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620 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 20, NO. 3, MAY 2005

A New Generation of Universal Contactless BatteryCharging Platform for Portable Consumer

Electronic EquipmentS. Y. R. Hui, Fellow, IEEE, and Wing. W. C. Ho, Member, IEEE

Abstract—This invention is related to a new planar inductivebattery charger for portable electronic equipment such as mobilephones, palm pilots and CD players. New multilayer printed-cir-cuit-board winding matrices of hexagonal structures that can gen-erate magnetic flux of almost even magnitude over the surface ofthe winding arrays have been developed. The new concept formsthe basis for a new generation of universal charging platform fora wide range of portable electronic equipment. Different types ofportable electronic equipment can be placed and charged simulta-neously on the charging platform, regardless of their positions andorientation. The principle and structure of the charging platformare explained and the feasibility has been confirmed with prac-tical measurements. The proposed universal charging platform hasbeen successfully used for mobile phones, MP3 players and elec-tronic dictionaries.

Index Terms—Battery chargers, planar transformers, printed-circuit-board (PCB) windings.

I. INTRODUCTION

PORTABLE electronic equipment such as mobile phones,handheld computers, personal data assistants is normally

powered by batteries. In many cases, rechargeable batteries arepreferred because of environmental and economical concerns.The most common way to charge rechargeable batteries is touse a conventional charger, which normally consists of an ac–dcpower supply (in case of using the ac mains) or an dc–dc powersupply (in case of using a car battery). A conventional chargeruses a cord (electric cable for a physical electrical connection)to connect the charger circuit (a power supply) to the battery lo-cated in the portable electronic equipment. The basic schematicof the conventional battery charger is shown in Fig. 1.

Inductive electronic chargers without direct physical elec-trical connection have been developed in some portable elec-tronic equipment such as electric toothbrushes and drills. Induc-tive chargers have also been proposed [1]–[3]. These inductivetype chargers, however, use traditional transformer designs withwindings wound around ferrite magnetic cores. The main mag-netic flux between the primary winding and secondary windinghas to go through the magnetic core materials (Fig. 2). Other

Manuscript received June 17, 2004; revised December 6, 2004. This workwas supported by the Hong Kong Research Grant Council under Project CERGCityU 1223/03E. Recommended by Associate Editor J. A. Ferreira.

S. Y. R. Hui is with the Department of Electronic Engineering, City Universityof Hong Kong, Hong Kong, China (e-mail: [email protected]).

W. W. C. Ho was with the City University of Hong Kong, Hong Kong, China,and is now with Artesyn Technologies Asia-Pacific Ltd., Hong Kong, China.

Digital Object Identifier 10.1109/TPEL.2005.846550

Fig. 1. Schematic of conventional battery charger with direct electricalconnection.

Fig. 2. Schematic of magnetic core-based transformers used in conventionalinductive battery charger system [4].

contactless charger [4] proposed also uses magnetic cores as themain structure for the coupled transformer windings.

Planar magnetic components are attractive in portable elec-tronic equipment applications such as the power suppliesand distributed power modules for notebook and handheldcomputers. As the switching frequency of power converterincreases, the size of magnetic core can be reduced. Whenthe switching frequency is high enough (e.g., a few hundredsof kilo-hertz), the magnetic core can be eliminated. Low-costcoreless PCB transformers for signal and low-power (a fewWatts) applications have been proposed [5]–[16]. In powertransfer applications, the PCB transformers have to be shieldedto comply with EMC regulations. An investigation of planartransformer shielded with ferrite sheets has been reported [14].A similar shielding structure using ferrite only for planar in-ductor has been demonstrated in [17]. However, it is practicallyshown that using only thin ferrite materials for EMI shielding[16] is not effective. The EM fields can penetrate the thin ferritesheets easily. Planar PCB transformer effectively shieldedwith both thin ferrite plates and thin copper sheets has beendemonstrated [16].

