Abstract:
A charging system for an Implantable Medical Device (IMD) is disclosed. The charging system features an electronics module connected to a charging coil by a cable. The charging system can be configured with a belt or harness that holds the charging coil position to charge the IMD and also providing a user with easy access to the electronics module. Resistance in the cable between electronics module and the charging coil is minimized by using multiple, individually insulated conductors to carry AC current.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Patent Application Ser. No. 62/351,198, filed Jun. 16, 2016, which is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to wireless external chargers for use in implantable medical device systems. 
       BACKGROUND 
       [0003]    Implantable stimulation devices are devices that generate and deliver electrical stimuli to body nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder subluxation, etc. The description that follows will generally focus on the use of the invention within a Spinal Cord Stimulation (SCS) system, such as that disclosed in U.S. Pat. No. 6,516,227. However, the present invention may find applicability in any implantable medical device system. 
         [0004]    As shown in  FIGS. 1A-1C , a SCS system typically includes an Implantable Pulse Generator (IPG)  10  (Implantable Medical Device (IMD)  10  more generally), which includes a biocompatible device case  12  formed of a conductive material such as titanium for example. The case  12  typically holds the circuitry and battery  14  ( FIG. 1C ) necessary for the IMD  10  to function, although IMDs can also be powered via external RF energy and without a battery. The IMD  10  is coupled to electrodes  16  via one or more electrode leads  18 , such that the electrodes  16  form an electrode array  20 . The electrodes  16  are carried on a flexible body  22 , which also houses the individual signal wires  24  coupled to each electrode. In the illustrated embodiment, there are eight electrodes (Ex) on each lead  18 , although the number of leads and electrodes is application specific and therefore can vary. The leads  18  couple to the IMD  10  using lead connectors  26 , which are fixed in a non-conductive header material  28 , which can comprise an epoxy for example. 
         [0005]    As shown in the cross-section of  FIG. 1C , the IMD  10  typically includes a printed circuit board (PCB)  30 , along with various electronic components  32  mounted to the PCB  30 , some of which are discussed subsequently. Two coils (more generally, antennas) are show in the IMD  10 : a telemetry coil  34  used to transmit/receive data to/from an external controller (not shown); and a charging coil  36  for charging or recharging the IMD&#39;s battery  14  using an external charger, which is discussed in detail later. 
         [0006]      FIG. 2  shows the IMD  10  in communication with an external charger  50  used to wirelessly convey power to the IMD  10 , which power can be used to recharge the IMD&#39;s battery  14 . The transfer of power from the external charger  50  is enabled by a primary charging coil  52 . The external charger  50 , like the IMD  10 , also contains a PCB  54  on which electronic components  56  are placed. Again, some of these electronic components  56  are discussed subsequently. A user interface  58 , including touchable buttons and perhaps a display and a speaker, allows a patient or clinician to operate the external charger  50 . A battery  60  provides power for the external charger  50 , which battery  60  may itself be rechargeable. The external charger  50  can also receive AC power from a wall plug. A hand-holdable housing  62  sized to fit a user&#39;s hand contains all of the components. 
         [0007]    Power transmission from the external charger  50  to the IMD  10  occurs wirelessly and transcutaneously through a patient&#39;s tissue  25 , via inductive coupling.  FIG. 3  shows details of the circuitry used to implement such functionality. Primary charging coil  52  in the external charger  50  is energized via charging circuit  64  with an AC current, Icharge, to create an AC magnetic charging field  66 . This magnetic field  66  induces a current in the secondary charging coil  36  within the IMD  10 , providing a voltage across coil  36  that is rectified ( 38 ) to DC levels and used to recharge the battery  14 , perhaps via a battery charging and protection circuitry  40  as shown. The frequency of the magnetic field  66  can be perhaps 80 kHz or so. When charging the battery  14  in this manner, is it typical that the housing  62  of the external charger  50  touches the patient&#39;s tissue  25 , perhaps with a charger holding device or the patient&#39;s clothing intervening, although this is not strictly necessary. 
