Patent Publication Number: US-2021194289-A1

Title: Implantable medical system with external power charger

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This Non-Provisional Utility application claims benefit to U.S. Provisional Application No. 62/949,747, filed Dec. 18, 2019, titled “EXTERNAL POWER CHARGER,” the entirety of which incorporated herein by reference. 
    
    
     BACKGROUND 
     The present disclosure relates generally to medical devices. More particularly, the present disclosure relates to devices that charge and may communicate with implantable medical devices. 
     Wireless power transfer or transmission is used to deliver power from a power source without a mechanical connection to electronic devices. Wireless power transfer systems are used in a variety of applications, such as, for recharging batteries in mobile computing devices such as smart phones or wearable devices. Wireless power transfer systems are also used to transmit power transcutaneously, or through the skin, to medical devices implanted in a patient either to directly power the implanted medical device or to recharge an energy storage system of the implanted medical device. The implantable medical device may be implanted within a patient and perform a task such as to monitor a parameter of the patient or to deliver a therapy to the patient. In one example, the implantable medical device is an implantable neurostimulator implanted into the patient and used to provide nerve stimulation via an electrical lead. Many implantable medical devices are designed to receive power directly from an energy storage system such as a battery or capacitor located with the implantable medical device, but the energy storage system often becomes depleted of energy long before the end of the useful life of the implantable medical device. The implantable medical device may include a rechargeable energy storage system such as a rechargeable battery to extend the life of the implantable medical device. A wireless charger may be applied to recharge a depleted battery in the implanted medical device. From time to time, the wireless charger and the implanted medical device may also wirelessly and transcutaneously exchange communication signals. 
     In some examples, transcutaneous charging is performed via inductive power transfer or transmission. The energy storage system of the implantable medical device can be recharged with an external charger configured to provide inductive power transfer. Inductive power transfer can be performed with an inductive coupling between coils of wire such as a primary coil in the charger and a secondary coil in the implantable medical device. Power is transferred between the coils with a magnetic field. An alternating current (AC) through the primary coil creates an oscillating magnetic field. The magnetic field passes through the secondary coil, and the magnetic field induces an alternating electromotive force, or EMF, (voltage), which creates an alternating current in the secondary coil. The induced alternating current may either directly drive a load in the implantable medical device, or be rectified to direct current (DC) by a rectifier in the implantable medical device, which drives the load. Resonant inductive coupling is a type of inductive coupling in which power is transferred by magnetic fields between two resonant circuits, one in the charger and one in the implantable medical device. Each resonant circuit includes a coil of wire connected to a capacitor, or a self-resonant coil or other resonator with internal capacitance. Resonant circuits, or tank circuits, are tuned to resonate at generally the same resonant frequency. The resonance between the coils can greatly increase coupling and power transfer between the charger and the implantable medical device. In this example, the external charger does not mechanically connect with the implantable medical device, and the external charger can be used to charge the implantable medical device from a relatively short distance away. 
     SUMMARY 
     Current wireless power chargers may include multiple internal compartments to separate the primary coil from the electronic components and battery. For example, U.S. Pat. No. 9,821,112 to Olson et al. (which is incorporated by reference into this disclosure) describes an example of an external charger having an external antenna with a primary coil separate from a charging unit having electronics and battery to drive the primary coil. Additional components are used in the charger to separate the primary coil from the electronic components and battery, which increases costs and creates a more cumbersome wireless power charger. Simply combining all the elements into a single compartment, however, can create undesirable effects such as loading to the primary coil from conductive elements of printed circuit boards, such as a ground plane, that generates a reflected impedance on the primary coil. For instance, the introduction of a ground plane in proximity with a primary coil may increase the resistance by a factor of ten leading to significant inefficiencies in power transfer that can increase the length of a recharge session and cause excessive heating of the primary recharger. 
     A disclosed external charger includes a primary coil within the same internal compartment as the electronics and battery. In one example, the charger includes a housing that defines an internal compartment that includes printed circuit board assemblies, a battery, and a recharge coil assembly. The recharge coil assembly can operate as a primary coil to deliver magnetic energy to recharge an energy storage system on an implantable medical device with a corresponding secondary coil. The recharge coil assembly in the example includes a flat, coreless recharge coil on a plastic or insulative bobbin with a flux guide such as a ferrite sheet disposed between the recharge coil and the printed circuit board assemblies. The recharge coil assembly also includes a flat telemetry coil that is concentric and coplanar with the recharge coil and spaced-apart from the recharge coil. In one example, the telemetry coil is wound on the plastic bobbin around the outer diameter of the recharge coil. The telemetry coil can exchange communication with the implanted medical device via inductive telemetry using an inductive telemetry protocol such as Telemetry N. The flux guide can include a ferrite sheet that is configured to reduce loading to the recharge coil assembly from the printed circuit board assemblies and to amplify magnetic flux towards the implantable medical device during a recharge session. 