Based on coreless PCB transformer technology, a contactlesscharger using a single primary printed winding without anyEMI shield has been proposed in [18]. However, the magnetic

0885-8993/$20.00 © 2005 IEEE

HUI AND HO: NEW GENERATION OF UNIVERSAL CONTACTLESS BATTERY CHARGING PLATFORM 621

Fig. 3. (a) Structure of a hexagonal spiral winding and (b) its simplified symbolused in this paper.

flux distribution of a single spiral winding has a major problemof nonuniform magnetic flux distribution. As illustrated withmeasurement later in this paper, the magnitude of the magneticfield in the center of the core of a spiral winging is highestand decreases from the center. This means that if the portableelectronic device is not placed properly in the central region,the charging effect is not effective. Without proper EMI shield,undesirable induced currents may flow in other metallic partsof the portable electronic equipment. In this new invention,we propose a new method that overcomes this problem. Moreimportantly, the proposed charging system allows more thanone equipment to be charged simultaneously, regardless oftheir orientations on the charging surface [19], [20].

II. MAGNETOMOTIVE FORCE (MMF) GENERATION

OF SPIRAL WINDINGS

In order to design a universal charging platform, it is neces-sary to generate a uniform mmf distribution over a planar sur-face. In this section, the mmf distribution of single layer planarwinding array is first addressed. Then the structure of the pro-posed multilayer winding arrays is explained.

First consider a spiral winding arranged in a hexagonal shapeas shown in Fig. 3(a). For simplicity, it will be represented as ahexagon as shown in Fig. 3(b). If a current passes through eachspiral winding pattern, a magnetomotive force (mmf), which isequal to the product of the number of turns and current

(i.e., ), is generated. Fig. 4(a) shows two spiral windingpatterns adjacent to each other. The per-unit mmf plot over thedistance (dotted line) can be linearized as shown in Fig. 4(b).It can be seen that the mmf distribution over the distance is notuniform. The maximum mmf occurs in the center of the patternand the minimum mmf occurs in the edge of the pattern.

Now consider three adjacent patterns in Fig. 5. The maximummmf region is labeled by a symbol “P” (which stands for mmfPeak). The minimum mmf region at the junction of two patternsis labeled as “V” (which stands for mmf Valley). Note that eachPeak is surrounded by six Valleys and each Valley surroundedby three Peaks.

III. UNIFORM MMF GENERATION BASED ON MULTILAYER

PCB WINDING ARRAYS

Many hexagonal spiral windings can be arranged as an arrayas shown in Fig. 6. These windings can be connected in parallel,in series or a combination of both to the electronic driving circuit[19], [20]. In Fig. 6, only the mmf peaks (P) are labeled. It shouldbe noted there are six mmf valleys (V) surrounding each peakat the six vertices of each hexagonal pattern.

Fig. 4. (a) Two adjacent hexagonal spiral winding patterns and (b) the mmfdistribution along distance between X and Y .

Fig. 5. Peak (P) and valley (V) positions of the PCB winding pattern.

In order to generate a uniform mmf distribution over theplanar charging surface, two more layers of PCB windingarrays should be added. This principle is explained firstly byadding a second layer of PCB winding array to the first one asshown in Fig. 7. The second layer is placed on the first one insuch a way that the peak mmf positions (P) of the patterns ofone layer is placed directly over the valley positions (V) of thepatterns in the other layer. Fig. 7 highlights the peak positionsof the patterns that are directly over the valley positions ofthe other layer for the two overlapped PCB layers. It can beobserved from Fig. 7 that the use of two layers of PCB windingarrays does not offer the optimal solution of generating uniformmmf over the inductive charging surface. For each hexagonalpattern in the two-layer structure, the peak positions occupy thecentral position and three (out of six) vertices of each hexagon.

The remaining three vertices are valley positions (V) thatneed to be filled by the third layer of PCB winding arrays. Thesevalley positions are shown in Fig. 7 as empty squares. Carefulexamination of Fig. 7 shows there are six peak positions (P)surrounding each valley position. Therefore, a third layer ofhexagonal PCB winding array can be used to fill up all these re-maining valley positions. By placing the central positions (peakmmf positions) of the hexagonal winding patterns of the thirdlayer of the PCB winding array over the remaining valley po-sitions of the two-layer structure, an optimal three-layer struc-ture is formed as shown in Fig. 8. Fig. 8 highlights the peakmmf positions of the three-layer structure. It can be observedthat all central positions and vertices of all hexagonal patternshave peak mmf.

622 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 20, NO. 3, MAY 2005

Fig. 6. Layer of hexagonal winding patterns (Each mmf peak “P” is surrounded by six valleys).

Fig. 7. Square with a “P” inside of it refers to peak mmf positions. Blank square refers to valley positions (to be filled by the peak positions of the third PCBwinding array) in the intermediate two-layer hexagonal-spiral PCB winding array structure.

