FERRITE ANTENNAS FOR WIRELESS POWER TRANSFER

Background

[0001] This application claims priority from provisional application number 61/059,241, filed June 5, 2008, the entire contents of which disclosure is herewith incorporated by reference.

[0002] The applicant's previous applications and provisional applications, including, but not limited to, US Patent application number 12/018,069, filed January 22, 2008, entitled "Wireless Apparatus and Methods", the disclosure of which is herewith incorporated by reference, describe wireless transfer of power. The transmit and receiving antennas of one exemplary embodiment are resonant antennas, which are substantially resonant, e.g., within 10% of resonance, 15% of resonance, or 20% of resonance. The antenna of an exemplary embodiment is of a small size to allow it to fit into a mobile, handheld device where the available space for the antenna may be limited. An exemplary embodiment describes a high efficiency antenna for the specific characteristics and environment for the power being transmitted and received. Antenna theory suggests that a highly efficient but small antenna will typically have a narrow band of frequencies over which it will be efficient. The special antenna described herein may be particularly useful for this kind of power transfer.

[0003] One exemplary embodiment uses an efficient power transfer between two antennas by storing energy in the near field of the transmitting antenna, rather than sending the energy into free space in the form of a travelling electromagnetic wave. This exemplary embodiment increases the quality factor (Q) of the antennas. This can reduce radiation resistance (R _r ) and loss resistance

[0004 ] In one exemplary embodiment, two high-Q antennas are placed such that they react similarly to a loosely coupled transformer, with one antenna inducing power into the other.

[0005] The antennas may have Qs that are greater than 200, although the receive antenna may have a lower Q caused by integration and damping.

[0006] The transmit and receiving antennas may be resonant antennas, which are substantially resonant, e.g., within 10% of resonance, 15% of resonance, or 20% of resonance. The antenna may be of a small size to allow it to fit into a mobile, handheld device where the available space for the antenna may be limited. An exemplary embodiment describes a high efficiency antenna for the specific characteristics and environment for the power being transmitted and received.

[0007 ] One exemplary embodiment uses an efficient power transfer between two antennas by storing energy in the near field of the transmitting antenna, rather than sending the energy into free space in the form of a travelling electromagnetic wave. This exemplary embodiment increases the quality factor (Q) of the antennas. This can reduce radiation resistance (R _r ) and loss resistance (Rt).

Brief Description of the Drawings

[0008] In the Drawings :

[0009] Figures 1 and 2 show equivalent circuits; [0010] Figures 3 and 4 illustrate the uses of Ferrites; [0011] Figures 5-8B show shows a measurement set up; and [ 0012 ] Figure 9 shows a shows a measurement set up.

Detailed Description

[0013] The word "exemplary" is used herein to mean "serving as an example, instance, or illustration." Any embodiment described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments. [0014 ] The detailed description set forth below in connection with the appended drawings is intended as a description of exemplary embodiments of the present invention and is not intended to represent the only embodiments in which the present invention can be practiced. The term "exemplary" used throughout this description means "serving as an example, instance, or illustration," and should not necessarily be construed as preferred or advantageous over other exemplary embodiments. The detailed description includes

specific details for the purpose of providing a thorough understanding of the exemplary embodiments of the invention. It will be apparent to those skilled in the art that the exemplary embodiments of the invention may be practiced without these specific details. In some instances, well-known structures and devices are shown in block diagram form in order to avoid obscuring the novelty of the exemplary embodiments presented herein. [0015]

The words "wireless power" is used herein to mean any form of energy associated with electric fields, magnetic fields, electromagnetic fields, or otherwise that is transmitted between from a transmitter to a receiver without the use of physical electromagnetic conductors.

[0016] An exemplary embodiment uses ferrites in antennas for transmission and reception of magnetic flux used as wireless power. For example, ferrite materials usually include ceramics formed OfMO-Fe ₂ Os, where MO is a combination of divalent metals such as zinc, nickel, manganese and copper oxides. Common ferrites may include MnZn, NiZn and other Ni based ferrites.

[ 0017 ] Ferrite structures concentrate magnetic flux lines into the structure, thereby creating a magnetic path/field with less interference and eddy current losses in device electronics. This in essence sucks in the magnetic flux lines, thereby improving the efficiency of the magnetic power distribution. An embodiment describes a ferrite rod- shaped antennas. These may provide compact solutions that are easy to integrate into certain kinds of packaging. Also, the properties of ferrites may improve wireless power transmission.

[0018] The resonance frequency of Ferrite rod antennas may be easier to tune. In one exemplary embodiment, the tuning may be carried out by mechanically adjusting the position of the coil on the rod.