     In addition to inductive recharge, the charger can provide for multiple telemetry schemes from the components in the internal compartment. For example, the telemetry schemes can include a near-field inductive telemetry such as Telemetry N, a distance or radiofrequency telemetry such as Telemetry M, and Bluetooth Low Energy for communication with external computing devices such as a smart phone. The radiofrequency telemetry antenna can be placed within the internal compartment. 
     In addition to the flux guide, the printed circuit board assemblies can be configured and stacked to reduce reflected impedance on the recharge coil assembly. Relays can be applied to the recharge coil and the inductive telemetry coil to decouple the respective tank circuits when inactive to eliminate magnetic coupling between the coils. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram illustrating an example implantable medical system including an implantable medical device and an external charger of the present disclosure. 
         FIG. 2  is another schematic diagram illustrating an example implantable medical system including an implantable medical device configured as implantable neurostimulator and an external charger having an internal compartment with a flux guide of the present disclosure, which may be included in the example implantable medical system of  FIG. 1 . 
         FIG. 3  is a block diagram illustrating physical components of the external charger of  FIG. 1 or 2 . 
         FIG. 4  is an exploded perspective view illustrating physical components of the external charger of  FIG. 3 . 
         FIG. 5  is a perspective view illustrating an example recharge coil assembly of the external charger of  FIG. 3 . 
         FIG. 6  is a perspective cross section view illustrating physical components including a recharge coil assembly of the external charger of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION 
     Aspects of the present disclosure provide for an external charger for implantable medical devices, methods of manufacturing such external charger, and implantable medical device systems including such external charger. 
       FIG. 1  illustrates an implantable medical device system  20 . System  20  includes an implantable medical device  30 , which can be fully implanted within a patient  22 . The implantable medical device  30  can include an energy storage system, such as a rechargeable battery, and circuitry within the implantable medical device  30  to apply energy from the battery. The system  20  also includes a charger  32 , which can colloquially be referred to as a wireless recharger, outside of the patient  22 , or across transcutaneous boundary  24  such as the surface of the patient&#39;s skin proximate the implantable medical device  30 . In one example, the charger  32  is placed against the patient  22  and proximate the implantable medical device  30  to inductively transfer energy and to replenish the battery of the implantable medical device  30 . The charger  32  can include a primary coil to inductively couple with a secondary coil in the implantable medical device  30  and provide an inductive power transfer to recharge the battery when placed proximate the implantable medical device  30 . 
     Systems of the present disclosure can optionally include addition components. For example, system  20  can also optionally include a handset programmer configured to wirelessly interface with the implantable medical device  30  or with the charger  32 . In one example, the handset programmer can be implemented as a software application hosted on a general-purpose computing device or mobile computing device. System  20  can include a charging dock, which can be plugged into a wall outlet and configured to charge an internal battery of the charger  32 . The charger  32  can also be used in conjunction with a fixation product of system  20  to keep the charger  32  in position proximate the implantable medical device  30  during a recharge session. The fixation product can include a fixation belt to be worn around a portion of the patient  22  such as the belt line for implantable medical device  30  in the abdomen, buttocks or flank of the patient  22 , or a fixation drape to be worn around the neck with a counterweight to balance the charger  32  for an implantable medical device  30  in the pectoral region of the patient  22 . The fixation product receives the charger  32  to hold the charger  30  in place with respect to the fixation product so that the charger  32 , in one example, does not rotate and generally does not move with respect to the implantable medical device  30  during the recharge session and to secure the charger  32  so as not to fall out unless purposefully removed from the fixation product 
     The implantable medical device  30  may be of various types, such as a device for producing electrical stimulation or for sensing physiological signals for various medical applications such as neurological or cardiac therapy. An example of such an implantable pulse generator is available under the trade designation Medtronic InterStim Neurostimulator from Medtronic, Inc. In one example, the implantable medical device  30  can be configured to provide a small form factor, e.g., a volume on the order of approximately three cubic centimeters in some examples, and generate desired stimulation signals over an extended lifetime. The implantable medical device  30  can be described as an implantable neurostimulator for illustration. For example, the implantable medical device  30  is configured to be useful or appropriate for providing stimulation therapy to the patient  22 , and in particular sacral neuromodulation. The implantable medical device  30  can serve as the power source of the sacral neuromodulation therapy. In such examples, the implantable medical device  30  delivers electrical stimulation to the sacral nerve. 
