prEN IEC 63758-8:2026

• Status: Not Published
• Publication Date: 23-Feb-2028
• Technical Committee: CLC/TC 100X - Audio, video and multimedia systems and equipment and related sub-systems
• Current Stage: 4020 - Enquiry circulated - Enquiry
• Start Date: 18-Sep-2026
• Completion Date: 18-Sep-2026
• Directive: 2014/53/EU - Directive 2014/53/EU of The European Parliament and of the Council of 16 April 2014 on the harmonisation of the laws of the Member States relating to the making available on the market of radio equipment and repealing Directive 1999/5/EC
• Mandate: M/607 - COMMISSION IMPLEMENTING DECISION of C(2025)1207 on a Standardisation Request M/607 as regards charging interface and charging communication protocol for radio equipment capable of being recharged by means of wireless charging in relation with Article 3(4), sixth subparagraph, of Directive 2014/53/EU of the European Parliament and of the Council.
• Ref Project: oSIST prEN IEC 63758-8:2026 - Qi Specification - Version 2.3 - Part 8: Foreign object detection (Fast Track)

prEN IEC 63758-8:2026 is the CLC document for Qi Specification - Version 2.3 - Part 8: Foreign object detection. It defines how a Qi Power Transmitter detects Foreign Objects, limits heating, and uses pre-power-transfer and in-power-transfer methods, with some methods requiring support from the Power Receiver.

What does prEN IEC 63758-8:2026 specify?

prEN IEC 63758-8:2026 specifies Foreign Object Detection (FOD) methods for Qi wireless power transfer. In practice, it tells implementers how a Power Transmitter can decide whether metal objects in its magnetic field are safe, and what action to take when they are not.

Clause 1 sets the general rules for reading the document, including mandatory wording, notation, tolerances, packet-field format, text-string handling, and shorthand notation. Clauses 2 to 5 then cover the background, the risk of heating Foreign Objects, the methods used before power transfer starts, and the methods used while power transfer is already in progress.

Annex A is normative and gives the method for determining reference FOD values. Annex B is informative and describes the open-air Q test, which is a supporting method for detecting an object on the surface before a Power Receiver is brought in.

Annex	What it covers
Annex A	Determining the reference FOD values for a Power Receiver Product
Annex B	Open-air Q test, including Q measurement, threshold setting, compensation, and implementation issues

What are the key requirements of prEN IEC 63758-8:2026?

prEN IEC 63758-8:2026 requires the Power Transmitter to prevent representative metal objects from reaching unsafe temperatures, and it expects the transmitter to use more than one detection approach where needed. The document makes clear that a single method is often not enough because “Foreign Objects” and integral “Friendly Metals” can look similar in practice.

Avoid unsafe heating

Clause 3 says the Power Transmitter Product shall ensure that Foreign Objects in the Operating Volume do not reach unsafe temperature levels. That may mean limiting power, stopping transfer, or refusing to start transfer at all.

Clause 3.1 defines Representative Foreign Objects and sets temperature limits for those examples. This matters because the standard uses those representative objects as the baseline for checking whether a transmitter is detecting the real heating risk, not just a theoretical one.

Use pre-power-transfer methods when the surface is empty or being loaded

Clause 4.1 describes the empty-surface test. The Power Transmitter first uses an Analogue Ping to look for a change in resonance, then uses a Digital Ping to distinguish a Power Receiver Product from a Foreign Object. In practice, this is the startup check that helps a transmitter avoid energizing a coin, key, or foil on the pad.

Clause 4.2 describes resonance-change detection. The Power Receiver sends the Reference Resonance Frequency and Reference Quality Factor, and the Power Transmitter compares those values with its own measured resonance properties. This allows the transmitter to detect a Foreign Object even when it arrives together with the Power Receiver, which is one of the hardest cases.

The resonance-change method is only available to EPP and EPP5 products, so system designers need to confirm that the product profile supports it before depending on it.

Use in-power-transfer monitoring to watch for later Foreign Objects

Clause 5.1 describes basic power-loss accounting. The Power Receiver regularly reports Received Power, and the Power Transmitter compares Received Power with Transmitted Power to estimate power dissipated in Foreign Objects. In practice, this is the ongoing monitoring method that helps catch an object inserted after charging has already begun.

Clause 5.1.2 requires the Power Receiver to overestimate its Received Power value within defined accuracy margins. That matters because the Power Transmitter uses the reported value to decide whether loss is acceptable, so the report must be conservative enough to avoid hiding a dangerous condition.

Clause 5.1.3 explains how the FOD threshold must be chosen using the accuracy of both sides. The transmitter has to account for measurement uncertainty so that an apparently safe reading is still safe in reality. If the loss appears too high for too long, the transmitter may request less power, change its operating point, or abort transfer.

Calibrate only when the operating volume is empty

Clause 5.2 adds calibrated power-loss accounting. This method is only available in the Extended Protocol and depends on a prior pre-power-transfer check that the operating area is empty. That makes sense in practice because calibration is only useful if the baseline is clean.

The Power Transmitter builds a calibration curve from one or more calibration points sent by the Power Receiver. The receiver steps through power levels, and the transmitter accepts the points only when the power level is stable. The document also warns not to use calibration outside the measured range, and to restart power transfer if the receiver changes operating mode or if the calibration assumptions no longer match the real setup.

Follow the document’s rules for receiver support and protocol handling

Across Clauses 4 and 5, some methods require the Power Receiver to send design information, reference values, or power reports. The Power Receiver shall provide that information when the chosen FOD method needs it.

Clause 1.3 also states that the provisions are mandatory unless they are explicitly marked as recommended, optional, note, example, or informative. For implementers, that means the default reading is “shall” unless the document clearly says otherwise.

What terms does prEN IEC 63758-8:2026 define?

• Foreign Object - A metallic object such as a coin, key, or foil that can dissipate power from the magnetic field and heat up.
• Friendly Metal - Metal that is part of the Power Receiver Product or Power Transmitter Product and can also dissipate power, even though it is not a Foreign Object.
• Operating Volume - The space in which the Power Transmitter and Power Receiver interact and where a Foreign Object can create a heating risk.
• Foreign Object Detection (FOD) - The set of methods used to detect and manage Foreign Objects so that power transfer does not create unsafe heating.
• Analogue Ping - A weak power signal used to probe the transmitter’s resonance and detect the presence of an object.
• Digital Ping - A follow-up ping used to check whether the object present is a Power Receiver Product rather than a Foreign Object.
• Reference Resonance Frequency - The reference resonance value sent by the Power Receiver for the resonance-change method.
• Reference Quality Factor - The matching reference quality-factor value sent by the Power Receiver for the resonance-change method.

Who uses prEN IEC 63758-8:2026?

prEN IEC 63758-8:2026 is used by engineers and product teams designing Qi wireless power systems, especially Power Transmitter and Power Receiver products. It is also relevant to test laboratories, validation engineers, and quality or compliance teams who need to verify that a design does not overheat Foreign Objects.

The document is aimed at systems that use flat-surface wireless charging, including products such as mobile phones and chargers. It matters to buyers and integrators because it shows what a Qi implementation must be able to detect, report, and control during real charging use.

Which standards are used with prEN IEC 63758-8:2026?

prEN IEC 63758-8:2026 refers to the Qi Specification, Communications Protocol for data packets, protocol states, negotiation, and timeout details. Many of the FOD methods in this part depend on that companion document for the communication sequence.

It also refers to the Qi Specification, Glossary for symbols used in Annex B. In addition, Clause 1.3 says the wording of provisions follows ISO/IEC Directives, Part 2, which governs how requirements, recommendations, permissions, and capabilities are expressed.

