Erick O. Torres

Electrostatic Energy-Harvesting and Battery-Charging CMOS System Prototype

The self-powering, long-lasting, and functional features of embedded wireless microsensors appeal to an ever-expanding application space in monitoring, control, and diagnosis for military, commercial, industrial, space, and biomedical applications. Extended operational life, however, is difficult to achieve when power-intensive functions like telemetry draw whatever little energy is available from energy-storage microdevices like thin-film lithium-ion batteries and/or microscale fuel cells. Harvesting ambient energy overcomes this deficit by continually replenishing the energy reservoir and indefinitely extending system lifetime. In this paper, a prototyped circuit that precharges, detects, and synchronizes to a variable voltage-constrained capacitor verifies experimentally that harvesting energy electrostatically from vibrations is possible. Experimental results show that, on average (excluding gate-drive and control losses), the system harvests 9.7 nJ/cycle by investing 1.7 nJ/cycle, yielding a net energy gain of approximately 8 nJ/cycle at an average of 1.6 ¿W (in typical applications) for every 200 pF variation. Projecting and including reasonable gate-drive and controller losses reduces the net energy gain to 6.9 nJ/cycle at 1.38 ¿W.

A 0.7-

Self-powered microsystems like wireless microsensors and biomedical implants derive power from in-package minibatteries that can only store sufficient energy to sustain the system for a short life. The environment, however, is a rich source of energy that, when harnessed, can replenish the otherwise exhausted battery. The problem is harvesters generate low power levels and the electronics required to transfer the energy to charge a battery can easily demand more than the power produced. This paper presents how a 1 × 1 mm2 0.7-μm BiCMOS vibration-supplied electrostatic energy-harvesting system IC produces usable energy. The IC charges and holds the voltage across a vibration-driven variable capacitor CVAR so that ambient kinetic energy can induce CVAR to generate current into the battery when capacitance decreases, as the plates separate. The precharger, harvester, monitoring, and control microelectronics draw enough power to operate, yet allow the system to yield (experimentally) 1.27, 2.14, and 2.87 nJ per vibration cycle for battery voltages at 2.7, 3.5, and 4.2 V, which at 30 Hz produce 38.1, 64.2, and 86.1 nW. Experiments further show that the harvester system prototype charges 1 μF (emulating a small thin-film Li Ion) from 3.5 to 3.81 V in 35 s.

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Modern portable micro-systems like biomedical implants and ad-hoc wireless transceiver micro-sensors continue to integrate more functions into smaller devices, which result in low energy levels and short operational lives. Researchers and industry alike are consequently considering harvesting energy from the surrounding environment as a means of offsetting this energy deficit. Even with power efficient designs, low duty-cycle operation, smart power-aware network architectures, and batteries with improved energy density, the stored energy in micro-scale systems is simply not sufficient to sustain extended lifetimes. Fortunately, the surrounding environment is a rich source of energy, from solar and thermal to kinetic, but harnessing it without dissipating much power in the process is challenging. In this paper, an electrostatic vibrational energy harvester circuit is proposed and evaluated. It harnesses energy from inherent vibrations in the system (e.g., engine-powered applications) by modulating the parallel-plate distance of a variable capacitor and channeling the resulting change in charge into a secondary Li-ion micro-battery. The varactor, in essence, behaves like a vibration-dependent current source. Simulations show that a 100-to-1 pF variable plate capacitor subjected to vibrations with a period of 15 mus produces an average harvesting current of 40.8 muA, an energy gain of 569 pj per cycle, and a net average power gain of 38 muW.

m BiCMOS Electrostatic Energy-Harvesting System IC

The potential application space for miniaturized systems like wireless microsensors is expansive, from reconnaissance mission work and remote sensors to biomedical implants and disposable consumer products. Conforming to microscale dimensions, however, constrains energy and power to such an extent that sustaining critical power-hungry functions like wireless communication is problematical. Harvesting ambient kinetic energy offers an appealing alternative, except the act of transferring energy requires power that could easily exceed what the harvester generates in the first place. This paper reviews piezoelectric and electrostatic harvester circuits, describes how to design low-power switched-inductor converters capable of producing net energy gains when supplied from piezoelectric and electrostatic transducers, and presents experimental results from prototype embodiments. In the electrostatic case shown, the controller dissipated 0.91 nJ per cycle and the switched-inductor precharger achieved 90.3% efficiency to allow the harvester to net a gain of 2.47 nJ per cycle from a capacitor that oscillated between 157 and 991 pF. The piezoelectric counterpart harnessed 1.6 to 29.6 μW from weak periodic vibrations with 0.05-0.16- m/s2 accelerations and 65.3 μJ from (impact-produced) nonperiodic motion.