     Sacral neuromodulation therapy can be indicated for the management of the chronic intractable functional disorders of the pelvis and lower urinary or intestinal tract including overactive bladder, fecal incontinence, and nonobstructive urinary retention. The organs involved in bladder, bowel, and sexual function receive much of their control via the second, third, and fourth sacral nerves, commonly referred to as S2, S3 and S4 respectively. Electrical stimulation of these various nerves has been found to offer some control over these functions. Several techniques of electrical stimulation may be used, including stimulation of nerve bundles within the sacrum. The sacrum, generally, is a large, triangular bone situated at the lower part of the vertebral column, and at the upper and back part of the pelvic cavity. The spinal canal runs throughout the greater part of the sacrum. The sacrum is perforated by the anterior and posterior sacral foramina that the sacral nerves pass through. 
     Sacral neuromodulation creates an electrical field near the sacral nerve to modulate the neural activity that influences the behavior of the pelvic floor, lower urinary tract, urinary and anal sphincters, and colon. The implantable medical device  30  is configured to use current controlled stimulation to generate an electric field to modulate the sacral nerve. Electrical stimulation is delivered using metal electrodes provided with an implantable medical lead (not shown) coupled to implantable medical device  30 . The implantable medical lead includes a proximal end of a lead body in which a series of electrical contacts are located. Each electrical contact has a corresponding conductor within the lead body that extends to a distal end where a series of electrodes are present. During use, the proximal end is inserted into the implantable medical device, establishing an electrical interface between the electrical contacts of the implantable medical lead and electrical connectors carried by the implantable medical device  30 . The implantable medical device  30  generates stimulation signals that are delivered to the distal end of the implantable medical lead and to targeted tissue, or signals sensed by the distal end of the implantable medical lead at the targeted tissue are delivered to the implantable medical device  30 . 
     The implantable medical lead includes an electrode and is configured to carry current in the form of electrons, to biological tissue, which carries current in the form of ions. An interface between the electrode and the tissue includes non-linear impedance that can be a function of the voltage across that interface. During current-controlled stimulation, an amount of current is regulated. The voltage is changed according to the actual value of impedance, such that changes in impedance will not affect the total amount of current delivered to the tissue. Current controlled waveforms can ensure that the electric field in the tissue is independent of electrode polarization or the voltage drop across the electrode-electrolyte interface. Alternatively, the systems of the present disclosure can be configured or programmed to use voltage-controlled stimulation. 
     In some examples, the implantable medical device  30  includes or defines a connector enclosure assembly, a main enclosure assembly, electrical circuitry, and a battery. The battery is electrically coupled to the electrical circuitry and maintained in the main enclosure assembly. The connector enclosure assembly is coupled to the main enclosure assembly and, in one example, includes conductor fingers that are electrically connected to individual circuitry components, and in particular contact pads of the electrical circuitry. The electrical circuitry generates electrical signals, which are delivered to the connector enclosure assembly via the conductor fingers. The connector enclosure assembly further forms or defines an entryway sized to receive the proximal end of the implantable medical lead. Electrical connectors provided with the connector enclosure assembly interface with the electrical contacts and are electrically connected to respective ones of the conductor fingers, which connects the electrical circuitry with implantable medical lead. 
     The main enclosure assembly can assume various forms appropriate to maintain the electrical circuitry and the battery, as well as for assembly with the connector enclosure assembly. The electrical circuitry can include various electrical components and connections appropriate to provide, in some examples, a pulse generator for therapy stimulation, e.g., a constant current stimulation engine, sensing circuitry for measuring physiological parameters, telemetry for communication with external devices, memory, and a recharge circuit including the secondary coil. For example, the electrical circuitry can deliver stimulation signals, and can process or act upon received sensed signals. The electrical circuitry optionally provides various stimulation signal parameters, for example current controlled amplitude with a resolution of 0.1 mA steps, an upper limit of 12.5 mA, and a lower limit of 0.0 mA; a rate of 3-130 kHz; pulse width increments of 10 μs steps with a maximum of 450 μs and a minimum of 20 μs. The battery can include a rechargeable battery that assumes various forms appropriate to provide power for generating desired stimulation signals and to store power provided from the recharge circuitry. For example, the battery can incorporate lithium ion (Li+) chemistry, i.e., a lithium ion battery. 
     The charger  32  is available in different configurations depending on recharge frequencies and communication schemes for use with the implantable medical device  30 . For example, a first configuration of the charger  32  may support a bidirectional inductive telemetry communication scheme and an 8.9 kHz recharge frequency, a second configuration of the charger  32  may support a radiofrequency telemetry and downlink inductive telemetry communication schemes and a 40 kHz recharge frequency, and a third configuration may support the bidirectional inductive telemetry communication scheme and a 110 kHz recharge frequency. Other recharge frequencies and combinations of recharge frequencies and communication schemes are contemplated. 
