120 • 2011 IEEE International Solid-State Circuits Conference

ISSCC 2011 / SESSION 6 / SENSORS & ENERGY HARVESTING / 6.9

6.9 A Self-Supplied Inertial Piezoelectric Energy Harvester with Power-Management IC

Ethem Erkan Aktakka, Rebecca L. Peterson, Khalil Najafi

University of Michigan, Ann Arbor, MI

Harvesting energy from ambient vibrations is a promising technology for fullyautonomous wireless sensor nodes, which can give birth to new applications inbiomedical, industrial, and environmental monitoring. There have been inde-pendent solutions in increasing the harvesting efficiency either on the mechani-cal harvester [1] or on its power management circuitry [2,3]. Recently, a piezo-electric MEMS harvester using AlN was demonstrated to generate enough ener-gy to autonomously power a wireless temperature sensor with a full-bridge rec-tifier built with off-the-shelf components [1]. Meanwhile, AC-DC converters forpiezoelectric harvesters have been designed to enable efficient power extraction[2], or efficient rectification of low-voltage outputs [3], and have been testedwith commercial meso-scale piezoelectric beams. However, to realize an efficientstand-alone energy generator platform, it is necessary to integrate these effortsinto a single low-volume system. This paper presents a self-supplied energygenerator, which includes a MEMS harvester hybridly integrated with its powermanagement circuitry for autonomous charging of an energy reservoir (Fig.6.9.1). The proposed packaging of the generator is <0.3cm3. Initial testingresults are obtained with an unpackaged MEMS harvester.

The MEMS harvester is fabricated with a recently developed process technology[4], which involves aligned solder bonding and thinning of bulk PZT ceramics onsilicon. This technology offers advantages in fabrication flexibility and deviceperformance over existing piezoelectric thin film deposition methods. Althoughthe introduced low-temperature PZT-on-Si process allows post-CMOS integra-tion of the piezoelectric MEMS and its circuitry on the same silicon die, hybridintegration is preferred due to the large difference in the harvester and circuit diesizes. The harvester is packaged with a Si bottom cap and Si-Glass top cap, bothincorporating a recess for free movement of the harvester proof mass. The leadsfrom the harvester are fed to the top glass substrate through vertical low-resis-tivity Si vias. The CMOS chip is wire-bonded on this top cap, and the connec-tions between surface-mount device (SMD) components and the chip is routedthrough 1µm thick aluminum interconnects on the glass.

The power-management IC utilizes the TSMC 0.18µm technology. The system iscompletely self-supplied by the harvester vibration energy, and has no depend-ence on a previously charged energy reservoir. Power management is achievedwith three sub-circuits, MOS switches and an active diode for low-dropout rec-tification, a shunt pass system to increase harvesting efficiency (similar aim withthe “bias flip rectifier” described in [2]), and a trickle battery charger (Fig. 6.9.2).A supply-independent bias [5] is used to act as current mirror to the compara-tors in the system, and limit the overall power consumption. This VBIAS is alsoused as the reference voltage (VREF) in the trickle charger to define the voltageregulation levels.

The rectification of the piezoelectric output is achieved by incorporating twoparts (Fig. 6.9.3). First, four CMOS switches output the modulus of VPIEZO byconverting the negative half cycles into positive ones (VMODULUS) [6]. Second, anactive diode rectifies this voltage to store the charge on a temporary energyreservoir (VSTORAGE), which is also used as the power supply of the active cir-cuitry (VDD). The PMOS gate used in the active diode is bulk-regulated in orderto connect its n-well to the highest potential available for minimum leakage [7].This two-stage active rectification scheme minimizes the voltage drop on thepath between VPIEZO and VSTORAGE, although there is a minimum input thresholdrequirement to drive the initial CMOS switches. Also, between these two stagesa voltage limiter, implemented with a series of on-chip diodes, is used to protectthe circuit against possible peaks from the harvester.

A parallel shunt system is placed across the inputs from the piezoelectric har-vester, in order to increase the harvesting efficiency (PIN-LOADED/PIN-UNLOADED).When the harvester’s output current, IPIEZO, changes polarity, the harvesterpower starts to be wasted to discharge and recharge its self-capacitance (CPIEZO)in reverse polarization. In order to partially avoid this situation, the charge onCPIEZO is shunted to zero whenever IPIEZO crosses zero in either direction. This

condition is verified by checking when VMODULUS drops down VSTORAGE. Becauseof the Schottky diodes (D1, D2) used in the shunt path, no tuning for precisetiming control is necessary to drive M1 and M2, as in [2]. Instead, one of thesegates remains on during the whole recharging period of CPIEZO.