What does the prEN IEC 63758-8:2026 document contain?

The document contains the general rules for reading Qi requirements, the introduction to Foreign Object heating risks, and separate sections for pre-power-transfer and in-power-transfer FOD methods. It also includes a normative annex for determining reference FOD values and an informative annex for the open-air Q test.

Its technical content includes packet-based negotiation, resonance measurements, power-loss accounting, calibration logic, and protocol state transitions. The annexes add measurement procedures, threshold-setting guidance, and compensation methods that support design verification and test planning.

Figures and tables explain resonance shift, Q-deflection, calibration curves, representative foreign objects, packet timing, and calibration sequences. For implementers, those diagrams are useful because they turn the text into concrete test setups and operating flows for design, testing, and certification work.

SLOVENSKI STANDARD
01-november-2026
Qi specifikacija - Različica 2.3 - 8.del: Odkrivanje, zaznavanje tujkov (hitri
postopek)
Qi Specification - Version 2.3 - Part 8: Foreign object detection (Fast Track)
Ta slovenski standard je istoveten z: prEN IEC 63758-8:2026
ICS:
29.240.99 Druga oprema v zvezi z Other equipment related to
omrežji za prenos in power transmission and
distribucijo električne energije distribution networks
33.160.99 Druga avdio, video in Other audio, video and
avdiovizuelna oprema audiovisual equipment
35.200 Vmesniška in povezovalna Interface and interconnection
oprema equipment
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

100/4551/CDV
COMMITTEE DRAFT FOR VOTE (CDV)
PROJECT NUMBER:
IEC 63758-8 ED1
DATE OF CIRCULATION: CLOSING DATE FOR VOTING:
2026-09-18 2026-12-11
SUPERSEDES DOCUMENTS:
IEC TC 100 : AUDIO, VIDEO AND MULTIMEDIA SYSTEMS AND EQUIPMENT
SECRETARIAT: SECRETARY:
Japan Ms Mahiru Uehara
OF INTEREST TO THE FOLLOWING COMMITTEES: HORIZONTAL FUNCTION(S):
TC 106,TC 108
ASPECTS CONCERNED:
SUBMITTED FOR CENELEC PARALLEL VOTING NOT SUBMITTED FOR CENELEC PARALLEL VOTING
Attention IEC-CENELEC parallel voting
The attention of IEC National Committees, members of
CENELEC, is drawn to the fact that this Committee Draft for
Vote (CDV) is submitted for parallel voting.
The CENELEC members are invited to vote through the
CENELEC online voting system.
This document is still under study and subject to change. It should not be used for reference purposes.
Recipients of this document are invited to submit, with their comments, notification of any relevant patent rights of which they
are aware and to provide supporting documentation.
Recipients of this document are invited to submit, with their comments, notification of any relevant “In Some Countries”
clauses to be included should this proposal proceed. Recipients are reminded that the CDV stage is the final stage for
submitting ISC clauses. (SEE AC/22/2007 OR NEW GUIDANCE DOC).

TITLE:
Qi Specification - Version 2.3 - Part 8: Foreign object detection (Fast Track)

PROPOSED STABILITY DATE: 2029
NOTE FROM TC/SC OFFICERS:
As this is a fast-track process offered by WPC, the Word file is not provided.

electronic file, to make a copy and to print out the content for the sole purpose of preparing National Committee positions.
You may not copy or "mirror" the file or printed version of the document, or any part of it, for any other purpose without
permission in writing from IEC.

QiSpecification
ForeignObjectDetection
Version2.3
PublishDate:08December2025
DISCLAIMER
Theinformationcontainedhereinisbelievedtobeaccurateasofthedateofpublication,butisprovided“asis”andmay
containerrors.TheWirelessPowerConsortiummakesnowarranty,expressorimplied,withrespecttothisdocument
anditscontents,includinganywarrantyoftitle,ownership,merchantability,orfitnessforaparticularuseorpurpose.
NeithertheWirelessPowerConsortium,noranymemberoftheWirelessPowerConsortiumwillbeliableforerrorsin
thisdocumentorforanydamages,includingindirectorconsequential,fromuseoforrelianceontheaccuracyofthis
document.Foranyfurtherexplanationofthecontentsofthisdocument,orincaseofanyperceivedinconsistencyor
ambiguityofinterpretation,contact:[e-mail].

Contents
1. General 5
1.1 StructureoftheQi Specification 5
1.2 Scope 6
1.3 Compliance 6
1.4 References 6
1.5 Conventions 7
1.5.1 Notationofnumbers 7
1.5.2 Tolerances 7
1.5.3 Fieldsinadatapacket 7
1.5.4 Notationoftextstrings 8
1.5.5 Short-handnotationfordatapackets 8
2. Introduction 9
3. AvoidanceofForeignObjectheating 10
3.1 RepresentativeForeignObjects 11
4. Pre-powertransferFODmethods 12
4.1 Emptysurfacetest 12
4.2 Resonancechange 13
4.2.1 Measuretheresonanceproperties 14
4.2.2 Obtainreferencevalues 15
4.2.3 DeterminethepresenceofaForeignObject 16
4.2.4 InformthePowerReceiver 16
4.2.5 Stopthepowertransfer 16
5. In-powertransferFODmethods 17
5.1 Basicpowerlossaccounting 17
5.1.1 Method 17
5.1.2 Receivedpoweraccuracy 19
5.1.3 FODthreshold 19
5.2 Calibratedpowerlossaccounting 21
5.2.1 CalibratedFODthreshold 21
5.2.2 Calibrationprotocol 22
5.2.3 Calibrationperformanceconsiderations 28
AnnexA: DeterminingthereferenceFODvalues(normative) 32
AnnexB: Open-airQtest(informative) 34
B.1 Generalflowfortheopen-airQtest 35
B.2 Measuringthequalityfactor 36
B.2.1 EstimateQfromthedecayoftheringingresponsetoapulse 36
B.2.2 ComputeQfromacoilimpedancemeasurement 37
B.3 ImpactofobjectsontheQdeflection 37

B.4 CompensatedQ-deflection 38
B.5 Choosingathreshold 39
B.6 Temperaturecompensation 40
B.7 Potentialimplementationissues 40

1. General
TheWirelessPowerConsortium(WPC)isaworldwideorganizationthataimstodevelopandpromoteglobalstandards
forwirelesspowertransferinvariousapplicationareas.Animportantapplicationareacomprisesflat-surfacedevices
suchasmobilephonesandchargersoperatingatpowerlevelsupto25W.
1.1 StructureoftheQi Specification
Generaldocuments
l
Introduction
l
Glossary,Acronyms,andSymbols
l
TechnicalSystemOverview
l
PowerProfiles
Systemdescriptiondocuments
l
Mechanical,Thermal,andUserInterface
l
PowerDelivery
l
CommunicationsPhysicalLayer
l
CommunicationsProtocol
l
ForeignObjectDetection
l
NFCTagProtection
l
AuthenticationProtocol
l
MPPCommunicationsProtocol
l
MPPMagneticsDesign
l
MPP PowerDelivery
l
MPP CommunicationsPhysicalLayer
l
MPP ForeignObjectDetection
l
MPP DesignGuidelines(Informative)
l
MagneticCovers
l
ActiveAlignmentSystem
l
ThermalManagementProtocol
1.2 Scope
TheQiSpecification,ForeignObjectDetection(thisdocument)definesmethodsforensuringthatthepowertransfer
proceedswithoutheatingmetalobjectsinthemagneticfieldofaPowerTransmitter.AlthoughthePowerTransmitter
mayoptionallyuseanyofthesemethods,someofthemrequireassistancebythePowerReceiver.
1.3 Compliance
AllprovisionsintheQiSpecificationaremandatory,unlessspecificallyindicatedasrecommended,optional,note,
example,orinformative.VerbalexpressionofprovisionsinthisSpecificationfollowtherulesprovidedinISO/IEC
Directives,Part2.
Table1-Verbalformsforexpressionsofprovisions
Provision Verbalform
requirement “shall”or“shallnot”
recommendation “should”or“shouldnot”
permission “may”or“maynot”
capability “can”or“cannot”
1.4 References
Forundatedreferences,themostrecentlypublisheddocumentapplies.ThemostrecentWPCpublicationscanbe
downloadedfromhttp://www.wirelesspowerconsortium.com.

