Abstract:
An external antenna with a plurality of concentric primary coils recharges an implantable medical device with a secondary coil when the primary coils are placed in proximity of the secondary coil. Selection circuitry determines which of the plurality of concentric primary coils has the most efficient coupling with the secondary coil and drive circuitry drives the selected primary coil with an oscillating current. During a recharge session, selection circuitry periodically checks at least some of the primary coils to determine whether the primary coil with the most efficient connection has changed. An antenna housing may hold the primary coils in a rigid planar relationship with each other or the primary coils may shift with respect to each other, forming a cup-shape around a bulge in the skin created by the implantable medical device.

Description:
FIELD 
     The present invention is related to implantable medical devices and, in particular, implantable medical devices having a rechargeable power source. 
     BACKGROUND 
     Implantable medical devices for producing a therapeutic result in a patient are well known. Examples of such implantable medical devices include, but are not limited to, implantable drug infusion pumps, implantable neurostimulators, implantable cardioverters, implantable cardiac pacemakers, implantable defibrillators and cochlear implants. Of course, it is recognized that other implantable medical devices are envisioned which utilize energy delivered or transferred from an external device. 
     A common element in many of these implantable medical devices is the need for electrical power in the implanted medical device. The implanted medical device may require electrical power to perform its therapeutic function whether it be driving an electrical infusion pump, providing an electrical neurostimulation pulse or providing an electrical cardiac stimulation pulse. This electrical power is derived from a power source. 
     Typically, a power source for an implantable medical device is a rechargeable power source. In this form, an internal power source, such as a battery, can be used for direct electrical power to the implanted medical device. When the battery has expended, or nearly expended, its capacity, the battery can be recharged transcutaneously, via inductive coupling from an external power source temporarily positioned on the surface of the skin. 
     The external power source typically may have an external antenna that is placed in the proximity of a corresponding internal antenna associated with the implantable medical device. 
     In order to charge or recharge the implantable medical device&#39;s rechargeable power source, it is typical for the user to place an external charger, or an antenna associated with an external charger, in the proximity of the implantable medical device, or in the proximity of an internal or secondary antenna or coil associated with the implantable medical device. Optimally, the primary coil of the external charger will be aligned as closely as possible with the secondary coil of the implantable medical device minimizing the distance between the two coils and providing a relatively efficient transfer of energy between the external charger and implantable medical device. 
     SUMMARY 
     It may sometimes be difficult for the user to exactly locate the external antenna, or more particularly, the primary coil in the proper location with respect to the internal secondary antenna of the implantable medical device for optimal charging or power transfer efficiency. Exact location is complicated by determining the exact proper location. Although a bulge created by the implantable medical device is commonly used to locate the external antenna, the center of the bulge may not be the proper location because the secondary coil associated with the implantable medical device may not be centered with respect to the implantable medical device and, hence, may not be centered with respect to the bulge. Further, it is often difficult to secure the external antenna in the proper location even if the proper location is known. Since charging is not an instantaneous procedure, a mechanism is generally employed to secure the external antenna in a location to conduct transcutaneous energy transfer. The securing mechanism may not precisely locate the external antenna or the external antenna may be subject to movement with respect to the patient as a result of the patient&#39;s movements. 
     If the primary coil of the external antenna is not optimally located with respect to the secondary coil of the implantable medical device, optimal efficiency of energy transfer and, hence, charging of the implantable medical device often is not achieved. 
     In an embodiment, a plurality of primary coils are utilized to provide a wider effective charging area for the external power source. The use of a plurality of concentric primary coils allows at least one of the primary coils to be energized. In particular, the primary coil that provides the best coupling and/or most efficient transcutaneous transfer of energy will be energized. If the external antenna is not accurately or nearly accurately aligned with the secondary coil, then the smallest concentric primary coil of the external power source may not be aligned with the secondary coil of the implantable medical device. More efficient transcutaneous energy transfer may result if a larger one of the plurality of concentric primary coils is energized for transcutaneous energy transfer. In addition, the availability of multiple, i.e., more than one, primary coils of varying diameters possibly results in greater comfort for the user and/or a greater likelihood of success in charging of the implantable medical device by the user. 