Electrostatic Energy Harvester and Li-Ion Charger Circuit for Micro-Scale Applications

Power management is an essential component of any electrical system, and nowadays a limiting factor in the miniaturization of portable electronic devices. Not only are the battery and power components difficult to integrate but their performance requirements in mobile environments are more stringent. And although point-of-load (PoL) regulation techniques and monolithic controllers are industry standards today, more integration is indispensable. To address these issues, system-in-package (SiP) self-renewable energy source and storage devices are proposed alongside an array of circuit techniques designed to circumvent the shortcomings of such a miniaturized environment, like smart load-sharing schemes, customizable and self-adaptive PoL regulators, active inductor and capacitor multipliers, and robust self-calibrating and self-stabilizing dc-dc converters. On their own, each seeks to push the limits of integration while maintaining and many times improving performance. As a whole, they promise the birth of a new generation of ICs

Harvesting Ambient Kinetic Energy With Switched-Inductor Converters

Miniature self-powered systems like wireless microsensors that rely only on easily exhaustible tiny in-package batteries suffer from short lifetimes. Harvesters, however, extend life by replenishing consumed energy with energy from the environment. The problem is harvesters generate considerably low power so producing a net gain with which to recharge a battery requires ultra low-energy circuits. This brief presents a 1.5 x1.5 mm2 0.7-μm BiCMOS self-tuning electrostatic energy-harvester integrated circuit (IC) that adapts to changing battery voltages (VBAT) to produce usable power from vibrations across VBATs entire operating range. The prototype holds CVARs voltage so that kinetic energy in vibrations can generate and steer current into the battery when capacitance decreases. Unlike in [13], the inductor-based precharger that charges CVAR to VBAT adapts to a constantly shifting VBAT target. Collectively, the precharger and its self-tuning reference, system monitors, and other control circuits draw sufficient power to operate yet dissipate low enough energy to yield a net gain. Experimentally, the harvester IC generates 1.93, 2.43, and 3.89 nJ per vibration cycle at battery voltages 2.7, 3.5, and 4.2 V, which at 30 Hz produce 57.89, 73.02, and 116.55 nW. Accordingly, the system charges 1 μF from 2.7 to 4.2 V (a thin-film lithium-ion range) in 69 s and harnesses 47.9% more energy than with a fixed reference in the same time frame.

SiP integration of intelligent, adaptive, self-sustaining power management solutions for portable applications

Wireless micro-sensors and similar technologies must derive their energy from micro-scale sources (e.g., thin-film Li Ions, etc.) to function in volume-constrained environments like the human body. Unfortunately, confining the source to small spaces limits the total energy available to such an extent that operational life is often impractically short. Ambient energy offers an alternate and virtually boundless source, except small volumes restrain harvesting power. Voltage-constrained electrostatic CMOS harvesters, for example, draw energy from the work done against the mechanical plates of a MEMS variable capacitor at relatively slow rates, producing low output power. This paper discusses how much energy is available in such a system before and after harvesting and offers energy-conservation schemes for increasing its net energy gain (i.e., power output) during all operational phases.

Self-Tuning Electrostatic Energy-Harvester IC

The potential application space for miniaturized systems like wireless microsensors is expansive, from reconnaissance mission work and remote sensors to biomedical implants and disposable consumer products. Conforming to microscale dimensions, however, constrains energy and power to such an extent that sustaining critical power-hungry functions like wireless communication is next to impossible. Harvesting ambient energy offers an appealing alternative, except the act of transferring energy requires power that could easily exceed what the transducer generates in the first place. This paper presents how to design low-power switched-inductor converters capable of producing net energy gains when supplied from low-power piezoelectric and electrostatic kinetic-harvesting sources.