         [0008]    The IMD  10  can also communicate data back to the external charger  50  during charging using reflected impedance modulation, which is sometimes known in the art as Load Shift Keying (LSK). This involves modulating the impedance of the charging coil  36  with data bits (“LSK data”) provided by the IMD  10 &#39;s control circuitry  42  to be serially transmitted from the IMD  10  to the external charger  50 . For example, and depending on the logic state of a bit to be transmitted, the ends of the coil  36  can be selectively shorted to ground via transistors  44 , or a transistor  46  in series with the coil  36  can be selectively open circuited, to modulate the coil  36 &#39;s impedance. At the external charger  50 , an LSK demodulator  68  determines whether a logic ‘0’ or ‘1’ has been transmitted by assessing the magnitude of AC voltage Vcoil that develops across the external charger&#39;s coil  52  in response to the charging current Icharge and the transmitted data, which data is then reported to the external charger&#39;s control circuitry  72  for analysis. Such back telemetry from the IMD  10  can provide useful data concerning charging to the external charger  50 , such as the capacity of the IMD&#39;s battery  14 , or whether charging of the battery  14  is complete and operation of the external charger  50  and the production of magnetic field  66  can cease. LSK communications are described further for example in U.S. Patent Application Publication 2013/0096652. 
         [0009]    External charger  50  can also include one or more thermistors  71 , which can be used to report the temperature (expressed as voltage Vtherm) of external charger  50  to its control circuitry  72 , which can in turn control production of the magnetic field  66  such that the temperature remains within safe limits. See, e.g., U.S. Pat. No. 8,321,029, describing temperature control in an external charging device. 
         [0010]    Vcoil across the external charger&#39;s charging coil  52  can also be assessed by alignment circuitry  70  to determine how well the external charger  50  is aligned relative to the IMD  10 . This is important, because if the external charger  50  is not well aligned to the IMD  10 , the magnetic field  66  produced by the charging coil  52  will not efficiently be received by the charging coil  36  in the IMD  10 . Efficiency in power transmission can be quantified as the “coupling” between the transmitting coil  52  and the receiving coil  36  (k, which ranges between 0 and 1), which generally speaking comprises the extent to which power expended at the transmitting coil  52  in the external charger  50  is received at the receiving coil  36  in the IMD  10 . It is generally desired that the coupling between coils  52  and  36  be as high as possible: higher coupling results in faster charging of the IMD battery  14  with the least expenditure of power in the external charger  50 . Poor coupling is disfavored, as this will require high power drain (e.g., a high Icharge) in the external charger  50  to adequately charge the IMD battery  14 . The use of high power depletes the battery  60  in the external charger  50 , and more importantly can cause the external charger  50  to heat up, and possibly burn or injure the patient. 
         [0011]    Charging the IMD  10  with the external charger  50  can be inconvenient if the IMD is implanted in a position that is difficult for the patient to reach. For example, an IMD used for spinal cord stimulation is typically implanted in the patient&#39;s upper buttock. A patient may have difficulty holding the external charger  50  in contact with their skin and in proper alignment for an adequate length of time to charge the battery  14 . Thus, there is a need in the art for more convenient and effective methods of charging the battery of an implanted medical device. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIGS. 1A-1C  show different views of an implantable pulse generator, a type of implantable medical device (IMD), in accordance with the prior art. 
           [0013]      FIG. 2  shows an external charger being used to charge a battery in an IMD, while  FIG. 3  shows circuitry in both, in accordance with the prior art. 
           [0014]      FIGS. 4A-4D  show an improved charging system having a charging coil assembly and an electronics module, in accordance with an example of the invention. 
           [0015]      FIGS. 5A and 5B  show a patient wearing a carrier containing an improved charging system, in accordance with an example of the invention. 
           [0016]      FIG. 6  shows a carrier for carrying an improved charging system, in accordance with an example of the invention. 
           [0017]      FIGS. 7A-7F  schematically represent the skin effect and the proximity effect. 
           [0018]      FIGS. 8  shows circuitry in an improved charging system, in accordance with an example of the invention. 
           [0019]      FIGS. 9A-9C  show embodiments of a multi-conductor cable. 
           [0020]      FIGS. 10A and 10B  illustrate Litz wire and a multi-conductor cable, respectively. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    An improved charging system  100  for an IMD  10  is shown in  FIG. 4A . Charging system  100  includes two main parts: an electronics module  104  and a charging coil assembly  102  which includes a charging coil  126 . The electronics module  104  and the charging coil assembly  102  are connected by a cable  106 . The cable  106  may be separable from both the electronics module  104  and the charging coil assembly  102  via a port/connector arrangement. Alternatively, the cable  106  may be permanently or semi-permanently attached to either of or both of the electronics module  104  and/or the charging coil assembly  102 . In the illustrated embodiment  100 , the cable  106  includes a connector  108  that can attach to and detach from a port  122  of the electronics module  104 . 
