5,053 research outputs found
Modelagem e projeto de conversores AC/DC de ultrabaixa tensão de operação
Tese (doutorado) - Universidade Federal de Santa Catarina, Centro Tecnológico. Programa de Pós-Graduação em Engenharia ElétricaEsta tese apresenta o desenvolvimento de um modelo analítico e muito simples do circuito retificador, considerando a lei corrente-tensão (exponencial) do diodo, tendo como mérito simplificar um problema relativamente complexo e não linear (retificador) com uma ótima precisão. O modelo mostra-se válido, mesmo para tensões abaixo da tensão térmica, tendo sido testado para um ampla variação de tensão e corrente. São apresentadas equações para a tensão DC de saída, ripple de tensão, transiente durante o startup e eficiência de conversão de potência. Para validação, o modelo é comparado à simulações realizadas em simulador SPICE e a resultados experimentais, mostrando uma ótima precisão. Comparando-se este modelo com outros citados nas referências bibliográficas, este possui a vantagem de ser analítico, mais simples e/ou mais preciso. O desenvolvimento deste modelo torna-se mais importante, à medida que cresce o interesse pela utilização de sensores remotos autoalimentados, e também pelo uso de dispositivos de identificação por rádiofrequência (RFID). O espaço de projeto do conversor AC/DC foi explorado por meio de equações simples e de uma metodologia de projeto desenvolvida para que, através de gráficos, o projetista possa de forma fácil, rápida e com boa precisão, determinar os principais elementos do conversor AC/DC e da rede de adaptação de impedâncias. Para alcançar potências menores na entrada do conversor AC/DC, a metodologia utiliza redes de adaptação de impedâncias para o casamento entre as impedâncias da antena (ou impedância da fonte geradora de sinal AC) e do conversor AC/DC. Além disso, esta metodologia pode ser utilizada para conversores AC/DC com diodos ou transistores conectados como diodos, mesmo que sua equação característica não seja a do diodo exponencial. Para a utilização do conversor AC/DC em circuitos integrados, são estudadas as possibilidades de uso do transistor MOS conectado como diodo operando na região de inversão fraca. Para obter suporte experimental, foram projetados multiplicadores de tensão, com rede de adaptação de impedâncias incorporada ao circuito integrado e também externa ao mesmo, com o objetivo de atingir a menor potência de entrada disponível.This thesis presents a simple analytical model of the rectifier circuit assuming that the diode is characterized by the exponential current-voltage law. The model shown is valid even for voltages below the thermal voltage and it has been tested for a wide range of voltages and currents. Equations are provided for the DC output voltage, ripple voltage, transient during startup and power conversion efficiency. For validation, the model is compared to simulations carried out in SPICE and experimental results, showing a good accuracy. Comparing this model with others cited in the references, this one has the advantage of being analytical, simpler, and more accurate. The development of this model becomes more relevant with the growing use of self powered remote sensors, and radio frequency identification devices (RFID). The design space of the AC/DC converter was explored using a graphic methodology. To operate with reduced power at the input, the methodology uses an impedance adaptation network for the matching between the impedances of the antenna (or the source impedance of the AC signal) and that of the AC/DC converter. Furthermore, this methodology can be used for AC/DC converters with diodes or transistors connected as diodes, even if their characteristic equations are not exponential. To obtain experimental support, voltage multipliers have been designed with impedance adaptation network incorporated into the integrated circuit and also external to it, in order to achieve the lowest possible power at the input
Breathers in Josephson junction ladders: resonances and electromagnetic waves spectroscopy
We present a theoretical study of the resonant interaction between dynamical
localized states (discrete breathers) and linear electromagnetic excitations
(EEs) in Josephson junction ladders. By making use of direct numerical
simulations we find that such an interaction manifests itself by resonant steps
and various sharp switchings (voltage jumps) in the current-voltage
characteristics. Moreover, the power of ac oscillations away from the breather
center (the breather tail) displays singularities as the externally applied dc
bias decreases. All these features can be mapped to the spectrum of EEs that
has been derived analytically and numerically. Using an improved analysis of
the breather tail, a spectroscopy of the EEs is developed. The nature of
breather instability driven by localized EEs is established.Comment: 15 pages, 13 figure
A market-based transmission planning for HVDC grid—case study of the North Sea
