A New Generation of Universal Contactless Battery Charging Platform ...

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

A New Generation of Universal Contactless Battery Charging Platform for Portable Consumer Electronic Equipment S. Y. R. Hui, Fellow, IEEE, and Wing. W. C. Ho, Member, IEEE

Abstract—This invention is related to a new planar inductive battery charger for portable electronic equipment such as mobile phones, palm pilots and CD players. New multilayer printed-circuit-board winding matrices of hexagonal structures that can generate magnetic flux of almost even magnitude over the surface of the winding arrays have been developed. The new concept forms the basis for a new generation of universal charging platform for a wide range of portable electronic equipment. Different types of portable electronic equipment can be placed and charged simultaneously on the charging platform, regardless of their positions and orientation. The principle and structure of the charging platform are explained and the feasibility has been confirmed with practical measurements. The proposed universal charging platform has been successfully used for mobile phones, MP3 players and electronic dictionaries.

Fig. 1. Schematic of conventional battery charger with direct electrical connection.

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

I. INTRODUCTION

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ORTABLE electronic equipment such as mobile phones, handheld computers, personal data assistants is normally powered by batteries. In many cases, rechargeable batteries are preferred because of environmental and economical concerns. The most common way to charge rechargeable batteries is to use a conventional charger, which normally consists of an ac–dc power supply (in case of using the ac mains) or an dc–dc power supply (in case of using a car battery). A conventional charger uses a cord (electric cable for a physical electrical connection) to connect the charger circuit (a power supply) to the battery located in the portable electronic equipment. The basic schematic of the conventional battery charger is shown in Fig. 1. Inductive electronic chargers without direct physical electrical connection have been developed in some portable electronic equipment such as electric toothbrushes and drills. Inductive chargers have also been proposed [1]–[3]. These inductive type chargers, however, use traditional transformer designs with windings wound around ferrite magnetic cores. The main magnetic flux between the primary winding and secondary winding has to go through the magnetic core materials (Fig. 2). Other

Manuscript received June 17, 2004; revised December 6, 2004. This work was supported by the Hong Kong Research Grant Council under Project CERG CityU 1223/03E. Recommended by Associate Editor J. A. Ferreira. S. Y. R. Hui is with the Department of Electronic Engineering, City University of 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. 2. Schematic of magnetic core-based transformers used in conventional inductive battery charger system [4].

contactless charger [4] proposed also uses magnetic cores as the main structure for the coupled transformer windings. Planar magnetic components are attractive in portable electronic equipment applications such as the power supplies and distributed power modules for notebook and handheld computers. As the switching frequency of power converter increases, the size of magnetic core can be reduced. When the switching frequency is high enough (e.g., a few hundreds of kilo-hertz), the magnetic core can be eliminated. Low-cost coreless PCB transformers for signal and low-power (a few Watts) applications have been proposed [5]–[16]. In power transfer applications, the PCB transformers have to be shielded to comply with EMC regulations. An investigation of planar transformer shielded with ferrite sheets has been reported [14]. A similar shielding structure using ferrite only for planar inductor has been demonstrated in [17]. However, it is practically shown that using only thin ferrite materials for EMI shielding [16] is not effective. The EM fields can penetrate the thin ferrite sheets easily. Planar PCB transformer effectively shielded with both thin ferrite plates and thin copper sheets has been demonstrated [16]. Based on coreless PCB transformer technology, a contactless charger using a single primary printed winding without any EMI shield has been proposed in [18]. However, the magnetic

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

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

flux distribution of a single spiral winding has a major problem of nonuniform magnetic flux distribution. As illustrated with measurement later in this paper, the magnitude of the magnetic field in the center of the core of a spiral winging is highest and decreases from the center. This means that if the portable electronic 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 parts of the portable electronic equipment. In this new invention, we propose a new method that overcomes this problem. More importantly, the proposed charging system allows more than one equipment to be charged simultaneously, regardless of their orientations on the charging surface [19], [20].