1.5 Conventions
1.5.1 Notationofnumbers
l
Realnumbersusethedigits0to9,adecimalpoint,andoptionallyanexponentialpart.
l
Integernumbersindecimalnotationusethedigits0to9.
l
Integernumbersinhexadecimalnotationusethehexadecimaldigits0to9andAtoF,andareprefixedby"0x"
unlessexplicitlyindicatedotherwise.
l
SinglebitvaluesusethewordsZEROandONE.
1.5.2 Tolerances
Unlessindicatedotherwise,allnumericvaluesintheQiSpecificationareexactlyasspecifiedanddonothaveany
impliedtolerance.
1.5.3 Fieldsinadatapacket
Anumericvaluestoredinafieldofadatapacketusesabig-endianformat.Bitsthataremoresignificantarestoredata
lowerbyteoffsetthanbitsthatarelesssignificant.Table2andFigure1provideexamplesoftheinterpretationofsuch
fields.
Table2-Exampleoffieldsinadatapacket(table)
b b b b b b b b
7 6 5 4 3 2 1 0
B (msb)
B 16-bitNumericDataField (lsb)
B OtherField
(msb)
B 16-bitNumericDataField (lsb)
Field
Figure1-Examplesoffieldsinadatapacket

1.5.4 Notationoftextstrings
TextstringsconsistofasequenceofprintableASCIIcharacters(i.e.intherangeof0x20to0x7E)enclosedindouble
quotes(").Textstringsarestoredinfieldsofdatastructureswiththefirstcharacterofthestringatthelowestbyte
offset,andarepaddedwithASCIINUL(0x00)characterstotheendofthefieldwherenecessary.
EXAMPLE: Thetextstring“WPC”isstoredinasix-bytefieldsasthesequenceofcharacters'W','P','C',NUL,NUL,and
NUL.Thetextstring“M:4D3A”isstoredinasix-bytefieldasthesequence'M',':','4','D','3',and'A'.
1.5.5 Short-handnotationfordatapackets
Inmanyinstances,theQiSpecificationreferstoadatapacketusingthefollowingshorthandnotation:
/
Inthisnotation,referstothedatapacket'smnemonicdefinedintheQiSpecification,Communications
Protocol,andreferstoaparticularvalueinafieldofthedatapacket.Thedefinitionsofthedatapacketsin
theQiSpecification,CommunicationsProtocol,listthemeaningsofthemodifiers.
Forexample,EPT/ccreferstoanEndPowerTransferdatapackethavingitsEndPowerTransfercodefieldsetto0x01.

2. Introduction
InanormalusecaseofapowertransferaccordingtotheQiSpecification,thePowerSignal(magneticfield)ofthePower
TransmitterinteractswiththePowerReceiverProductonly.However,sometimesauseraccidentallyplacesmetallic
objectssuchascoins,paperclips,keys,orpiecesofaluminiumfoilnexttoorunderneaththePowerReceiverProduct,
eitherbeforethepowertransferstarts,orwhileitisongoing.TheQiSpecificationreferstosuchobjectsasForeign
Objects.
AproblemwithForeignObjectsisthattheycandissipatepowerfromthemagneticfield,andasaresultheatupto
unsafetemperaturelevels.Thesystemshouldthereforenotinitiatethepowertransfer,limitthepowerlevel,orstopthe
powertransferwhenitdetectsthatoneormoreForeignObjectsarepresent.
Figure2-PowertransfersystemincludingaForeignObject
AfactorcomplicatingForeignObjectDetection(FOD)isthepresenceofFriendlyMetalsinthemagneticfield.AFriendly
MetalissimilartoaForeignObjectinthesensethatitcandissipatepowerfromthemagneticfield.However,unlikea
ForeignObject,itisanintegralpartofthePowerReceiverProductorPowerTransmitterProduct.Inmanycases,itis
hardforaPowerTransmittertodistinguishproperlybetweenForeignObjectsandFriendlyMetals.Typically,nosingle
methodissufficienttosolvetheproblem.Accordingly,thePowerTransmittershouldusemultiplemethodstomaximize
theprobabilityofdetectingForeignObjects,whileminimizingtheprobabilityoffalsealarms.

3. AvoidanceofForeignObjectheating
AsexplainedinSection2,Introduction,thePowerSignalcanheatupForeignObjectsthatarepresentintheOperating
Volume.Therefore,aPowerTransmitterProductshallensurethatsuchForeignObjectsdonotreachunsafe
temperaturelevels.Thismayinvolvelimitingorterminatingthepowertransfer.
ThePowerTransmittercanuseseveralapproachestopreventexcessiveofheatingForeignObjectsandapplythose
beforestartingthepowertransferand/orwhilethelatterisinprogress.ThemainusecasesthatFODshouldaddress
include:
l
AuserplacingaForeignObjectbeforeplacingaPowerReceiverProduct
l
AuserplacingaForeignObjecttogetherwithaPowerReceiverProduct
l
AuserplacingaForeignObjectafterplacingaPowerReceiverProduct
ThemethodsdescribedinSection4,Pre-powertransferFODmethods,addressthefirsttwousecases.Detectinga
ForeignObjectbeforestartingthepowertransferletsthePowerTransmittertakeoneormoreofthefollowingactions.
l
Warntheuserofapotentialunsafesituation
l
RefusetostartthepowertransferuntilauserhasremovedtheForeignObject
l
Proceedtotransferpowerbutatareducedlevel
ThemethodsdescribedinSection5,In-powertransferFODmethods,addressthethirdusecase.Theyalsoaddressthe
usecaseinwhichthePowerTransmitterproceedswiththepowertransfereventhoughitsuspectsaForeignObjectis
present.Ingeneral,thesemethodsenablethePowerTransmittertolimitthepowerlosstoForeignObjects(byreducing
thepowerlevel),withtheaimtolimittheirheating.
IfthePowerTransmitterdoesnotdetectaForeignObjectbeforestartingthepowertransfer,itmayusethatknowledge
tocalibratethesystemtoimproveitssensitivityforpowerloss(seeSection5.2,Calibratedpowerlossaccounting,for
details).However,whenitdetectsaForeignObjectandproceedstotransferpoweratareducedlevelanyway,itshould
ensurethatanyofsuchcalibrationsitmayperformdonotdegradeitssensitivity.
SomeofthemethodsdescribedinSection4,Pre-powertransferFODmethods,andSection5,In-powertransferFOD
methods,involvethePowerReceiversendinginformationaboutitsdesignpropertiestothePowerTransmitter.The
PowerReceivershallprovidethedesigninformationassociatedwithallthosemethods.
InadditiontothemethodsdescribedinSection4,Pre-powertransferFODmethods,andSection5,In-powertransferFOD
methods,thePowerTransmittercanmonitorthetemperatureofitsInterfaceSurfaceforhotspots.Moreover,itcan
activelycoolitsInterfaceSurfacetodrainheatawayfromthePowerReceiverandForeignObjects.

