Influence of Magnetic Design Choices on theQuality Factor of Off-the-Shelf Wireless Power

Transmitter and Receiver CoilsStijn Wielandt and Nobby Stevens

Abstract—In the design process of inductive wireless powersystems, the quality factors of transmitter and receiver coils playan important role in the optimization of the link efficiency. Inthis work, the experimental evaluation of the quality factor ofseveral commercially available transmitter and receiver coils isperformed. The influence on the Q-factor of the following designchoices is evaluated: an alignment aid magnet in the transmittercoil, the presence of ferrite at the receiver coil, electrical screeningat both the transmitter and receiver coil and finally the effect ofthe proximity of the transmitter and receiver. The results of thisresearch provide a clear overview of the impact of the studiedmagnetic design choices.

Index Terms—Inductive power transmission, Q measurement.

I. INTRODUCTION

S INCE a couple of years, the wireless charging of elec-tronic devices by use of inductive coupling has received

increased attention. The advantages are numerous. There isno need for galvanic or physical contact. It is obvious thatcharging the battery of biological implants is simplified sig-nificantly by this approach [1]. Another field of applicationthat uses the principle of energy transfer by magnetic fieldcoupling is inductive RFID [2]. Here, the transferred energyis directly employed to power a chip that broadcasts its uniqueidentification.

Recently, we see a trend to provide more standard consumerelectronic devices with the technology on board. This is anatural consequence since more of these devices are usedoutdoors, in contrast with the traditional laptops. The fact thatthe technology allows the mobile device to be waterproof (nogalvanic charging port required), is obviously an advantagefor outdoor usage of devices such as smart phones, dataloggers,. . . Also the medical industry can benefit from thisproperty, due to the increased hygiene when no connectors arerequired to provide the energy [3]. Another potential advantageis the opportunity to get rid of annoying charging cables andmanufacturer dependent adapters, as increased standardizationmust allow the wireless charging of electronic devices of allkind. Important steps towards standardization have been takenin the industry. For example, there is the Qi-standard fromthe Wireless Power Consortium [4] (abbreviated as “WPC”),which had 140 members at the beginning of 2013. Besides theWPC, there is also the Alliance for Wireless Power [5] andthe Power Matters Alliance [6].

Apart from the numerous advantages of wireless chargingby inductive coupling, there is a technological price to pay

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Fig. 1. Typical trend of the equivalent resistance R and quality factor Q.

for this form of contactless charging. Compared to galvaniccharging, more circuitry is required and the overall efficiencyof energy transfer decreases. The overall efficiency is oftendominated by the magnetic link efficiency. In this work, weevaluate the impact of several design choices on the magneticlink efficiency. The transmitter and receiver coils we selectedto do the evaluation are commercially available.

II. QUALITY FACTOR EVALUATION

The maximal link efficiency [1] that can be obtained isdescribed by equation (1) .

ηlink =k2QTxQRx(

1 +√1 + k2QTxQRx

)2 (1)

Here, k is the coupling coefficient between the primary andsecondary coil. It is the ratio of the amount of flux thatthrough the secondary coil to the flux generated by the primarycoil. k decreases rapidly as the distance between the coilsincreases and is also strongly dependant on the orientation ofthe coils [7]. Besides the coupling factor k, it is clear that thequality factors of the transmitter and receiver coil determinethe magnetic link performance. The quality factor (abbreviatedas “Q” from here on) of a coil is defined by equation (2) [1].

Q =ωL

R(2)

Here, L is the self inductance of the coil and R is the equiva-

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lent series resistance. In the frequency range that is consideredin this paper, from 75 to 700 kHz, the L remains constant whilethe equivalent series resistance R increases significantly, due toproximity effects, the skin-effect and potentially eddy currentlosses. A typical trend of the equivalent series resistance and Qas a function of the frequency is depicted in Fig. 1, where theR increases quadratic (in the case Litze wire is employed [8])and L remains constant. We have normalized the valuesto the maximum value, to emphasize the typical trend thatoccurs. At higher frequencies, the influence of stray capacitiesbecomes relevant and the imaginary part of the impedanceis modified significantly. For the coils we studied, this effectoccurs at several MHz, which is well beyond the frequencyrange we consider. Remark that the Qi-standard is defined totransfer power at frequencies between 100 and 205 kHz. Themeasurement of the very low resistance at these frequenciesis rather challenging and requires specialized measurementequipment. We have used the Agilent Technologies 4285aRLC meter [9], which performed very well.

The goal of this work is not to indicate which commercialtransmitter and receiver coils have the highest Q. We ratherwant to compare the Q of coils that are identical except fora typical design parameter that is significantly modified. Thisprocedure enables us to study the influence of these designparameters on the quality factor Q.