There is significant interest in building HVDC transmission to carry out transnational power exchange and deliver cheaper electricity from renewable energy sources which are located far from the load centers. This paper presents a market-based approach to solve a long-term TEP for meshed VSC-HVDC grids that connect regional markets. This is in general a nonlinear, non-convex large-scale optimization problem with high computational burden, partly due to the many combinations of wind and load that become possible. We developed a two-step iterative algorithm that first selects a subset of operating hours using a clustering technique, and then seeks to maximize the social welfare of all regions and minimize the investment capital of transmission infrastructure subject to technical and economic constraints. The outcome of the optimization is an optimal grid design with a topology and transmission capacities that results in congestion revenue paying off investment by the end the project's economic lifetime. Approximations are made to allow an analytical solution to the problem and demonstrate that an HVDC pricing mechanism can be consistent with an AC counterpart. The model is used to investigate development of the offshore grid in the North Sea. Simulation results are interpreted in economic terms and show the effectiveness of our proposed two-step approach
Assessment of thermal instabilities and oscillations in multifinger heterojunction bipolar transistors through a harmonic-balance-based CAD-oriented dynamic stability analysis technique
We present a novel analysis of thermal instabilities and oscillations in multifinger heterojunction bipolar transistors (HBTs), based on a harmonic-balance computer-aided-design (CAD)-oriented approach to the dynamic stability assessment. The stability analysis is carried out in time-periodic dynamic conditions by calculating the Floquet multipliers of the limit cycle representing the HBT working point. Such a computation is performed directly in the frequency domain, on the basis of the Jacobian of the harmonic-balance problem yielding the limit cycle. The corresponding stability assessment is rigorous, and the efficient calculation method makes it readily implementable in CAD tools, thus allowing for circuit and device optimization. Results on three- and four-finger layouts are presented, including closed-form oscillation criteria for two-finger device
Superlattice nonlinearities for Gigahertz-Terahertz generation in harmonic multipliers
Semiconductor superlattices are strongly nonlinear media offering several
technological challenges associated with the generation of high-frequency
Gigahertz radiation and very effective frequency multiplication up to several
Terahertz. However, charge accumulation, traps and interface defects lead to
pronounced asymmetries in the nonlinear current flow, from which high harmonic
generation stems. This problem requires a full non-perturbative solution of
asymmetric current flow under irradiation, which we deliver in this paper
within the Boltzmann-Bloch approach. We investigate the nonlinear output on
both frequency and time domains and demonstrate a significant enhancement of
even harmonics by tuning the interface quality. Moreover, we find that
increasing arbitrarily the input power is not a solution for high nonlinear
output, in contrast with materials described by conventional susceptibilities.
There is a complex combination of asymmetry and power values leading to maximum
high harmonic generation.Comment: 13 pages, 7 figures, Accepted for Nanophotonics (De Gruyter
Controlling the harmonic conversion efficiency in semiconductor superlattices by interface roughness design
In semiconductor superlattices, when Bragg oscillating electrons interact
with an input electromagnetic field, frequency multiplication is possible. An
ideal superlattice has a purely antisymmetric voltage current response and can
thus produce only odd harmonics. However, real world superlattices can also
have even harmonic response and that increases the range of possible output
frequencies. These effects have been recently explained with a predictive model
that combines an Ansatz solution for the Boltzmann Equation with a
Nonequilibrium Green's Functions approach. This predictive tool, coupled with
recent progress on GHz input sources, support the growing interest in
developing compact room temperature devices that can operate from the GHz to
the THz range. The natural question to ask is what efficiencies can be
expected. This paper addresses this issue by investigating power-conversion
efficiency in irradiated semiconductor superlattices. Interface imperfections
are consistently included in the theory and they strongly influence the power
output of both odd and even harmonics. Good agreement is obtained for predicted
odd harmonic outputs with experimental data for a wide frequency range. The
intrinsic conversion efficiency used is based on the estimated amplitude of the
input field inside the sample and thus independent of geometrical factors that
characterize different setups. The method opens the possibility of designing
even harmonic output power by controlling the interface quality
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