3.1 RepresentativeForeignObjects
ForeignObjectscanhavemanydifferentsizes,shapes,andmaterialcompositions.Toaddressthisdiversity,theQi
Specification,ForeignObjectDetection(thisdocument)definestherequiredFODcapabilitiesofaPowerTransmitterin
termsofasetofRepresentativeForeignObjects.Table3liststheseobjects.
Table3-RepresentativeForeignObjects
Designator Shape Material Dimensions Limit/C°
RFO#1 Disk RFe160 ø15mm, 60
1mmthick
(Steel1.1011)
accordingto
DIN EN 10027-2
RFO#2 Ring ENAW-6082(3.2315) ø20mm(inner) 60
accordingto ø22mm(outer)
1mmthick
DIN EN 573
RFO#3 Foil ENAW-1050A(3.0255) ø20mm, 80
0.1mmthick
accordingto
DIN EN 573
RFO#4 Disk ENAW-6082(3.2315) ø22mm, 60
1mmthick
accordingto
DIN EN 573
WhenoneoftheRepresentativeForeignObjectsispresentintheOperatingSpace,thePowerTransmittershallnotheat
ittoatemperatureabovethelimitassociatedwiththatobject.

4. Pre-powertransferFODmethods
APowerTransmittercanusedifferentmethodstodetectForeignObjectsbeforeinitiatingapowertransfertoaPower
Receiver.SomeofthesemethodsdependonthePowerReceiverprovidinginformationaboutitsdesignproperties.
WiththemethoddescribedinSection4.1,Emptysurfacetest,thePowerTransmitterensuresthatitwillonlystartthe
powertransferifitsInterfaceSurfacewasemptyjustbeforeauserplacedaPowerReceiverProduct.Itdoessoby
waitingforausertoplaceanobject,determiningifitisaForeignObject,andifthatisthecase,waitingfortheuserto
removeit.
ThePowerTransmittertypicallyusesanAnaloguePingtodeterminewhetherthereisanobjectinitsOperatingVolume.
ItappliesaweakPowerSignalandlooksforchangestoitsresonancepropertiesasasignofthepresenceofanobject.If
anobjectispresent,thePowerTransmitterfollowsupwithaDigitalPing,towhichonlyaPowerReceiverProductwill
respond.IfthereisnoresponsetotheDigitalPing,thePowerTransmitterassumesthattheobjectonitssurfaceisa
ForeignObject.
Theempty-surfacetestcannotdetectaForeignObjectthatarrivessimultaneouslywithaPowerReceiverProduct(e.g.,a
pieceofaluminiumfoilstickingtothePowerReceiverProduct).TodistinguishtheForeignObjectfromFriendlyMetal
withinthePowerReceiverProduct,thePowerTransmittercanquantifythechangesinitsresonancepropertiesand
lookfordifferenceswiththeexpectedchangesthatthepresenceofthePowerReceiverProductalonewouldinduce.
Accordingly,withthemethoddescribedinSection4.2,Resonancechange,thePowerReceiverprovidestheexpected
inductanceofareferencetankcircuitwhenthePowerReceiverProductisplacedonthePowerTransmitterProduct.
ThePowerTransmittercanusethisinformationtodetermineifthereisaForeignObjectbetweenitselfandthePower
ReceiverProduct.Tomakeasufficientlyaccuratedetermination,thePowerTransmittershouldaccountfordifferences
ofitsdesignwiththereferencetankcircuit.
NOTE:BecausethePowerReceiverprovidestherelevantinformationinthenegotiationphaseoftheEnhancedProtocol(seetheQi
Specification,CommunicationsProtocol,fordetails),theresonancechangemethodisavailabletoEPPandEPP5productsonly.
4.1 Emptysurfacetest
Figure3showsaconceptualflowdiagramthatdescribeshowtheempty-surfacetestfitsinthepre-powerstartupflow.
Basically,theflowcontainstwoloops.InthefirstloopthePowerTransmitterwaitsforausertoplaceanobject.Next,if
theobjectrespondstoaDigitalPing,theobjectisaPowerReceiverProduct,andthePowerTransmitterproceedsto
deliverpower.Otherwise,theobjectisaForeignObject,andinthesecondloop,thePowerTransmitterwaitsforthe
usertoremoveit(backinloop1).Accordingly,atthestartofthefirstloop,theInterfaceSurfaceisempty.
Figure3-Conceptualflowdiagramoftheempty-surfacetestatstartup

Practicalimplementationsshouldenhancethisconceptualflowtoaddresscornercasessuchas
l
TheuserinitiallyplacingthePowerReceiverProductinamisalignedposition,whereitdoesnotrespondtoa
DigitalPing,andsubsequentlymovingittoabetter-alignedposition
l
TheAnaloguePingnotbeingabletodetectthepresenceofaPowerReceiverProductwhenthelatterhasonlya
weakimpactonthePowerTransmitter'sproperties
l
CommunicationserrorscorruptingtheresponsetotheDigitalPing
Examplesofenhancementstoaddressthesecornercasesincludetheuseoftimeouts,flags,multiplethresholds,and
more.Inmoredetail,whenwaitingforausertoplaceaPowerReceiverProduct,thePowerTransmittercanissue
DigitalPingsoccasionallytoensurethatitsAnaloguePingsdidnotmissdetectingaPowerReceiverProduct.This
approachcanalsohelptodealwithinitiallymisalignedPowerReceiverProductswhencombinedwithcontinuous
monitoringofanychangesinthePowerTransmitter'sproperties.
WheninitiatinganAnaloguePing,thePowerTransmittergeneratesaweakPowerSignalandmeasuresthequality
factorofitstankcircuit.Ifthemeasuredvalueissufficientlydifferentfromtheemptypadvalue(asstoredinthePower
Transmitter),anobjectispresent.AnnexB ,providesdetailsonhowtomeasurethequalityfactor,howtoevaluatethe
impactofanobjectonthemeasuredvalue,howtochooseanappropriatethreshold,andhowtocompensateforvarious
driftsinthesystem.
4.2 Resonancechange
WhenauserplacesaPowerReceiverProductinaPowerTransmitter'sOperatingVolume,theinductanceofthePower
Transmitter'scoiltypicallyincreasesduetotheproximityofthePowerReceiver'sShielding.Thisresultsinadecreaseof
thePowerTransmitter'sresonancefrequency.Atthesametime,powerabsorptionintheFriendlyMetalsofthePower
ReceiverProductcausesthequalityfactoroftheresonancetodecrease.
Figure4showsanexampleofthiseffect,withthecurvesinthediagramillustratingthebehaviouroftheresonance
undervariousconditions.WhentheOperatingVolumeisempty,theresonanceoccursatthefrequency (darkcurve).
WhenaPowerReceiverProductispresentintheOperatingVolume,theresonanceshiftstoa(typicallylower)
frequency .ThemagnitudeoftheshiftdependsonthedesignpropertiesofthePowerTransmitterandthePower
ReceiverProduct,aswellasonthepositionofthelatterintheOperatingVolume.Forexample,theshiftistypically
smallerforaphoneinaprotectivecasethanforthesamephonewithoutsuchacase.Thereasonistheincreased
distancebetweenthephone'sShieldingandthePowerTransmitter'scoil.
Figure4-ResonanceshiftcausedbyaPowerReceiverProductandForeignObjects
ThedashedcurvesinFigure4showtheshiftoftheresonancecurvewhenaForeignObjectispresentintheOperating
VolumeinadditiontothePowerReceiverProduct.Typically,theForeignObjectcounterstheshiftinducedbythePower
ReceiverProduct,andreducesthestrengthoftheresonance(i.e.thequalityfactor).ThereasonisthattheForeign
ObjectintroducesapowerlossandshieldsferritesinthePowerReceiverProductfromthePowerTransmitter'scoil.