Fig. 4. (a) Two adjacent hexagonal spiral winding patterns and (b) the mmf and . distribution along distance between

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II. MAGNETOMOTIVE FORCE (MMF) GENERATION OF SPIRAL WINDINGS In order to design a universal charging platform, it is necessary to generate a uniform mmf distribution over a planar surface. In this section, the mmf distribution of single layer planar winding array is first addressed. Then the structure of the proposed multilayer winding arrays is explained. First consider a spiral winding arranged in a hexagonal shape as shown in Fig. 3(a). For simplicity, it will be represented as a hexagon as shown in Fig. 3(b). If a current passes through each spiral winding pattern, a magnetomotive force (mmf), which is equal to the product of the number of turns and current (i.e., ), is generated. Fig. 4(a) shows two spiral winding patterns adjacent to each other. The per-unit mmf plot over the distance (dotted line) can be linearized as shown in Fig. 4(b). It can be seen that the mmf distribution over the distance is not uniform. The maximum mmf occurs in the center of the pattern and the minimum mmf occurs in the edge of the pattern. Now consider three adjacent patterns in Fig. 5. The maximum mmf region is labeled by a symbol “P” (which stands for mmf Peak). The minimum mmf region at the junction of two patterns is labeled as “V” (which stands for mmf Valley). Note that each Peak is surrounded by six Valleys and each Valley surrounded by three Peaks. III. UNIFORM MMF GENERATION BASED ON MULTILAYER PCB WINDING ARRAYS Many hexagonal spiral windings can be arranged as an array as 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 should be noted there are six mmf valleys (V) surrounding each peak at the six vertices of each hexagonal pattern.

Fig. 5.

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

In order to generate a uniform mmf distribution over the planar charging surface, two more layers of PCB winding arrays should be added. This principle is explained firstly by adding a second layer of PCB winding array to the first one as shown in Fig. 7. The second layer is placed on the first one in such a way that the peak mmf positions (P) of the patterns of one layer is placed directly over the valley positions (V) of the patterns in the other layer. Fig. 7 highlights the peak positions of the patterns that are directly over the valley positions of the other layer for the two overlapped PCB layers. It can be observed from Fig. 7 that the use of two layers of PCB winding arrays does not offer the optimal solution of generating uniform mmf over the inductive charging surface. For each hexagonal pattern in the two-layer structure, the peak positions occupy the central position and three (out of six) vertices of each hexagon. The remaining three vertices are valley positions (V) that need to be filled by the third layer of PCB winding arrays. These valley positions are shown in Fig. 7 as empty squares. Careful examination of Fig. 7 shows there are six peak positions (P) surrounding each valley position. Therefore, a third layer of hexagonal PCB winding array can be used to fill up all these remaining valley positions. By placing the central positions (peak mmf positions) of the hexagonal winding patterns of the third layer of the PCB winding array over the remaining valley positions of the two-layer structure, an optimal three-layer structure is formed as shown in Fig. 8. Fig. 8 highlights the peak mmf positions of the three-layer structure. It can be observed that all central positions and vertices of all hexagonal patterns have peak mmf.

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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 PCB winding array) in the intermediate two-layer hexagonal-spiral PCB winding array structure.

In order to understand that the mmf over the surface has uniform mmf distribution, one can consider any distance between any two adjacent peak mmf positions as illustrated in Fig. 9. If the winding patterns are excited in the same manner and polarity so that the mmf generated by each layer of the winding array are always in the same direction at any moment, the resultant mmf is simply the sum of the mmf generated by each layer. Fig. 9 shows that the resultant mmf over the distance between any two adjacent peak positions in Fig. 9 is equal to 1.0 per unit. This indicates that the proposed three-layer

PCB winding array structure can be used to generate uniform mmf over the inductive charging surface. When used as a contactless, inductive charging surface, this uniform mmf distribution feature ensures that, for a given airgap, a secondary PCB coupling winding can always couple the same amount of 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 the same over the inductive charging surface. The four-layer PCB winding array structure can be constructed in a three-layer


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

A. Evaluation of Magnetic Flux Distribution

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

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

IV. EXPERIMENTAL VERIFICATION Special multi-layer PCB’s based on the hexagonal spiral winding design (Fig. 8) have been made and tested. To confirm that the theory of uniform mmf distribution is correct, the multi-layer PCB winding structure has been tested by several steps. For initial tests, the windings are connected in series and are excited by a RF power amplifier in the frequency range of 300 kHz–1 MHz. In later tests, a switched mode power supply has been successfully designed for driving the charging platform.