In order to understand that the mmf over the surface hasuniform mmf distribution, one can consider any distance be-tween any two adjacent peak mmf positions as illustrated inFig. 9. If the winding patterns are excited in the same mannerand polarity so that the mmf generated by each layer of thewinding array are always in the same direction at any moment,the resultant mmf is simply the sum of the mmf generated byeach layer. Fig. 9 shows that the resultant mmf over the distancebetween any two adjacent peak positions in Fig. 9 is equalto 1.0 per unit. This indicates that the proposed three-layer

PCB winding array structure can be used to generate uniformmmf over the inductive charging surface. When used as acontactless, inductive charging surface, this uniform mmf dis-tribution feature ensures that, for a given airgap, a secondaryPCB coupling winding can always couple the same amountof magnetic flux regardless of the position of the secondary(coupling) PCB on the inductive charging surface. In addition,the voltage induced in the secondary winding would be thesame over the inductive charging surface. The four-layer PCBwinding array structure can be constructed in a three-layer

HUI AND HO: NEW GENERATION OF UNIVERSAL CONTACTLESS BATTERY CHARGING PLATFORM 623

Fig. 8. Structure of three-layer of hexagonal-spiral PCB winding arrays (with peak mmf positions highlighted with symbol P).

Fig. 9. Sum of mmf over distance, indicating the uniform mmf distribution ofthe three-layer PCB winding array structure.

PCB, with one of the four layers accommodating the returnpaths of the spiral windings to the electronic driving circuit.

IV. EXPERIMENTAL VERIFICATION

Special multi-layer PCB’s based on the hexagonal spiralwinding design (Fig. 8) have been made and tested. To confirmthat the theory of uniform mmf distribution is correct, themulti-layer PCB winding structure has been tested by severalsteps. For initial tests, the windings are connected in series andare excited by a RF power amplifier in the frequency rangeof 300 kHz–1 MHz. In later tests, a switched mode powersupply has been successfully designed for driving the chargingplatform.

A. Evaluation of Magnetic Flux Distribution

1) Test 1: Excitation of Only One Layer of PCB Winding: Inthis test, only one layer of the three PCB winding array structureis excited at high-frequency. The PCB is placed on a PrecisionEMC scanner so that the magnetic field of the PCB can bemeasured. Fig. 10(a) shows the measured 2-D magnetic fluxdistribution superimposed on a photograph of the PCB. Themagnitude of the mmf over the surface is plotted in Fig. 10(b).As predicted, the magnitude of the mmf is highest (peak) inthe center of the hexagonal winding pattern. The presence ofpeaks and valleys of mmf are confirmed.

2) Test 2: Excitation of Two Layers of PCB Windings: Thesecond test is conducted with two layers of PCB windings ex-cited by the high-frequency ac voltage source. Fig. 11(a) showsthe measured 2-D magnetic flux distribution superimposed ona photograph of the PCB. The magnitude of the mmf over thesurface is plotted in Fig. 11(b). As expected, half of the valleysare now filled with mmf peaks by the additional layer.

3) Test 3: Excitation of Three Layers of PCB Windings: Thethird test is conducted with three layers of PCB winding ar-rays excited by the high-frequency ac voltage source. Fig. 12(a)shows the measured 2-D magnetic flux distribution superim-posed on a photograph of the PCB. The magnitude of the mmfover the surface is plotted in Fig. 12(b). As expected, all the mmfvalleys are now filled with mmf peaks. These results show thatthe proposed three-layer hexagonal winding array structure canbe used to generate a magnetic field with uniform magnitudeover the planar surface. This feature is essential for a universalcharging platform because this uniform mmf distribution en-sures that the charged electronic equipment can be placed any-where on the charging surface.

624 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 20, NO. 3, MAY 2005

(a) (b)

Fig. 10. (a) Measured mmf scan superimposed on the photograph of the PCB and (b) the magnitude plot of the mmf (for a one-layer hexagonal PCB windingstructure).

(a) (b)

Fig. 11. (a) Measured mmf scan superimposed on the photograph of the PCB and (b) the magnitude plot of the mmf (for a two-layer hexagonal PCB windingstructure).

(a) (b)

Fig. 12. (a) Measured mmf scan superimposed on the photograph of the PCB and (b) the magnitude plot of the mmf (for a three-layer hexagonal PCB windingstructure).