ThePowerTransmittercanusethechangeintheresonancefrequencytodeterminewhetheraForeignObjectispresent
intheOperatingVolume.However,itneedshelpfromthePowerReceivertodoso.ThisisbecauseonePowerReceiver
ProductcanproducethesamechangeasanotherPowerReceiverProductandaForeignObjectcombined.Tosupport
theresonancechangeFODmethod,thePowerReceivershallsendaReferenceResonanceFrequency anda
ReferenceQualityFactor usingFODdatapacketsinthenegotiationphaseofthecommunicationsprotocol.
TheReferenceResonanceFrequencyandtheReferenceQualityFactoraretheresonancefrequencyandqualityfactorof
areferencetankcircuitloadedwiththePowerReceiverProduct.SeeSection4.2.2,Obtainreferencevalues,fordetails.
ToensurethatcontributionsofForeignObjectstotheresonancechangedominateoverthoseofFriendlyMetals,anEPP
PowerReceiverProductshallhaveaReferenceQualityFactorof .
NOTE:TheReferenceResonanceFrequencyandtheReferenceQualityfactorarenotpropertiesoftheresonanceinthePower
Receiver'stankcircuit.Instead,theyreflecthowFriendlyMetalsandferritesinthePowerReceiverProductaffectthe
resonanceinthePowerTransmitter'stankcircuit.
Theresonance-changebasedFODmethodthereforeconsistsofthefollowingsteps.
1. Measuretheresonanceproperties,i.e. and .
2. Obtainreferencevaluesforthesequantities,i.e. and .
3. DeterminetheprobabilitythataForeignObjectispresent.
4. InformthePowerReceiveriftheprobabilityexceedsathreshold.
5. StopthepowertransferiftheriskofheatingaForeignObjecttoanunsafetemperatureistoohigh.
Thefollowingsectiondescribestepsofthemethodindetail.
4.2.1 Measuretheresonanceproperties
ThePowerTransmittershouldmeasureitsresonanceproperties—asaffectedbythepresenceofaPowerReceiverin
theOperatingVolume—beforeexecutingaDigitalPingandwakingupthePowerReceiver.IfthePowerReceiverwould
wakeup,theadditionalloadaddstotheQfactor,yieldingaspuriousresult.Accordingly,thePowerTransmittershould
useaslowaPowerSignalaspossible.
NOTE:ThePowerSignalislowenoughifthePowerTransmitterkeepstherectifiedvoltageofTPR#MP3below0.85V.
Therecommendedmethodofmeasuringtheresonancepropertiesistomakeafrequencysweep,whilemeasuringthe
voltage appliedtothetankcircuitaswellastheresultingvoltage acrossthecoil.Theratio / ofthesetwo
voltagesatthehighestpointoftheshiftedresonanceyieldsthequalityfactor .
Figure5-Measuringtheresonanceproperties

