FIELD OF THE INVENTION

The present invention is in the field of wireless power transfer systems. More particularly, but not excl usively, the invention relates to magnetical ly permeable cores incorporated into transmitters and receivers in wireless power transfer systems.

BACKGROUND OF THE INVENTION

Wireless power transfer systems are a wel l known area of both establ ished and developing technology. Typical ly, a primary side (or transmitter) generates a time-varying magnetic field from a transmitting coil or coils. This magnetic field induces an alternating current in a suitable receiving coil in a secondary side (or receiver) that can then be used to charge a battery or power a load, such as a portable device.

A basic problem that must be overcome in wireless power transfer system design is ensuring that power can be transferred over sufficient displacements (i.e. between the primary side and secondary side), while maintaining a sufficient amount of power transfer.

It is known that introducing magnetical ly permeable elements into either the transmitting coils or receiving coils can improve the performance of the system. Magnetically permeable elements increase the inductance of the transmitter or receiver. This means that less coil turns are required to achieve the same inductance val ue as a transmitter or receiver without magnetically permeable elements. Having fewer coils turns results in a decrease in losses due to resistance in the coil wire. Magnetical ly permeable elements can also be configured to 'shape' the magnetic field, which can be directed from the transmitter to the receiver. By directing the magnetic field, the coupling factor between the transmitter and receiver can be increased, thus improving the performance of the system.

For wireless power transfer systems, the magnetically permeable element may be in the form of a planar sheet underneath a layer of windings. In other applications, the magnetically permeable element may be a core, about which the windings of the transmitting coils or receiving coils are wound.

SUMMARY OF THE INVENTION

The present invention may provide an improved magnetically permeable core for use in transmitters or receiver, which may improve the tolerable displacement between the transmitter and receiver, or which may at least provides the public with a useful choice.

According to one exemplary embodiment there is provided a wireless power transfer system including: a. a power transmitter including: i. a magnetically permeable transmitter core having a base portion and an axial core portion extending away therefrom along a magnetic field propagation axis, wherein the base portion extends further away from the magnetic field propagation axis than the axial core portion; ii. transmitter windings provided about the axial core portion; and iii. an AC power supply driving the windings to produce an oscillating magnetic field; and b. a power receiver including: i. a magnetically permeable receiver core having a base portion and an axial core portion extending away therefrom along a magnetic field reception axis, wherein the base portion extends further away from the magnetic field reception axis than the axial core portion; ii. receiver windings provided about the axial core portion; and iii. a power receiver circuit receiving an AC current from the windings induced by the oscillating magnetic field.

The base portion of the transmitter and/or receiver core may be a disk.

The base portion of the transmitter and/or receiver core may include at least one discontinuity to inhibit eddy currents.

The discontinuity may extend from an edge of the base portion of the transmitter and/or receiver core towards its centre.

The opening may be in the form of a slot having a radiused termination.

The discontinuity may be in the form of an opening that allows access from one side of the base portion of the transmitter and/or receiver core, remote from the axial core portion, to the other side, proximate the axial core portion.

The base portion and axial core portion of the transmitter and/or receiver core may include a channel to permit a communications component to pass from one side of the transmitter and/or receiver core to another side of the transmitting and/or receiver core.

The base portion and an axial core portion of the transmitter and/or receiver core may be separate pieces. The magnetically permeable transmitter and/or receiver core may include an outer portion extending from the base portion about the axial portion, wherein the axial portion extends further from the base than the outer portion.

The axial portion may extend at least 20 percent further from the base than the outer portion. The base portion may include an opening that allows access from one side of the base portion of the transmitter and/or receiver core, remote from the axial portion, to a space between the axial portion and outer portion of the transmitter and/or receiver core.

The power transmitter core and windings may be contained within a first wireless power connector and the power receiver core and windings may be contained within a second wireless power connector and wherein the connectors may be interconnectable so as to align the magnetic field propagation and reception axes.

One or more cores may include a graduated transition between the base portion and the axial core portion where the direction of the main magnetic flux path changes.

Both the transmitter and receiver cores may include a graduated transition between the base portion and the axial core portion where the direction of the main magnetic flux path changes. Each graduated transition may be in the form of a curve. Each graduated transition may be in the form of one or more straight transition sections disposed at an angle with respect to the base portion and axial core portion.