     In general, the charger  32  delivers magnetic energy to a corresponding implantable device  30  at the preselected frequency with a resonant inductor-capacitor (LC) tank circuit to generate an H-field. The tank circuit includes a recharge coil in series with a recharge capacitor. Various configurations of the charger  32  can share a common coil design, and the preselected recharge frequency is determined via a selected tank capacitance of the recharge capacitor. The tank circuit can oscillate at a resonant frequency. A phase locked loop in the tank circuit is created via pulsing an applied tank voltage in phase with a tank current. During resonance, the tank current is approximately or generally sinusoidal over time. The tank circuit can achieve maximum tank power when a tank voltage pulse is aligned in time with the tank current. Recharge power can be adjusted by altering the magnitude and duty of the tank voltage pulse input to the tank circuit. 
       FIG. 2  illustrates an implantable medical device system  50 , which can correspond with system  20 . The system  50  includes an example implantable medical device  60 , which can correspond with implantable medical device  30 , that is configured as an implantable neurostimulator. The system  50  further includes an example external charger  62 , or wireless charger, which can correspond to the charger  32 , to inductively transfer energy to the example implantable medical device  60 . The example implantable medical device  60  is configured to use current controlled stimulation to generate an electric field to modulate the sacral nerve. Electrical stimulation is delivered using metal electrodes provided with an implantable medical lead  64  coupled to implantable medical device  60 . In one example, the system  50  can include an additional component  66 , such as additional components, that can include a handset programmer to wirelessly interface with the implantable medical device  60  or with the charger  62 , a charging dock to charge the charger  62 , or a fixation product to hold the charger  62  in place against a patient, such as patient  22 . 
     The example implantable medical device  60  includes a rechargeable power source  72 , electronic components  74  coupled to the rechargeable power source  72 , and a recharge system  76  coupled to the rechargeable power source  72  within an enclosure  78 . The electronic components  74  deliver a therapy to or monitor a parameter of a patient, such as via electrical stimulation. In one example, the electronic components  74  include a communication module to communicate with the charger  62  and can be configured to communicate with a handset programmer, which may be included in a mobile computing device, in additional component  66 . In one example, the rechargeable power source  72  is a rechargeable battery. The recharge system  76  includes a secondary coil to receive power via an inductive power transfer from the charger  62 . 
     The external charger  62  includes a housing  80  forming an internal compartment  82 , recharger electronic components  84  disposed in the internal compartment  82 , and a recharge coil assembly  86  disposed within the internal compartment  82 . In one example, the recharger electronic components  84  are disposed on a printed circuit board assembly in the internal compartment  82 . The recharger electronic components  84  may include or be coupled to a power source such as a rechargeable battery. The recharge coil assembly  86  includes a recharge coil  88  to provide power to the secondary coil in the recharge system  76  of the example implantable medical device  60  via the inductive power transfer. The recharge coil assembly  86  also includes a flux guide  90  having a ferrite sheet disposed between the recharge coil  88  and the printed circuit board assembly of the electronic components  84 . 
     In one example, the recharge coil assembly  86  includes an insulative bobbin having a first major surface and an opposite second major surface, the recharge coil  88 , a telemetry coil, and the flux guide  90 . The recharge coil  88  is disposed on the first major surface of the insulative bobbin and coupled to the recharger electronic components  84  to form a resonant recharge tank circuit to provide power to the secondary coil of the example implantable medical device  60  via inductive power transfer. The telemetry coil can be disposed concentric to the recharge coil  88  on the first major surface of the insulative bobbin and operably coupled to the electronic components  84  to form a resonant telemetry tank circuit to provide inductive telemetry with the implantable medical device  60 . The flux guide  90  having the ferrite sheet is disposed on the second major surface of the insulative bobbin and between the recharge coil  88  and telemetry coil and the main printed circuit board assembly. 
       FIG. 3  illustrates a charger  100 , which can correspond with charger  32 . The charger  100  includes a housing  102  that forms a common internal compartment  104 . The internal compartment  104  includes the components  106  of the charger  100 . In one example, the internal compartment  104  within the housing  102  includes a first housing assembly  108  and a second housing assembly  110  with the components  106 , although examples are contemplated in which components  106  are included on one or more housing assemblies within the internal compartment  104 . The first housing assembly  108  can include a main printed circuit board assembly  120 , a recharge coil assembly  122 , a flex antenna  124 , and a temperature sensor flex assembly  126 . The second housing assembly  110  may include a user interface printed circuit board assembly  130  and user interface components  132 . 