4.2.2 Obtainreferencevalues
OncethePowerTransmitterhasmeasuredthepropertiesofitsresonance,itshouldexecuteaDigitalPingtowakeup
thePowerReceiver.ThelattershallsendFOD/rfandFOD/qfdatapacketsinthenegotiationphaseofthe
communicationsprotocoltoprovideitsReferenceResonanceFrequency andReferenceQualityFactor ;see
theQiSpecification,CommunicationsProtocol,fordetails.
TheReferenceResonanceFrequencyandReferenceQualityFactorarethepropertiesofareferencecoilassemblyas
affectedbytheproximityofthePowerReceiverProduct.Figure6providesanillustrationofthereferencecoilassembly,
whichconsistsofaferrite,acoil,andacover.Table4definesitsproperties.Fordetailsaboutdetermining and
,seeSectionAnnexA,DeterminingthereferenceFODvalues(normative).
NOTE:ThereferencecoilassemblyisbasedontheA10andMP-A1PowerTransmitterdesigns.
Figure6-Referencecoilassembly
Table4-Propertiesofthereferencecoilassembly
Dimension Value Unit
Ferrite:relativepermeability =650+ ×25
µ
Length 53.3 mm
Width 53.3 mm
Thickness 2.54 mm
Coil;centeredontheferrite;105×40AWGtype2litzwire
Outerdiameter 43 mm
Innerdiameter 20.5 mm
Thickness 2.1 mm
Numberofturnsperlayer 10 N/A
Numberoflayers 2 N/A
Cover:magneticallypassivematerial
Thickness 2.5 mm
Atanoperatingfrequencyof100kHz,theinductanceofthereferencecoilassemblyisabout =25µHwitha
resistanceofabout =100mΩ.
4.2.3 DeterminethepresenceofaForeignObject
ThePowerTransmittershouldusethemeasuredresonancefrequency ,themeasuredqualityfactor ,thereceived
ReferenceResonanceFrequency ,andthereceivedReferenceQualityFactor todeterminetheprobability
thataForeignObjectispresentinitsOperatingVolume.Inthecalculationsinvolved,thePowerTransmittershould
accountforitsdesigndifferenceswiththereferencecoilassemblyusedtodeterminethereferencevalues.
4.2.4 InformthePowerReceiver
ThePowerTransmittershallinformthePowerReceiverabouttheprobabilitythataForeignObjectispresentinthe
OperatingVolume.Iftheprobabilityisbelowathreshold,itshallrespondtotheFODStatusdatapacketwithACK.Ifthe
probabilityisabovethethreshold,itshallrespondwithNAK.SeetheQiSpecification,CommunicationsProtocol,for
detailsaboutabortingthepowertransferwhenthePowerTransmitterdiscoversaForeignObject.
NOTE:WhenthePowerTransmitterhasnotyetreceivedbothreferencevalues,itmaynotbeabletoconfidentlydeterminethe
probabilityofaForeignObjectbeingpresent.Inthatcase,itmayrespondwithACKtoFODStatusdatapacketsuntilithasall
valuesitneeds.
4.2.5 Stopthepowertransfer
UponreceivingaNAKresponsetoanFODdatapacket,aPowerReceivershallswitchtothepowertransferphaseofthe
BaselineProtocol(seetheQiSpecification,CommunicationsProtocol)limitingitsLoadPowerlevelto5Worless.Ifthe
PowerTransmitterassessesthattheriskofheatingaForeignObjecttounsafetemperaturesistoohigh,itmayremove
thePowerSignal.
5. In-powertransferFODmethods
APowerTransmittercanuseseveralmethodstodetectForeignObjectswhilethepowertransfertoaPowerReceiveris
inprogress.MostofthesemethodsdependonthePowerReceiverprovidinginformationabouttheongoingpower
transfer.
OnemethodinvolvesestimatingthepowerlosstoForeignObjectsbybalancingtheTransmittedPowerandReceived
Powerlevels.Toenablethismethod,thePowerReceivershallprovidesufficientlyaccurateReceivedPowerleveldatato
thePowerTransmitteronaregularbasis.BecausethisFODmethodinvolvesone-waycommunicationsfromthePower
ReceivertothePowerTransmitteronly,itappliestoproductsinallpowerprofiles.Section5.1,Basicpowerloss
accounting,describesthismethodindetail.
Animprovementofthisbasicmethodiscalibratedpowerlossaccounting.Thismoreadvancedmethodisavailableto
EPPandEPP5productsonlybecauseitusespropertiesoftheEnhancedProtocol(seetheQiSpecification,
CommunicationsProtocol,fordetails).Moreover,itassumesthatatleastonepre-powertransferFODmethodisavailable
aswell.Typically,thePowerTransmitterandPowerReceivercalibratetheestimatedpowerlossatthestartofthe
powertransferphase.Section5.2,Calibratedpowerlossaccounting,describesthismethodindetail.
5.1 Basicpowerlossaccounting
InthisFODmethod,thePowerTransmitterestimatestheamountofpower dissipatedinForeignObjectsusingthe
ReceivedPowerleveldataitreceivesfromthePowerReceiver.Iftheestimatedpowerlossexceedsathreshold
forsomeperiod,thereisariskofheatingForeignObjectstounsafetemperatures.Inthatcase,Power
Transmittermaytakeoneofthefollowingactions.
l
RequestthePowerReceivertoreduceitspowerconsumption(ExtendedProtocolonly)
l
IgnorethePowerReceiver'sCEdatapacketsandchangeitsoperatingpointtoreducetheTransmittedPower
level(andthereforetheamountofpowerdissipatedintheForeignObjects)
l
Abortthepowertransfer
NOTE:ExperimentsandsimulationsofthetemperatureriseinForeignObjectsofvarioussizes,shapesandmaterialscompositions
haveshownthatapowerdissipationofuptoabout500mWsuchobjectsisacceptableinmostcases.
5.1.1 Method
Thepower thatForeignObjectsdissipatefromthePowerSignalisequaltothedifferenceoftheTransmittedPower
andReceivedPower ,i.e.
Inthisequation, representstheinputpowertothePowerTransmitter; theoutputpowerfromthePower
Receiver; thepowerlossinthePowerTransmitter;and thepowerlossinthePowerReceiver.Figure7
illustratestheequation.
Figure7-Powerlossaccounting
ThepowerlossincurredinaPowerReceiverProducttypicallyincludesthefollowingcontributions.
l
Thepowerlossinthetankcircuit
l
Thepowerlossintherectifier
l
Thepowerlossinferrite(s)thatservetoconstrainthemagneticfield
l
ThepowerlossinmetalpartsofthePowerReceiverProductthatareexposedtothemagneticfield
Bymeasuringitsoperatingcurrent,thePowerReceivercandetermineitstankcircuitandrectifierlosseswithrelatively
goodaccuracy.However,itismoredifficultforthePowerReceivertomeasureitsferriteandFriendlyMetallosses.
Moreover,thelattercandependstronglyonthepositionofthePowerReceiverProductintheOperatingVolume.Asa
result,thePowerReceivercantypicallydetermineitsferriteandFriendlyMetallosseswithmuchlessaccuracythanits
circuitlosses.
ThepowerlossincurredinaPowerTransmitterProducttypicallyincludesasimilarsetofcontributions.
l
Thepowerlossintheinverter
l
Thepowerlossinthetankcircuit
l
Thepowerlossinferrite(s)thatservetoconstrainthemagneticfield
l
ThepowerlossinmetalpartsofthePowerTransmitterProductthatareexposedtothemagneticfield
LikeaPowerReceiver,aPowerTransmittercandetermineitsinverterandtankcircuitlosseswithrelativelygood
accuracy,anditsferriteandFriendlyMetallosseswithmoredifficulty.However,sincethelattertypicallydependonly
weaklyonthepositionofthePowerReceiverProductintheOperatingVolume,thePowerTransmittercandetermine
themwithrelativelygoodaccuracyaswell—andinmostcaseswithmuchbetteraccuracythanaPowerReceivercan
determineitsferriteandFriendlyMetallosses.
Inthepowerlossaccountingmethod,thePowerTransmittermonitorstheForeignObjectloss whilethepower
transferisongoing.Hereto,thePowerReceivershalldetermineitsReceivedPowerlevelandregularlysendthis
informationtothePowerTransmitter.ThereportedReceivedPowerlevelistheaverageoveratimewindowpreceding
thedatapacketsusedtocommunicateit.TheoffsetandsizeofthiswindowareelementsofthePowerTransfer
Contract.Fordetails,seetheQiSpecification,CommunicationsProtocol.
ToestimatetheForeignObjectloss asaccuratelyaspossible,thePowerTransmittershoulddetermineits
TransmittedPowerlevelinatimewindowthatmatchesthePowerReceiver'stimewindowaswellaspossible.Because
thePowerTransmitterdoesnotknowwhenthePowerReceiverwillreportitsReceivedPowerlevelnext,itshould
determineitsTransmittedPowerlevelinslidingwindowsandselectthebestmatch.SeeFigure8foranillustration.
Figure8-SlidingwindowsfordeterminingtheTransmittedPowerlevel
Inthisexample,thePowerTransmitterstartstomeasureitsTransmittedPowerlevelafterreceivingadatapacket.It
usesawindowsizeequaltotheoneinthePowerTransferContract,andanappropriateslidingoffset .Smaller
slidingoffsetsyieldslargeroverlapsbetweenthePowerTransmitterandPowerReceiverwindows.Forexample,a
slidingoffsetof25%ofthewindowsizeyieldsanoverlapof75%intheworst-casemisalignment.WhenthePower

Transmitterdetectsthepreambleofadatapacket,itstopstakingmeasurements.IfthereceiveddatapacketwasanRP8
orRPdatapacket,ittakesthelastmeasuredTransmittedPowervaluefromtheappropriateslidingwindowand
calculatestheForeignObjectloss.Ifthislossisbelowthethreshold,itsendsanACKresponse(mode0,1,and2ofRP
datapacketsonly)andstartsmeasuringitsTransmittedPowerlevelagain.Ifthelossisabovethethresholdseveral
timesinarow,thePowerTransmittershouldconcludethataForeignObjectispresentandtakeappropriateactionto
preventheatingit.
5.1.2 Receivedpoweraccuracy
TheeffectivenessofthepowerlossaccountingmethoddependsontheaccuracywithwhichthePowerTransmitterand
PowerReceivercandeterminetheirTransmittedPowerandReceivedPowerlevels.AsdiscussedinSection5.1.1,Method
,thePowerReceivertypicallydoesnotdetermineitsReceivedPowerlevelbymeasuringitdirectly,butinsteadby
calculatingitfromameasuredoutputpower(totheLoad),anestimateofitscircuitloss(tankcircuit,rectifier,etc.),and
anestimateofitsferriteandFriendlyMetallosses.
ThePowerReceivershallreportanoverestimatedReceivedPowervalue initsRP8andRPdatapacketssuchthat
where representstheReceivedPowerlevel,and representsamarginasprovidedinTable5.
Table5-EstimatedReceivedPoweraccuracy
EstimatedReceivedPower Unit
≤ 5W 350 mW
5W< ≤10W 500 mW
10W< 750 mW
NOTE:Table5impliesthatareportedamountofReceivedPower =5W,impliesanactualamountofReceivedPower in
therangeof4.65Wuptoandincluding5W.
Itishard—ifnotimpossible—foraPowerReceivertoestimatethecontributionsofthepowerlossinitsFriendlyMetals
totheReceivedPowerlevelwithanaccuracythatisindependentofitspositionandorientationintheOperating
Volume.Theestimatethereforesuffersfromasystematicerrororbiasthatdependsonthepositionandorientation.If
thelatterrepresenttoolargeamisalignmentbetweenthePowerTransmitterandPowerReceiver,theEstimated
ReceivedPoweraccuracymayfailtheaboverequirement.
5.1.3 FODthreshold
ThediagramsinFigure9illustratetheimpactoftheestimatedReceivedPowerandTransmittedPowerlevelaccuracies
ontheFODthreshold.Thehorizontalaxesrepresentthepowerlevel,whichincreasestowardstheright.Accordingto
therequirementgiveninsection5.1.2,ReceivedPoweraccuracy,theactualReceivedPowerlevelissomewhereinthe
verticallyhatchedareatotheleftoftheEstimatedReceivedPowerlevel .AssumingthatthePowerTransmitter
estimatesitsTransmittedPowerlevelwithanaccuracyof ,theactualTransmittedPowerlevelissomewherein
thehorizontallyhatchedareacenteredontheestimatedTransmittedPowerlevel  .
AForeignObjectthatcanheatuptounacceptablyhightemperaturesispresentintheOperatingVolumeiftheactual
ForeignObjectloss exceedsathreshold ,i.e.
SincethePowerTransmitteronlyknowstheestimatedpowerlevels,itshouldusethefollowingcondition