The angle may be about 45 degrees. According to a further exemplary embodiment there is provided a magnetical ly permeable core for use in a wireless power transfer system, incl uding a base having first and second portions extending away therefrom, wherein the first portion extends further from the base than the second portion such as to maintain an effective flux l inkage throughout a range of relative displacement of a receiving core from a transmitting core and wherein the core incl udes one or more graduated transitions between the base portion and the axial core portion where the direction of the main magnetic fl ux path changes.

Each graduated transition may be in the form of a curve.

Each graduated transition may be in the form of one or more straight transition sections disposed at an angle with respect to the base and first or second core portions.

The angle is about 45 degrees.

It is acknowledged that the terms "comprise", "comprises" and "comprising" may, under varying j urisdictions, be attributed with either an excl usive or an incl usive meaning. For the purpose of this specification, and unless otherwise noted, these terms are intended to have an inclusive meaning - i.e. they will be taken to mean an incl usion of the l isted components which the use directly references, and possibly also of other non-specified components or elements. Reference to any prior art in this specification does not constitute an admission that such prior art forms part of the common general knowledge. BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings which are incorporated in and constitute part of the specification, illustrate embodiments of the invention and, together with the general description of the invention given above, and the detailed description of embodiments given below, serve to explain the principles of the invention.

Figure 1 shows a magnetically permeable core according to one embodiment of the present invention;

Figure 2 shows an exploded view of the magnetically permeable core of Figure 1 ;

Figure 3 shows a top view of the magnetically permeable core of

Figure 1 ;

Figure 4 shows a cross-section of the magnetically permeable core of Figure 1 ;

Figure 5 shows a cross-section of a transmitter and receiver pair;

Figure 6 shows a cross-section of a magnetically permeable core;

Figure 7 shows an exploded view of a magnetically permeable core and a bobbin;

Figure 8a shows a cross-section of a transmitter according to one embodiment of the present invention; Figure 8b shows a cross-section of a transmitter having a 'pot core' type core;

Figures 9a to 9c show cross-sections through transmitter and receiver pairs having different combinations of cores;

Figures 10a to 10i show cross-sections through the transmitter and receiver pair of Figure 9a for an array of relative displacements;

Figures 11a to 1 1 i show cross-sections through the transmitter and receiver pair of Figure 9b for an array of relative displacements;

Figure 12 shows a connector according to one embodiment of the present invention;

Figures 13a to 13c show magnetically permeable cores having different types of openings;

Figure 14 shows a cross-sectional view of a wireless power transfer system according to another embodiment; and

Figures 15 to 18 show cores with graduated transitions.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Figure 1 shows a magnetically permeable core 1 . Such a core may be adapted for incorporation into transmitters or receivers for use in wireless power transfer systems. The core includes a base 2 from which extends a first portion 3 and a second portion 4. The base connects the first portion to the second portion. Importantly, the first portion extends further from the base than the second portion. It is this difference in length between the first portion and the second portion that ensures an effective fl ux linkage is maintained for a range of displacements between a transmitting core and receiving core. This will be discussed in more detail in a later section. In one embodiment, the first portion may extend at least 20 percent further from the base than the second portion.

In the core 1 of Figure 1 , the base is a circular planar disk 2. The first portion is a column 3 extending perpendicularly from the centre of the disk and the second portion is a cyl inder 4 extending from the periphery of the disk. The col umn and cyl inder are concentric. The col umn extends further from the disk than the cyl inder. The remainder of the description wil l refer to, and describe in more detail, the column (being the first portion), the cyl inder (being the second portion) and the disk. However, those skil led in the art will appreciate that there are many other possible geometries that do not depart from the invention. For example:

• the base may be another shape besides circular;

• the first portion may not be circular;

• the second portion may not be a complete cyl inder, i.e. only partial ly surrounding the first portion; or

• the second portion may be a col umn extending from the centre of the base, and the first portion may be a cyl inder extending from the periphery of the disk.

