1000 cycles, and nanosilver sintered IGBT on Cu packages showed almost no change on Zth after 800 cycles. For active power cycling test (Tj = 45 - 175 C), nanosilver sintered IGBT on Cu assembly had a lifetime over 48,000 cycles. The failure point of the assembly was the detachment of the wirebonds. Great reliability performances of nanosilver sintered die-attachment on Cu were mainly due to the low mismatch of coefficient of thermal expansion (CTE) between sintered-Ag and Cu. Meanwhile, low inter-diffusion rate between Ag and Cu prevented the interface from the reliability issue related to Kirkendall voids, which often took place in tin (Sn) -based solder joints.
- A diffusion-viscous analysis and experimental verification of the drying behavior in nanosilver-enabled low-temperature joining techniqueXiao, Kewei (Virginia Tech, 2014-01-23)The low-temperature joining technique (LTJT) by silver sintering is being implemented by major manufacturers of power electronics devices and modules for bonding power semiconductor chips. A common die-attach material used with LTJT is a silver paste consisting of silver powder (micron- or nano-size particles) mixed in organic solvent and binder formulation. It is believed that the drying of the paste during the bonding process plays a critical role in determining the quality of the sintered bond-line. In this study, a model based on the diffusion of solvent molecules and viscous mechanics of the paste was introduced to determine the stress and strain states of the silver bond-line. A numerical simulation algorithm of the model was developed and coded in the C++ programming language. The numerical simulation allows determination of the time-dependent physical properties of the silver bond-line as the paste is being dried with a heating profile. The properties studied were solvent concentration, weight loss, shrinkage, stress, and strain. The stress is the cause of cracks in the bond-line and bond-line delamination. The simulated results were verified by complementary experiments in which the formation of cracks in bond-line and interface delamination was observed during the pressure-free drying of a die-attach nanosilver paste. Furthermore, the important drying parameters, such as drying pressure, low temperature drying time and temperature ramp rate of nanosilver LTJT process, are experimentally studied and analyzed with the numerical simulation. The simulated results were consistent with the experimental findings that the quality of sintered silver bond-line increases with increasing external drying pressure, with increasing low temperature drying time, and with decreasing temperature ramp rate. The insight offered by this modeling study can be used to optimize the process profile that enable pressure-free, low-temperature sintering of the die-attach material to significantly lower the cost of implementing the LTJT in manufacturing.
- Electrical and Thermal Characterizations of IGBT Module with Pressure-Free Large-Area Sintered JointsJiang, Li (Virginia Tech, 2013-10-17)Silver sintering technology has received considerable attention in recent years because it has the potential to be a suitable interconnection material for high-temperature power electronic packaging, such as high melting temperature, high electrical/thermal conductivity, and excellent mechanical reliability. It should be noted, however, that pressure (usually between three to five MPa) was added during the sintering stage for attaching power chips with area larger than 100 mm2. This extra pressure increased the complexity of the sintering process. The maximum chip size processed by pressure-free sintering, in the published resources, was 6 x 6 mm2. One objective of this work was to achieve chip-attachment with area of 13.5 x 13.5 mm2 (a chip size of one kind of commercial IGBT) by pressure-free sintering of nano-silver paste. Another objective was to fabricate high-power (1200 V and 150 A) multi-chip module by pressure-free sintering. In each module (half-bridge), two IGBT dies (13.5 x 13.5 mm2) and two diode dies (10 x 10 mm2) were attached to a DBC substrate. Modules with solder joints (SN100C) and pressure-sintered silver joints were also fabricated as the control group. The peak temperature in the process of of pressure-free sintering of silver was around 260oC, whereas 270oC for vacuum reflowing of solder, and 280oC under three MPa for pressure-sintering of silver. The process for wire bonding, lead-frame attachment, and thermocouple attachment are also recorded. Modules with the above three kinds of joints were first characterized by electrical methods. All of them could block 1200 V DC voltage after packaging, which is the voltage rating of bare dies. Modules were also tested up to the rated current (150 A) and half of the rated voltage (600 V), which