Patent Publication Number: US-2017361112-A1

Title: External Charger for an Implantable Medical Device Having a Multi-Layer Magnetic Shield

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a non-provisional of U.S. Provisional Patent Application Ser. No. 62/350,626, filed Jun. 15, 2016, to which priority is claimed, and which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to wireless external chargers for use in implantable medical device systems. 
     BACKGROUND 
     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, including a Deep Brain Stimulation (DBS) system. 
     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. 
     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 . Two coils (more generally, antennas) are shown 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. 
       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. 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. 
     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 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, it is 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. 
     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. 
     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. 
     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 . Generally speaking, if the external charger  50  is well aligned with the IMD  10 , then Vcoil will drop as the charging circuitry  64  provides the charging current Icharge to the charging coil  52 . Accordingly, alignment circuitry  70  can compare Vcoil, preferably after it is rectified  76  to a DC voltage, to an alignment threshold, Vt. If Vcoil&lt;Vt, then external charger  50  considers itself to be in good alignment with the underlying IMD  10 . If Vcoil&gt;Vt, then the external charger  50  will consider itself to be out of alignment, and can indicate that fact to the patient so that the patient can attempt to move the charger  50  into better alignment. For example, the user interface  58  of the charger  50  can include an alignment indicator  74 . The alignment indicator  74  may comprise a speaker (not shown), which can “beep” at the patient when misalignment is detected. Alignment indicator  74  can also or alternatively include one or more Light Emitting Diodes (LED(s); not shown), which may similarly indicate misalignment. 
     Providing the user with some indication of alignment 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. 
     The coupling between coils  52  and  36  is also improved through the use of a shield  80  that is positioned to focus the magnetic field  66  toward the coil  36 . The shield  80  is constructed from a ferromagnetic material having a high magnetic permeability. Such materials can include iron, cobalt, nickel, manganese, chromium, as well as oxides, alloys, and other combinations of these metals for example. The shield  80  increases the coupling between the coils  52  and  36  in three ways. First, the magnetic permeability of the shield  80 , being substantially higher than the magnetic permeability of air and other non-ferromagnetic materials, increases the permeance of magnetic flux paths generated as a result of the energization of the coil  52 . For a given magnetomotive force (e.g., a given current through the fixed number of turns in the coil  52 ), magnetic flux is proportional to the permeance of the magnetic circuit. Thus, an increase in the permeance of the magnetic flux paths results in an increase in the magnetic flux through any cross-sectional area perpendicular to the paths, most importantly through the coil  36 . Second, as shown in  FIGS. 4  (which shows the flux paths with no shield) and  5  (which shows the flux paths with shield  80 ), the shield  80  alters the shape of the magnetic field  66  such that the length of the flux path is shortened. For a given magnetomotive force, magnetic flux is inversely proportional to the length of the flux path. Thus, decreasing the length of the flux path also increases the flux through the coil  36 . Third, the shield  80  alters the shape of the magnetic field  66  such that the components of the charger  50  above the shield  80  are substantially unexposed to the field  66 . Exposure of any conductive components (such as, perhaps, the components  56  or the battery  60 ) to the field  66  results in the generation of eddy currents that produce a magnetic field in an opposite direction of the field  66 . By substantially reducing the generation of eddy currents that oppose the field  66 , the shield  80  additionally increases the flux through the coil  36 . Reducing the generation of eddy currents also has the beneficial effect of reducing the generation of heat in the charger  50 . Because the shield  80  improves coupling between the coils  52  and  36 , its use enables the charger  50  to operate at lower charging currents or for the coil  52  to be designed with a smaller number of turns (or some combination of both). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A-1C  show different views of an implantable pulse generator, a type of implantable medical device (IMD), in accordance with the prior art. 
         FIG. 2  shows an external charger being used to charge a battery in an IMD, in accordance with the prior art. 
         FIG. 3  shows circuitry in an external charger and an IMD, in accordance with the prior art. 
         FIG. 4  shows the magnetic flux paths created by an external charger that does not include a shield, in accordance with the prior art. 
         FIG. 5  shows the magnetic flux paths created by an external charger that includes a shield, in accordance with the prior art. 
         FIG. 6  includes charts that show the relationship of magnetic permeability and coil inductance as a function of charging current, in accordance with the use of a prior art shield. 
         FIG. 7  shows an external charger that includes a multi-layer shield, in accordance with an example of the invention. 
         FIG. 8  shows the layers of a modified shield, in accordance with an example of the invention. 
         FIG. 9  is a chart that shows the magnetic permeabilities of the layers of a multi-layer shield as a function of magnetic field intensity, in accordance with an example of the invention. 
         FIG. 10  shows the magnetic flux paths created by an external charger that includes a multi-layer shield, in accordance with an example of the invention. 