HUI AND HO: NEW GENERATION OF UNIVERSAL CONTACTLESS BATTERY CHARGING PLATFORM 625

Fig. 13. Schematic of the test circuits.

Fig. 14. Photograph of charging platform prototype (enclosed area on theplatform was energized and tested).

B. Practical Evaluation as a Universal Charging Platform

Fig. 13 shows the schematic of the primary and secondarycircuit of the planar battery charging system. The full-bridge in-verter is fed with a dc voltage source (typically chosen within therange from 10 V to 30 V) at high frequency (chosen within therange from 100 to 500 kHz). The primary planar PCB windingarrays are connected in series in this test, although they can inprinciple be connected in series, in parallel or a combination ofboth [19], [20]. They are driven by a power inverter, the oper-ating frequency of which is controlled by a standard PWM con-trol IC. Fig. 14 shows a photograph of the charging platformwith the cover removed. The PCB winding array can be seen.In this particular test, only a portion of the area (enclosed in therectangular box in Fig. 14) is energized for evaluation.

The following tests were carried out under the conditionslisted as follows.

DC supply voltage: 25 V.Inverter frequency: 130 kHz.Secondary Load: A secondary PCB winding loadedwith a voltage regulator and an electronic load.Electronic load: Electronic load set at 10 .Primary winding: two columns of four spiral hexag-onal windings with 25 turns are connected in series;DC blocking capacitor 4.7 nF;Parallel capacitor 16.8 nF.Secondary winding: one spiral circular winding with18 turns and an outer diagram of 3.9 cm is used.Parallel capacitor 0.5 F.

The bottom of the PCB is shielded with a thin ferrite sheetand a thin layer of copper [16]. With this experimental setup,the output regulated dc voltage of the secondary circuit is 4 V.

Fig. 15. Photograph of charging platform prototype (enclosed area on theplatform was energized and tested).

Fig. 16. Two views of the measured induced rms secondary winding voltageover the charging surface. (a) Measured rms secondary winding voltage (V).(b) Measured rms secondary winding voltage (V).

With a 10- load, the expected ideal load power is 1.6 W. Fig. 15shows a photograph of the experimental setup. The secondaryvoltage and current, the regulator’s output voltage and the loadpower were measured and recorded over the charging surface.Measurements were made in the – plane and were recordedevery 0.5-cm apart in both directions. The -axis is dividedinto 25 measurement points and the -axis is divided into 14measurement points. So each set of 3-D magnitude plot consistsof 25 14 350 measurements. A plastic sheet with the –coordinates was placed on top of the PCB in order to facilitatethe measurements. The secondary voltage was measured by aTektronix differential voltage probe P5205 and the current by

626 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 20, NO. 3, MAY 2005

Fig. 17. Two views of the measured secondary load power over the chargingsurface. (a) Measured secondary load power on the charging surface (W).(b) Measured secondary load power on the charging surface (W).

a Tektronix current probe. A high-speed (500 MHz) Tektronixdigital storage oscilloscope was used for the voltage and currentmeasurements. Its mathematical function is used to calculateaverage power values.

Fig. 16(a) and (b) show two views of the measured secondaryvoltage plot when the secondary PCB planar winding is placedover the primary charging surface. It can be seen that over5 V rms can be induced in the secondary winging within theexcited area. It is noted that the induced voltage in the centralarea is slightly lower than that near the edges. Fig. 17(a) and(b) show two views of the measured secondary load powerover the charging surface. It can be observed that the loadpower over the surfaced is in the range from 1.3 W to 1.58 W,which is close to the ideal power of 1.6 W in the design.It is interesting to note that the load power absorbed in thecentral area of the surface is somewhat less than that near theedges of the charging surface. This phenomenon is probablydue to the loading effect and the magnetic flux distributionwhen the load is placed in the central charging area. Thecharging platform in its full version (entire surface) has beenused to charge several portable electronic equipment (suchas mobile phones, MP3 player and Hard disc player) usinga secondary charging module with a conventional chargingconnector. Fig. 18 shows a photograph of a mobile phonebeing charged by the universal charging platform.

Fig. 18. Photograph showing a mobile phone being charged through apatent-pending secondary charging module. (Note: winding array of the wholearea is excited in this particular test.)