The core 1 is made from a magnetical ly permeable material. This may incl ude ferrite or another suitable material. The core may be formed as a single piece, or, as shown in the exploded view of Figure 2, made from separate pieces. In Figure 2, the col umn 3, cylinder 4 and disk 2 are three separate pieces. In another embodiment, the col umn and disk may be formed as a single piece and the cyl inder as another piece. Upon assembly, these pieces may be fixed together in some way (for example, by adhesive) or they may be held in proximal position by some other means. Those skil led in the art wil l appreciate that having the core formed as a single piece wil l improve the inductance value of the core. Conversely, having the core formed from separate pieces may simpl ify manufacture. Further, having a division between the pieces (even where those pieces are directly abutting) may prevent the onset of magnetic saturation in the core. It is possible that the component pieces (i.e. the col umn, cyl inder and disk) may themselves consist of separate pieces. For example, the col umn may be segmented into a 'stack' of shorter col umns (not shown). This may also prevent the onset of magnetic saturation. The col umn 3 and disk 2 may incl ude a channel 5. In the core shown in Figure 2, this channel consists of a hole 6 in the centre of the disk that al igns with a bore 7 that passes through the length of the col umn (i.e. the col umn is hol lowed). As wil l be discussed later, such a channel may permit communication systems or similar to pass from one side of the core to the other. In another embodiment, there may be no channel (i.e. there may be no central hole in the disk and the col umn may be sol id). Though this may obstruct communication systems, it may al low the col umn to be narrower, while having the same cross-sectional area as its hol lowed counterpart. It will be appreciated that such a channel may occupy space that could otherwise be fil led with magnetical ly permeable material. This effectively lessens the inductance val ue of the column, which may have to be compensated for in some way - for example, by making the col umn longer or wider.

The disk 2 may incl ude discontinuities in the form of openings 8 that al low access from one side of the disk to the space between the col umn 3 and the cyl inder 4. Such an opening may be provided to al low wire for the windings to enter and exit the 'inside' of the core 1 . In Figure 2, there are two openings 8 for each end of the wire. The openings may be holes that pass through the disk or they may be 'cut-outs' 8 (as shown in Figure 2) that extend to the edge of the disk 2. Where the disk and cyl inder 4 are formed together, the cut-outs may extend al l the way to the edge of the cyl inder (effectively creating a slot through the cylinder). As wil l be discussed i n more detail later, such cut-outs may be preferable to holes as they el iminate an interfering fl ux path that would otherwise encircle the opening. The slots reduce eddy currents induced by conductors passing through the slots to supply or receive power from the windings. Additional narrow slots may be provided to further inhibit eddy currents. Providing radiused terminations of the slots also reduces losses. Such slots may be left open as air gaps or be fil led with non-metal l ic low permeabil ity (preferably close to air) insulating material.

Figure 3 shows a cross-section of the core 1 in a plane paral lel with the disk 2. It shows the disk, and a cross-section of the column 3 and the cylinder 4. The channel 5 and openings 8 discussed previously are also shown. The cross- section shows that the thickness of the cyl inder and hol lowed col umn may be the same. A magnetic field must pass through the cylinder and the column (via the disk). A key consideration wil l be the relative cross-sectional areas, as the fl ux may be l imited by the total cross-sectional area of a particular part. In the core 1 shown in Figure 3, the cross-sectional area of the col umn 3 is the smal lest, and it is therefore this which may l imit the amount of magnetic fl ux that is able to be generated without the core overheating. Those skil led in the art wil l appreciate how the core dimensions wil l need to be configured with this in mind. Figure 3 also shows that in this particular embodiment the core has a generally circular cross-section. This may be suitable where the core needs to be rotational ly symmetric. Figure 4 shows a cross-section of the core 1 in a plane perpendicular to the disk 2. It shows the disk, cylinder 4 and column 3. It also shows how the channel 5 passes through the disk and the column. It is helpful to identify three volumes within the boundaries that are defined by the core:

• the volume provided between the column and the cylinder (Volume A');

• the volume around the first portion further from the disk than the cylinder (Volume B'); and

· the volume that would be taken up by the cylinder were it to extend the same distance from the disk as the column (Volume C).

As will be described in more detail later, each of these three volumes may be used to accommodate windings.