         FIG. 11  is a chart that illustrates the magnetic flux through shields of varying types, in accordance with an example of the invention. 
         FIG. 12  is a chart that shows the inductance of an external charger&#39;s charging coil as a function of charging current for shields of varying types, in accordance with an example of the invention. 
         FIGS. 13A and 13B  show different views of a charging system that incorporates a multi-layer shield, in accordance with an example of the invention. 
         FIG. 14  shows circuitry of the charging system illustrated in  FIGS. 13A and 13B , in accordance with an example of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     While the shield  80  improves the coupling between the coils  52  and  36 , its use in the charger  50  also creates certain challenges. As is known, the magnetic permeability of ferromagnetic materials such as those from which the shield  80  is constructed varies as a function of the intensity H of the magnetic field (e.g., field  66 ) of a flux path through the material. Thus, the magnetic permeability of the shield  80  varies with the charging current Icharge. As shown in the chart in  FIG. 6 , the magnetic permeability of the material increases from its vacuum permeability (μ0) when no current is flowing through the coil (H=0) to a maximum permeability (μsat) at the material&#39;s “saturation point” before dropping sharply as the magnetic field intensity further increases. As also shown in  FIG. 6 , the sharp decrease in permeability of the shield results in a corresponding sharp decrease in the inductance Lcoil of coil  52  at charging currents above the saturation current Isat. This problem is exacerbated by the fact that materials having the highest magnetic permeability (i.e., the materials that most significantly increase coupling between the coils  52  and  36 ) generally saturate at a lower magnetic field intensity. Thus, a material having a more desirable (i.e., higher) magnetic permeability generally saturates at a lower charging current and a material having a more desirable (i.e., higher) saturation point generally has a lower magnetic permeability. 
     The inventor has recognized that it would be beneficial for the charger&#39;s control circuitry  72  to control the magnitude of Icharge to obtain a desired charging rate of the IMD  10 &#39;s battery  14  and to control the frequency of Icharge so that the coil  52  operates at its resonant frequency. These values of the magnitude and frequency of Icharge are affected by the orientation of the charger  50  with respect to the IMD  10  due to changes in the mutual inductance between the coils  52  and  36 . The inventor has also recognized that while the control circuitry  72  is capable of adjusting the frequency of the charging current to correct gradual changes in the resonant frequency, the change in the resonant frequency caused by the sharp decrease in Lcoil as a result of the magnetic saturation of the shield  80  is extremely difficult to control and often requires a “reset” of the charge control scheme whereby the charging current is substantially reduced and then gradually increased back to desired levels. Such control “resets” are time-consuming and inefficient as they can substantially increase the amount of time that is required to charge the battery  14 . 
     There are a few ways in which this problem can be avoided, but each has its own drawback. The thickness of the shield  80  can be increased to increase the value of Isat, but that undesirably increases the size and weight of the charger. The maximum value of Icharge can be limited so that it cannot exceed Isat, but that limits the rate at which the IMD  10  can be recharged, especially for non-ideal orientations of the charger  50  relative to the IMD  10 . The shield  80  can be eliminated altogether, but that forgoes the beneficial effects that the shield provides. 
     The inventor has conceived of a modified shield  80 ′ that strikes a balance between magnetic permeability and saturation and beneficially directs the magnetic field  66  towards the IMD  10 .  FIG. 7  illustrates a modified charger  50 ′ that includes a multi-layer shield  80 ′. The shield  80 ′ may be held in its position between the coil  52  and the PCB  54  by adhering the shield  80 ′ to the PCB  54  (i.e., affixing the shield  80 ′ to the PCB  54  with an adhesive) or by mechanically affixing the shield  80 ′ (e.g., to the case  62  or other component of the charger  50 ′) using a clamp or similar support structure (not shown). While the shield  80 ′ is illustrated as being positioned between the coil  52  and the PCB  54 , this positioning is not strictly necessary and the shield  80 ′ may be positioned at other locations so long as the charging coil  52  is positioned between the shield  80 ′ and the external surface of the charger that is configured to be placed towards the patient&#39;s tissue. For example, the shield  80 ′ could be mounted against the top portion of the case  62  directly opposite the user interface  58 . Other than the replacement of the shield  80  with the modified shield  80 ′, the other components and circuitry of the modified charger  50 ′ are unchanged from the charger  50 . 
