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Considerations When Designing a Wireless Charging SystemWHITE PAPER

Considerations When Designing a Wireless Charging System Abracon LLCLLCWireless charging is expected to grow from500 million units in 2017 to 750 million units in2018; consumer electronic devices lead this 33%increase back in 2015 and accelerated with theiPhone 8 and iPhone X releases thanks to the builtin wireless charging.Figure (1) – Wireless Power Market ForecastWireless Power Market .0100%500.050%2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 2025ReceiversTransmittersReceivers (y-o-y Growth)0%Transmitters (y-o-y Growth)Source: IHS MarkitThis growth has also been the catalyst for ancillaryproduct designs that now incorporatewireless charging. For many designers, this is theirfirst exposure to a wireless charging design.charging market. PowerMat, which created thealternative standards body PWA, has now joinedforces with the WPC to focus on Qi systems.(*) Qi (pronounced Chee, is the wireless charging standardfrom the Wireless Power Consortium (WPC).The Qi Wireless Charging TechnologyQi is the Wireless Power Consortium (WPC)standard for close coupled inductive wirelesscharging, and involves coupling EMF from atransmitter coil with a closely coupled Rx coilplaced over the top of it.Figure (2) - Inductive Charging vs. ResonantChargingcoil diameter Dcoil diameter DInductive Charging:Closely coupled andslightly “off resonance”Resonant Charging:Loosely coupledand “at resonance”Receiver coilcoil distance Zcoil distance ZTransmitter coiltightly coupled coils: Z much smaller than DThe wireless charging technology often leads toseveral questions:- Which wireless charging coil technology should Ichoose?- What size coil should I use?- How do I match transmitter and receiver coils foroptimal efficiency?- What power and efficiency could I expect from mydesign?- What type of receiver coils can I use in mydesign?Choosing the right wireless charging coiltechnologyloosely coupled coils: Z similar to DTightly coupled, o -resonance coils (left) allow formaximum power transfer, while loose, resonantcoils (right) can be placed anywhere in the field.Figure 2 shows the basic differences between thetwo technologies.Qi technology, as mentioned, is seeing new heightsof growth and is now mainstream with siliconsuppliers and product developers; but chargingat a distance is still the Holy Grail of wirelesscharging.Resonant charging is the other alternative offeringloosely coupled charging so items being chargedhave more freedom. Many advantages can beThe wireless charging market was until recentlyharnessed from resonant wireless chargingsplit into two standard bodies Wireless Powertechnology. For example, allowing multiple devicesConsortium (WPC) and Power Matters Alliance(PMA), but this has now seen convergence into one to be charged simultaneously, offering differentcharging ranges for applications and allowingcompany. CES 2018 showed a tidal wave of Qi(*)control over which devices are charged first.charging pods dominating the wirelessPage 25101 Hidden Creek Ln Spicewood TX 78669 512.371.6159 www.abracon.com

Considerations When Designing a Wireless Charging System Abracon LLCLLCWhile charging efficiency of resonant chargingWired versus inductive close coupled wirelesswill be less than the close coupled version,charging:different user applications can still benefit, offeringFigure (3) – Typical Wall Wired Charging Systeminnovative solutions. Examples such as charginginteractive customer displays in supermarketsDC Out Aor background charging of devices in your livingDiodeDCRectificationroom can be enabled by this approach. gercharging isn’t the cheapest or most efficient, butBridgeLossesLossesFrom wallRectifierDC Out BTransformerboth technologies will find their niche driven by theRegulationElectronicsend application.HF AC110VAC/240VACDC Out CWireless Charging TechnologyComparisonsAs we have already shown, Qi (closely coupledinductive charging) is the dominant technology, but thebest wireless charging technology will be decided byapplication factors.Inductive ChargingTightly coupled means much greater efficiency,but less spatial freedom.Can only charge a single mobile device.If used the alignment magnet reduces “k”(coupling factor) as well as inductance andlowers efficiency.High power transfers possible forexample: Semtech TSDMRX-19V20W-EVM(19V/20W), using Abracon AWCCA-RX350300101 Rx coil.Faster charge rates than resonant chargingrelative to the size of the coil.Resonant ChargingLooser Coupled efficiency, but greater spatialfreedom.Can charge multiple mobile devices and does not needalignment aids.Lower power transfer (8W / 1.6A), example IDT P90385V Resonant Power Transmitter.The new high power near field WattUp transmittercapable of 10W charging.BatteryEfficiency at different points in the wall chargingsystem:At (DC Out A) – output of wireless power receiverefficiency 80% to 90%At (DC Out B) – output of wireless power receiverefficiency 60% to 76%At (DC Out C) – output of wireless power receiverefficiency 50% to 64%When the efficiency of the wire ( 95%) is included,the system efficiency can be 72% to the chargerand as low as 47% to the batteryMinimum distance at which both coils can maintainresonant operation. If the resonating coils are movedtoo close, their mutual inductance causes the oscillatingmagnetic fields to “collapse” and power transfer ceases.Figure (4) - Typical Wireless Charging systemCoils tend to be larger to provide the power transferand have the Q needed. Higher Q demands lowimpedance, also driving thicker lower resistive wire.DC Out ADC Out BDC Out CFigures 3 and 4 compare wired charging toresonant charging, showing how efficiency varieswhere the DC Out is measured.Page 32This is the most important measurement parameterto decide which wireless charging technology isbest suited for a particular application.Measurement of efficiency of any chargingsystem, including wireless charging systems, canbe computed from the basic efficiency formula:Efficiency Pout / (Pout-Ploss), but when thesemeasurements are made, it is important tounderstand the total system efficiency.Battery Charger1EfficiencyBridge DriverWPT ReceiverDCInputSynchronous DC-DCBuck RegulatorWPT TransmitterEfficiency at different points in the wireless chargingsystem:At (DC Out A) – output of wireless power receiverefficiency 89%At (DC Out B) – output of wireless power receiverefficiency 75%At (DC Out C) – output of wireless power receiverefficiency 60%So it can be shown that inductive charging can beas efficient as a wired charger when measuredfrom end to end of the system.5101 Hidden Creek Ln Spicewood TX 78669 512.371.6159 www.abracon.com