Having described the underlying geometry of the core, it is appropriate to now consider a core in the context of a transmitter or receiver, which will show the benefits of the core's underlying geometry. Figure 5 shows a cross-section of a transmitter 9 and a receiver 10. The transmitter and receiver are generally the same geometries, both including a magnetically permeable core 1 (as described above), windings 11, 12 and circuitry 13, 14. In the case of the transmitter 9, the circuitry 13 will be transmitter circuitry that is adapted to connect to a suitable power supply 15 and to output an alternating current into the windings 11, which in turn will generate a magnetic field. Those skilled in the art will appreciate that there are any number of approaches to such transmitter circuitry, and the invention is not limited in this respect. Similarly, in the receiver 10, the circuitry 14 will be receiver circuitry that is adapted to receive power from the windings 12, and to output power, that may subsequently be used to power a load or charge a battery 16. Those skilled in the art will appreciate that there are any number of approaches to such receiver circuitry, and the invention is not limited in this respect.

The transmitter 9 and receiver 10 include the core 1 , 1 ', consisting of a column 3, 3', base 2, 2' and cylinder 4, 4'; and windings 1 1 , 12. The windings consist of a length of wire, wound in a series of loops. The windings are configured to occupy volume A, volume B and volume C within the core. As will be readily appreciated, the number of loops will be related to the gauge of wire, the relative dimensions of the core and the power requirements for the transmitter or receiver. Preferably, there will be an even number of layers as this simplifies the winding process. Figure 6, shows one possible approach to winding. The winding begins with layer 1 , and then follows the order indicated by the numbers.

In one embodiment, as shown in Figure 7 the windings (not shown) may be wound on a bobbin 1 7, which can then be inserted into the core 1 . Such a bobbin may include partitions 18 to separate the bobbin into zones, corresponding to the volumes inside the core. The bobbin may include slots 1 9 to allow the wire to move between zones. When an alternating current is supplied to the windings, a magnetic field is generated. It will be appreciated that the magnetically permeable core not only increases the inductance of the transmitter (or receiver) but also 'guides' the field. Figure 8a shows a cross-section through a transmitter 9 having a core 1 and windings 1 1 , and the field 20 generated by a transmitter when there is no receiver present. For comparison, Figure 8b shows a cross-section through a transmitter 21 having a core 22 and windings 23 that occupy the same vol ume, but where the col umn and cyl inder extend the same distance. This type of core 22 is sometimes cal led a 'pot core'.

As wil l be seen when comparing the fields 20, 24 in Figure 8a and 8b, the field 20 of the core 1 of the present invention is further from the core. Conversely, the field 24 of the pot core 22 remains relatively close to the core. (It wil l be appreciated that, in fact, a field extends to infinity, therefore the field lines in Figures 8a and 8b represent the part of the field that may be used for power transfer and represent the comparative shape of the field, for il l ustrative purposes.) The reasons for this difference incl ude:

• Having a shorter cyl inder provides a vol ume that can be occupied by additional windings (vol ume C), and more windings increases the size of the field; and

• Having a shorter cylinder means that the field l ines tend to pass around the windings in vol ume C, which results in the field l ines going further from the core.

Though this shows how the field generated by a transmitter 9 may be 'improved' by the core 1 of the present invention, the way in which the core maintains an effective fl ux l inkage for a range of relative displacements between a transmitting core and a receiving core are best understood by looking at the fields establ ished between a transmitter and receiver pair.

Figures 9a to 9c show cross-sections through transmitter and receiver pairs, and a comparison of fields generated for a range of core types. For the sake of comparison, each transmitter and receiver are al igned (i.e. their cyl inders are col l inear) with the same separation. It wil l be appreciated that, in fact, a field extends to infinity, therefore the field l ines in Figures 9a to 9c represent the part of the field that may be used for power transfer and represent the comparative shape of the field for il l ustrative purposes.

