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

Antenna and rectifier designs for miniaturized radio frequency energy scavenging systems

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