oSIST prEN IEC 63758-8:2026

• Status: Not Published
• Public Enquiry End Date: 29-Nov-2026
• ICS: 29.240.99 - Other equipment related to power transmission and distribution networks
• ICS: 33.160.99 - Other audio, video and audiovisual equipment
• ICS: 35.200 - Interface and interconnection equipment
• Technical Committee: AVM - Audio, video and multimedia systems and equipment
• Current Stage: 4020 - Public enquire (PE) (Adopted Project)
• Start Date: 28-Sep-2026
• Due Date: 15-Feb-2027
• 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: prEN IEC 63758-8:2026 - Qi Specification - Version 2.3 - Part 8: Foreign object detection (Fast Track)

oSIST prEN IEC 63758-8:2026 is the Slovenian adoption of prEN IEC 63758-8:2026 for Qi Specification - Version 2.3 - Part 8: Foreign object detection. It specifies how a Qi Power Transmitter detects metal foreign objects in the magnetic field of wireless power transfer and how the Power Receiver may need to support that detection so heating can be avoided or limited. It is aimed at product designers, firmware engineers, test labs, and quality teams working on Qi Power Transmitter and Power Receiver products.

What does oSIST prEN IEC 63758-8:2026 specify?

oSIST prEN IEC 63758-8:2026 specifies methods for ensuring that power transfer proceeds without heating metal objects in the magnetic field of a Power Transmitter. The document covers both before-start detection and detection while power transfer is already in progress, and some methods require information from the Power Receiver.

The document is organized around a general front section, the FOD methods, and two annexes.

Annex	What it covers
Annex A	Determining the reference FOD values for a Power Receiver Product. This is the normative measurement procedure.
Annex B	Open-air Q test. This is an informative method for detecting objects on the interface surface before a Power Receiver is brought into the Operating Volume.

Clause 1 gives the general rules for structure, scope, compliance language, references, numbering, tolerances, and data-packet notation. Clause 2 introduces the foreign-object problem and the role of Friendly Metal. Clause 3 states the heating-avoidance objective and lists representative foreign objects. Clause 4 covers pre-powertransfer methods, and Clause 5 covers in-powertransfer methods.

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

The transmitter has to prevent unsafe heating

Clause 3 states that a Power Transmitter Product shall ensure that Foreign Objects in the Operating Volume do not reach unsafe temperature levels. In practice, that means the transmitter must either limit power, stop power transfer, or avoid starting transfer at all when a risky object is present.

Clause 3.1 defines representative foreign objects. Table 3 uses four representative objects, including steel and aluminium shapes, each with a temperature limit. This matters because the document measures FOD capability against specific test objects rather than against a vague general case.

Pre-powertransfer detection can stop the transfer before it starts

Clause 4.1 describes the empty-surfacetest. The Power Transmitter uses an Analogue Ping to sense whether something is on the surface, then a Digital Ping to distinguish a Power Receiver Product from a Foreign Object. If the object does not respond as a Power Receiver, the transmitter treats it as a Foreign Object and waits for removal.

This method matters when a coin, key, or foil piece is placed before charging begins. The text also notes corner cases such as misalignment, weak resonance impact, and corrupted responses, so implementations need timeouts, repeated checks, and similar safeguards.

Resonance-change FOD uses transmitter and receiver data together

Clause 4.2 describes the resonance-change method. The Power Receiver shall send FOD/rf and FOD/qf data packets with its Reference Resonance Frequency and Reference Quality Factor during the negotiation phase. The transmitter then compares measured resonance properties with those reference values to judge whether a Foreign Object is present.

This method is available to EPP and EPP5 products only. In practice, it helps when the receiver itself contains Friendly Metal that also affects the resonance. If the probability of a Foreign Object is high, the transmitter informs the receiver with NAK, and the receiver shall limit Load Power to 5 W or less.

In-powertransfer detection relies on received-power accounting

Clause 5.1 describes basic power loss accounting. The Power Receiver shall report Received Power level data regularly, and that reported value shall be sufficiently accurate and intentionally overestimated within the document’s margin rules. The Power Transmitter compares Received Power to Transmitted Power over matching time windows and uses the difference as an estimate of Foreign Object loss.

