Harvesting Ultra-Low Power Wireless Signals in the GHz Range

We present methods for harvesting wireless energy as low as -30 dBm (1uW) from the 2.4 GHz frequency range (e.g. WiFi signals) with discrete components. We have constructed a proof-of-concept device which is capable of operating at -18.8 dBm (13.2 uW) with no onboard power sources, relying solely on the 2.4 GHz energy source. The device is constructed on a PCB and consists of an impedance matching network, a rectifier, and a DC-DC converter. The impedance matching network matches a 2.4 GHz 50 Ohm input source to the high impedance rectifier and provides a passive boost. The rectifier converts the AC signal from the impedance matching network to a DC signal. This DC signal feeds into the DC-DC converter subsystem which boosts the voltage from about 45 mV DC to a clean 95 mV DC output

Power conditioning optimization for ultra low voltage wearable thermoelectric devices using self-sustained multi-stage charge pump

Waste heat energy recovery from human body utilizing the thermoelectric generator (TEG) has shown potential in the generation of electrical energy. However, the level of heat source from the human body restricts the temperature deviation as compared to ambient temperature (approximately 3~10 °C in difference), thereby yielding an ultra-low voltage (ULV) normally less than 100 mV. This research aims at generating power from the TEG by harnessing human body temperature as the heat source to power up wearable electronic devices realizing a self-sustain system. However, power conversion of the TEG has typically low efficiency (less than 12%), requiring proper design of its power regulation system. The generated ULV marked the lowest energy conversion factor and improvement is therefore required to validate the use of ULV generated from human body temperature. This problem was addressed by proposing an improved solution to the power regulation of the ULV type TEG system based on the DC-DC converter approach, namely a multi-stage charge pump, with specifications restricted at the ULV source. Performances of the TEG connected in multiple array configurations with the generated source voltage fed into fabricated charge pump circuit to boost and regulate the voltage from the ULV into the low voltage (LV) region were analyzed. The maximum source voltage (20 mV) was referred and simulated in the LT Spice software and used as a benchmark to be compared with the voltage generated by the fabricated charge pump circuits. Error performances of the fabricated charge pump circuits were further analyzed by manipulating the circuits’ parameters, namely, the switching frequency and the capacitance values. It was found that the proposed method was able to handle the ULV source voltage with proper tuning on its component parameters. The overall power conversion efficiency of 26.25% was achieved based on the performance evaluation values for components applied in this research. Hence, this proved the viability of thermoelectric applications in ULV using the proposed power regulation system

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