where helpsthePowerTransmittertoensurethattheactualForeignObjectloss does
notexceedthethreshold ,regardlessoftheuncertaintyintheestimatedpowerloss.Seetheexamplebelowfor
additionaldetails.
Figure9-AccuracyandFODthreshold
EXAMPLE: AssumeaPowerTransmitteraccuracyof andanEstimatedReceivedPowerlevelofat
most5W.Asshownindiagrams(a)and(c),thePowerTransmittershoulduse
toensurethattheForeignObjectlossdoesnotgoabove
undetected.Inthiscase,theaccuracyrangesdonotoverlap.Diagrams(b)and(d)
showthatifthePowerTransmitterhasanaccuracyof itshouldset
toensurethesame.However,inthiscase,theaccuracy
rangesoverlap,andfalse-positiveFODtriggerscanoccurasexplainedlaterinthissection.Toavoidthe
latterissues,thePowerTransmittercanuse with obtaining
thesituationofdiagram(a).However,inthiscaseForeignObjecttemperaturescanrisetohigherlevels
withoutthePowerTransmitterbeingabletodetectsuch.
OncloserinspectionofthediagramsinFigure9,itfollowsthattheFODsensitivitydependsonbiasesinthepowerlevel
estimates.ThesebiasesaretypicallydifferentfordifferentpositionsandorientationsofthePowerReceiverinthe
OperatingVolume.Forexample,forpositionsandorientationsofthePowerReceiverintheOperatingVolumewherethe
actualReceivedPowerlevelisclosetotheEstimatedReceivedPowerlevel,andtheactualTransmittedPowerlevelis
closetothelowerboundoftheaccuracyrange,theFODsensitivityishigh.Inthiscase,thePowerTransmitterdetectsa
ForeignObjectlossclosetozero.Ontheotherhand,forpositionsandorientationswheretheactualReceivedPower
levelisclosetolowerboundofthetolerancerange,andtheTransmittedPowerlevelisclosetotheupperbound,the
PowerTransmitterdetectsForeignObjectlossesabove only,i.e.theFODsensitivityislow.
Asafinalobservation,ifthePowerTransmitterusesoverlappingaccuracyrangestokeeptheuncertaintyinthe
estimatedForeignObjectlossbelowthethreshold ,asshownindiagram(b)and(d)ofFigure9,thereisarisk

offalse-positiveFODtriggers.Forexample,inthecrosshatchedareaofdiagram(d),theactualReceivedPowerleveland
theactualTransmittedpowerlevelcanbeequal,i.e.noForeignObjectispresent,whiletheestimatedlevelsindicatean
overrunoftheFODthreshold.Asshownindiagram(c),suchfalsepositivesdonotoccurwithnon-overlappingaccuracy
ranges,becauseincaseofanFODthresholdoverrun,theactualTransmitterPowerlevelalwaysexceedstheactual
ReceivedPowerlevel.
5.2 Calibratedpowerlossaccounting
BecauseitisdifficultforaPowerReceivertodetermineitsferriteandFriendlyMetallossesaccurately,itislikelythat
itsestimateoftheReceiverPowerlevelhasasystematicerrororbias.Moreover,thisbiastypicallydependsstronglyon
thepositionandorientationofthePowerReceiverProductintheOperatingVolume.APowerTransmittercanenhance
itsFODsensitivityandrobustnessbycalibratingthisbiasaway.However,beforedoingsoitshouldverifythatthe
OperatingVolumeisemptyofForeignObjects.Otherwise,itrisksreducingitsFODperformanceratherthanenhancing
it.
PowerlevelcalibrationisavailableonlyintheExtendedProtocol.Inthisprotocol,thePowerTransmittercanusethe
methodsdiscussedinSection4,Pre-powertransferFODmethods,toverifythatitissafetocalibratetheestimatedpower
levels.
5.2.1 CalibratedFODthreshold
WhencalibratingtheestimatedForeignObjectloss,thePowerTransmitterhasdeterminedthattheOperatingVolumeis
emptyofForeignObjects,andthereforeitknowsthattheTransmittedPowerandReceivedPowerlevelsareequal,i.e.
;seeFigure10(a).However,theestimatedTransmittedPowerandEstimatedReceivedPowerlevelsare
typicallynotequal.ThehorizontallyhatchedareasinFigure10indicateswheretheestimatedTransmittedPowerlevel
canberelativetotheactualTransmittedPowerlevel.TheverticallyhatchedareaindicatesthesamefortheEstimated
ReceivedPowerlevel.Clearly,theestimatedForeignObjectlosstypicallyhasanegativevalueatcalibration,i.e.
.Onlywhenbothestimatedpowerlevelsareinthecross-hatchedarea,i.e.theoverlap
betweenthehorizontallyandverticallyhatchedareas,theestimatedPowerlosstobeintherangefrom to
.
NOTE:IfthePowerTransmitterobtainsavalueoutsidethisrangewhencalibrating,toshouldconcludethatsomethingiswrong,and
refrainfromusingthecalibrationdata.Forexample,atanEstimatedReceivedPowerLevelof8W,theassociatedaccuracyis
= 500 mW.AssumingthePowerTransmitter'saccuracyis150mW,thecalibrationdatashouldbeintherangefrom-
650mWupto150mW.Topreventanendlessloopofrestarts,thePowerTransmittershouldNAKtheCFG/epdatapacketin
arestart,forcingthesystemtooperateusingtheBaselineProtocolandlimitingthepowerlevelto5W.
Figure10-CalibratingtheestimatedForeignObjectloss
Usingthecalibrationdata,thePowerTransmittershouldassumethataForeignObjectispresentintheOperating
VolumeiftheestimatedForeignObjectloss exceedstheeffectiveFODthreshold ,i.e.