A. Line-out magnet at the transmitter coil

The first parameter we consider is the presence of a discshaped bonded Neodymium alignment aid magnet at thetransmitter coil, as defined in section 3.2.1.1.4 of the SystemDescription Wireless Power Transfer [10], published by theWPC. It is meant to provide an effective alignment means. Themagnet is centered within the transmitter coil, and has its northpole oriented towards the receiver coil. The transmitter coil westudied is manufactured by Laird Technologies (part numberRWC5353EJ240-501 [11]) and is compatible with the abovementioned Qi-standard (Design A1 of [10]). This correspondsto 105 strands of AWG40 wire and the ferrite is 28 Stewardmaterial which is specified by the WPC. A photograph isshown on Fig. 3(a). We performed two measurements onthe Q of the transmitting coil, one with the magnet presentand the other one with the magnet removed (design A10 of[10] shown on Fig. 3(b)). Remark that we only removed thecentral magnet, the underlying ferrite layer was not modified.The results are shown on Fig. 2. It is clear that the magnetsignificantly reduces (almost a reduction of factor 2) the QTx.Apart from that, the frequency at which the maximal Q isobtained, is also shifted when the magnet is present. Thefrequency region where the maximal Q occurs, is between100 and 205 kHz, which corresponds to the region of interestas defined by the Qi-standard.

B. Screening at the receiver coil

For EMC reasons, it may be required to apply an electricallyconducting screen on the transmitter and the receiver coil. Wehave evaluated the impact of the screening on the Q for atransmitter and receiver coil. The transmitter coil we selected

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QT

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Fig. 2. Influence of the alignment magnet on the QTx.

(a) Laird transmitter coil with magnet. (b) Laird transmitter coil without magnet.

(c) Wurth receiver coil with ferrite. (d) Wurth receiver coil without ferrite.

(e) Elec & Eltek pancake receiver coil. (f) Elec & Eltek transmitter coil.

Fig. 3. The transmitter and receiver coils that were measured.

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QImpact screening on transmitter and receiver coils

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Fig. 4. Influence of the screening on the Q for a representative transmitterand receiver coil.

was the coil of Laird Technologies of Fig. 3(b) (thus withoutthe alignment aid magnet). For the receiver coil, we selected acoil from Elec & Eltek (prod. nr. Y31-60019F [12]), which isalso Qi-compatible. A photograph of this coil is depicted onFig. 3(e). The results are shown on Fig. 4. This graph clearlyshows a decreased Q for the coils with a screen. Moreover, thisnegative influence is more pronounced when the ferrite layeris thinner (which is the case for the receiver coil). The ferritelayer has a thickness of 2.5 mm at the side of the transmitterand only 0.54 mm at the receiver side. Fig. 4 also shows thatthe Q for the receiver coil is generally lower than the one ofthe transmitter coil. Also, the frequency where a maximum Qcan be achieved, is significantly higher than 205 kHz, whichis the maximum frequency for Qi-compatible devices.

C. Ferrite usage with the receiver coil

A third design choice we consider is the presence of a ferritelayer. It is clear that the ferrite increases the L of the coils.Apart from that, it can also be applied as a guide for themagnetic field in order to increase the coupling k betweentwo coils. On the other hand, additional, frequency dependantjoule losses are introduced due to the presence of the ferrite.The receiver coil we studied is a coil from Wurth Elektronik(760308201 [13]) and is depicted on Fig. 3(c). Remark thatthe central cylindrical part with the label “WE” also consistsof ferrite. This coil is also compatible with the Qi-standard. Toevaluate the impact of the ferrite on the Q, we removed all theferrite. This is shown on Fig. 3(d). On Fig. 5, the results areshown. It is clear that the increased self inductance dominatesthe additional losses caused by the presence of the ferrite.

D. Coils at Operating Distance

Until now, we have considered the transmitter and receivercoils as individual entities and we have studied the impact ofseveral design choices. For configurations that have a strongcoupling (order 0.1 or higher), relation (1) should be treated

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Fig. 5. Ferrite influence on the Q for the Wurth receiver coil.