         [0022]    Electronics module  104  preferably includes within its housing  105  a battery  110  and active circuitry  112  needed for charging system operation. Electronics module  104  may further include a port  114  (e.g., a USB port) to allow its battery  110  to be recharged in conventional fashion, and/or to allow data to be read from or programmed into the electronics module, such as new operating software. Housing  105  may also carry a user interface, which as shown in the side view of  FIG. 4B  can include an on/off switch  116  to begin/terminate generation of the magnetic field  66 , and one or more LEDs  118   a  and  118   b.  In one example, LED  118   a  is used to indicate the power status of the electronics module  104 . For example, LED  118   a  may be lit when its battery  110  is charged, and may blink to indicate that the battery  110  needs charging. More complicated user interfaces, such as those incorporating a speaker and a display, could also be used. User interface elements can be included on other faces of the electronic module&#39;s housing  105 , and may be placed such that they are easily viewed for the therapeutic application at hand (e.g., SCS, DBS). Electronics are integrated within the housing  105  of the electronics module  104  by a circuit board  120 . 
         [0023]    Charging coil assembly  102  preferably contains only electronic components that are stimulated or read by active circuitry  112  within the electronics module  104 . Such components can include the primary charging coil  126  already mentioned, which as illustrated comprises a winding of copper wire and is energized by charging circuitry in the electronics module  104  to create the magnetic charging field  66 . One or more passive coils can be included within the charging coil assembly  102 , which are used to determine the position and/or alignment of the charging coil  126  (charging coil assembly  102 ) with respect to the IMD  10  being charged, and more specifically whether the charging coil  126  is aligned and/or centered with respect to an IMD  10  being charged. Additionally, or alternatively, the charging coil assembly  102  may contain one or more coils for sending and receiving telemetry to/from the IMD  10 . In the embodiment shown in  FIG. 4B , the one or more passive coils are formed using one or more traces in a circuit board  124 , which circuit board  124  is also used to integrate the electronic components within the charging coil assembly  102 . Circuit board  124  is shown in isolation in  FIG. 4C . While in some embodiments the charging coil  126  comprises a wire winding and the one or more passive coils comprise traces within the circuit board  124 , this is not strictly necessary: the charging coil  126  can also be formed from circuit board traces, and the one or more passive coils can comprise wire windings. Note that the charging coil  126  and the one or more passive coils, as well as being concentric, are also formed in planes that are parallel and can also be formed in the same plane. 
         [0024]    Further passive components preferably included within the charging coil assembly  102  include tuning capacitors  131  coupled to the primary coil  126  and to each one or more passive coils, which is used to generally tune each coils&#39; resonance to that of the magnetic field  66 . One skilled in the art will understand that the value of the capacitor  131  (C) connected to the charging coil  126  and to each sense coil will be chosen depending on the inductance (L) of that coil, in accordance with the equation fres=1/sqrt(2πLC). The charging coil assembly  102  can further include one or more thermistors  136 , which can be used to report the temperature of the charging coil assembly  102  to the electronics module  104  ( FIG. 8 , Vtherm). Such temperature data can in turn control production of the magnetic field  66  such that the temperature remains within safe limits. See, e.g., U.S. Pat. No. 8,321,029, describing temperature control in an external charging device. 
         [0025]    Electronic components within the charging coil assembly  102  can be integrated differently. In  FIGS. 4B and 4C , a single circuit board  124  is used, with the charging coil  126  mounted to the patient-facing side of the circuit board  124 , and with wires  134  in the cable  106  preferably coupled to the circuit board  124 . In  FIG. 4D  however, two circuit boards  124   a  and  124   b  are used. Circuit board  124   b  is outside of the area of the charging coil  126 , and includes the capacitors  131 . Circuit board  124   a  is within the area of the charging coil  126 , and includes the one or more passive coils and the thermistors  136 . In the two-circuit-board  124   a  and  124   b  arrangement of  FIG. 5D , notice in the cross section that the charging coil  126  and circuit boards  124   a  and  124   b  can be generally located in the same plane, which allows for a thinner construction of the charging coil assembly  102 . In  FIG. 4D , the wires  134  within the cable  106  can connect to both circuit boards  124   a  and  124   b  to allow communication between the components and the electronics module  104 . The two circuit boards  124   a  and  124   b  can also have connections between them (not shown). 
