Design and Optimization of Resonance-Based

48IEEETRANSACTIONSONBIOMEDICALCIRCUITSANDSYSTEMS,VOL.5,NO.1,FEBRUARY2011DesignandOptimizationofResonance-BasedEfficientWirelessPowerDeliverySystemsforBiomedicalImplantsAnilKumarRamRakhyani,StudentMember,IEEE,ShahriarMirabbasi,Member,IEEE,andMuChiao,Member,IEEEAbstract—Resonance-basedwirelesspowerdeliveryisaneffi-cienttechniquetotransferpoweroverarelativelylongdistance.Thistechniquetypicallyusesfourcoilsasopposedtotwocoilsusedinconventionalinductivelinks.Inthefour-coilsystem,theadverseeffectsofalowcouplingcoefficientbetweenprimaryandsecondarycoilsarecompensatedbyusinghigh-quality()factorcoils,andtheefficiencyofthesystemisimproved.Unlikeitstwo-coilcounterpart,theefficiencyprofileofthepowertransferisnotamonotonicallydecreasingfunctionoftheoperatingdistanceandislesssensitivetochangesinthedistancebetweentheprimaryandsecondarycoils.Afour-coilenergytransfersystemcanbeopti-mizedtoprovidemaximumefficiencyatagivenoperatingdistance.Wehaveanalyzedthefour-coilenergytransfersystemsandout-linedtheeffectofdesignparametersonpower-transferefficiency.Designstepstoobtaintheefficientpower-transfersystemarepre-sentedandadesignexampleisprovided.Aproof-of-conceptpro-totypesystemisimplementedandconfirmsthevalidityofthepro-posedanalysisanddesigntechniques.Intheprototypesystem,forapower-linkfrequencyof700kHzandacoildistancerangeof10to20mm,usinga22-mmdiameterimplantablecoilresonance-basedsystemshowsapower-transferefficiencyofmorethan80%withanenhancedoperatingrangecomparedtoefficiencyachievedbyaconventionaltwo-coilsystem.IndexTerms—Biomedicalimplants,couplingcoefficient,induc-tivewirelesspowerlinks,powertransmissionefficiency,resonance-basedpowerdelivery,telemetry,wirelesspowertransfer.I.INTRODUCTIONIMPLANTABLEdevicesarebecomingmoreandmorepop-ularinhealthandmedicalapplicationsduetotheirabilitytolocallystimulateinternalorgansand/ormonitorandcom-municatetheinternalvitalsignstotheouterworld.ThepowerrequirementofbiomedicalimplantsdependsontheirspecificManuscriptreceivedApril30,2010;revisedJuly25,2010;acceptedAugust11,2010.DateofpublicationOctober07,2010;dateofcurrentversionJanuary26,2011.ThisworkwassupportedinpartbyaresearchfundfromtheNaturalSciencesandEngineeringCouncilofCanada(NSERC),inpartbytheCanadaResearchChair(CRC)Program,andinpartbyinfrastructurefundingfromtheCanadaFoundationofInnovation(CFI)andBritishColumbiaKnowledgeDe-velopmentFund(BCKDF).ThispaperwasrecommendedbyAssociateEditorB.-D.Liu.A.K.RamRakhyaniandS.MirabbasiarewiththeDepartmentofElectricalandComputerEngineering,UniversityofBritishColumbia,VancouverV6T1Z4,BCCanada.M.ChiaoiswiththeDepartmentofMechanicalEngineering,UniversityofBritishColumbia,Vancouver,BCV6T1Z4,Canada.Colorversionsofoneormoreofthefiguresinthispaperareavailableonlineat[1]–[4]toafewtensofmilliwatts[5]–[8].Providingrequiredpowertoimplanteddevicesinareliablemannerisofparamountim-portance.Someimplantsuse(rechargeable)batteries;however,theirapplicationsarelimitedduetothesizeand/orlongevityofthebatteries.Wirelesspower-transferschemesareoftenusedinimplantabledevicesnotonlytoavoidtranscutaneouswiring,butalsotoeitherrechargeorreplacethedevicebattery.Wire-lesspowertransferisalsousedinotherapplicationdomainswhereremotepoweringisrequired,forexample,contactlessbatterycharging[9]andradio-frequencyidentification(RFID)tags[10].Apopulartechniqueforwirelesspowertransfer,particularlyinbiomedicalimplants,isinductivecoupling,whichwasfirstusedtopoweranartificialheart[11],[12]andsincethenhascommonlybeenusedinimplantabledevices[1]–[8],[13],[14].Aninductivelycoupledpower-transfersystemconsistsoftwocoilsthataregenerallyreferredtoasprimaryandsecondarycoils.Inthesesystems,power-transferefficiencyisastrongfunctionofthequalityfactor()ofthecoilsaswellasthecou-plingbetweenthetwocoils.Hence,theefficiencydependsonthesize,structure,physicalspacing,relativelocation,andthepropertiesoftheenvironmentsurroundingthecoils.Thecou-plingbetweenthecoilsdecreasessharplyasthedistancebe-tweenthecoilsincreasesandcausestheoverallpower-transferefficiencytodecreasemonotonically.Inductivepowertransferina(co-centric)2-coilsystemisextensivelyanalyzedinthelit-erature[5],[8],[10],[15].Thepower-transferefficiency,,versusnormalizeddistance(i.e.,theratiooftheseparationbetweenthecoils)()andthegeometricmeanoftheprimaryandthesecondarycoilradii()isacommonlyusedperformancemetricforcomparingdifferentdesigns.Duetothelow-factor(duetothesourceandloadresistances)andlowcouplingofthecoilsinthetwo-coilsystem,two-coil-basedpower-transfersystemssufferfromrelativelylow-power-transferefficiency,typically,40%for[5],[6],[16],andgenerallydropsex-ponentiallywithdistance()for.Inimplantabledevices,thesizeoftheimplantedcoilisconstrainedbytheim-plantsite.Typically,anexternalcoilcanbemadebigenoughtoimprovethepower-transferrange.Ascouplingbetweencoilsdependsonamountofmagneticfluxlinkagebetweenprimaryandsecondarycoils,foragivenoperatingrangeandsmallsizeoftheimplanted(secondary)coil,couplingreducesa