[0019] However, Ferrite rod antennas may suffer from Q degradation at higher magnetic field strengths (higher receive power levels) due to increasing hysteresis losses in Ferrite material. Also, Ferrites may only be practical at relatively low frequencies. At these low frequencies, however, ferrites have the advantage of channelling the magnetic

field through the core, rather than through the electronics on the board near the core. This can help to increase the resulting Q of the circuit.

[0020] The present application describes use of special ferrite antennas to carry out wireless transfer of power. One exemplary embodiment describes ferrite antennas for low frequency, e.g., 135 Khz.

[ 0021 ] Exemplary embodiments describe that the hysteresis losses in ferrite material may occur at higher power receive levels and higher magnetic field strengths. In addition, increasing the magnetic field strength may actually shift the resonance frequency, especially in certain materials where there are nonlinear B-H characteristics in the ferrites. In addition, harmonics emissions can be generated to in due to inherent nonlinearity. This nonlinearity becomes more important at lower Q factors.

[ 0022 ] Figure 1 is a block diagram of an inductively coupled energy transmission system between a source 100, and a load 150. The source includes a power supply 102 with internal impedance Z _s 104, a series resistance R ₄ 106, a capacitance Cl 108 and inductance Ll 110. The LC constant of capacitor 108 and inductor 110 causes oscillation at a specified frequency.

[0023] The secondary 150 also includes an inductance L2 152 and capacitance C2 154, which may be matched to the capacitance and inductance of the primary. A series resistance R2 156. Output power is produced across terminals 160 and applied to a load ZL 165 to power that load. In this way, the power from the source 102 is coupled to the load 165 through a wireless connection shown as 120. The wireless communication is set by the mutual inductance M.

[ 0024 ] Figure 2 shows an equivalent circuit to the transmission system of Figure 1. The power generator 200 has internal impedance Zs 205, and a series resistance Rl 210. Capacitor Cl 215 and inductor Ll 210 form the LC constant. A current 11 215 flows through the LC combination, which can be visualized as an equivalent source shown as 220, with a value Ul .

[0025] This source induces into a corresponding equivalent power source 230 in the receiver, to create an induced power U2. The source 230 is in series with inductance L2 240, capacitance C2 242, resistance R2 244, and eventually to the load 165.

[0026] The electronic circuits may be connected to a coupling loop that couples the power electronically to a main inductive loop. The equivalent circuits noted above may refer to the main inductive loop, which is electrically disconnected from any circuit parts. [ 0027 ] One exemplary embodiment is to compare the performance of these antennas, at different power levels and other different characteristics. By doing this, information about the way these materials operate in different characteristics is analyzed. Exemplary embodiments describe large (L), medium (M), and small (S) ferrite antennas, and their performances.

[0028] Ferrite Rod materials are normally used in communication receiver applications at small signal levels such as at or below 1 mW. No one has suggested using these materials at large power levels, e.g. up to 2 W or for power transfer.

[0029] Our applications have shown that antennas that are based on ferrites, or very magnetic materials may have certain advantages when used in transmitting and/or receiving wireless power. However, these antennas may be extremely application-specific: for example, while the ferrite can be used in many different applications and produce significant advantages in the wireless transmission of power, there uses are really limited to very constrained situations where the ferrites produce advantages. The present application also describes, however, that elongated shape of the ferrite Rod may in fact be advantageous at for some applications. Proper shaping of this elongated shape may actually improve the operation of the ferrite Rod, and as such may be important. Therefore, the exemplary embodiments define that this shape and location of a ferrite Rod used for and an antenna in a wireless power system as well is the materials used for that ferrite Rod, maybe crucially important.

[ 0030 ] Coils Ll and L2 in figure 1 and figure 2 may be tubular ferrite elements, with a coil wrapped there around. The tubular ferrite element has an elongated shape, meaning that the length of the element is larger wider than the width of the element as shown in figure 3.

[ 0031 ] Figure 3 also illustrates how the ferrite in essence concentrates the magnetic field lines such as 300, where the field lines over an area 301 prior to the ferrite device are concentrated into an area 302, corresponding to the diameter of the ferrite device.

[0032] This magnification may improve the operation.

[0033] Through experimentation, the magnetic field was found to be equal

[ 0034 ] where u is the magnetic permeability of the material, and H is the magnetic field in amps per meter.

[0035] Figure 4 illustrates a plot between the magnetic field H in amps per meter and the resultant magnetic fields B. This shows a hysteresis effect, and one exemplary embodiment defines maintaining the desired B and H fields at specified levels where the hysteresis losses are minimized.

[0036] Moreover, since the magnetic fields are concentrated in the ferrite, this provides less degradation due to neighborhood effects.

