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Design of an Inductively Coupled Wireless Power System for Moving Receivers

Bart Thoen, Stijn Wielandt, Jeroen De Baere, Jean-Pierre Goemaere, Lieven De Strycker and Nobby Stevens

DraMCo, Faculty of Engineering Technology, ESAT, KU Leuven Gebroeders De Smetstraat 1, 9000 Gent, Belgium

info@dramco.be

Abstract—Present-day commercial wireless inductive power transfer systems rely on a low frequency alternating magnetic field, generated by a transmitter coil and picked up by a receiver coil. In general, these inductively coupled coils are perfectly aligned, in order to obtain a constant high coupling factor. In some cases though, the receiver can move on a regular basis above the transmitter. This paper discusses a method to wirelessly power a device moving on a surface. The transmitter consists of a single coil that is significantly smaller than the operation radius of the moving receiver. Therefore, the receiver is not always in the range of the transmitter and the power transfer occurs intermittent. In order to provide the receiver with a constant energy supply, a supercapacitor buffer is implemented. When the receiver approaches the transmitter, the buffer is charged in a short period of time. Furthermore, a communication channel is set up from the power receiver to the power transmitter, enabling receiver identification and power management.

Keywords-inductive coupling; coil design; inductive link

I. INTRODUCTION

Wireless power transfer with resonant inductive coupling has gained in significance over the past years. An increasing number of smartphone manufacturers integrates wireless charging into their products. Because smartphones need to be charged regularly, it is more user-friendly to drop them on a charging pad instead of plugging in a connector. Most of the present-day charging systems are based on the Qi-standard developed by the Wireless power consortium [1], but other alliances are emerging, for example the Alliance for wireless power and the Power matters alliance. All these systems are designed with a stationary receiver as boundary condition. As a consequence, none of these is suitable to power a constantly moving device.

In this paper, we propose a method to wirelessly power a planarly moving receiver. As a specific example, a wireless mouse will be used to demonstrate the solution. The device can move in the xy-plane as seen in Fig. 1. The distance between the two coils remains constant along the z-axis. The transmitter consists of a single planar coil in the working plane. In order to take care of the dead zones in the working plane, energy buffers are used. In this configuration, power needs to be transferred during the short periods of time when the power receiver maneuvers over the power transmitter. To make the power transmitter intelligent, communication is used to detect the power receiver.

Figure 1. Setup of the transmitter and the receiver

This paper is organized as follows. In section II, some theoretical considerations are shown. In section III the coil design is studied. Afterwards the hardware design of the system is considered. This includes an overall description of the system, the receiver and the transmitter, together with a justification of the design choices. In section V, the experimental results are discussed and the conclusion is drawn in section VI.

II. THEORY

A. Energy buffer

To buffer the dead zones in the working area supercapacitors or supercaps are used. Compared to batteries, supercaps charge more efficiently and can be recharged millions of times [2]. When charging a supercap the equivalent load is low, especially when the capacitor is discharged [2].

B. Link efficiency

An important part of the magnetic design involves the maximization of the energy transfer efficiency. This can be achieved by creating two inductively coupled resonant LC-tanks [3]. Either series or parallel resonance can be employed to achieve optimal coupling efficiency [4]. The choice of the proper resonance configuration depends on the load in the secondary circuit (i.e. the receiver) [3]. In case of a supercap which has a low equivalent load, a series resonant circuit is appropriate in order to achieve the maximum link efficiency. This configuration behaves like a current-controlled circuit [3].

The maximum link efficiency can be expressed by (1) [7][8]. This equation shows that the maximum achievable link efficiency ( ηlinkmax ) depends on three parameters. More precisely the coupling factor (k), the quality factor of the power receiver coil (QLs2) and the power transmitter (QLs1) coil.

The authors would like to thank the Flemish Agency for Innovation by Science and Technology (IWT), the German Federation of Industrial Research Associations (AIF) and the European Collective Research Networking (CORNET) group to support this research.