     In an aspect of the present invention, a plurality of primary coils, concentrically arranged, are utilized in the antenna of the external power source. The external power source may select one of the primary coils, for example, to be used to more efficiently transfer energy to the implantable medical device. 
     In an embodiment, the present invention provides an external power source for an implantable medical device having therapeutic componentry and a secondary coil operatively coupled to the therapeutic componentry. A plurality of concentric primary charging coils are each capable of transcutaneously inductively energizing the secondary coil when externally placed in proximity of the secondary coil. Drive circuitry selectively couples to each of the plurality of concentric primary coils for energizing a selected one of the plurality of concentric primary coils. 
     In an embodiment, the selected one of the plurality of concentric primary coils is a single selected one of the plurality of concentric primary coils. 
     In an embodiment, the selected one of the plurality of concentric primary coils is determined by efficiency of energy transfer. 
     In an embodiment, the selected one of the plurality of concentric primary coils is determined to be one of the plurality of concentric primary coils providing a greatest efficiency of energy transfer between the selected one of the plurality of concentric primary coils and the secondary coil. 
     In an embodiment, selection circuitry determines which of the plurality of concentric primary coils is selected to be the selected one of the plurality of concentric primary coils. 
     In an embodiment, the selection circuitry determines the selected one of the plurality of concentric primary coils based on which of the plurality of concentric primary coils provides a greatest efficiency of energy transfer between the selected one of the plurality of concentric primary coils and the secondary coil. 
     In an embodiment, the selection circuitry periodically checks an efficiency of energy transfer between each of the plurality of primary coils and the secondary coil. 
     In an embodiment, each of the plurality of concentric primary coils has an inside diameter and an outside diameter, wherein the secondary coil has an outside diameter and wherein a distance between the outside diameter of one of the plurality of concentric primary coils to the inside diameter of a next larger one of the plurality of concentric primary coils is not greater than the outside diameter of the secondary coil. 
     In an embodiment, the plurality of concentric primary coils lie in a plane. 
     In an embodiment, a plane of one of the plurality of concentric primary coils is offset from a plane of another of the plurality of concentric primary coils whereby the plurality of concentric primary coils may more easily form over a bulge created by the implantable medical device than if the plurality of concentric primary coils were planar. 
     In an embodiment, the implantable medical device further has a rechargeable power source operatively coupled to the secondary coil and wherein the selected one of the plurality of concentric primary coils charges the rechargeable power source. 
     In an embodiment, the present invention provides a method of energizing a secondary coil of an implantable medical having therapeutic output componentry coupled to the secondary coil. An array of a plurality of concentric primary charging coils is positioned in proximity of the secondary coil, each of the plurality of concentric primary charging coils being capable of transcutaneously inductively energizing the secondary coil. One of the plurality of concentric primary coils is selected to be energized. The selected one of the plurality of concentric primary coils is energized. 
     In an embodiment, only a single one of the plurality of concentric primary coils is selected. 
     In an embodiment, selecting is determined, at least in part, by an efficiency of energy transfer between the plurality of concentric primary coils and the secondary coil. 
     In an embodiment, the one of the plurality of concentric primary coils having a greatest efficiency of energy transfer with the secondary coil is selected. 
     In an embodiment, one of the plurality of concentric primary coils is periodically reselected. 
     In an embodiment, one of the plurality of concentric primary coils is reselected at least once every minute. 