were the test conditions in the datasheet for commercial modules with the same voltage and current ratings. I-V characteristics of packaged devices were similar (on-resistance less than 0.5 mohm). All switching waveforms at transient stage (both turn-on and turn-off) were clean. Six switching parameters (turn-on delay, rise time, turn-off delay, fall time, turn-on loss, and turn-off loss) were measured, which were also similar (<9%) among different kinds of modules. The results from electrical characterizations showed that both static characterizations and double-pulse test cannot be used for evaluating the differences among chip-attach layers. All modules were also characterized by their thermal performances. Transient thermal impedances were measured by gate-emitter signals. Two setups for thermal impedance measurement were used. In one setup, the bottoms of modules were left in the air, and in the other setup, bottoms of modules were attached to a chiller (liquid cooling and temperature controlled at 25oC) with thermal grease. Thermal impedances of three kinds of modules still increased after 40 seconds for the testing without chiller, since the thermal resistance of heat convection from bottom copper to the air was included , which was much larger than the sum of the previous layers (from IGBT junction, through the chip-attach layer, to the bottom of DBC substrate). In contrast, thermal impedances became almost stable (less than 3%) after 15 seconds for all modules when the chiller was used. Among these three kinds of modules, the module with pressure sintered joints had the lowest thermal impedance and the thermal resistance (tested with the chiller) around 0.609oK/W, In contrast, the thermal resistance was around 964oK /W for the soldered module, and 2.30oK /W for pressure-free sintered module. In summary, pressure-free large-area sintered joints were achieved and passed the fabrication process for IGBT half-bridge module with wiring bonding. Packaged devices with these kinds of joints were verified with good electrical performance. However, thermal performances of pressure-free joints were worse than solder joints and pressure-sintered joints.
- Electrical Characterization of Gallium Nitride Drift Layers and Schottky DiodesAllen, Noah P. (Virginia Tech, 2019-10-09)Interest in wide bandgap semiconductors such as silicon carbide (SiC), gallium nitride (GaN), gallium oxide (Ga 2 O 3 ) and diamond has increased due to their ability to deliver high power, high switching frequency and low loss electronic devices for power conversion applications. To meet these requirements, semiconductor material defects, introduced during growth and fabrication, must be minimized. Otherwise, theoretical limits of operation cannot be achieved. In this dissertation, the non-ideal current- voltage (IV) behavior of GaN-based Schottky diodes is discussed first. Here, a new model is developed to explain better the temperature dependent performance typically associated with a multi-Gaussian distribution of barrier heights at the metal-semiconductor interface [Section 3.1]. Application of this model gives researches a means of understanding not only the effective barrier distribution at the MS interface but also its voltage dependence. With this information, the consequence that material growth and device fabrication methods have on the electrical characteristics can be better understood. To show its applicability, the new model is applied to Ru/GaN Schottky diodes annealed at increasing temperature under normal laboratory air, revealing that the origin of excess reverse leakage current is attributed to the low-side inhomogeneous barrier distribution tail [Section 3.2]. Secondly, challenges encountered during MOCVD growth of low-doped GaN drift layers for high-voltage operation are discussed with focus given to ongoing research characterizing deep-level defect incorporation by deep level transient spectroscopy (DLTS) and deep level optical spectroscopy (DLOS) [Section 3.3 and 3.4]. It is shown that simply increasing TMGa so that high growth rates (>4 µm/hr) can be achieved will cause the free carrier concentration and the electron mobilities in grown drift layers to decrease. Upon examination of the deep-level defect concentrations, it is found that this is likely caused by an increase in 4 deep level defects states located at E C - 2.30, 2.70, 2.90 and 3.20 eV. Finally, samples where the ammonia molar flow rate is increased while ensuring growth rate is kept at 2 µm/hr, the concentrations of the deep levels located at 0.62, 2.60, and 2.82 eV below the conduction band can be effectively lowered. This accomplishment marks an exciting new means by which the intrinsic impurity concentration in MOCVD-grown GaN films can be reduced so that >20 kV capable devices could be achieved.