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ηlinkmax=k2QLs1QLs2

1+ 1+k2QLs1QLs2

2 (1)

III. COIL DESIGN

There are several ways to design transmitter coils, for example a coil matrix [8], cooperative flux generators [9] or a single transmitter coil. The two first options require a complex driver circuit with multiplexers in order to activate the coil(s) with the best coupling. If a single transmitter coil is used, the driver will be less complex. In this single coil design there are also several options. The transmitter coil can be positioned around the whole working area. This results in a weak coupling factor depending on the size of the area. Another option is to use a single coil in the working area. This method improves the coupling factor in the vicinity of the transmitter coil, but there will be points in the working area where the magnetic field is to weak. This last method is used in this design. Now the overall system can be built with relatively small coils, which reduces the cost of copper and shielding materials.

With the example of the wireless mouse, the secondary coil has to fit inside a double AA-battery compartment.

A. Ferrite copper shielding

If a coil is placed on a metal surface or when it is located close to metallic objects, eddy currents are being induced, reducing the magnetic field [11]. In order to substantially decrease the influence of metal in the vicinity of the transmitter and receiver coil, a ferrite copper shield can be used [5][6]. In this way the system becomes immune for any material behind the shield [11].

B. Receiver coil

In order to reach a high link efficiency, the quality factors of the coils must be high. For example, k2QLs1 and QLs2 need to be higher than 10, when the coupling factor is ± 30 %, in order to achieve a maximal link efficiency of 50 % (1). Litz wire was used to increase the Q-factor, since this type of wire reduces the resistance increase by the skin effect [10]. Further maximization can be obtained by using the total available area. Furthermore, a ferrite core was used in order to increase both the Q and coupling factor. This structure is placed on top of a ferrite copper shield. A picture of the receiver coil is depicted in Fig. 2. A summary of the specifications can be found in Table I.

Figure 2. Picture of the receiver coil

TABLE I. SPECIFICATIONS RECEIVER COIL

Parameter Value

Ferrite 3F4

Ferrite core 3C90

Copper shield 35 µm on FR4 substrate

Inside diameter 10x36 mm

Outside diameter 28x50 mm

Number of turns 9

Wire 2x36 strands, 0.6 mm

Inductance 9.47 µH

C. Transmitter coil

A planar arrangement of the transmitter coil is desired to limit the thickness off the total system to an absolute minimum. To achieve this a pancake coil is used. In order to select the internal diameter of this coil, simulations were performed in CST EM-studio. When an outer diameter of 80 mm is selected, a circular working range with a diameter of approximately 40 mm is obtained, which is the maximum internal diameter of the receiver coil. As seen in Fig. 3, an internal diameter of 20 mm results in a high coupling factor in the center, though it decreases quickly as the receiver moves away from the center position. When the inner diameter becomes too large, the coupling factor in the center drops significantly (internal diameter 70 mm). The higher the coupling factor, the easier it is to obtain a high link efficiency. But the power receiver moves over the transmitter coil, so a relatively constant coupling factor over the total working area is desired as this improves the predictability of the total system. This is especially important in terms of dimensioning the receiver electronics. Therefore a coil with an internal diameter of 40 mm is used. In this configuration, the coupling factor can be assumed to be constant over an area with a diameter of 40 mm. A picture of the transmitter coil is visualized in Fig. 4 and the specifications are summarized in Table II.

Figure 3. Simulation of pancake coils with variable diameter

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Figure 4. Picture of the transmitter coil

TABLE II. SPECIFICATIONS TRANSMITTER COIL

Parameter Value

Ferrite 3F4

Copper shield 35 µm on FR4 substrate

Inside diameter 40 mm

Outside diameter 80 mm

Number of turns 14

Wire 660 strands, 1.35 mm

Inductance 27.88 µH

IV. HARDWARE OVERVIEW

A. Overview of the total system

Figure 5. Block diagram of the total system

The total system consists of a power receiver circuit, a power transmitter circuit, two coupled coils and a power supply with two power outputs. The power receiver sends data to the power transmitter. In this way, the power transmitter can identify the power receiver and reduce the power output when the power receiver is not in its vicinity [1]. The power transmitter drives the primary coil by generating an AC-signal and demodulates the data to enable communication. When the power receiver doesn't need power or if the power receiver is out of range, the power to the primary coil is reduced.

The system operates at a frequency of 120 kHz, which lies within the frequency range of the Qi-standard (100 to 200 kHz) [1].