    
    
     
       DRAWINGS 
         FIG. 1  illustrates an implantable medical device implanted in a patient; 
         FIG. 2  is a block diagram of an implantable medical device; 
         FIG. 3  is a detailed block diagram of an implantable medical device implanted subcutaneously and an associated external power source or charging device; 
         FIG. 4  illustrates a top view of an embodiment of the primary coil arrangement of an external antenna associated with an external power source; 
         FIG. 5  illustrates a side cross-sectional view of the embodiment of the primary coil arrangement of an external antenna associated with an external power source illustrated in  FIG. 4 ; 
         FIG. 6  illustrates a side cross-sectional view of another embodiment of the primary coil arrangement of an external antenna associated with an external power source illustrated in  FIG. 4 ; 
         FIG. 7  illustrates a top view of another embodiment of the primary coil arrangement of an external antenna associated with an external power source; 
         FIG. 8  is a block diagram schematic representation of an external power source including an external antenna; 
         FIG. 9  is a flow chart illustrating a method of energizing a secondary coil of the implantable medical device using an external antenna having plurality of primary coils; and 
         FIG. 10  is a flow chart illustrating in more detail a method of energizing a secondary coil of the implantable medical device using an external antenna having plurality of primary coils. 
     
    
    
     DETAILED DESCRIPTION 
     In order to achieve effective and efficient energy transfer to an implantable medical device and the effective and efficient charging of a rechargeable power source, such as a battery, a proper alignment of a primary coil associated with an external antenna and an external power source with a secondary coil of an implantable medical device is desired. Unfortunately, it is often difficult to achieve the precise alignment desired to obtain the most effective and, possibly, efficient result. Too often, the primary coil of the external device may not be precisely aligned with the secondary coil of the implantable medical device. When this happens, a less than optimal transcutaneous transfer of energy may result. 
     In an embodiment, a plurality of primary coils are utilized to provide a wider effective charging area for the external power source. The use of a plurality of concentric primary coils allows at least one of the primary coils to be energized, in particular, the primary coil that provides the best coupling and/or most efficient transcutaneous transfer of energy. If the external antenna is accurately or nearly accurately positioned, i.e., laterally aligned, with the secondary coil, then the secondary coil will be most directly aligned with the smallest of the concentric primary coils and that coil may be energized and utilized for transcutaneous energy transfer. If however, the external antenna is not accurately or nearly accurately aligned with the secondary coil, then the smallest concentric primary coil of the external power source may not be aligned with the secondary coil of the implantable medical device. More efficient transcutaneous energy transfer may result if a larger one of the plurality of concentric primary coils is energized for transcutaneous energy transfer, since a larger primary coil, although not accurately aligned, may still cover, or partially cover, the secondary coil resulting in a more efficient transcutaneous transfer of energy than if the smallest primary coil had been energized, which could be completely misaligned with the secondary coil. In addition, the availability of multiple, i.e., more than one, primary coils of varying diameters gives rise to efficient transcutaneous energy transfer with the antenna of the external power source having a larger range of positions, possibly resulting in greater comfort for the user and/or a greater likelihood of success in charging of the implantable medical device by the user. 
     Use of an external power source having an antenna with a plurality of concentric primary coils can generally be illustrated by the generic system in  FIG. 1 , which shows implantable medical device  16 , for example, a neurological stimulator, implanted in patient  18 . The implantable medical device  16  is typically implanted by a surgeon in a sterile surgical procedure performed under local, regional, or general anesthesia. Before implanting the medical device  16 , a lead  22  is typically implanted with the distal end position at a desired therapeutic delivery site  23  and the proximal end tunneled under the skin to the location where the medical device  16  is to be implanted. Implantable medical device  16  is generally implanted subcutaneously at depths, depending upon application and device  16 , of from 1 centimeter (0.4 inches) to 2.5 centimeters (1 inch) where there is sufficient tissue to support the implanted system. Once medical device  16  is implanted into the patient  18 , the incision can be sutured closed and medical device  16  can begin operation. 
     Implantable medical device  16  can be any of a number of medical devices such as an implantable therapeutic substance delivery device, implantable drug pump, electrical stimulator, cardiac pacemaker, cardioverter or defibrillator, as examples. 