[ 0037 ] An exemplary embodiment describes different prototype exemplary embodiments and describes testing of different ferrite sizes and materials. [ 0038 ] Figures 5 and 6 represents a first ferrite Rod which was tested as a prototype, called size L. Size L has a size of 87 mm x 10 mm diameter. The ferrite Rod itself is shown in figure 6. The Rod 600 can be made of any ferrite material, but in an exemplary embodiment is made of the ferroxcube 4B2.

[0039] The ferrite Rod also has a main coil part 605 which slides relative to the ferrite Rod 600. The main coil is formed of litz wire, or more generally stranded contact wire, of a size 72 x .5 mm. A coupling winding is also mechanically coupled to the sliding main coil 605. The coupling winding 610 is electrically unconnected to the main coil 605, and may be formed of 1 to 10 coils of wire.

[0040] In operation, coupling winding receives power from the source, and couples that power to the main coil. The overall length of the sliding winding, litz wire plus coupling wire, is 29 mm, which slides along on the 87 mm ferrite Rod.

[0041] Exemplary embodiments described herein show different numbers of windings of main coil inductance, coupling coil inductance, and also show the effect of the value "d" which is shown as 650.

[ 0042 ] The connection to the assembly is shown via a BMC connector 620 that connects only to the coupling winding 610 via flexible connection cables 622. The cables are long enough such that the sliding coil 601 can slide anywhere on the sliding coil body 600.

[0043] The main coil 605 is also in parallel with a capacitor 610, forming the LC constant. In one exemplary embodiment, the connection between a capacitor and the coil may be made of flexible wires, or alternatively the capacitor can be mounted such that it moves whenever the coil is moved.

[0044 ] The coil/capacitor connection is connected to an output line 659, which itself is connected to a wireless power circuit 660. This circuit, for example, may be a wireless power transmitter that produces a modulated power signal at a frequency (e.g., 135 Khz), or a receiver that receives the wireless power and rectifies it.

[0045] The size L. may provide a transmit or receive power up to 2 W for example. [0046] Figure 7A and 7B shows another exemplary embodiment of size M where the medium size ferrite Rod is used 40 mm x 10 mm diameter. This may provide for receive power of 1 W or less, for example.

[0047 ] Figures 8 A and 8B show the size small Rod, of 22 mm x 8 mm in diameter. The rod itself, shown in figure 8B at 800, also has a sliding coil assembly 805 formed of a main coil and a coupling coil main coil 806 and a coupling coil 807. The device is formed in parallel with a capacitor. The size small device may have lost some of the size medium device may be usable for a power level of less less than Vi W in an exemplary embodiment. [0048] A number of tests were made for the different antennas, and different information can be found from these tests. For the ferrite antenna of size L, a number of different size devices could be used.

[0049] A first antenna tested had a main coil of approximately 24 mm in length, formed of 72 by 0.5 mm lits (stranded) wire. This forms an inductance of about 88.1 uH with a Q of 273 as 100 mV. In this exemplary embodiment, the capacitance used was 15.77 uF for a frequency of 135 kHz.

[ 0050 ] The test values described herein may use the test setup shown in figure 9 by using different capacitance and voltage values. According to one exemplary embodiment,

the measurement may be carried out at by using transmit antennas, and assuming reciprocity as a receiving antenna. According to one exemplary embodiment, the Q value is used to determine a limit for the amount of power applied.

[0051] According to one exemplary embodiment, the characteristics of a ferrite Rod antenna are evaluated based on the following parameters →Coillength ^χ 2Amm Coilmaterial - Litzell x 0,05mm →lnduc tan ceL=$$,lμHbisd = 8,5mm/ Q = 273 →C = \5,lluF resonantfrequency / ₀ = \35kHz(\00mV)

[0052 ] With a capacitor of C = \5uF

[0053] These results are summarized in Table 1, which shows those results for the L antenna, 87 mm length, 10 mm diameter. The main coil is 24 mm long formed of 37 turns of wire. A 15.77 nf capacitor is used, and the measured inducatance (without the ferrite) is 6.3I uH: [0054 ]

Table 1

[0055] The measurements used the test setup shown in Figure 9, where a waveform generator 900 applied to the coupling loop 610, and the main loop is tested via a high impedance probe 905. [0056] Additional Test results were as follows :

Table 2

Another exemplary embodiment tested the "L" antenna prototype with the same characteristics as above, with a 3 turn coupling coil. The results were:

Table 4

Note the Q varying from around 50-160, and depending on the frequency, with frequencies closer to 135Khz (resonance) working more effectively.