Figure 9a shows a transmitter 9 and receiver 10 which both include the core 1 , 1 ' of the present invention (as shown also in Figure 5). As can be seen, the field l ines l ink from the transmitter col umn 3 to receiver col umn 3', through the receiver disk 2', then from the receiver cyl inder 4' to the transmitter cyl inder 4. This is because this path has lower rel uctance (and is therefore preferred) to the path from the transmitter col umn 3 to transmitter cyl inder 4 (as shown by the dotted l ines). For comparison, Figure 9b shows a transmitter

21 and receiver 1 0, where the transmitter incl udes a regular pot core 22, while the receiver incl udes the core 1 ' of the present invention. In this instance, despite their being the same separation between the transmitter and receiver as Figure 9a, there is no fl ux l inkage from the transmitter col umn 25 to the receiver col umn 3', receiver disk 2', receiver cyl inder 4' and back to the transmitter cylinder 26. This is because is the path directly from the transmitter col umn 25 to transmitter cyl inder 26 has a lower rel uctance (and is therefore preferred) to the path via the receiver (as shown by the dotted l ine). Also, as with the explanation of Figures 8a and 8b, the core 1 of the present invention provides a vol ume that can be occupied by additional windings (volume C), and more windings increases the strength and size of the field. This demonstrates how the core of the present invention maintains a fl ux l inkage for larger separations. Figure 9c shows a transmitter 9 and receiver 27 where the transmitter incl udes the core 1 of the present invention, while the receiver incl udes a regular pot core 22'. As with Figure 9a, the field l ines may link from the transmitter col umn 3 to receiver column 25', through the receiver core 22', then from the receiver cyl inder 26' to the transmitter cyl inder 4. However, due to the longer receiver cyl inder (compared to the receiver cyl inder 4' of Figures 9a and 9b), the field lines may go directly from the receiver column 25' to the receiver cylinder 26' without passing through the bulk of the receiver core 22'. This behaviour is demonstrated by two of the field lines 28. Therefore, having a pot core in the receiver may not be as effective as the core of the present invention.

Figures 10a to 10i and Figures 11a to 11i show a range of fields for two transmitter and receiver pairs, over an array of relative displacements. Figure 10a to 10i corresponds to the transmitter 9 and receiver 10 pair of Figure 9a and Figure 11a to 11 i corresponds to the transmitter 21 and receiver 10 pair of Figure 9b. As will be seen by comparing the two sets of figures, the core of the present invention enables an effective flux linkage to be maintained for a larger range of relative displacements between a receiving core and a transmitting core.

Relative displacement may include lateral displacement (i.e. displacement in a plane parallel to the disk), lengthwise displacement (i.e. displacement perpendicular to a plane parallel to the disk) or a combination of both.

An effective flux linkage may be considered the flux linkage between a transmitter and receiver that is sufficient to transfer power. What is considered 'sufficient' will be dependent on the particular application, including:

• the power requirements of the load; and

• the tolerable amount of energy loss (i.e. required level of efficiency).

Therefore, if the field lines shown in the figures represent the upper limit of the part of the field that may be used for power transfer, then the field passing through the receiver indicates that there is an effective flux linkage. For example, Figures 10a and 11a show an effective flux linkage, whereas Figures 10i and 11i do not. Those skilled in the art will appreciate that the use of singular field lines on Figures 10a-10i and 11a-11i does not convey the complexity of the actual field, and the field l ines used in the figures are drawn merely as ill ustrative.

The range of relative displacements is the range of relative displacement between the transmitting core and receiving core where there is stil l sufficient power transfer. The lower bound for the range of relative displacements wil l be zero - that is to say, the case where the transmitting core and receiving core are mutual ly al igned with no separation between them. However, the upper l imit of the range of relative displacements is dependent upon the characteristics of the particular transmitter and receiver pair. In particular, the upper l imit may be dependent on at least some of the fol lowing interrelated factors:

• The volume of the core;

· The inductance of the core;

• The number of windings in the core;

• The dimensions of the windings;

• The current suppl ied to the transmitter windings;

• The type of core used in the receiver;

· The relative geometry of the parts of the core;

• The relative angle between the windings of the transmitter and the windings of the receiver.

Someone skilled in the art wil l appreciate that a transmitter and receiver pair wil l be designed with these factors considered, and they may be weighted differently depending on the priorities of the particular case. For example, where a transmitter must fit inside a certain vol ume, this wil l determine the vol ume of the core. Then the thickness of the parts of the core (and therefore, the core's inductance) will need to be balanced against the number of windings able to fit inside the core to ensure there is sufficient power transfer up to a tolerable upper l imit. In another example, the transmitter and receiver pair may be designed to ensure a large upper limit, which will require a larger core with a larger number of windings. These two examples demonstrate that the upper l imit of the range of the relative displacements is dependent on these factors and the required operating characteristics of the transmitter and receiver pair.