     If implantable medical device  16  is a drug infusion device, for example, implantable medical device  16  operates to infuse a therapeutic substance into patient  18 . Implantable medical device  16  can be used for a wide variety of therapies such as pain, spasticity, cancer, and many other medical conditions. The therapeutic substance contained in implantable medical device  16  is a substance intended to have a therapeutic effect such as pharmaceutical compositions, genetic materials, biologics, and other substances. Pharmaceutical compositions are chemical formulations intended to have a therapeutic effect such as intrathecal antispasmodics, pain medications, chemotherapeutic agents, and the like. Pharmaceutical compositions are often configured to function in an implanted environment with characteristics such as stability at body temperature to retain therapeutic qualities, concentration to reduce the frequency of replenishment, and the like. Genetic materials are substances intended to have a direct or indirect genetic therapeutic effect such as genetic vectors, genetic regulator elements, genetic structural elements, DNA, and the like. Biologics are substances that are living matter or derived from living matter intended to have a therapeutic effect such as stem cells, platelets, hormones, biologically produced chemicals, and the like. Other substances may or may not be intended to have a therapeutic effect and are not easily classified such as saline solution, fluoroscopy agents, disease diagnostic agents and the like. Unless otherwise noted in the following paragraphs, a drug is synonymous with any therapeutic, diagnostic, or other substance that is delivered by the implantable infusion device. 
     If implantable medical device  16  is an electrical stimulator, as in the embodiment of  FIG. 1 , therapy module  28  ( FIG. 2 ) may deliver an electrical stimulus, such as an electrical pulse, or series of electrical pulses, either mono-polar or bi-polar, through one or more electrical leads  22  and/or electrodes to provide specific or general benefit to that patient such as pain relief or muscular control. 
     In  FIG. 2 , implantable medical device  16  has a rechargeable power source  24 , such as a Lithium ion battery, powering electronics  26  and therapy module  28  in a conventional manner. Therapy module  28  is coupled to patient  18  through one or more therapy connections  30 , which is also conventional. Rechargeable power source  24 , electronics  26  and therapy module  28  are contained in hermetically sealed housing  32 . Secondary charging coil  34  is attached to the exterior of housing  32 . Secondary charging coil  34  is operatively coupled through electronics  26  to rechargeable power source  24 . In an alternative embodiment, secondary charging coil  34  could be contained in housing  32  or could be contained in a separate housing umbilically connected to electronics  26 . Electronics  26  help provide control of the charging rate of rechargeable power source  24  in a conventional manner. Magnetic shield  36  is positioned between secondary charging coil  34  and housing  32  in order to protect rechargeable power source  24 , electronics  26  and therapy module  28  from electromagnetic energy when secondary charging coil  34  is utilized to charge rechargeable power source  24 . 
     Rechargeable power source  24  can be any of a variety power sources including a chemically based battery or a capacitor. Rechargeable power source may be a well known lithium ion battery. 
       FIG. 3  illustrates an alternative embodiment of implantable medical device  16  situated under cutaneous boundary  38 . Implantable medical device  16  is similar to the embodiment illustrated in  FIG. 2 . However, charging regulator  42  is shown separate from electronics  26  controlling therapy module  28 . Again, charging regulation and therapy control is conventional. Implantable medical device  16  also has internal telemetry coil  44  configured in conventional manner to communicate through external telemetry coil  46  to an external programming device (not shown), charging unit  50  or other device in a conventional manner in order to both program and control implantable medical device  16  and to externally obtain information from implantable medical device  16  once implantable medical device  16  has been implanted. In an embodiment, internal telemetry coil  44  is rectangular in shape with dimensions of 1.85 inches (4.7 centimeters) by 1.89 inches (4.8 centimeters) constructed from 150 turns of 43 AWG wire and is sized to be larger than the diameter of secondary charging coil  34 . In this embodiment, secondary coil  34  is located in internal antenna  68  and is constructed with 182 turns of 30 AWG wire with an inside diameter of 0.72 inches (1.83 centimeters) and an outside diameter of 1.43 inches (3.63 centimeters) with a height of 0.075 inches (0.19 centimeters). Magnetic shield  36  is positioned between secondary charging coil  34  and housing  32  and sized to cover the footprint of secondary charging coil  34 . 