This matters because it lets the transmitter spot heating risk while power is flowing. If the estimated loss stays above the threshold, the transmitter may request lower consumption, reduce its own operating point, or abort power transfer.

Calibrated power-loss accounting improves sensitivity

Clause 5.2 describes calibrated power loss accounting. This method is available only in the Extended Protocol and assumes that at least one pre-powertransfer method has already confirmed an empty operating volume. The calibration exchange uses RP/1, RP/2, RP/0, RP/4, CE, CHS, PROP, ACK, and NAK states and packets.

In practice, the transmitter and receiver build a calibration curve from one or more stable operating points at the start of the power transfer phase. The document gives timing constraints, a calibration timeout, and rules for handling unstable positions, changes in operating mode, and recalibration. If the curve would have to be extrapolated outside the calibrated range, the receiver is expected to restart the transfer or negotiate a lower level so the FOD threshold stays meaningful.

Annex A gives the reference-measurement procedure

Annex A defines how to obtain the Reference Resonance Frequency and Reference Quality Factor for a Power Receiver Product. The procedure uses a reference coil assembly, an LCR meter or similar equipment, a 100 kHz measurement frequency, and measurements at five positions.

This annex matters for design validation and conformance testing because the receiver’s reference values feed the resonance-change FOD method. The document also specifies averaging of the five-position results, so the numbers used by the transmitter are not based on a single placement.

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

• Foreign Object Detection (FOD) - The set of methods used to detect objects that could heat up in the magnetic field during Qi power transfer.
• Foreign Object - A metallic object such as a coin, key, paper clip, or foil that can dissipate power from the magnetic field and become unsafe.
• Friendly Metal - Metal that is part of the Power Receiver Product or Power Transmitter Product and is therefore not treated as a foreign object, even though it can also absorb power.
• Power Transmitter - The device that generates the power signal and performs FOD decisions during Qi power transfer.
• Power Receiver - The device that receives power and, for several methods, provides design or power-reporting data to the transmitter.
• Operating Volume - The volume around the transmitter where the magnetic field interacts with the receiver and any foreign objects.
• Reference Resonance Frequency - The resonance-frequency value that the Power Receiver provides for the resonance-change FOD method.
• Reference Quality Factor - The quality-factor value that the Power Receiver provides for the resonance-change FOD method.

Who uses oSIST prEN IEC 63758-8:2026?

This standard is used by companies and laboratories that design, test, or certify Qi wireless power products. That includes engineers working on Power Transmitter Products, Power Receiver Products, firmware for packet handling, and thermal or electrical test methods.

It is particularly relevant for flat-surface Qi products such as mobile phones and chargers, and for products in the broader Qi ecosystem that use power-transfer negotiation, received-power reporting, and calibration. Quality managers and buyers use it to check whether a supplier’s transmitter or receiver implementation can detect Foreign Objects, manage false alarms, and control heating risk.

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

• Qi Specification, Communications Protocol - Provides the packet types, state transitions, ACK/NAK behavior, Baseline Protocol, Extended Protocol, and timing details that this part depends on.
• Qi Specification, Glossary - Provides the symbols and terminology used, especially in Annex B.
• ISO/IEC Directives, Part 2 - Defines the verbal forms used for requirements, recommendations, permissions, and capabilities.
• DIN EN 10027-2 - Supplies the steel designation used for one of the representative foreign objects.
• DIN EN 573 - Supplies the aluminium alloy designations used for the representative foreign objects.

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

The document contains clause-level rules for compliance language, number notation, tolerances, packet notation, and shorthand packet references. It also contains a technical explanation of why Foreign Objects and Friendly Metals are hard to distinguish, plus flow descriptions for pre-powertransfer and in-powertransfer FOD.

Several figures, tables, and worked procedures support implementation and testing. These include the representative foreign-object table, the reference coil assembly data, packet-sequence examples for calibration, timing constraints for calibration packets, and diagrams showing resonance shift, power-loss accounting, and calibration curves.

Annex A gives a practical measurement procedure for reference values, while Annex B gives the open-air Q test, methods for measuring Q, guidance on Q-deflection, threshold choice, temperature compensation, and implementation issues. Together, these parts are intended for design validation, protocol implementation, and conformance testing of Qi FOD behavior.

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
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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
...