28 IEEE TRANSACTIONS ON BIOMEDICAL CIRCUITS AND SYSTEMS, VOL. 1, NO. 1, MARCH 2007Feedback Analysis and Design of RF Power Links forLow-Power Bionic SystemsMichael W. Baker, Student Member, IEEE, and Rahul Sarpeshkar, Member, IEEEInvited PaperAbstract—This paper presents a feedback-loop technique foranalyzing and designing RF power links for transcutaneous bionicsystems, i.e., between an external RF coil and an internal RFcoil implanted inside the body. The feedback techniques shedgeometric insight into link design and minimize algebraic ma-nipulations. We demonstrate that when the loop transmission ofthe link’s feedback loop is1, the link is critically coupled, i.e.,the magnitude of the voltage transfer function across the linkis maximal. We also derive an optimal loading condition thatmaximizes the energy efficiency of the link and use it as a basis forour link design. We present an example of a bionic implant systemdesigned for load power consumptions in the 1–10-mW range, alow-power regime not significantly explored in prior designs. Suchlow power levels add to the challenge of link efficiency, becausethe overhead associated with switching losses in power amplifiersat the link input and with rectifiers at the link output significantlydegrade link efficiency. We describe a novel integrated Class-Epower amplifier design that uses a simple control strategy tominimize such losses. At 10-mW load power consumption, wemeasure overall link efficiencies of 74% and 54% at 1- and 10-mmcoil separations, respectively, in good agreement with our theo-retical predictions of the link’s efficiency. At 1-mW load powerconsumption, we measure link efficiencies of 67% and 51% at1- and 10-mm coil separations, respectively, also in good accordwith our theoretical predictions. In both cases, the link’s rectifiedoutput dc voltage varied by less than 16% over link distances thatranged from 2 to 10 mm.Index Terms—Biomedical power supplies, feedback systems,implantable biomedical devices, low-power systems, power trans-former, RF powering, transcutaneous power transfer.I. INTRODUCTIONIMPLANTED electronics are used in medical devices for di-agnosis as well as for treatment of a wide variety of condi-tions—pacemakers for cardiac arrhythmia, retinal implants forthe blind, cochlear implants for deafness, deep-brain stimula-tors for Parkinson’s disease, spinal-cord stimulators for controlof pain, and brain-machine interfaces for paralysis prosthetics.Such devices need to be small and operate with low power tomake chronic and portable medical implants possible. They areManuscript received December 18, 2006; revised January 15, 2007. Thispaper was recommended by Editor-in-Chief T. S. Lande.The authors are with the Research Laboratory for Electronics, MassachusettsInstitute of Technology, Cambridge, MA 02139 USA (e-mail: [email protected]).Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TBCAS.2007.893180most often powered by inductive RF links to avoid the need forimplanted batteries, which can potentially lose all their chargeor necessitate resurgery if they need to be replaced. Even whensuch devices have implanted batteries, an increasing trend in up-coming fully implanted systems, wireless recharging of the bat-tery through RF links is periodically necessary.Fig. 1 shows the basic structure of an inductive power linksystem for an example implant. An RF power amplifier drivesa primary RF coil which sends power inductively across theskin of the patient to a secondary RF coil. The RF signal onthe secondary coil is rectified and used to create a power supplythat powers internal signal-processing circuits, electrodes andelectrode-control circuits, signal-sensing circuits, or telemetrycircuits depending on the application. The power consumptionof the implanted circuitry is eventually borne by external bat-teries that power the primary RF coil; if an RF link is energyefficient, most of the energy in the primary RF coil will betransported across the skin and dissipated in circuits in the sec-ondary. It is also important for an RF link to be designed suchthat the power-supply voltage created in the secondary is rela-tively invariant to varying link distances between the primaryand secondary, due to patient skin-flap-thickness variability, de-vice-placement, and device variability.Recent advances in signal processing and electrode designhave reduced power dissipation in internal circuits consid-erably. For example, a cochlear implant processor with only250W of signal-processing power [1], [2] can be combinedwith electrodes that dissipate 750W of power via loweredimpedance strategies or low-power stimulation strategies [3] tocreate cochlear-implant systems that dissipate 1 mW of power.Pacemaker systems often run on power levels that range from10W to 1 mW depending on their complexity. RF power linksfor such systems need to achieve good energy efficiency suchthat needless amounts of external power are not used to poweran efficient internal system. This paper explores the design ofsuch RF links and builds on prior work in relatively high-powersystems. Small losses that are important in low-power systems,may be insignificant in higher-power systems. For example,the retinal-implant design described in [4] is geared towardssystems that dissipate near 250 mW; it dissipates 40 mWin its closed-loop Class-E power amplifiers alone, which isprohibitive for our intended applications but acceptable inthe retinal-implant design. As another example, the designdescribed in [5] is geared towards a link system that is capableof driving amperes of current into the primary portion of the1932-4545/$25.00 © 2007 IEEEAuthorized licensed use limited to: IEEE Xplore. Downloaded on March 20, 2009 at 10:56 from IEEE Xplore. Restrictions apply.BAKER AND SARPESHKAR: FEEDBACK ANALYSIS AND DESIGN OF RF POWER LINKS 29Fig. 1. Example of a low-power bionic implant system is shown.link such that a reasonable amount of power may be receivedin several tiny secondaries.A theoretical model of RF links has been described in [6] whofocused on operating at conditions of critical coupling. In criti-cally coupled conditions, the magnitude of the voltage transferfunction from the primary to the secondary is maximized andthe voltage is relatively invariant to varying link distances. How-ever, the energy-transfer efficiency has a theoretical
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