1) Test 1: Excitation of Only One Layer of PCB Winding: In this test, only one layer of the three PCB winding array structure is excited at high-frequency. The PCB is placed on a Precision EMC scanner so that the magnetic field of the PCB can be measured. Fig. 10(a) shows the measured 2-D magnetic flux distribution superimposed on a photograph of the PCB. The magnitude of the mmf over the surface is plotted in Fig. 10(b). As predicted, the magnitude of the mmf is highest (peak) in the center of the hexagonal winding pattern. The presence of peaks and valleys of mmf are confirmed. 2) Test 2: Excitation of Two Layers of PCB Windings: The second test is conducted with two layers of PCB windings excited by the high-frequency ac voltage source. Fig. 11(a) shows the measured 2-D magnetic flux distribution superimposed on a photograph of the PCB. The magnitude of the mmf over the surface is plotted in Fig. 11(b). As expected, half of the valleys are now filled with mmf peaks by the additional layer. 3) Test 3: Excitation of Three Layers of PCB Windings: The third test is conducted with three layers of PCB winding arrays excited by the high-frequency ac voltage source. Fig. 12(a) shows the measured 2-D magnetic flux distribution superimposed on a photograph of the PCB. The magnitude of the mmf over the surface is plotted in Fig. 12(b). As expected, all the mmf valleys are now filled with mmf peaks. These results show that the proposed three-layer hexagonal winding array structure can be used to generate a magnetic field with uniform magnitude over the planar surface. This feature is essential for a universal charging platform because this uniform mmf distribution ensures that the charged electronic equipment can be placed anywhere on the charging surface.

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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 winding structure).

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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 winding structure).

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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 winding structure).


Fig. 13.

Schematic of the test circuits.

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

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

B. Practical Evaluation as a Universal Charging Platform Fig. 13 shows the schematic of the primary and secondary circuit of the planar battery charging system. The full-bridge inverter is fed with a dc voltage source (typically chosen within the range from 10 V to 30 V) at high frequency (chosen within the range from 100 to 500 kHz). The primary planar PCB winding arrays are connected in series in this test, although they can in principle be connected in series, in parallel or a combination of both [19], [20]. They are driven by a power inverter, the operating frequency of which is controlled by a standard PWM control IC. Fig. 14 shows a photograph of the charging platform with the cover removed. The PCB winding array can be seen. In this particular test, only a portion of the area (enclosed in the rectangular box in Fig. 14) is energized for evaluation. The following tests were carried out under the conditions listed as follows. DC supply voltage: 25 V. Inverter frequency: 130 kHz. Secondary Load: A secondary PCB winding loaded with a voltage regulator and an electronic load. Electronic load: Electronic load set at 10 . Primary winding: two columns of four spiral hexagonal 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 with 18 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 sheet and a thin layer of copper [16]. With this experimental setup, the output regulated dc voltage of the secondary circuit is 4 V.

Fig. 16. Two views of the measured induced rms secondary winding voltage over 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. 15 shows a photograph of the experimental setup. The secondary voltage and current, the regulator’s output voltage and the load power were measured and recorded over the charging surface. Measurements were made in the – plane and were recorded every 0.5-cm apart in both directions. The -axis is divided into 25 measurement points and the -axis is divided into 14 measurement points. So each set of 3-D magnitude plot consists of 25 14 350 measurements. A plastic sheet with the – coordinates was placed on top of the PCB in order to facilitate the measurements. The secondary voltage was measured by a Tektronix differential voltage probe P5205 and the current by

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

V. CONCLUSION

Fig. 17. Two views of the measured secondary load power over the charging surface. (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) Tektronix digital storage oscilloscope was used for the voltage and current measurements. Its mathematical function is used to calculate average power values. Fig. 16(a) and (b) show two views of the measured secondary voltage plot when the secondary PCB planar winding is placed over the primary charging surface. It can be seen that over 5 V rms can be induced in the secondary winging within the excited area. It is noted that the induced voltage in the central area is slightly lower than that near the edges. Fig. 17(a) and (b) show two views of the measured secondary load power over the charging surface. It can be observed that the load power 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 the central area of the surface is somewhat less than that near the edges of the charging surface. This phenomenon is probably due to the loading effect and the magnetic flux distribution when the load is placed in the central charging area. The charging platform in its full version (entire surface) has been used to charge several portable electronic equipment (such as mobile phones, MP3 player and Hard disc player) using a secondary charging module with a conventional charging connector. Fig. 18 shows a photograph of a mobile phone being charged by the universal charging platform.