Another prototype used a 4 turn coupling coil, and had slightly higher Q values, as shown in

Table 5.

Table 6

This shows how q is increased for the higher voltages and larger values of d (representing the distance 650) shown in Figure 7B.

In a similar way, tests were made on the M device.

The tested values for this device showed:

Where "d" is the distance from the left edge, shown in Figure 7B, and using a resonant frequency of 135 Khz.

A median calculation showed:

Therefore, two, 22uF capacitors were used to bring this system to resonance.

Table 8 shows measured and calculated values:

Tables 10-12 show the effect of changing d in the test prototype, 40 mm long, 10 mm diameter, 25 turns on the main coil over 17mm, and 4 turns on the coupling coil.

In a similar way, tests were made on the S device. 3 different flavors of the tested rod are used:

This has an inductance of 2.08 uH. b) 12 winding antenna

c) Z-Antenna

d) Z-Antenna Number 94

e) Z-Antenna No. 93

f) Z-Antenna (No. 97)

g)

h) A i) Antenna x

For the tested device with length 21.5 mm, 8 mm in diameter, 12 turn main coil, and d of lmm from the end of the rod; wire type 72, and a 252 nF capacitor, the following values were obtained via testing:

Another prototype of the 45 winding, 0.26 mm diameter antenna was tested with a 19.22 uF capacitor, which were tested to have the following values:

Table 14

Yet another prototype has a 21.5mm rod of 8 mm diameter. The main coil used 45 turns at 7mm. The coupling coil used 2 turns, started .2mm from the end of the coil. The results as a function of "d", the number of mm from the end of the coil, were

Yet another prototype relates the number of windings to the precise tuning of the inductor. This exemplary embodiment uses a 30x3 lmm antenna.

The number of windings, N, can be related to the distance d in mm:

inductance and resonance values were found as:

In one exemplary embodiment, the number of windings of the coupling loop was related to the impedance. Many times, a 50 ohm impedance is desireable. For a 43 winding secondary, a 7 winding coupling loop may be optimal. For this exemplary embodiment,

-^ 1 Windings is the optimal value for the coupling loop

characteristics : 43/7 Windings (43 windings main loop, 7 windings coupling loop) , resonant frequency : 129 - 139kHz

Table 17

The inductance creates a 15 uH inductance (in air, without the ferrite). Values of this exemplary embodiment, using a 20 nF capacitor (2 * 10 nF) were found as:

As can be seen from the above, this system may optimize the coupling inductive loop separately from the main inductive loop (that is in parallel with the capacitor).

[0057 ] Those of skill would further appreciate that the various illustrative logical blocks, modules, circuits, and algorithm steps described in connection with the embodiments disclosed herein may be implemented as electronic hardware, computer software, or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the exemplary embodiments of the invention.

[ 0058 ] The various illustrative logical blocks, modules, and circuits described in connection with the embodiments disclosed herein may be implemented or performed with a general purpose processor, a Digital Signal Processor (DSP), an Application Specific Integrated Circuit (ASIC), a Field Programmable Gate Array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general purpose processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration.

[0059] The steps of a method or algorithm described in connection with the embodiments disclosed herein may be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. A software module may reside in Random Access Memory (RAM), flash memory, Read Only Memory (ROM), Electrically Programmable ROM (EPROM), Electrically Erasable Programmable ROM (EEPROM), registers, hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art. An exemplary storage medium is coupled to the processor such that the processor can read information from, and write information to, the storage medium. In the alternative, the storage medium may be integral to the processor. The processor and the storage medium may reside in

an ASIC. The ASIC may reside in a user terminal. In the alternative, the processor and the storage medium may reside as discrete components in a user terminal.

[0060] In one or more exemplary embodiments, the functions described may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored on or transmitted over as one or more instructions or code on a computer-readable medium. Computer-readable media includes both computer storage media and communication media including any medium that facilitates transfer of a computer program from one place to another. A storage media may be any available media that can be accessed by a computer. By way of example, and not limitation, such computer-readable media can comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium that can be used to carry or store desired program code in the form of instructions or data structures and that can be accessed by a computer. Also, any connection is properly termed a computer-readable medium. For example, if the software is transmitted from a website, server, or other remote source using a coaxial cable, fiber optic cable, twisted pair, digital subscriber line (DSL), or wireless technologies such as infrared, radio, and microwave, then the coaxial cable, fiber optic cable, twisted pair, DSL, or wireless technologies such as infrared, radio, and microwave are included in the definition of medium. Disk and disc, as used herein, includes compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk and blu-ray disc where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Combinations of the above should also be included within the scope of computer-readable media.

[0061] The previous description of the disclosed exemplary embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these exemplary embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.