- Electrical Integration of SiC Power Devices for High-Power-Density ApplicationsChen, Zheng (Virginia Tech, 2013-10-24)The trend of electrification in transportation applications has led to the fast development of high-power-density power electronics converters. High-switching-frequency and high-temperature operations are the two key factors towards this target. Both requirements, however, are challenging the fundamental limit of silicon (Si) based devices. The emerging wide-bandgap, silicon carbide (SiC) power devices have become the promising solution to meet these requirements. With these advanced devices, the technology barrier has now moved to the compatible integration technology that can make the best of device capabilities in high-power-density converters. Many challenges are present, and some of the most important issues are explored in this dissertation. First of all, the high-temperature performances of the commercial SiC MOSFET are evaluated extensively up to 200 degree C. The static and switching characterizations show that the device has superior electrical performances under elevated temperatures. Meanwhile, the gate oxide stability of the device - a known issue to SiC MOSFETs in general - is also evaluated through both high-temperature gate biasing and gate switching tests. Device degradations are observed from these tests, and a design trade-off between the performance and reliability of the SiC MOSFET is concluded. To understand the interactions between devices and circuit parasitics, an experimental parametric study is performed to investigate the influences of stray inductances on the MOSFETs switching waveforms. A small-signal model is then developed to explain the parasitic ringing in the frequency domain. From this angle, the ringing mechanism can be understood more easily and deeply. With the use of this model, the effects of DC decoupling capacitors in suppressing the ringing can be further explained in a more straightforward way than the traditional time-domain analysis. A rule of thumb regarding the capacitance selection is also derived. A Power Electronics Building Block (PEBB) module is then developed with discrete SiC MOSFETs. Integrating the power stage together with the peripheral functions such as gate drive and protection, the PEBB concept allows the converter to be built quickly and reliably by simply connecting several PEBB modules. The high-speed gate drive and power stage layout designs are presented to enable fast and safe switching of the SiC MOSFET. Based on the PEBB platform, the state-of-the-art Si and SiC power MOSFETs are also compared in the device characteristics, temperature influences, and loss distributions in a high-frequency converter, so that special design considerations can be concluded for the SiC MOSFET. Towards high-temperature, high-frequency and high-power operations, integrated wire-bond phase-leg modules are also developed with SiC MOSFET bare dice. High-temperature packaging materials are carefully selected based on an extensive literature survey. The design considerations of improved substrate layout, laminated bus bars, and embedded decoupling capacitors are all discussed in detail, and are verified through a modeling and simulation approach in the design stage. The 200 degree C, 100 kHz continuous operation is demonstrated on the fabricated module. Through the comparison with a commercial SiC phase-leg module designed in the traditional way, it is also shown that the design considerations proposed in this work allow the SiC devices in the wire-bond structure to be switched twice as fast with only one-third of the parasitic ringing. To further push the performance of SiC power modules, a novel hybrid packaging technology is developed which combines the small parasitics and footprint of a planar module with the easy fabrication of a wire-bond module. The original concept is demonstrated on a high-temperature rectifier module with SiC JFET. A modified structure is then proposed to further improve design flexibility and simplify module fabrication. The SiC MOSFET phase-leg module built in this structure successfully reaches the switching speed limit of the device almost without any parasitic ringing. Finally, a new switching loop snubber circuit is proposed to damp the parasitic ringing through magnetic coupling without affecting either conduction or switching losses of the device. The concept is analyzed theoretically and verified experimentally. The initial integration of such a circuit into the power module is presented, and possible improvements are proposed.
- Energy storage for power factor correction in battery charger for electric-powered vehicles(United States Patent and Trademark Office, 2018-03-13)Switches of a switching circuit used to control operation of an electric motor such as in an electrically powered vehicle connect respective windings of the electric motor as a single phase inductor during battery charging. The inductor can then store inherent low frequency, second order ripple power and return that power to a load presented by a battery during battery charging to deliver substantially constant current. Storage of ripple power in the inductor allows the capacitance value, size, weight and cost of a filter capacitor of a power factor correction circuit providing input power to a battery charger to be reduced by an order of magnitude or more. Direction of current flow through the inductor is periodically reversed to avoid magnetizing the motor.
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