V. CONCLUSION

The concepts of a novel (patent-pending) universal chargingplatform and a method of using multilayer PCB spiral windingmatrices to generate uniform magnetomotive force over a planarsurface are described in this paper. Its feasibility as a batterycharger has been practically demonstrated and confirmed. Thepresent prototype has been tested successfully as a contactlessbattery charger for a range of modified consumer electronicproducts. This new invention has an important feature that sev-eral electronic devices can be placed and charged on the plat-form simultaneously, regardless of their positions and orien-tation on the effective charging surface. By designing appro-priate secondary circuits to meet the charging requirements ofdifferent types of portable electronic equipment, this new inno-vation forms the basis of a new generation of universal contact-less planar battery charger. In principle, the winding arrays canbe arranged into groups and excited by individual inverters sothat localized charging can occur only in the region in which thecharged equipment is placed [19], [20].

ACKNOWLEDGMENT

The authors wish to thank P. W. Chan for his efforts in col-lecting the measurements for this paper.

REFERENCES

[1] K. Oguri, “Power supply coupler for battery charger,” U.S. Patent 6 356049, Dec. 5, 2000.

[2] Y. Yang and M. Jovanovic, “Contactless electrical energy transmissionsystem,” U.S. Patent 6 301 128, Feb. 9, 2000.

[3] H. J. Brockmann and H. Turtiainen, “Charger with inductive powertransmission for batteries in a mobile electrical device,” U.S. Patent 6118 249, Aug. 17, 1999.

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[7] S. Y. Hui, H. S.-H. Chung, and S. C. Tang, “Coreless printed circuitboard (PCB) transformers for power MOSFET/IGBT gate drive cir-cuits,” IEEE Trans. Power Electron., vol. 14, no. 3, pp. 422–430, May1999.

HUI AND HO: NEW GENERATION OF UNIVERSAL CONTACTLESS BATTERY CHARGING PLATFORM 627

[8] S. C. Tang, S. Y. Hui, and H. S.-H. Chung, “Coreless printed circuitboard (PCB) transformers with multiple secondary windings for com-plementary gate drive circuits,” IEEE Trans. Power Electron., vol. 14,no. 3, pp. 431–437, May 1999.

[9] S. Y. Hui, S. C. Tang, and H. S.-H. Chung, “Optimal operation of core-less PCB transformer-isolated gate drive circuits with wide switchingfrequency range,” IEEE Trans. Power Electron., vol. 14, no. 3, pp.506–514, May 1999.

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S. Y. (Ron) Hui (F’03) was born in Hong Kong in1961. He received the B.Sc. degree (with honors)from the University of Birmingham, Birmingham,U.K., in 1984, and the D.I.C. and Ph.D. degrees fromthe Imperial College of Science and Technology,University of London, London, U.K., in 1987.

He was a Lecturer in power electronics at the Uni-versity of Nottingham, Nottingham, U.K., from 1987to 1990. In 1990, he took up a lectureship at the Uni-versity of Technology, Sydney, Australia, where hebecame a Senior Lecturer in 1991. He joined the Uni-

versity of Sydney in 1993 and was promoted to Reader of Electrical Engineeringin 1996. Presently, he is a Chair Professor of Electronic Engineering at the CityUniversity of Hong Kong. He has published over 150 technical papers, includingover 100 refereed journal publications.

Dr. Hui received the Teaching Excellence Award in 1999, the Grand AppliedResearch Excellence Award in 2001 from the City University of Hong Kong,and the Hong Kong Award for Industry-Technological Achievement and theConsumer Design Award, in 2001 and 2004, respectively. He is a Fellow of theIEE. He has been an Associate Editor of the IEEE TRANSACTIONS ON POWER

ELECTRONICS since 1997. He has been an At-Large Member of the IEEE PELSAdCom since October 2002. He was appointed an IEEE Distinguished Lecturerby IEEE PELS in 2005.

Wing W. C. Ho (M’86) received the B.E.Sc. degree in electrical engineeringand the B.Sc. degree in applied mathematics from the University of Western On-tario, London, ON, Canada, in 1986, the Ph.D. degree in electrical engineeringfrom the University of Hong Kong in 1997, and the M.B.A. degree from theUniversity of Newcastle, Newcastle, Australia, in 2003.

He is presently an Assistant Product Design Manager with Artesyn Technolo-gies AP, Ltd. He spent over 10 years with ASTEC AMPSS, Hong Kong, as aSenior Engineer.

Dr. Ho is a Chartered Engineer in Britain.