The concepts of a novel (patent-pending) universal charging platform and a method of using multilayer PCB spiral winding matrices to generate uniform magnetomotive force over a planar surface are described in this paper. Its feasibility as a battery charger has been practically demonstrated and confirmed. The present prototype has been tested successfully as a contactless battery charger for a range of modified consumer electronic products. This new invention has an important feature that several electronic devices can be placed and charged on the platform simultaneously, regardless of their positions and orientation on the effective charging surface. By designing appropriate secondary circuits to meet the charging requirements of different types of portable electronic equipment, this new innovation forms the basis of a new generation of universal contactless planar battery charger. In principle, the winding arrays can be arranged into groups and excited by individual inverters so that localized charging can occur only in the region in which the charged equipment is placed [19], [20]. ACKNOWLEDGMENT The authors wish to thank P. W. Chan for his efforts in collecting the measurements for this paper. REFERENCES [1] K. Oguri, “Power supply coupler for battery charger,” U.S. Patent 6 356 049, Dec. 5, 2000. [2] Y. Yang and M. Jovanovic, “Contactless electrical energy transmission system,” U.S. Patent 6 301 128, Feb. 9, 2000. [3] H. J. Brockmann and H. Turtiainen, “Charger with inductive power transmission for batteries in a mobile electrical device,” U.S. Patent 6 118 249, Aug. 17, 1999. [4] C.-G. Kim, D.-H. Seo, J.-S. You, J.-H. Park, and B. H. Cho, “Design of a contactless battery charger for cellular phone,” IEEE Trans. Ind. Electron., vol. 48, no. 6, pp. 1238–1247, Dec. 2001. [5] S. Y. R. Hui, S. C. Tang, and H. Chung, “Coreless printed circuit board (PCB) transformers—Fundamental characteristics and application potential,” IEEE Circuit Syst. Mag., vol. 11, no. 3, pp. 3–15, Sep. 2000. [6] , “Coreless printed-circuit board transformers for signal and energy transfer,” Electron. Lett., vol. 34, no. 11, pp. 1052–1054, 1998. [7] S. Y. Hui, H. S.-H. Chung, and S. C. Tang, “Coreless printed circuit board (PCB) transformers for power MOSFET/IGBT gate drive circuits,” IEEE Trans. Power Electron., vol. 14, no. 3, pp. 422–430, May 1999.


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S. Y. (Ron) Hui (F’03) was born in Hong Kong in 1961. 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 from the Imperial College of Science and Technology, University of London, London, U.K., in 1987. He was a Lecturer in power electronics at the University of Nottingham, Nottingham, U.K., from 1987 to 1990. In 1990, he took up a lectureship at the University of Technology, Sydney, Australia, where he became a Senior Lecturer in 1991. He joined the University of Sydney in 1993 and was promoted to Reader of Electrical Engineering in 1996. Presently, he is a Chair Professor of Electronic Engineering at the City University of Hong Kong. He has published over 150 technical papers, including over 100 refereed journal publications. Dr. Hui received the Teaching Excellence Award in 1999, the Grand Applied Research Excellence Award in 2001 from the City University of Hong Kong, and the Hong Kong Award for Industry-Technological Achievement and the Consumer Design Award, in 2001 and 2004, respectively. He is a Fellow of the IEE. 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 PELS AdCom since October 2002. He was appointed an IEEE Distinguished Lecturer by IEEE PELS in 2005.

Wing W. C. Ho (M’86) received the B.E.Sc. degree in electrical engineering and the B.Sc. degree in applied mathematics from the University of Western Ontario, London, ON, Canada, in 1986, the Ph.D. degree in electrical engineering from the University of Hong Kong in 1997, and the M.B.A. degree from the University of Newcastle, Newcastle, Australia, in 2003. He is presently an Assistant Product Design Manager with Artesyn Technologies AP, Ltd. He spent over 10 years with ASTEC AMPSS, Hong Kong, as a Senior Engineer. Dr. Ho is a Chartered Engineer in Britain.