This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. IEEE JOURNAL OF SOLID-STATE CIRCUITS 1 A Parallel-SSHI Rectifier for Piezoelectric Energy Harvesting of Periodic and Shock Excitations Daniel A. Sanchez, Student Member, IEEE, Joachim Leicht, Student Member, IEEE, Friedrich Hagedorn, Eduardas Jodka, Elham Fazel, and Yiannos Manoli, Senior Member, IEEE Abstract— Piezoelectric harvesters are capable of generating energy out of ambient vibrations. Dedicated interface circuits can significantly increase the harvesting capabilities compared with passive rectifiers. This paper presents an autonomous piezoelectric energy harvesting system in a 0.35-μm CMOS process. The implemented interface is based on the parallel-SSHI technique and can harvest from periodic and shock excitations. Regular operation is enabled for input voltages as low as 670 mV. It extracts up to 6.81 times more power compared with an ideal full-bridge rectifier depending on the generator characteristics and excitation conditions. The device is capable of cold startup and provides a stable output voltage for powering an application. Index Terms— Bias-flip rectifier, common-gate comparator, energy harvesting, frequency up-conversion, hysteretic buck converter, inductor sharing, piezoelectric, shock excitation, SSHI. I. I NTRODUCTION E NERGY harvesting allows scavenging energy from ambi- ent sources like thermal gradients, wind, light, or kinetic energy. Kinetic energy harvesting allows power extraction out of energy sources like vibrations (1–500 Hz), motion ( <2 Hz), shock (1–10 g), and water flow (1–50 1/min). Some examples include a car in motion [1], a person walking [2], railroad tracks [3], machines [4], [5], and a remote water meter [6]. The most popular kinetic energy harvesting approaches that have been documented are magnetic [7], piezoelectric [8], [9], and capacitive [10]. Piezoelectric energy harvesters (PEHs) are popular because of their high power density [11], ease of scaling, and their relative high output voltage [12]. They con- vert vibration-induced mechanical strain into electrical charge by means of the direct piezoelectric effect [13]. Commonly, PEHs are cantilever based, in which one or multiple layers of Manuscript received April 29, 2016; revised June 24, 2016 and August 16, 2016; accepted September 19, 2016. This paper was approved by Guest Editor Edgar Sanchez-Sinencio. This work was supported in part by the Mexican National Council of Science and Technology and in part by the German Academic Exchange Service. D. A. Sanchez, J. Leicht, F. Hagedorn, and E. Fazel are with the Fritz Huettinger Chair of Microelectronics, Department of Microsys- tems Engineering–IMTEK, University of Freiburg, 79110 Freiburg im Breisgau, Germany (e-mail: daniel.sanchez@imtek.de; joachim.leicht@ imtek.de; friedrich.hagedorn@imtek.de; elham.fazel@imtek.de). E. Jodka was with the University of Freiburg-IMTEK, Freiburg, Germany, and is currently with Texas Instruments, 85356 Freising, Germany (e-mail: e-jodka@ti.com). Y. Manoli is with the Fritz Huettinger Chair of Microelectronics, Department of Microsystems Engineering–IMTEK, University of Freiburg, 79110 Freiburg im Breisgau, Germany. He is also with the Hahn-Schickard Institute of Micromachining and Information Technology, 78052 Villingen- Schwenningen, Germany (e-mail: yiannos.manoli@imtek.de). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSSC.2016.2615008 Fig. 1. Cantilever-beam-based PEH and its equivalent spring–mass–damper system. Fig. 2. Equivalent electromechanical model of a piezoelectric harvester. piezoelectric material are mounted on a beam carrier. A deflec- tion at the tip of the cantilever, as shown in Fig. 1, produces mechanical strain at the top and bottom surfaces, and thus the PEH generates a charge that can be extracted and stored to power applications. The energy extraction is mechanically optimized when the PEH is continuously excited in resonance. However, this is rarely achieved using ambient vibrations, where changes in excitation frequencies and magnitudes are common, or shock excitations occur [1], [2], [4], [9], [14]. Once the PEH is excited, an internal alternating piezo- electric current I M charges and discharges the inherent capacitor C p , generating a voltage V p across the PEH termi- nals (Fig. 2). In order to power electronics, an interface is necessary to rectify V p . Using the simple full-bridge diode rectifier (FBR) shown in Fig. 3, energy can be extracted independently of the excitation type and frequency, as long as V p exceeds the output buffer voltage V buf . However, this energy extraction is only optimized when V buf equals half the PEH open-circuit voltage peak ˆ V p,oc [8], [9], [15]. A maximum power point tracking system [15] controls V buf to ˆ V p,oc /2 for different excitation amplitudes, but the power extraction capabilities are limited to the ones of an ideal FBR. For low-to-medium electromechanically coupled PEHs, sev- eral interfaces have reported increased harvesting capabilities compared with the FBR [8], [9], [16]–[19], but some require 0018-9200 © 2016 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.