The energy stored in the temporary reservoir (CSTORAGE) is dumped into a finalreservoir by a trickle charger (Fig. 6.9.4). Just to illustrate the operation princi-ple, assume that the system starts with zero voltage in both reservoirs. With avibration input, the MEMS harvester starts to supply power, and there is a start-up period where CSTORAGE is charged passively. When there is enough vibrationenergy to charge this capacitor up to a minimum level (V0), the active circuitbecomes operational, and a temporary dropout occurs in VSTORAGE. Now, CSTOR-

AGE continues to be charged, but more efficiently with the active diode and shuntsystem fully operational. When VSTORAGE reaches a certain value (V2), the bulk-regulated PMOS gates between the temporary and final energy reservoirs are on,and the scavenged energy is transferred to the battery through a current pulse,until VSTORAGE drops down to the level of either V1 or VBATTERY. Then, the gatesare turned off, and CSTORAGE is charged again back to the level of V2. This charg-ing scheme continues till VBATTERY is fully charged to V3 value. At this point, thegates turn off not to overcharge the battery above its rated voltage. In order toavoid the system to be locked at this stable operation point, VSTORAGE is put intoscanning between V1 and V4 levels, so the system can check whether the bat-tery needs to be recharged again. When the input vibration ends, VSTORAGEdecays down to zero, and the back-flow of battery charge is prevented with aNAND gate powered by the battery. The power consumption of this NAND gateadded with the leakage back to the CSTORAGE results in a near-zero (<5pA) stand-by current draw. For testing flexibility, the battery’s final charged voltage levelcan be adjusted by the option to use an external signal instead of internally gen-erated VBIAS for the comparison signal VREF. Alternatively, a different set of resis-tors, which are used to obtain fractions of VSTORAGE, can be chosen to define thenew voltage-regulation levels.

The overall system is tested by charging a final energy reservoir at differentvibration levels (Fig. 6.9.5). When the MEMS harvester is connected to an opti-mum resistive load without any power-management circuitry, it can supply24.1µW and 63.9µW under 0.5g and 1.0g vibration levels, respectively. TheNormalized Power Density (Power / Volume / Acceleration2), and bandwidth ofthe generator is compared with the current state-of-the-art micro-fabricatedinertial harvesters (Fig. 6.9.6). When the harvester is connected with its powermanagement IC, it can charge a 20mF ultra-capacitor up to 1.31V in 20minunder 0.5g vibration, and in ~8min under 1.0g vibration input.

Acknowledgements:This work is supported by the DARPA Hybrid Insect MEMS program under Grant# N66001-07-1-2006.

References:[1] R. Elfrink, et al., “First Autonomous Wireless Sensor Node Powered by aVacuum-Packaged Piezoelectric MEMS Energy Harvester,” IEEE InternationalElectron Devices Meeting, pp. 543-546, Dec. 2009.[2] Y.K. Ramadass and A.P. Chandrakasan, “An Efficient Piezoelectric EnergyHarvesting Interface Circuit Using a Bias-Flip Rectifier and Shared Inductor,”ISSCC Dig. Tech. Papers, pp. 296-297, Feb. 2009.[3] D. Kwon and G.A. Rincon-Mora, “A Single-Inductor AC-DC PiezoelectricEnergy-Harvester/Battery-Charger IC Converting ±(0.35 to 1.2V) to (2.7 to4.5V),” ISSCC Dig. Tech. Papers, pp. 494-496, Feb. 2010.[4] E.E. Aktakka, R.L. Peterson, and K. Najafi, “A CMOS Compatible PiezoelectricVibration Energy Scavenger Based on the Integration of Bulk PZT Films onSilicon,” IEEE International Electron Devices Meeting, Dec. 2010. [5] E. Dallago, et al., “Active Self Supplied AC-DC Converter for PiezoelectricEnergy Scavenging Systems with Supply Independent Bias,” IEEE Int. Symp.Circuits and Systems, pp. 1448-1451, May 2008.[6] C. Peters, et al., “A CMOS Integrated Voltage and Power Efficient AC/DCConverter for Energy Harvesting Applications,” Journal of Micromechanics andMicroengineering, vol. 18, no. 10, pp. 104005+, Sept. 2008.[7] M. Ghovanloo and K. Najafi, “Fully Integrated Wideband High-CurrentRectifiers for Inductively Powered Devices,” IEEE J. Solid-State Circuits, vol.39,pp. 1976-84, Nov. 2004.

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Figure 6.9.1: Photo of the generator, with MEMS harvester, its packaging,placement of the chip and SMD components, and chip micrograph.

Figure 6.9.2: Overall view of the power-management circuitry, showing theconnections between sub-circuits and off-chip components.

Figure 6.9.3: Active rectification is used to allow low drop-out, and the shunt-pass to increase the power extraction from the harvester.

Figure 6.9.5: Charging of a final energy reservoir under different vibration levels.

Figure 6.9.6: Performance metrics, and comparison of the results with thestate-of-the-art.

Figure 6.9.4: The trickle charger allows the system to charge a large energyreservoir, while it is self-supplied from the temporary reservoir.

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