Nevertheless, Figures 10a to 10i and Figures 1 1 a to 1 1 i demonstrate that for a core of fixed vol ume, the core of the present invention is an improvement, and provides a larger range of relative displacements.

For example, for a particular lengthwise displacement both a standard core and core of the present invention maintain an effective fl ux l inkage. This is shown by Figure 1 0a and Figure 1 1 a. For a longer lengthwise displacement a standard core may no longer maintain an effective fl ux linkage, whereas the core of the present invention wil l. This difference is shown by a comparison of Figure 1 1 d with Figure 10d. Then, for a yet longer lengthwise displacement (the upper l imit of the range of relative displacements) the core of the present invention may no longer maintain an effective fl ux l inkage. This threshold exists somewhere between Figure 1 0d and Figure 10g.

Thus it has been shown that having the col umn extend further from the disk than the cylinder enables an effective fl ux l inkage to be maintained for a range of relative displacements between a receiving core and a transmitting core, where that range wil l be larger than a similar core having a col umn not extend further.

A further benefit arises from the geometry of the core in that the core acts a shield, minimising the amount of fl ux that is 'behind' the core and windings (being the non-transmitting or non-receiving side). This is shown in Figure 8a by the lack of field below the transmitter. Such shielding has two main benefits: • It minimises losses due to eddy currents arising in metal l ic components adjacent to the core and windings; and

· It protects electronic components from interference due to leaked magnetic fields.

Such a transmitter or receiver may be incorporated into a connector 29 as shown in Figure 1 2. Such a connector may include a suitable cable 30 that l inks the end of the connector to further electronic components (not shown).

The connector may house al l or part of the circuitry 1 3, 14 for control l ing the transmitter 9 or receiver 1 0. The connector may incl ude potting 31 to encase the core 1 and windings 1 1 . Potting ensures the core and windings are protected and potting also serves to draw away heat.

As previously mentioned, the transmitter and receiver may be adapted to accommodate communication systems that may be used to communicate from transmitter to receiver and vice versa. Those skil led in the art wil l appreciate that there are any number of communication systems that are suitable for establ ishing such a data l ink, such as: optical systems, radio systems, near-field communication (NFC) systems, and systems that rely on modulating the signal appl ied to the windings. For those systems that rely on l ine of sight (optical) or an antenna, it may not be practical to have the communication system disposed behind the core and windings. In particular, the core may block a l ine of sight connection or it may shield a field produced by an antenna.

Further, some systems may rely on a close proximity between antennas (for example, NFC). Therefore, the communication system, or part of the communication system, may reside on the transmitting or receiving side of the core, with a channel in the core providing access to the non-transmitting or non-receiving side of the core. The circuitry for control l ing the communication systems may be incorporated into the circuitry for the transmitter and receiver.

Returning to Figure 5, a channel 5 in the core 1 , 1 ' through the disk 2, 2' and col umn 3, 3' provides access for an antenna 32, 32'. The antenna is located on the transmitting side and receiving side of the core, whilst the remainder of the communication system is at some position on the other side of the core. The transmitter antenna 32 is adapted to connect to the transmitter circuitry 1 3, whilst the receiver antenna 32' is adapted to connect to the receiver circuitry 14.

Another aspect of the core that has been previously mentioned is the openings provided in the disk to al low the windings to enter into the core. Figures 1 3a to 1 3c show the field in various core configurations. Figure 1 3a shows the field in the core 1 of Figure 1 . The field goes along the cyl inder 4, before spreading radial ly inwards in the disk, and then going along the col umn 3 and returning to the cyl inder. At the cut-outs 8, the field passes around the cut-outs. By having the cut-out extend to the edge 33 of the disk, the field wil l not be incl ined to encircle the opening. For comparison, Figure 1 3b shows a similar core 34, but where the openings are holes 35. These holes produce an interfering fl ux path, whereby the magnetic field encircles the hole. This field causes heating in the core and results in energy loss. It wil l be noted that both cores in Figure 1 3a and Figure 1 3b incl ude a central hole 6, 36. In this instance, the central hole does not cause interfering fl ux paths, since the hole is not in the path of the field. In other words, the core can be said to incl ude openings, and if those openings are in the path of magnetic field, the openings should extend to an edge.