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Fig. 6. Product of the individual Q’s and the combined Q’s.

with great caution. Placing a coil in the vicinity of anothercoil (i.e. at operating distance), can significantly affect theQ factor due to additional energy losses or a change in selfinductance of the coils. To illustrate this, we have measuredthe Q of a transmitter and receiver coil at great distance (nocoupling). A second measurement was performed on both coilswhen they were aligned coaxially at a distance of 4 mm.The results are shown in Fig. 6. We used a transmitter coilof Elec & Eltek without an alignment aid magnet, depictedin Fig. 3(f) [12]. The considered receiver coil was the Elec& Eltek pancake coil of Fig. 3(e). Both the transmitter andreceiver coil had an electric screening on the backside of theferrite. It is clear that in formula 1, we should use the Q valuesof the closely coupled coils. Using the individually measuredQ factors clearly overestimates the ηlink in this case.

III. CONCLUSION

When building a wireless power system, one should usecoils with high quality factors if a high efficiency is desired.

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Experiments with commercially available wireless power coilsgave us insight in the influence of magnetic design choiceson the quality factor. The use of an alignment aid magnet atthe transmitter coil results in a significant reduction of thequality factor. Electrically conducting screens on transmitteror receiver coils also result in a reduced Q factor, especiallywhen the present ferrite layer is thin. The use of ferrite in acoil set-up generally results in higher quality factors in theconsidered frequency domain. However, when two coils areclosely coupled, quality factors drop again. These results canhelp in the optimization of the link efficiency during the designprocess of a wireless power system. Future work could involvethe study of different wire types, coil shapes or ferrite types.

ACKNOWLEDGMENT

The authors would like to thank the Flemish FundingAgency IWT for providing the means that allowed us toperform this research.

REFERENCES

[1] K. Van Schuylenbergh and R. Puers, Inductive Powering: Basic Theoryand Application to Biomedical Systems, ser. Analog circuits and signalprocessing. Springer London, Limited, 2009.

[2] S. Preradovic, N. Karmakar, and I. Balbin, “RFID transponders,” Mi-crowave Magazine, IEEE, vol. 9, no. 5, pp. 90–103, Oct. 2008.

[3] D. Kurschner and C. Rathge, “Contactless energy transmission systemswith improved coil positioning flexibility for high power applications,”in Power Electronics Specialists Conference, 2008. PESC 2008. IEEE,june 2008, pp. 4326 –4332.

[4] The Wireless Power Consortium, 2012. [Online]. Available: http://www.wirelesspowerconsortium.com

[5] Alliance for Wireless Power, 2012. [Online]. Available: http://www.a4wp.org

[6] Power Matters Alliance, 2012. [Online]. Available: http://www.powermatters.org

[7] C. Zierhofer and E. Hochmair, “Geometric approach for couplingenhancement of magnetically coupled coils,” Biomedical Engineering,IEEE Transactions on, vol. 43, no. 7, pp. 708 –714, july 1996.

[8] C. Sullivan, “Optimal choice for number of strands in a litz-wiretransformer winding,” Power Electronics, IEEE Transactions on, vol. 14,no. 2, pp. 283 –291, mar 1999.

[9] Agilent Technologies 4285A, 2012. [Online]. Available: http://cp.literature.agilent.com/litweb/pdf/5963-5395E.pdf

[10] Wireless Power Consortium, “System DescriptionWireless Power Transfer,” March 2012. [On-line]. Available: http://www.wirelesspowerconsortium.com/downloads/wireless-power-specification-part-1.html

[11] Laird Technologies, 2012. [Online]. Available: http://www.lairdtech.com/

[12] Elec & Eltek magnetic products Ltd, 2012. [Online]. Available:http://www.eemagnetic.com/

[13] Wurth Elektronik receiver coil, 2012. [Online]. Available: http://katalog.we-online.de/pbs/datasheet/760308201.pdf

Stijn Wielandt received a Master’s degree in Industrial Sciences, Electronicsin 2011 at the Catholic University College of Ghent, Belgium, associatedwith the KU Leuven. Subsequently, he started working as a researcher atthe DraMCo (wireless and mobile communications) research group of KULeuven, where he has been working on projects related to RFID and wirelesspower technology.

Nobby Stevens received the Master’s degree in Physical Engineering fromGhent University, Belgium, in 1997, the DEA degree from the InstitutNational Polytechnique de Grenoble, Grenoble, France, in 1997, and thePh.D. degree from Ghent University, Belgium, in 2004. From the end of1997 to August 1998, he was a Product Development Engineer with Philips.Beginning in August 1998, he performed research on numerical modelling ofelectromagnetic fields interacting with the human body with the Department ofInformation Technology, Ghent University. In June 2004, he joined AgilentEEsof, Santa Rosa, CA, as a Research and Development Engineer. SinceNovember 2008, he is a professor at the Catholic University College Ghent andassociated researcher at the KU Leuven, Belgium, where he is also a memberof the DraMCo (wireless and mobile communications) research group. Hisresearch is focused on wireless communications and near field interactions.

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