         [0026]    Components in the charging coil assembly  102  are integrated within a housing  125 , which may be formed in different ways. In one example, the housing  125  may include top and bottom portions formed of hard plastic that can be screwed, snap fit, ultrasonic welded, or solvent bonded together. Alternatively, housing  125  may include one or more plastic materials that are molded over the electronics components. One side of the housing  125  may include an indentation  132  to accommodate the thickness of a material (not shown) that can be useful to affixing the charging coil assembly  102  to the patient, to the patient&#39;s clothes, or within a holding device such as a charging belt or harness. Such material may include Velcro or double-sided tape for example. 
         [0027]      FIGS. 5A and 5B  show front and back views, respectively, of a patient  500  using a holding device in the form of a belt  501  to hold the electronics module  104  and the charging coil assembly  102  during a charging session. The belt  501  fastens around the patient&#39;s waist, and can be secured by a fastening device  502 , such as a buckle, clasp, snaps, Velcro, etc. The belt  501  can be adjustable to fit patients with different waist sizes. The belt  501  can include a pouch generally located on the belt  501  in a position where the IMD  10  is implanted in the patient, such as the back of the patient proximate to the buttocks in an SCS application. Alternatively, the charging coil assembly  102  can be held in place by clips, Velcro, etc. Affixing the electronics module  104  and the charging coil assembly  102  to the patient allows the patient to move or walk during a charging session and thus allows charging “on the go.” 
         [0028]      FIG. 6  shows an example of a holding device  600 . In the example shown, the holding device comprises a belt  601  that may be worn by an IMD patient. The belt  601  may be formed of cloth such as nylon for example. As shown, belt  601  includes a loop  602  at first end  603   a  through which the second end  603   b  of the belt may be passed to fasten the belt  601  to the patient. The second end  603   b  may include opposing Velcro surfaces  604   a  and  604   b  to allow the second end to be secured after passage through the loop. Such fastening means are just one example, and many other well-known fastening means (e.g., buckles, clasps, snaps, hooks, etc.) could be used as well. 
         [0029]    Additionally, the holding device  600  need not be fastenable at its ends  603   a  and  603   b  to form a closed loop. Instead, the ends  603   a  and  603   b  may remain unconnected while still wearable by the patient. This is particularly useful if the holding device  600  comprises a collar draped around a patient&#39;s neck, as is useful in a Deep Brain Stimulation (DBS) application for example. The holding device  600  may alternatively be wearable by being affixable to the patient or his clothing by an adhesive for example. 
         [0030]    The charging coil assembly  102  can be integrated within the belt  601 . Such integration of the charging coil assembly  102  may be effectively permanent, with the assembly  102  stitched between the inner and outer pieces of belt cloth. Alternatively, the belt  601  may be formed of a rubberized material and molded around the charging coil assembly  102 . Integration may also be semi-permanent, in which the charging coil assembly  102  is insertable within the belt  601  and thereafter largely left there, although also removeable from time to time (such as to wash the belt, or to switch out the charging coil assembly  102 ). In this regard, the belt  601  can include a slot into which the charging coil assembly  102  can be inserted between the inner and outer pieces of belt cloth. Such a slot may be openable and closeable, and may include a Velcro flap in one example. The belt  601  may include a flared portion  605  if the charging coil assembly  102  is larger than the width of the belt. Still alternatively, the charging coil assembly  102  may be removeably affixed to the belt  601  with clips, adhesive, Velcro, etc. The electronics module  104  can also be attached to the belt  601  with clips, adhesive, Velcro, etc. The cable  106  connecting the electronics module  104  to the charging coil assembly may be permanently or semi-permanently integrated into the belt  601  using any of the options described above for integrating the charging coil assembly into the belt. Alternatively, the cable  106  may be removeably attached to the belt by running the cable through loops in the belt, or using, hooks, clips, etc. 
         [0031]    As shown in  FIGS. 5 and 6 , the cable  106  connecting the electronics module  104  and the charging coil assembly  102  must be long enough to wrap partially around a patient&#39;s body. Thus, the cable  106  may be on the order of a meter in length in some instances. A cable of substantial length may give rise to some difficulties related to powering a coil, such as charging coil  126 , for reasons explained here. 