     Internal telemetry coil  44 , having a larger diameter than secondary coil  34 , is not completely covered by magnetic shield  36  allowing implantable medical device  16  to communicate with the external programming device with internal telemetry coil  44  in spite of the presence of magnetic shield  36 . 
     Rechargeable power source  24  can be charged while implantable medical device  16  is in place in a patient through the use of external charging device  48 . In an embodiment, external charging device  48  consists of charging unit  50  and external antenna  52 . For purposes of illustration in  FIG. 3 , external charging device or external power source  48  is illustrated with single primary coil  54 . More specific illustrations of external antenna  52  with a plurality of concentric primary coils will be illustrated more specifically in later Figures. Charging unit  50  contains the electronics necessary to drive primary coil  54  with an oscillating current in order to induce current in secondary coil  34  when primary coil  54  is placed in the proximity of secondary coil  34 . Charging unit  50  is operatively coupled to primary coil by cable  56 . In an alternative embodiment, charging unit  50  and antenna  52  may be combined into a single unit. Antenna  52  may also optionally contain external telemetry coil  46  which may be operatively coupled to charging unit  50  if it is desired to communicate to, or from, implantable medical device  16  with external charging device  48 . Alternatively, antenna  52  may optionally contain external telemetry coil  46  which can be operatively coupled to an external programming device, either individually or together with external charging unit  48 . 
       FIG. 4  illustrates a more specific embodiment of primary coil  54  utilized in external antenna  52 . Instead of a single primary coil  54  as illustrated in  FIG. 3 , an embodiment utilizes a plurality of concentric primary coils  72 ,  74  and  76 . Since primary coils  72 ,  74  and  76  are concentric, each of primary coils  72 ,  74  and  76  having different diameters. In particular, primary coil  72  is the smallest and is approximately equal in diameter to primary coil  54  illustrated in  FIG. 3 . Primary coil  74  has a larger diameter than primary coil  72 . In effect, primary coil  72  nests inside of primary coil  74 . Likewise, primary coil  76  has a larger diameter than primary coil  74 . Primary coil  72  and primary coil  74  nest inside of primary coil  76 . 
     In an embodiment, primary coils  72 ,  74 ,  76  are constructed from 100 to 150 turns of 40 AWG wire. Primary coils  72 ,  74 ,  76  may be driven between 30 and 50 kiloHertz. In an embodiment, primary coil  76  has an outside diameter of approximately six-and-a-half inches (16.5 centimeters) and an inside diameter of approximately five-and-a-half inches (14.0 centimeters), primary coil  74  has an outside diameter of approximately four-and-a-half inches (11.4 centimeters) and an inside diameter of approximately three-and-a-half inches (8.9 centimeters), and primary coil  72  has an outside diameter of approximately two-and-a-half inches (6.35 centimeters) and an inside diameter of approximately one-and-a-quarter inches (3.2 centimeters). However, alternative diameters for primary coils  72 ,  74 ,  76  are envisioned depending on a variety of factors, such as the dimensions of implantable medical device  16  and physical characteristics of patient  18  that may be conducive to relatively larger or smaller primary coils  72 ,  74 ,  76 . 