In the core 1 of Figure 1 3a, the disk 2 and cyl inder 4 are separate. If they were formed together, then the opening 8 would no longer be a cut-out but another hole (leading to the problems identified above). Therefore, the opening could be made to extend to the edge of the cylinder by including a slot 37 in the cylinder 4, as shown in Figure 13c. In this way, the opening would not provide an interfering flux path. The cylinder would then be segmented into two half cylinders.

Figure 14 shows a cross-sectional view of a wireless power transfer system including a transmitter 38 and a receiver 39. The transmitter and receiver cores have the same general geometry and include magnetically permeable cores 42, 43; 46, 47 windings 44, 48 an AC source 41 receiving power from power supply 40 and a power converter 50 converting the AC current from winding 48 to the form required by load 51 .

In this case the transmitter core consists of a base portion 42 in the form of a disc and an axial portion 43 in the form of a cylinder. Likewise the receiver core consists of a base portion 46 in the form of a disc and an axial portion 47 in the form of a cylinder. The axial portions 43 and 47 are coaxial with magnetic field propagation and reception axes of the transmitter and receiver.

The cores may be formed of a magnetically permeable material such as ferrite. The base and axial portions may be separately or integrally formed. Channels

52 and 53 are provided through portions 42, 43, 46 and 47 to provide conduits for conductors to antennas 45 and 49.

In this embodiment the transmitter and receiver cores do not include outer portions (as per outer portions 1 and 1 ' in Figure 5) . Whilst the absence of the outer portions 1 and 1 ' will result in increased fringing flux (i.e. flux not constrained to a path between the transmitter and receiver) this construction has a simpler core design and is easier to wind (either by directly winding onto the axial core or winding onto a conventional bobbin that is placed on the axial core). The available winding area is also increased by the removal of the outer portions 1 and 1 '. Further the flux field pattern makes this design less sensitive to lateral offset between the transmitter and receiver cores. Whether this simplified design or the design of figure 5 is employed will depend upon the requirements of the particular application.

As per Figure 2 the base portions 42 and 46 may include discontinuities to inhibit eddy currents. These may be in the form of narrow cuts from an edge of a base portion towards its centre or in the form of a slot having a radiused termination as shown by slots 8 in Figure 2. The slot may provide an opening that allows access to the windings from one side of a base portion to the other side.

The power transmitter core and windings may be contained within a first wireless power connector and the power receiver core and windings may be contained within a second wireless power connector that may be interconnectable so as to align the magnetic field propagation and reception axes.

Figures 15 to 1 8 show modified core designs with soft transitions which reduce core losses and core heating.

Figures 1 5 and 16 show cores of the type employed in the embodiment of Figure 14. In the embodiment of Figure 1 5 the core 52 includes a graduated transition in the form of a curve 53 between the base portion 52b and an axial core portion 52a in the region where the direction of the main magnetic flux path changes. Figure 16 shows a variant in which the graduated transition is in the form of a straight transition 55 at about 45 degrees to the base portion 54b and axial core 54a. It will be appreciated that compound transitions may also be employed having multiple straight transition sections incrementally inclined to each other. The graduated transitions shown in Figures 1 5 or 1 6 may be applied to one or both of the cores shown in figure 14.

Figures 1 7 and 1 8 show core variants suitable for use in cores of the type shown in Figures 1 to 1 3c. Figure 1 7 shows a core 56 including graduated transitions in the form of curved portions 57 between the base portion 56b and first portion 56a and second portion 56c where the direction of the main magnetic flux path changes. Figure 1 8 shows a core 58 including graduated transitions in the form of straight sections 59 between the base portion 58b and first portion 58a and second portion 58c where the direction of the main magnetic flux path changes. The straight transition sections 59 may be disposed at an angle of about 45 degrees with respect to the base 58b and first portion 58a and second portion 58c. While the present invention has been illustrated by the description of the embodiments thereof, and while the embodiments have been described in detail, it is not the intention of the Applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, representative apparatus and method, and illustrative examples shown and described. Accordingly, departures may be made from such details without departure from the spirit or scope of the Applicant's general inventive concept.