         [0032]    The electronics module  104  typically powers the charging coil  126  by providing an AC current to the coil  126  via conductors in the cable  106 . The frequency of the AC current is generally on the order of about  80  KHz, a frequency that generates magnetic fields that can efficiently penetrate a patient&#39;s tissue and transcutaneously charge an IMD  10 . A person of skill in the art will appreciate that a conductor carrying an AC current having a sufficiently high frequency exhibits a physical phenomenon referred to as “skin effect.” The skin effect is illustrated in  FIGS. 7A and 7B , which shows a section of a conductor  700  in longitudinal view ( FIG. 7A ) and cross sectional view ( FIG. 7B ). In a conductor  700  carrying a high frequency AC current  701 , most of the current becomes preferentially distributed near the surface of the conductor, illustrated by the shaded region  702 . The interior portion  703  of the conductor carries much less current. As a result, the current is carried by a smaller effective cross section of conductor, effectively increasing the resistance of the conductor. 
         [0033]    A potential improvement for addressing the skin effect is to use stranded conductor, i.e., conductor having multiple strands instead of a single strand. For a given cross sectional area of conductor, the ratio of surface to interior area is greater for a stranded conductor compared to a single strand. However, stranded cable gives rise to another effect, referred to as “proximity effect.” The proximity effect is illustrated in  FIGS. 7C-7F , which shows two pairs of conductors  704  and  705  carrying AC current of the same polarity ( FIGS. 7C and 7D ) and of the opposite polarity ( FIGS. 7E and 7F ), respectively. The alternating magnetic field caused by the AC current in one conductor creates eddy currents in the neighboring conductor(s), which warp the AC current in that conductor. When the proximate conductors carry current having the same polarity, as shown in  FIGS. 7C and 7D , the eddy currents cause the AC currents to become preferentially distributed on the surface of the conductors away from each other, as illustrated by the shaded area  706 . In other words, the AC current is not only preferentially confined to the surface of the conductor; it is also preferentially confined to an area of the surface that is distant from neighboring AC carrying conductors. As illustrated in  FIGS. 7E and 7F , when proximate conductors carry AC current of opposite polarity, the proximity effect causes the current to preferentially concentrate in areas of the surface  707  nearest to each other. Both the proximity effect and the skin effect constrain the current to a small region of the conductors, thereby increasing the resistance of the conductors. That increase in resistance can cause a large portion of the power delivered by the electronics module  104  to be lost in the cable  106  and therefore not delivered to the charging coil  126 . 
         [0034]    One method of reducing the resistance of the such a cable is by using Litz wire for carrying current between the coil and the electronics module. Litz wire utilizes many thin, individually insulated wire strands woven or twisted together in a prescribed pattern designed to cancel the proximity effect. However, implementing Litz wire in a cable such as cable  106 , which typically contains multiple conductors for in addition to the power conductors, can be difficult and requires a substantial amount of customization as well as customized cable terminations or connectors. 
         [0035]    The inventors have found that a multi-conductor cable, such as a standard ribbon cable, can be used as cable  106  in a configuration that achieves some of the same benefits as Litz wire but without custom cable and connector requirements.  FIG. 8  shows a schematic diagram of the electronics module  104 , the charging coil assembly  102 , and a ribbon cable  806  (implemented as cable  106 , as illustrated in  FIG. 4 ). The electronics module  104  may include (as part of circuitry  112 ;  FIG. 4A ) control circuitry  72  that controls charging circuitry  64  to generate an AC charging current. The charging circuitry  64  includes two terminals Vcharge+ and Vcharge−, which supply current Icharge+ and Icharge−, respectively, to the charging coil  126 . The polarity of Vcharge+/− and Icharge+/− alternate at a frequency that is typically on the order of 80 KHz. A plurality of traces  801  in the circuit board  120  connect the Vcharge+ to connectors at terminal port  122 . Four traces  801  are shown in  FIG. 8  but the number of traces can vary. Likewise, a plurality of traces  802  connect the Vcharge− terminal to connectors at port  122 . The Icharge+ current is carried on a plurality of conductors represented as dashes (---) within cable  106 . The Icharge− current is carried on a plurality of conductors represented as dash-dots (-.-). Complimentary pluralities of traces  803  and  804  within the charging coil assembly  102  conduct the Icharge+/− current to the coil  126 , providing voltage Vcoil+/− across the coil, energizing the charging coil to produce the magnetic field  66 . Using multiple small, individually insulated conductors within ribbon cable  806  instead of one large conductor to carry the Icharge+/− currents decreases the resistance of the of the cable  806  by minimizing the skin effect and spacing the conductors apart from each other minimizes the resistance due to the proximity effect and the skin effect. 