     The main printed circuit board assembly  120  includes a main microcontroller unit (MCU)  140  that can be coupled to follower microcontrollers such as programmed recharge MCU  142  and a programmed inductive telemetry MCU  144 , such as a Telemetry N MCU. The main MCU  140  can also be coupled to a radiofrequency telemetry module  146 , such as Telemetry M radiofrequency module if the charger supports a radiofrequency, or a Telemetry M, communication feature. The main printed circuit board assembly  120  can include an additional communication module  148 , such as a Bluetooth Low Energy module, to communicate with a remote handset programmer if included with the system  20 . The main printed circuit board assembly  120  also includes a power management circuit  150 , which is also operably coupled to the main MCU  140 . The power management circuit  150  can be coupled to a charger power source  152 , such as a rechargeable lithium ion battery and to charger pins  156 . In one example, charger pins  156  may interface with a charging dock to receive power to recharge the battery  152  is the charging dock is included with the system  20 . The main MCU  140  can also be coupled to the temperature sensor flex assembly  124 , that is strategically located in the charger  100  to provide a temperature measurement signal to the main MCU  140 . 
     The recharge coil assembly  122  is operably coupled to the main printed circuit board assembly  120  within the internal compartment  104  and as part of the first housing assembly  108 . The recharge coil assembly  122  includes a recharge coil and tuned LC circuit  160 , a telemetry coil and tuned LC circuit  162 , such as a coil for inductive telemetry, or Telemetry N, and a flux guide  164 . The recharge coil and tuned LC circuit  160  serves as the primary coil to generate the H-field and charge the implantable medical device  30 . The recharge coil and tuned LC circuit  160  is coupled to components on the main printed circuit board assembly  120  such as the programmed recharge MCU  142  to receive a voltage duty cycle signals and provide return signals to the recharge MCU  142  that can be used to moderate the voltage duty cycle signal. The telemetry coil and tuned LC circuit  162  is also coupled to components on the main printed circuit board assembly  120  such as the inductive telemetry MCU  144  to receive communication signals from the inductive telemetry MCU  144  and to transmit a Telemetry-N communication signal to the implantable medical device  30  as well as receive a communication Telemetry-N communication signal from the implantable medical device  30  and provide the signal to the inductive telemetry MCU  144 . 
     The radiofrequency telemetry module  146  is coupled to a flex antenna  124  and configured to generate a signal to communicate commands with the implantable medical device  30 , such as Telemetry-M commands that are exchanged between the radiofrequency telemetry module  146  and the implantable medical device  30 . 
       FIG. 4  illustrates an exploded view of a charger  200 , which can correspond with charger  100  and demonstrate an example of arrangement of components of the charger within the internal compartment  104 . The charger  200  includes a first housing portion  202  and a second housing portion  204 . The first and second housing portions  202 ,  204  are included in an outer shell of the of the charger  200 , or housing  102 , and serve to define the internal compartment  104  within the housing  102 . 
     The first housing portion  202  includes a main wall  206  that can provide a surface to interface with a transcutaneous boundary  24 . During a recharge session with the implantable medical device  30 , the main wall  206  is placed against the patient  22  and over the implantable medical device  30 . The first housing portion  202  can include an upstanding edge  208  that can surround the main wall  206 . The main wall  206  can be configured to interface directly against the patient  22  or through a fixation product such as a fixation belt or fixation drape of the system  20 . 
     Charger  200  further includes a main printed circuit board assembly  210 , which can correspond with the main printed circuit board assembly  120  and include respective electrical components. The main printed circuit board assembly  210  can include electrical components to couple with a temperature flex assembly  212  including a temperature sensor  214  disposed against the main wall  206 . Additionally, if the charger  200  includes a radiofrequency telemetry feature, such as Telemetry-M, a corresponding electrical component on the main printed circuit board assembly  210 , such as the radiofrequency telemetry module  146 , is coupled to a flex antenna  216 , which can be attached to and upstanding along the upstanding edge  208  at a back of the first housing portion  202 . If the charger does not include a feature to perform the communication via the radiofrequency telemetry feature, the corresponding electrical components may not be populated on the main printed circuit board  210  and the flex antenna  216  may not be included. In the example in which the radiofrequency telemetry feature is Telemetry M, which uses the Medical Device Radiocommunications Service (MedRadio), formerly Medical Implant Communication Service (MICS), frequency band for communication with the implantable medical device  30 . The radiofrequency telemetry module can be provided in a land grid array package with a radiofrequency shield. A monopole antenna based on a λ/4 radiator at 400 MHz is 18.75 cm, but the radiofrequency telemetry flex antenna is of a shorter length due to constraints of the internal compartment. 