Considerations When Designing a Wireless Charging System Abracon LLCLLCInductive close coupled versus resonantwireless charging:Figure (5) – Inductive and resonant chargingefficiency versus load (battery) currentEfficiency ComparisonFigure (7) – Power efficiency versus Q and DynamicRatio (Dr) between Tx and Rx CoilsPower Efficiency65.0160.055.0Efficiency, Pout / (Pout Ploss)Power Transfer Efficiency70.0losses, but due to the nature of resonant chargingloosely coupled solutions, even at 20mm, showrelatively little coupling 8Battery CurrentResonant11.20.1Dr 1, Q 100Dr 0.3, Q 100Dr 0.1, Q 100Dr 0.03, Q 1000.01Dr 1, Q 1000Dr 0.3, Q 1000Dr 0.1, Q 1000Dr 0.03, Q 10001.4Dr 0.01, Q 10000.0010.010.101.0010.00Axial Position. z/DThe purpose of the comparison in Figure (5) isto show the trade offs between inductive andresonant charging technologies.Figure (6) – Charge-cycle efficiency calculationplotting total energy versus time over the batterycharge cycle.70000Energy to battery65.7 kJ 39.6%Source energy, Loosely coupled60000Source energy, Closely coupledEnergy flow (Joules)5000043.8 kJ 59.4%4000030000Considerations when picking a Rx Coil forInductive Charging – Coupling Factor20000100000020406080100120140160time (minutes)The inductive closely coupled system used 50%less energy than the loosely coupled resonantsystem over the battery’s 2100mAH chargeperiod. Comparing the two technologies, the highfrequency, loosely coupled system will often useGaN output transistors and zero-voltage switching,which results in significant transmitter switchingPage 4Figure (7) shows some of the limitations of inductivecharging, which requires coils to be aligned andwell matched. The plots show that power transferefficiency is a function of the Q of the system andthe distance between the power transmitter andpower receiver. Some of the data shows Q at 1000,but this is not achievable in practice due to wire orwinding losses, so Q’s of 20s to 100 are normal.The tuning of the coils helps to improve Q. Theselimitations point to the benefits of loosely coupledresonant charging where alignment of coils bydesign cannot be planar.The “transformer” in an inductive, close coupledwireless charging system is two separateinteractive devices: a Tx Coil and a Rx Coil. Whenplaced over each other, they couple inductivelyand are modeled as a 2-coil transformer with an aircore.5101 Hidden Creek Ln Spicewood TX 78669 512.371.6159 www.abracon.com

Considerations When Designing a Wireless Charging System Abracon LLCLLCFigure (8) – Inductive Coil CouplingFigure (10)k 0.2 to 0.7SendEnergyReceiveEnergyTransmitter (Tx)Receiver (Rx)The ferrite material used is important. Soft ferrite ispreferred. Ferrites can be divided into two familiesThe shielding on the Tx and Rx coils is essentialbased on their magnetic coercivity (their resistanceand provides a magnetic flux short, allowing theto being demagnetized). Soft ferrites have lowflux fields to be contained within the two cores. See coercivity and are best for shielding.the concentrations of flux lines inside the ferritesheets of the coils.The ferrite thickness is an important consideration;thicker shields absorb more flux and are lessTypical coupling factors are much lower (k 0.2 susceptible to saturation (Bs), so the thickness and0.7) compared to a traditional transformer (k 0.95O/D around the outside of the coil is an important 0.99). Some of this weaker coupling can beconsideration.mitigated by the series resonant cap on the Tx andBsRx coils that increases Q. This means the efficiencyis limited to about 85%.Coupling factor (k) -k L12L11 L22Br-HcConsiderations when picking a Rx Coil forInductive Charging – The Shield Hc0BrA Typical B-H LoopBsInductive wireless charging coils will normally beseen with a ferrite shield. Figure (10) shows the coilpositioned on the black ferrite shield. The ferriteSaturation magnetization (Bs) represents thehas important properties that shield electronicssaturation limit of flux density, and Remanence (Br)behind the assembly.is the residual flux even after the withdrawal of theinducing field. Coercivity (Hc) is the magnetic fieldThe shield has 2 major functions:required in the opposite direction to demagnetize-Provide a “short path” for the magnetic flux so the ferrite.that it limits the heating of other componentsbehind the shield, focusing EMF into the ferrite.-Improve inductance so that coils can be woundwith less windings, saving excessive resistance.Note: the shield should extend beyond theouter edge of the coil to reduce EMF fromescaping and reduce the saturation point.Page 55101 Hidden Creek Ln Spicewood TX 78669 512.371.6159 www.abracon.com