         [0036]    The cable  806  must have an adequate number of discrete conductors to carry the AC powering current and any other AC or DC signals required for a particular electronics module/charging coil assembly pair. For example, the electronics module  104  illustrated in  FIG. 8  includes an LSK demodulator  68  that can demodulate LSK data  69  encoded upon the reflected impedance of the charging coil  126 . Two terminals, Vcoil+ and Vcoil, provide the input to the LSK Demodulator  68 . Those two terminals may be connected (directly or indirectly) to the pluralities of traces  801  and  802  or they may be connected to other pluralities of traces with complementary pluralities of independent conductors within cable  806 . 
         [0037]    The charging coil assembly  102  illustrated in  FIG. 8  also includes a passive coil  128 , which, in that embodiment, is an alignment coil. Examples of alignment coils are described, for example, in commonly owned U.S. Patent Application Ser. No. 62/350,451, filed Jun. 15, 2016. The passive coil  128  includes terminals Va+ and Va−, which connect to corresponding terminals Va+ and Va− of an analog to digital converter  142  of the electronics module  104 . The output of the analog to digital converter  142  is provided to an alignment circuit  140 , as described in the &#39;XXX application. The illustrated cable  106  includes single conductors for each of Va+ and Va−. However, multiple discrete conductors could be used for the passive coil  128  in a manner similar to the multiple conductors used for the charging coil  126 . A plurality of traces would connect each of Va+ and Va− to port  122 . 
         [0038]    The illustrated charging coil assembly  102  also includes a thermistor, which generates a DC voltage Vtherm as a function of temperature. Vtherm is provided to the microcontroller  72 , which may be programmed to adjust charging based on the measured temperature. Typically, only a single conductor, as included in the illustrated cable  106 , is needed to communicate Vtherm between the charging coil assembly  102  and the electronics module  104  because Vtherm is a DC voltage. The illustrated cable  806  also includes a single conductor GND providing a ground between the electronics module  104  and the charging coil assembly  102 . 
         [0039]    In the illustrated cable  806  of  FIG. 8 , the DC conductors (solid lines) are grouped together in one part of the cable  806  and the AC Icharge+ and Icharge− conductors are interspersed with each other in a different part of the cable. However, many different configurations are possible.  FIGS. 9A , B, and C show three alternative configurations,  901 ,  901  and  903 , respectively. For clarity, only the cable  806  is shown. In configuration  901  the Icharge+ conductors (---) are grouped together and the Icharge− conductors (-.-) are grouped together. In configuration  902 , the DC conductors Va+, Va−, Vtherm, and GND are interspersed between the AC conductors Icharge+ and Icharge− (note that only four AC conductors are shown, but the number can vary). In configuration  903 , grounded dummy conductors (represented as dotted lines . . . ) are interspersed between the AC conductors Icharge+ and Icharge−. Many other configurations are possible. 
         [0040]    An advantage of the illustrated cable  806  is that each of the conductors are physically the same. In other words, it need not be the case that some conductors are made of Litz wire, some of single stranded wire, some of stranded wire, etc. Thus, the cable need not be customized for each particular configuration of an electronics module and/or charging coil assembly, so long as the cable has an adequate number of conductors. Numerous multi-conductor cables are available, such as ribbon cable, serial cable, USB cable, and the like. For example, ribbon cable having 4, 6, 8, 9, 10, 14, 15, 16, 18, 20, and up to about 80 conductors are available, each with standard connectors. Likewise, serial cables having 4, 9, 25 and other numbers of conductors, with standard connectors. Moreover, according to some embodiments, the cable  106  can be permanently attached (i.e., hardwired) to one or both the electronics module  104  and/or the charging coil assembly  102 . In such a case, the conductors of the cable may be soldered to soldering pads on the PCB. 
         [0041]      FIGS. 10A and 10B  illustrate how using multiple individually insulated conductors to carry AC current, as described above, differs from simply using Litz wire.  FIG. 10A  shows the conductors  1002  of a Litz wire  1001  bonded to a bonding pad  1003  of a PCB  1004 . Even though the conductors  1002  of the Litz wire  1001  are individually insulated with insulation  1005 , they are still held intimately close together within the Litz wire  1001 . They are therefore susceptible to the proximity effect. By contrast, the multi-conductor cable  1006  maintains separation d of conductors  1007 . Each of the conductors are connected to separate connectors  1008  (solder pads in the illustration) on PCB  1010 . 
         [0042]    Although particular embodiments of the present invention have been shown and described, it should be understood that the above discussion is not intended to limit the present invention to these embodiments. It will be obvious to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the present invention. Thus, the present invention is intended to cover equivalents that may fall within the spirit and scope of the present invention as defined by the claims.