       FIG. 5  illustrates a cross-sectional view of the embodiment of primary coil  54  shown in  FIG. 4 . Antenna housing  78  contains primary coil  72  nested inside of primary coil  74 , and both primary coil  72  and primary coil  74  are nested inside of primary coil  76 . As depicted, in an embodiment primary coil  72 , primary coil  74  and primary coil  76  are on substantially the same plane. In this embodiment, antenna housing  78  is made of a substantially inflexible plastic or similar material known in the art and, thus, is substantially rigid. Primary coil  72 , primary coil  74  and primary coil  76  are largely fixed within antenna housing  78  and are not enabled to shift relative to each other, with each primary coil  72 ,  74 ,  76  maintaining the same position relative to each other coil and antenna housing  78 . 
       FIG. 6  illustrates a cross-sectional view of an embodiment in which antenna housing  78  is pliable and flexible, and primary coils  72 ,  74  and  76  may shift with respect to each other. Rather than being comprised of a rigid material, antenna housing  78  is made from a substantially flexible material such as fabric or nylon. Primary coil  72 , primary coil  74  and primary coil  76  are not fixed in relation to each other, though primary coil  72  is connected to primary coil  74 , and primary coil  74  is connected to primary coil  76 , by flexible couplers  80 , allowing primary coils  72 ,  74  and  76  to shift with respect to each other, but to maintain proximity with each other, and maintain approximately the same distance between each primary coil  72 ,  74 ,  76 . In an embodiment, flexible couplers  80  may be comprised of an insulating material to prevent shorting one primary coil  72 ,  74 ,  76  with another. Conductive materials, such as metal wires, may be included in flexible couplers  80  to operatively couple charging unit  50  to an intended destination primary coil  72 ,  74  and  76 . However, insulating materials may still be used to prevent conductive materials from coming into contact with any other than the intended destination primary coil  72 ,  74 ,  76 . 
     In an embodiment, the distance between the outside diameter of primary coil  72 ,  74  to the inside diameter of primary coil  74 ,  76 , respectively, is not greater than the outside diameter of secondary coil  34 . 
     This embodiment allows external antenna  52  to form a cup-like shape, conforming to the bulge created in cutaneous boundary  38  by implantable medical device  16 . As can be seen in  FIG. 6 , the creation of a cup-like shape may bring one or more of primary coil  72 ,  74 ,  76  into closer proximity of secondary coil  34 , thereby potentially creating a more effective and efficient energy transfer than would naturally be attainable if antenna housing  78  were rigid. Advantageously, this embodiment creates an external antenna  52  that offers increased patient comfort, due to its ability to conform to the contours of the patient&#39;s body. In an embodiment, patient comfort may be enhanced, while the heating of tissue at cutaneous boundary  38  may be reduced, by making antenna housing  78  of a porous or breathable material, or by omitting antenna housing  78  material in some places between primary coils  72 ,  74 ,  76 . 
       FIG. 7  illustrates a top view of the flexible external antenna  52  depicted in  FIG. 6 . Primary coils  72 ,  74  and  76  remain concentric relative to each other, with primary coil  72  connected to primary coil  74 , and primary coil  74  connected to primary coil  76 , via flexible couplers  80 . In an embodiment, void areas  82  may be left free of antenna housing  78  material, or antenna housing  78  material in void areas  82  may be comprised of porous or breathable material, such as nylon mesh. 
       FIG. 8  shows a block diagram of an embodiment of external charger  48 . Charging unit  50  includes drive circuitry  86  that drives primary coils  72 ,  74 ,  76  with an oscillating current. Charging unit  50  further includes selection circuitry  88  that determines which primary coil  72 ,  74 ,  76  drive circuitry  88  will drive. Selection circuitry  88  may be comprised of standard, off-the-shelf componentry, including a processor and memory units. Alternatively, selection circuitry  88  may be comprised of other standard components known in the art, such as comparators. Selection circuitry  88  further comprises a switch, which may variably conduct the oscillating current generated by drive circuitry  86  to the intended destination primary coil  72 ,  74 ,  76 . 
