This paper presents a low power boost converter for thermoelectric energy harvesting that demonstrates an efficiency that is 15% higher than the state-of-the-art for voltage conversion ratios above 20. This is achieved by utilizing a technique allowing synchronous rectification in the discontinuous conduction mode. A low-power method for input voltage monitoring is presented. The low input voltage requirements allow operation from a thermoelectric generator powered by body heat. The converter, fabricated in a 0.13 μm CMOS process, operates from input voltages ranging from 20 mV to 250 mV while supplying a regulated 1 V output. The converter consumes 1.6 (1.1) μW of quiescent power, delivers up to 25 (175) μW of output power, and is 46 (75)% efficient for a 20 mV and 100 mV input, respectively.

A 20 mV Input Boost Converter With Efficient Digital Control for Thermoelectric Energy Harvesting

This work presents the optimization of antenna captured low power radio frequency (RF) to direct current (DC) power converters using Schottky diodes for powering remote wireless sensors. Linearized models using scattering parameters show that an antenna and a matched diode rectifier can be described as a form of coupled resonator with different individual resonator properties. The analytical models show that the maximum voltage gain of the coupled resonators is mainly related to the antenna, diode and load (remote sensor) resistances at matched conditions or resonance. The analytical models were verified with experimental results. Different passive wireless RF power harvesters offering high selectivity, broadband response and high voltage sensitivity are presented. Measured results show that with an optimal resistance of antenna and diode, it is possible to achieve high RF to DC voltage sensitivity of 0.5 V and efficiency of 20% at −30 dBm antenna input power. Additionally, a wireless harvester (rectenna) is built and tested for receiving range performance.

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Optimization of Passive Low Power Wireless Electromagnetic Energy Harvesters

This paper describes a simple analytical model of the rectifier circuit valid down to very low voltage operation (input voltage below the thermal voltage). Steady-state and transient analyses of the rectifier with the diode described by an exponential I-V characteristic are presented. We provide formulas for the dc output voltage level, the output voltage ripple, and the power conversion efficiency. We also give a compact expression for the charging time of the load capacitor. The models of the rectifier and the voltage doubler for input voltages down to the thermal voltage are verified with simulations and measurements using circuits built with off-the-shelf 1N4148 diodes.

Analysis of the Rectifier Circuit Valid Down to Its Low-Voltage Limit

The key factor in widespread adoption of Radio Frequency Identification (RFID) technology is tag cost minimization. This paper presents the first low-cost, ultra-low power, passive RFID tag, fully integrated on a single substrate in a standard CMOS process. The system combines a 24 GHz, dual on-chip antenna, RF front-end, and a C-Flash based, rewritable, non-volatile memory module to achieve full on-chip system integration. The complete system was designed and fabricated in the TowerJazz 0.18 mu m CMOS technology without any additional mask adders. By embedding the RF, memory, and digital components together upon a single substrate in a standard digital process, the low-cost aspirations of the "5-cent RFID tag" become feasible. Design considerations, analysis, circuit implementations, and measurement results are presented. The entire system was fabricated on a 3.6 mm x 1.6 mm (6.9 mm(2)) die with the integrated antennas comprising 82% of the silicon area. The total read power was measured to be 13.2 mu W, which is sufficiently supplied by the on-chip energy harvesting unit.

A Low-Power Low-Cost 24 GHz RFID Tag With a C-Flash Based Embedded Memory

A DC-DC boost converter for ultra-low-voltage thermal energy harvesting is presented herein. The focus of this brief is to provide both self-startup and efficient conversion at ultra-low input voltages. The converter startup is achieved through the use of a cold starter based on an ultra-low-voltage oscillator and a charge pump. The inductive boost converter architecture uses two low-side switches in order to independently optimize the low-voltage startup and the steady-state conversion efficiency. The model of the developed converter allows optimization of the boost switches as a function of the TEG internal resistance and input voltage. A zero current switching circuit with accurate detection for low input voltages is designed and implemented. The prototype fabricated in 130 nm standard CMOS technology achieves startup for an input voltage of 11 mV and, once started, the system can perform the boost operation for input voltages as low as 7.3 mV, delivering 50% of end-to-end efficiency at 10.5 mV and peak end-to-end efficiency of 85% at 140 mV.

A 7.5-mV-Input Boost Converter for Thermal Energy Harvesting With 11-mV Self-Startup

In this paper, we present measurement results from a prototype chip (fabricated in 130 nm CMOS technology), designed to extract maximum power from a Thermoelectric Generator (TEG). From analytical expression, we prove that the maximum extracted power is around 75% of the available power in a TEG, without using a closed loop maximum peak power tracking to regulate the input voltage. In our measurements, the TEG is modeled by a voltage source (50mV-200mV) with a series resistance of 5 Ohms. The prototype is fully electric, starts from 50 mV and can extract 60% (at 50 mV) to 65% (at 200 mV) of the available power. Hence the measurement result closely agrees with the analytical expression.

An energy harvesting chip designed to extract maximum power from a TEG

This paper presents a simple analytical model of the basic rectifier circuit for very low voltage operation based on the Shockley (exponential) law of the diode. The main alternatives for the implementation of diodes in CMOS technologies and the calculation of the equivalent diode parameters for low voltage operation are summarized. The model of the half-wave rectifier for very low voltage is verified with measurements on a circuit built with a 1N4148 diode. The main diode implementations using MOS transistors of a 0.13 µm technology operating in weak inversion are analyzed and a very simple design procedure is suggested.

Design of very low voltage CMOS rectifier circuits

A relaxation oscillator is presented that makes use of a current-mode Schmitt trigger to reduce the effects of process, voltage and temperature (PVT) variations. A detailed analysis of the oscillator, including the temperature performance, is presented and verified by experimental results. A test chip with a typical frequency of 32 kHz was fabricated in a 0.18 μm standard CMOS process. The measured frequency variations were +/− 30 ppm/°C for temperature variation from −20 °C to 80°C and +/− 500 ppm/V for supply voltage variation from 0.7 V to 1.8 V. The short term stability is 66 ppm (2 ns) of jitter while the long term stability is 500 ppm of Allan deviation after 10 seconds. A careful design results in a total area of 0.1 mm2 and a power consumption of 150 nW.

Carlos Galup Montoro

Papers

An energy harvesting chip designed to extract maximum power from a TEG

Design of very low voltage CMOS rectifier circuits

A 150nW 32 kHz mobility-compensated relaxation oscillator with +/−30ppm/°C temperature stability