Figure10(b)showshowthePowerTransmittercandeterminetheactualForeignObjectloss quiteaccurately
usingthecalibrationdata.Inthisexample,theestimatedForeignObjectloss isfarsmallerthantheactual
ForeignObjectloss.
InordertoenhanceitsFODreliabilityoverawiderrangeofpowerlevels,thePowerTransmittershouldcollect
calibrationdataatmultipleoperatingpointsandconstructacalibrationcurve.ThePowerReceivershouldenablethe
PowerTransmittertocollectseveralcalibrationdatapointsinthefirstfewsecondsofthepowertransferphaseby
steppingthroughseveralpowerlevels.SeeSection5.2.2,Calibrationprotocol,forthedetails.
Figure11showsanexampleofacalibrationcurve.Inthisexample,thePowerTransmitterhascollectedsixcalibration
datapoints(bluecircles).TheshadedareasofthediagramindicatetheaccuracyrangesofthePowerReceiver(blue)
andPowerTransmitter(gray).Alldatapointsareinthisarea,asexplainedatthetopofthissection.ThePower
Transmittershoulduseabest-fitlinethroughthedatapoints(dashedblueline)todeterminecalibrationvaluesat
powerlevelswhereitdoesnothaveactualmeasurementdata.ThecalibratedFODthreshold(solidredline)follows
fromaddingthislinetotheuncalibratedthreshold (thedashedredline).
NOTE:InsteadofcalibratingtheestimatedForeignObjectloss,thePowerTransmittermaycalibratetheEstimatedReceivedPower,
Inthatcase,theaboveequationreadsas ,with thepower-level
independentthreshold.
Figure11-CalibrationcurveandeffectiveFODthreshold
WhendeterminingtheFODthreshold,thePowerTransmittershouldnotonlyaccountforinductionheatingcausedby
theForeignObjectloss,butalsoforheatconductionfromthePowerTransmitterProductand/orPowerReceiver
ProducttoForeignObjects.Typically,thismeansthatthePowerTransmittershoulduseanFODthresholdthat
decreaseswithincreasingpowerlevel(becausethesurfacetemperaturesofPowerTransmitterProductsandPower
ReceiverProductsaregenerallyhigherathighertransferredpowerlevels).ThedashedredlineinFigure11should
thereforenotbehorizontalbuthaveanegativeslope.
5.2.2 Calibrationprotocol
AsshowninFigure12,thePowerReceivershallinitiatethecalibrationprotocolfromstate11atthestartofthepower
transferphase.Accordingly,thefirstRPdatapacketaPowerReceiversendsshallbeRP/1.IfthePowerTransmitterhas

confirmeditssupportforrecalibration(i.e.ACK'edanSRQ/rcsdatapacket),thePowerReceivermayinitiatethe
calibrationprotocolmultipletimes.Otherwise,thePowerReceivershallnotinitiatethecalibrationprotocolagaininthe
powertransferphase.SeeSection5.2.3,Calibrationperformanceconsiderations,foradditionalinformation.Figure13
providesthedetailsofthecalibrationprotocol.
Figure12-Calibrationprotocol
5.2.2.1 Transitionsfromstate11
State11isthemainstateofthepowertransferphase.SeetheQiSpecification,CommunicationsProtocol,fordetailsand
additionaltransitions.
l
RP/1.ThePowerReceiverhassentthefirstcalibrationdatapoint,initiatingthecalibrationprotocol.The
ReceivedPowerlevelreportedinthisdatapacketshouldbeatmost10%oftheReferencePowerlevelcontained
inthePowerTransferContract.
5.2.2.2 Transitionsfromstate13
SeetheQiSpecification,CommunicationsProtocol,fordetails.
Figure13-Calibrationprotocoldetails
5.2.2.3 Transitionsfromstate11-1
Instate11-1,thePowerReceiverprovidesthefirstcalibrationdatapoint.Thefollowingtransitionsarepossiblefrom
thisstate.
NOTE:State11-1isarestrictedformofstate11.
l
RP/1.ThePowerReceiverhassentanRP/1datapacket.TheReceivedPowerlevelreportedinthisdatapacket
shouldbeatmost10%oftheReferencePowerlevelcontainedinthePowerTransferContract.
l
CE.ThePowerReceiverhassentaCEdatapacket.
l
PTx:CHS.ThePowerTransmitterhasreceivedaCHSdatapacket.
NOTE:Thistransitionservestoensurebackwardscompatibilitywithversion1.2PowerReceivers.
l
PTx:PROP.ThePowerTransmitterhasreceivedaPROPdatapacket.ThePowerTransmittershallignoreall
unsupportedPROPdatapackets.
l
PTx:reserved.ThePowerTransmittershallignoreallreserveddatapackets.
l
PTx:calibration-timeout.ThePowerTransmitterhasnotbeenabletoconstructacalibrationcurvebeforethe
calibrationtimeout .Seebelowforthevalueof .
l
PTx:illegaldatapacket.ThePowerTransmitterhasreceivedadatapacketotherthanaCE,CHS,PROP,RP/1,or
reserveddatapacket.
5.2.2.4 Transitionsfromstate12-1
Thisstateisidenticaltostate12inthepowertransferphase.SeetheQiSpecification,CommunicationsProtocol,for
details.
5.2.2.5 Transitionsfromstate13-1
Instate13-1,thePowerTransmitterprovidesfeedbackaboutthefirstcalibrationdatapoint.Thefollowingtransitions
arepossiblefromthisstate.
NOTE:State13-1isarestrictedformofstate13.
l
ACK.ThePowerTransmitterhasacceptedthefirstcalibrationdatapoint.ItisreadyforthePowerReceiverto
stepupitspowerlevel.ThePowerTransmittershouldwaitforthepowerleveltostabilizebeforeusingthis
transition.ItmayconsiderthepowerleveltobestablewhenithasreceivedtwoconsecutiveCE/0datapackets
(i.e.twoconsecutiveCEdatapacketscontainingaControlErrorValueof0).
l
NAK.ThePowerTransmitterdoesnotacceptthefirstcalibrationdatapointyet.
l
PRx:response-timeout.ThePowerReceiverhasnotdetectedaresponsetotheRP/1datapacketbeforethe
timeout .SeetheQiSpecification,CommunicationsProtocol,forthetimeoutvalue.
5.2.2.6 Transitionsfromstate11-2
Instate11-2,thePowerReceiversendsanextcalibrationdatapoint.Thefollowingtransitionsarepossiblefromthis
state.
NOTE:State11-2isarestrictedformofstate11.
l
RP/2.ThePowerReceiverhassentanRP/2datapacket.TheReceivedPowerlevelreportedinthefinalRP/2
datapacketshouldbeclosetotheReferencePowerlevelcontainedinthePowerTransferContract.
l
CE.ThePowerReceiverhassentaCEdatapacket.
l
PTx:CHS.ThePowerTransmitterhasreceivedaCHSdatapacket.
NOTE:Thistransitionservestoensurebackwardscompatibilitywithversion1.2PowerReceivers.
l
PTx:PROP.ThePowerTransmitterhasreceivedaPROPdatapacket.ThePowerTransmittershallignoreall
unsupportedPROPdatapackets.
l
PTx:reserved.ThePowerTransmittershallignoreallreserveddatapackets.
l
PTx:calibration-timeout.ThePowerTransmitterhasnotbeenabletoconstructacalibrationcurvebeforethe
calibrationtimeout .Seebelowforthevalueof .
l
PTx:illegaldatapacket.ThePowerTransmitterhasreceivedadatapacketotherthanaCHS,PROP,CE,RP/1,
RP/2,orreserveddatapacket.
l
PTx:RP/1.ThePowerTransmitterhasreceivedanRP/1datapacket.ThiscanhappenifthePowerReceiverhas
timedoutontheACKResponsePatternfollowingtheprecedingRP/1datapacket.

5.2.2.7 Transitionsfromstate12-2
Thisstateisidenticaltostate12inthepowertransferphase.SeetheQiSpecification,CommunicationsProtocol,for
details.
5.2.2.8 Transitionsfromstate13-2
Instate13-2,thePowerTransmitterprovidesfeedbackaboutthenextcalibrationdatapoint.Thefollowingtransitions
arepossiblefromthisstate.
NOTE:State13-2isequivalenttostate13.
l
ACK.ThePowerTransmitterhasacceptedthenextcalibrationdatapoint.ItisreadyforthePowerReceiverto
stepupitspowerlevel.ThePowerTransmittershouldwaitforthepowerleveltostabilizebeforeusingthis
transition.ItmayconsiderthepowerleveltobestablewhenithasreceivedtwoconsecutiveCE/0datapackets
(i.e.twoconsecutiveCEdatapacketscontainingaControlErrorValueof0).
l
NAK.ThePowerTransmitterdoesnotacceptthenextcalibrationdatapointyet.
l
PR
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