     In the example, electronic components on the main printed circuit board assembly  210 , such as a power management circuit  150 , can interface with a charging dock of system  20  via pogo pins  218  operably coupled to the main printed circuit board assembly and extending through the housing such as through the first housing portion  202 . 
     The charger  200  includes a recharge coil assembly  220 , which can correspond with the recharge coil assembly  122 . The recharge coil assembly  220  is disposed between the main wall  206  and the main printed circuit board assembly  210 . In the example, the recharge coil assembly  220  includes a generally planar flat recharge coil  222  having a concentric winding around a plastic bobbin  224 . The recharge coil  222  is generally parallel to a plane generally defined by the main wall  206 . The example recharge coil  222  does not include a magnetic core. Rather, the bobbin  224  can define an air core for the recharge coil  222 . The recharge coil  222  serves as a primary coil in a resonant tank circuit to generate the H-field. In one example, a single wire is wrapped concentrically around the plastic bobbin  224  to form the recharge coil  222 . Each end of the wire can be formed into or attached to pins  226  or receptacles that are coupled to an electrical component on the main printed circuit board assembly  210  to form the tank circuit. The plastic bobbin  224  can be formed to include an inner annular channel (not shown) to receive the recharge coil  222 . 
     The recharge coil assembly  220  can also include a generally planar flat inductive telemetry coil  230 , such as a Telemetry-N coil, to provide for inductive communication with the implantable medical device using the Telemetry-N protocol. In one example, the inductive telemetry MCU  144  can decode standard amplitude-shift keying (ASK) decoding by receiving an amplitude modulated burst with a carrier of 175 kHz through the inductive telemetry coil  230 . The burst is then sent through a band pass/amplifier circuit included in the electronic components on the main printed circuit board assembly  210 . From the amplifier, the signal is passed to a logarithmic amplifier circuit to form an envelope of the signal. The signal passes through a low pass filter and then to a comparator to convert the base band data into a digital signal. The signal is then sent to a microprocessor for decoding. The transmit circuit is an H-Bridge configuration that generates modulated bursts at a carrier frequency of 175 kHz and a data rate of 4.4 kbps. In the example, the inductive telemetry coil  230  is formed concentrically around and spaced apart from the recharge coil  222 , such as in an outer annular channel (not shown) on the plastic bobbin  224  spaced apart from the inner annular channel. In one example, a single wire is wrapped concentrically around the plastic bobbin  224  to form the inductive telemetry coil  230 . Each end of the wire can be attached to or formed into pins  232  or receptacles that are coupled to an electrical component on the main printed circuit board assembly  210 , such as an H-bridge in the inductive telemetry MCU  144 . 
     The recharge coil assembly  220  includes a planar flux guide  240  disposed alongside the planar recharge coil  222  and between the recharge coil  222  and the main printed circuit board assembly  210 . The flux guide  240  includes a ferrite shield to concentrate magnetic flux and reduce the height of a H-field generated on the side of the main printed circuit board assembly  210 , or opposite the recharge coil  222 . The inclusion of the flux guide  240  results in more flux as measured in Webers (Wb) for a given electrical current through the recharge coil  222  than without the flux guide  240 , and increases the total inductance of the recharge coil  222 . The flux guide  240  further reduces losses in the recharge coil  222  as a result of, for example, conductivity in the ground plane of the circuit board assemblies  210 , which is discussed below. A flux guide  240  is of a size to cover both the recharge coil  222  and the first telemetry coil  230  and can be circular, or generally circular in shape. In one example, the flux guide  240  is constructed from three ferrite sheets of approximately 0.3 mm thick each adhered together to form a single flux guide sheet of approximately 1 mm thick. Each ferrite sheet can be constructed from porous NiCuZn Ferrite that may include a 3 mm square grid pattern of score lines to provide some flexibility in the ferrite sheet. 
     In the example, a foam padding  242  is disposed between the recharge coil assembly  220  and the main printed circuit board assembly  210 . The foam padding  242  can be a flat sheet of foam that includes a silhouette formed to the shape of the recharge coil assembly  220 . The foam padding  242  can be included to space the recharge coil assembly  220  from the main printed circuit board assembly  210 , urge the recharge coil assembly against the main wall, or to protect the components in the internal compartment of the housing from shock. 