     An embodiment of a recharge session is described in  FIG. 9 . When a user elects to conduct a charging session, external antenna  52  is positioned ( 910 ) relative to implantable medical device  16  such that primary coils  72 ,  74 ,  76  are in proximity of secondary coil  34 , and external telemetry coil  46  is in proximity of internal telemetry coil  44 . Selection circuitry  88  selects ( 912 ) a primary coil  72 ,  74 ,  76  (which process is described in  FIG. 10 ), and drive circuitry  86  energizes ( 914 ) selected primary coil  72 ,  74 ,  76 . After selected primary coil  72 ,  74 ,  76  has been energized for some period of time, in an embodiment one minute, it is determined whether the recharging session should end ( 916 ). If not, selection circuitry  88  again selects ( 912 ) a primary coil  72 ,  74 ,  76 , and drive circuitry  86  energizes ( 914 ) the selected primary coil  72 ,  74 ,  76 . This process is repeated until it is determined that the process should end, at which point recharging is stopped ( 918 ). 
     In an embodiment described in  FIG. 10 , when external telemetry coil  46  is in proximity of internal telemetry coil  44 , selection circuitry  88  initially selects ( 1010 ) one primary coil  72 , and energizes ( 1012 ) primary coil  72 . Selection circuitry  88  then measures ( 1014 ) and records the efficiency of the connection between primary coil  72  and secondary coil  34  by comparing the power delivered to primary coil  72  and the power generated in secondary coil  34 , as reported to external charging device  48  via internal telemetry coil  44  and external telemetry coil  46 . Selection circuitry  88  then selects ( 1016 ) and energizes ( 1018 ) primary coil  74 , and again measures ( 1020 ) and records the efficiency of the connection between primary coil  74  and secondary coil  34 . The efficiency may be reported to external charging device, or another external device, in the manner discussed above. Selection circuitry  88  then selects ( 1022 ) and energizes ( 1024 ) primary coil  76 , and again measures ( 1026 ) and records the efficiency of the connection between primary coil  76  and secondary coil  34  and reports the efficiency in the above-described manner. In embodiments with other than three primary coils  72 ,  74 ,  76 , this procedure may be expanded or contracted to correspond to the number of primary coils  72 ,  74 ,  76 , such that the efficiency of the connection between each primary coil  72 ,  74 ,  76  and the secondary coil is measured and recorded. In another embodiment, the efficiencies of the various primary coils may be recorded within an internal storage device of implantable medical device  16  and transferred to external charging device  48  all at once after all measurements have been completed. 
     After the efficiency between each primary coil  72 ,  74 ,  76  and secondary coil  34  has been measured and recorded, selection circuitry  88  determines ( 1028 ) which primary coil  72 ,  74 ,  76  has the best, most efficient connection with secondary coil  34 . Where primary coil  72  has the most efficient connection, primary coil  72  is energized ( 1030 ). Where primary coil  74  has the most efficient connection, primary coil  74  is energized ( 1032 ). Where primary coil  76  has the most efficient connection, primary coil  76  is energized ( 1034 ). The selected coil may remain energized until a period of time has elapsed, in an embodiment one minute, at which point energizing stops ( 1036 ), and the process begins again by selecting ( 1010 ) primary coil  72 . The process may repeat until the recharge session has been completed ( FIG. 9 ). 
     In an embodiment, a plurality of primary coils  72 ,  74 ,  76 , for example two of primary coils  72 ,  74 ,  76 , could be energized simultaneously. In secondary coil  34  is not exactly aligned with one of primary coils  72 ,  74 ,  76  but rather is aligned, for example, between primary coils  74  and  76 , then it may be desirable to energize both of primary coils  74  and  76  than having to choose only one of primary coils  72 ,  74 ,  76 . 
     Thus, embodiments of concentric primary coils for inductively charging an implantable medical device, external power source and method are disclosed. One skilled in the art will appreciate that the present invention can be practiced with embodiments other than those disclosed. The disclosed embodiments are presented for purposes of illustration and not limitation, and the present invention is limited only by the claims that follow.