Osciladores de ultra-baixa-tensão com aplicação em circuitos de captação de energia

Tese (doutorado) - Universidade Federal de Santa Catarina, Centro Tecnológico, Programa de Pós-Graduação em Engenharia Elétrica, Florianópolis, 2014Abstract: This thesis describes the analysis and design of oscillators and charge pumps that can operate with very low supply voltages. The focus is on operation of the MOS transistor in the triode region owing to the limited voltage options available. Special attention has been given to the properties of the zero-VT transistor due to its high drive capability at low voltage. In order to investigate the minimum supply voltage for MOSFET oscillators, three topologies were studied. Two of them, namely the enhanced swing ring and the enhanced swing Colpitts oscillators, can operate with supply voltages below the thermal voltage, kT =q. Simplified theoretical expressions for the minimum supply voltage, oscillation frequency and minimum transistor gain of the oscillators were derived. Measurement results obtained using prototypes built with zero-VT transistors verified the operation of the oscillators for a supply voltage as low as 30 mV and 3.5 mV with high swing amplitude for arrangements built with integrated and off-theshelfinductors, respectively. The application of the ultra-low-voltage oscillators to energy harvesting circuits was addressed in this work. In order to convert the ac signal of the oscillator into a dc signal, the popular Dickson charge pump converter was employed. Expressions for the output voltage, input resistance and power converter efficiency of the Dickson charge pump operating at ultra-low voltages were derived. Experimental results obtained with prototypes built with the enhanced swing ring oscillator and the Dickson charge pump confirmed the feasibility of obtaining a dc output equal to 1 V at current consumptions of 100 nA and 1 µA from input voltages of 10 mV and 23 mV, respectively.O presente trabalho apresenta a análise, projeto e experimentação de osciladores e conversores dc-dc elevadores operando a muito baixas tensões de alimentação. Devido aos baixos valores de tensão de alimentação de interesse deste trabalho, especial atenção foi dada à operação do transistor MOS na região triodo e às propriedades do transistor zero-VT, graças a sua alta capacidade de corrente para baixas tensões. Com o objetivo de investigar a mínima tensão de alimentação de osciladores a MOSFET, três topologias foram estudadas. Duas delas, chamadas de oscilador em anel com elevada excursão desinal e oscilador Colpitts com elevada excursão de sinal, podem trabalhar com tensões de alimentação inferiores à tensão térmica, kT /q. Expressões simplificadas para a mínima tensão de alimentação, frequência de oscilação e mínimo ganho do transistor foram derivadas para cada topologia. Resultados experimentais obtidos com protótipos implementados com transistores zero-VT comprovam a operação dos osciladores com tensões tão baixas quanto 30 mV e 3,5 mV em circuitos construídos com indutores integrados e discretos, respectivamente. A aplicação dos osciladores a circuitos de captação de energia (energy harvesting circuits) a partir de fontes de alimentação de ultra-baixa-tensão foi estudada neste trabalho. Com o propósito de converter tensões ac geradas pelos osciladores em sinais dc, o clássico conversor Dickson foi utilizado. Expressões para a tensão de saída, resistência de entrada e eficiência de conversão de potência do conversor Dickson operando a ultra-baixas-tensões foram derivadas. Resultados experimentais obtidos com protótipos construídos com o oscilador em anel com elevada excursão de sinal e com o conversor Dickson, provaram a possibilidade de se obter uma tensão dc na saída de 1 V para correntes de carga de 100 nA e 1 µA a partir de tensões de entrada de 10 mV e 23 mV, respectivamente

Fast-waking and low-voltage thermoelectric and photovoltaic CMOS chargers for energy-harvesting wireless microsensors

The small size of wireless microsystems allows them to be deployed within larger systems to sense and monitor various indicators throughout many applications. However, their small size restricts the amount of energy that can be stored in the system. Current microscale battery technologies do not store enough energy to power the microsystems for more than a few months without recharging. Harvesting ambient energy to replenish the on-board battery extend the lifetime of the microsystem. Although light and thermal energy are more practical in some applications than other forms of ambient energy, they nevertheless suffer from long energy droughts. Additionally, due to the very limited space available in the microsystem, the system cannot store enough energy to continue operation throughout these energy droughts. Therefore, the microsystem must reliably wake from these energy droughts, even if the on-board battery has been depleted. The challenge here is waking a microsystem directly from an ambient source transducer whose voltage and power levels are limited due to their small size. Starter circuits must be used to ensure the system wakes regardless of the state of charge of the energy storage device. The purpose of the presented research is to develop, design, simulate, fabricate, test and evaluate CMOS integrated circuits that can reliably wake from no energy conditions and quickly recharge a depleted battery. Since the battery is depleted during startup, the system must use the low voltage produced by the energy harvesting transducer to transfer energy. The presented system has the fastest normalized wake time while reusing the inductor already present in the battery charger for startup, therefore, minimizing the overall footprint of the system.Ph.D