     A second printed circuit board assembly  250 , such as a user interface printed circuit board assembly, can be disposed between the main printed circuit board assembly  210  and the second housing portion  204 . The second printed circuit board assembly can support user interface elements  254  and be operably coupled to the main printed circuit board assembly. User interface elements  254  can include lights, buttons, displays, or other features to receive user inputs or provide information to the user of the charger  200 . For example, user interface elements  254  include a light  256  having components including a light pipe  258  and shelter  260  and a power button  262  having components including a power button actuator  264  and a seal  266 . The light  256  and power button  262  can be coupled to electronic components on the second printed circuit board assembly  250 . 
     An internal battery  270  can be disposed against or proximate the back of the first and second housing portions  202 ,  204 , such as proximate the second telemetry flex antenna  216 . The internal battery can be disposed alongside the main printed circuit board assembly  210  and the recharge coil assembly  220 . In the example, a foam padding  272  is coupled to the battery  270 . The battery  270  can be operably coupled to electronic components on the main printed circuit board assembly  210 , such as a power management circuit  150 . 
     A ground plane on a printed circuit board is generally a large area or layer of a conductive foil such as copper foil connected to the ground, which may include a terminal of the power supply. The ground plane serves as the return path for current from many different components. Typically, the ground plane is made as large as possible, covering most of the area of the printed circuit board which is not occupied by circuit traces. In multilayer circuit boards, the ground plane is often a separate layer covering the entire circuit board. This serves to make circuit layout easier, allowing the designer to ground any component without having to run additional traces. Electronic component leads that are to be grounded are routed directly through a hole in the board to the ground plane on another layer. The large area of the foil also conducts the large return currents from many components without significant voltage drops, which permits for a consistent reference potential. The ground plane, however, contributes to loading the primary coil and to generating reflected impedance in the primary coil that are significant. 
     In one example, the configuration or distribution of the electronic components on the main printed circuit board assembly  210  can be selected to reduce the reflected impedance. For example, the circuit board of the main printed circuit board assembly  210  can be selected to be generally annular and in the shape of the recharge coil to fit over the recharge coil assembly  220 . In another example, or in addition a selected shape of the main printed circuit board assembly  210 , the ground plane can be manufactured to include various cut outs or slits of different sizes, lengths, or configurations to reduce reflected impedance. In still a third configuration, multiple, layered printed circuit board assemblies, such as the second printed circuit board assembly  250  disposed above the main printed circuit board assembly  210  within the internal compartment can reduce reflected impedance. 
       FIG. 5  illustrates the recharge coil assembly  220  as viewed from a first major exterior  280 , which interfaces with the main wall  206  of the first housing portion  202  in the assembled charger  200 . The recharge coil assembly  220  in the example includes the plastic bobbin  224 , the recharge coil  222 , the inductive telemetry coil  230 , and the planar flux guide  240 . 
     The example recharge coil assembly  220  includes the plastic bobbin  224  having a generally circular internal ridge  282  and a generally circular external ridge  284  on first major surface  286  forming a generally circular internal annular channel having a generally flat surface between the internal ridge  282  and the external ridge  284 . Additionally, the plastic bobbin  224  can include a generally circular external annular channel having a generally flat surface outside of the external ridge  282 . In one example, the generally flat surface of the external annular channel is in the plane of the generally flat surface of the internal annular channel on the first major surface  286 . 
     The flat recharge coil  222  is disposed into the internal annular channel between the internal ridge  282  and the external ridge  284 . The recharge coil assembly  220  does not include a core, such as magnetic core within the internal ridge  282 , and the recharge coil  222  is coreless. In one example, the recharge coil  222  is formed from  127  turns of twenty-five strand, thirty-eight American Wire Gauge (AWG) litz wire, and includes an inner diameter of about 45 mm, an outer diameter of about 91 mm, and a depth of about 3 mm. In one example, the recharge coil  222  in the assembled recharge coil assembly includes an inductance of approximately 2.05 mH. 
     The recharge coil assembly  220  can be included in various models or configurations of the charger  200  to deliver magnetic energy at a preselected frequency. For example, a charger can deliver magnetic energy to the implanted medical device  30  at one of 9 kHz, 40 kHz, and 110 kHz. The recharge frequency can be determined by selecting an appropriate tank capacitor. A tank capacitor in series with the recharge coil  222  having an inductance L for the primary coil tank circuit can include a tuning or tank capacitance C based from the selected resonant frequency f as 
         C= 1/ L ·(2π f ) 2  
 
     The recharge coil  222  is configured to deliver magnetic energy at a recharge frequency over a range of recharge frequencies based on a tank capacitor having a selected tank capacitance over a range of tank capacitances. In one example, the selected tank capacitor can be included on the main printed circuit board assembly  210 , and included in the same location on the main printed circuit board assembly  210  so as to be populated during manufacturing. During manufacturing, a selected main printed circuit board assembly with a particular capacitance value for the tank capacitor can be coupled to the recharge coil  222 , or recharge coil assembly  220 , to provide the selected recharge frequency. This permits the use of a single design and configuration of the recharge coil assembly  220  to be used for multiple models of chargers  200  for use with different recharge frequencies. 