B. Power receiver

Figure 6. Block diagram of the power receiver

The power receiver converts the received AC-signal into the voltage required to power the device. In order to enable

communication with the power transmitter, load modulation is applied [1][3]. This communication technique uses the existing coupled coils and can be implemented with few extra components. With load modulation the resonance frequency of the secondary tank is changed. This produces an ASK-modulated signal over the primary resonance capacitor. A resonant secondary tank is used to charge the supercap buffer. The AC-signal is rectified using a full-wave rectifier [3]. Simulations showed that this type of rectifier can transfer twice the amount of power to the energy buffer in the same time interval, compared to a half-wave rectifier. Since the voltage level on the energy buffer varies, a switching voltage regulator can be used to acquire a stable output voltage.

If the microcontroller would always derive its power from the supercap, it would be impossible to transmit data when the supercap is discharged. However, it is necessary to send a data packet to the power transmitter in order to start charging the supercap. That's why the supercap is initially disconnected. During initial start-up, the microcontroller in the power receiver transmits a high power request to the power transmitter and afterwards connects the supercap.

C. Power transmitter

Figure 7. Block diagram of the power transmitter

The power transmitter consists of a class-D driver. This driver is suitable for frequencies below 1 MHz and works best if the coupling factor of the coils is large (> 10 %) [3]. For proper use of this driver there must be a capacitor in series with the primary coil. This capacitor cancels out the leakage inductance of the coil [3]. The power transmitter can switch between low and high power output modes by changing the DC-input voltage of the driver. The low power mode enables communication and initial start-up of the microcontroller in the power receiver. The high power mode is needed for quickly charging the supercap. The power receiver can decide in which power mode the power transmitter should operate, and transmits this choice in a data packet. The detection of the amplitude variation over the resonance capacitor is being performed with an AM-demodulator.

V. MEASUREMENT RESULTS

To test the design a wireless mouse was chosen, because it is a mobile device. The average power consumption of this device is 12 mW. The maximum power transfer of the transmitter is 5 W. The supercap buffer has a capacity of 1.65 F at a maximal voltage of 5 V. With this setup the empty supercap can be charged in about 10 s. The device can be fed from a fully charged supercap for 13 minutes. To guaranty a

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continuous operation the device needs to move across the power transmitter only 1 % of the time.

A. Start-up of the device

In Fig. 8 the initial start-up of the system is visualized. In this initial status, the supercap is completely discharged. Curve (a) displays the DC-voltage at the receiver, curve (b) represents the power supply voltage of the driver and curve (c) shows the demodulated communication in the power transmitter. When the supercap is discharged, the microcontroller will disconnect the supercap from the receiver DC-voltage. When the power receiver stirs over the power transmitter in this initial mode, the low power output of the primary coil will induce a secondary voltage that is just enough to power the microcontroller, which is visible in the waveform (a) at 0 s. After a small delay, the power receiver transmits a data packet, asking for high power (visualized in curve (c)). The driver voltage is increased (curve (b)). At the receiver side the supercap is connected and starts to charge, causing a voltage drop over the microcontroller, as seen in curve (a) at 1.272 s. After a while, when the voltage over the supercap has reached a sufficient level, the microcontroller restarts and operates in the normal operating mode, sending feedback about the status of the buffer to the power transmitter and regulating the output voltage of the power receiver. When the supercap is fully charged, a low power command is sent to the power transmitter.

Figure 8. Waveforms start-up of the device

VI. CONCLUSIONS AND FUTURE WORK

In this paper, we presented a design that can be used to wirelessly power a planarly moving receiver. To power the device, a single transmitter coil is placed in the working area. To guaranty a non-intermittent energy supply to the load of the moving receiver, a supercap is integrated .

The influence of the inner diameter of a pancake coil on the coupling factor is simulated. The smaller the inner diameter of

the transmitter coil, the higher the maximum coupling factor, but this coupling factor drops rapidly as transmitter and receiver are misaligned. When the transmitter coil is designed with a large inner diameter, the coupling factor is generally lower, but also exhibits a lower gradient as the coils are misaligned.

With the example of the wireless mouse and the given specifications, the device needs to be 1 % of the operation time above the transmitter coil.

The proposed setup can be applied in multiple applications. For example, moving devices that operate in a fixed setting. It can also be used for devices operating in an environments where conventional charging methods are forbidden.

To extend the range of the mobile devices, it is possible to strategically expand the number of transmitter coils over a certain area, increasing the working space.

Future work could consist in making the system usable for devices that consume more energy. This implies that the buffer size needs to increase, as well as the transferred power or the coil area.

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