     The flat inductive telemetry coil  230  is disposed into the external annular channel external to external ridge  284 . The recharge coil assembly  220  does not include a core, such as magnetic core within the internal ridge  282  or otherwise, and the inductive telemetry coil  230  is coreless. In one example, the inductive telemetry coil  230  is formed from twenty-five turns of thirty American Wire Gauge (AWG) litz wire, and includes an inner diameter that is greater than the outer diameter of the recharge coil  222 . At 175 kHz, a free space antenna inductance is 211 μH and a resistance is 5Ω. In one example, the charger  200  can use the inductive telemetry coil  230  to both send and receive communication. A receiver bandpass filter can be tuned to greater than 175 kHz to avoid environmental noise such as radio-frequency identification (RFID). In one example, the tuning capacitor of the inductive telemetry tank circuit can be 3300 pF. 
     The recharge coil  222  and inductive telemetry coil  230 , being concentric and proximate each other, have a relatively strong mutual magnetic coupling. An active one of the recharge coil  222  and the inductive telemetry coil  230  can produce sympathetic current in an inactive one of the recharge coil  222  and the inductive telemetry coil  230 . The sympathetic current in the inactive coil is undesirable in that it wastes energy in coil resistance and the sympathetic current can cancel a portion of the desired H-field of the active coil. To remove coil interaction effects, electronic components include relays to open the respective tank circuit of the inactive coil. Capacitors for the recharge and inductive telemetry tank circuits can be selected to account for stray capacitance from the relays. 
     The flux guide  240  is coupled to a second major surface  288  (indicated in  FIG. 3 ) of the plastic bobbin  224 . The second major surface  288  is opposite the first major surface  286 , and the recharge coil  222  and inductive telemetry coil  230  are spaced apart and electrically insulated from the flux guide  240 . The flux guide  240  is sized and shaped to cover the recharge coil  222  and inductive telemetry coil  230 . In one example, the flux guide  240  is sized and shaped to be opposite the internal annual channel and the external annular channel of the plastic bobbin  224 . In the illustrated example, the flux guide  240  covers the second major surface  288  of the plastic bobbin  224 . 
       FIG. 5  illustrates an example of the assembled charger  200 . The first housing portion  202  and second housing portion  204  are attached together to form a housing  302  having an internal compartment  304 , which includes the components of the charger  200 . The first major exterior  280  of the recharge coil assembly  220  interfaces with the main wall  206  of the first housing portion  202 . In the example, the internal ridge  282  and external ridge  284  of the first major surface  286  of the plastic bobbin  224  are urged against the main wall  206 . The recharge coil  222  and inductive telemetry coil  230  are coupled to the first major surface  286  in the respective annular channels to interface with the main wall  206 . The flux guide  240  is disposed on the second major surface  288  of the recharge coil assembly  220  and extends over the recharge coil  222  and the inductive telemetry coil  230  opposite the first major surface  286 . The main printed circuit board assembly  210  and the second interface printed circuit board assembly  250  are included in the internal compartment  304  between the recharge coil assembly  220  and the second housing portion  204 . The battery  270  is included adjacent the recharge coil assembly  220  and the main and second printed circuit board assemblies  210 ,  250 . In the example, the flex antenna  216  is disposed against the housing  302  proximate the battery  270 . 
     During a recharge session, the charger  200  can be placed against a patient at a transcutaneous boundary such that an axis extending generally perpendicular to the first major surface of  286  of the plastic bobbin  224  and within the internal ridge  282  extends through the implantable medical device  30 . 
     The external charger can be constructed by selecting a recharge coil assembly comprising a flat recharge coil having a selected inductance and coupling a recharge capacitor to the flat recharge coil to form a recharge tank circuit. The recharge capacitor includes a capacitance selected from one of a plurality of capacitances, such as three capacitances, configured to be coupled to the flat recharge coil to provide the recharge tank circuit with a resonant frequency based on the selected recharge capacitor. The recharge capacitor can be coupled to a main printed circuit board assembly. A flux guide having a ferrite sheet is disposed between the recharge coil and the printed circuit board assembly. The recharge coil assembly and the main printed circuit board assembly are assembled together within a common internal compartment of a housing. 
     All patents referenced in the disclosure are incorporated by reference in their entireties into this disclosure. 
     Although the present disclosure has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes can be made in form and detail without departing from the spirit and scope of the present disclosure.