     As shown in  FIG. 8 , the shield  80 ′ includes multiple ferromagnetic layers (i.e., layers that include ferromagnetic materials) that saturate at different magnetic field intensities. While two layers  80 A and  80 B are shown, the shield  80 ′ can employ a greater number of layers. In one embodiment, each layer may be approximately 1-2 mm thick. The layers  80 A and  80 B are illustrated as the same thickness, but this is not strictly necessary and the thicknesses of the layers can be either the same or different. In a preferred embodiment, the shield  80 ′ has the same general shape as the coil  52 . For example, the shield  80 ′ and the coil  52  may both be generally circular, generally rectangular, generally square, or some other shape, but the shield  80 ′ can also take a different shape than the coil. Further, the shield  80 ′ and coil  52  may be concentric (i.e., share the same center), although this is also not necessary. The shield  80 ′ may also be generally the same size as the coil  52 , although it can also be larger or smaller than the coil  52 . In one embodiment, each layer may be cut to the desired size and shape from a sheet of ferromagnetic material. For example, each layer may be cut to size using a die cutting machine. In one embodiment, the layers of the shield  80 ′ are affixed to each other by an adhesive applied between the layers. 
     While the layers  80 A and  80 B can be constructed from any ferromagnetic material, in a preferred embodiment one or more of the layers may comprise a ferrite material. Such ferrite materials are generally rigid, and a rigid ferrite material having the desired dimensions (i.e., relatively large compared to thickness) may be relatively brittle and subject to cracking. While the layers  80 A and  80 B can be formed from a ferrite material in this rigid form, because a significant crack in any layer can substantially reduce the effectiveness of the shield  80 ′, in a preferred embodiment, the ferrite materials for one or more layers may be pre-scored (e.g., to create a grid of small squares) and held together by a component such as a polymer film, for example. These types of pre-scored materials are more flexible and less susceptible to cracking. Examples of the types of ferrite materials that can form the various layers can include manganese-zinc ferrite, nickel-zinc ferrite, strontium ferrite, barium ferrite, and cobalt ferrite. While the ferromagnetic materials of the various layers may be positioned in direct contact with each other, the layers may also be separated by a thin barrier, such as the film that holds a pre-scored ferrite sheet together. 
     The layers are arranged such that the layer closest to the coil  52  saturates at the highest magnetic field intensity (i.e., has the highest magnetic saturation point). That is, the saturation point of the first layer  80 A occurs at a higher magnetic field intensity (and thus a higher Icharge) than the saturation point of the second layer  80 B (i.e., HsatA&gt;HsatB), and so on for any additional layers. As described above, while not an absolute law, materials that saturate at a higher magnetic field intensity generally have a lower magnetic permeability. Therefore, the above saturation point relationship of the layers (i.e., HsatA&gt;HsatB) generally corresponds to the opposite magnetic permeability relationship (i.e., μA&lt;μB). This general relationship is illustrated in  FIG. 9 , which shows example magnetic permeabilities of the layers  80 A and  80 B as a function of magnetic field intensity. The shield  80 ′ only makes sense where the general relationship between saturation and magnetic permeability applies. This is true because if the layer  80 A, which saturates at a higher magnetic field intensity than the layer  80 B, is also superior to layer  80 B in terms of magnetic permeability, then nothing is gained by adding the layer  80 B. Instead, a single layer shield  80  constructed of the material of the first layer  80 A would be superior to a multi-layer shield. However, as shown below, where the general relationship between magnetic permeability and saturation applies, a multi-layer shield  80 ′ can provide a beneficial balance between permeability and saturation. 
     As illustrated in  FIG. 10 , the flux paths having the highest magnetic intensity H (i.e., those that have the shortest path around the coil  52 ) flow through the first layer  80 A in the multi-layer shield  80 ′. The magnetic permeability of the layer  80 A acts to increase the flux through the coil  36  in the same way as the single layer shield  80  described above. The layer  80 A also acts to shield the layer  80 B such that the flux paths flowing through the layer  80 B are at an intensity level H that is below the saturation point of layer  80 B. As a result, the multi-layer shield  80 ′ also takes advantage of the even higher magnetic permeability of the layer  80 B in a manner that avoids the risk of layer  80 B exceeding its saturation point. As will be understood, the number of layers and the thicknesses of the various layers can be selected to obtain the desired flux through the layers over a range of operating conditions (i.e., a range of Icharge values). For example, the thicknesses of the layers can be adjusted such that at the maximum magnetic intensity that can be generated by the coil (i.e., the maximum Icharge), the flux paths that flow through a layer closer to the coil  52  (e.g., layer  80 A) and into a layer further from the coil  52  (e.g., layer  80 B) have a magnetic intensity that is just below the saturation point of the layer further from the coil  52 . 
     The balance between magnetic permeability and saturation that is achieved by the shield  80 ′ is illustrated in  FIG. 11 , which shows example plots of the magnetic flux B through a single layer shield formed entirely of the material of layer  80 A, a single layer shield formed entirely of the material of layer  80 B, and a multi-layer shield  80 ′ formed of the layers  80 A and  80 B. Because flux B is related to intensity H according to the equation B=μH, the saturation point (i.e., the point at which magnetic permeability μ is maximized) occurs at the inflection point of the plot of flux vs. magnetic intensity (i.e., the point at which the curve transitions from convex to concave). As shown, the single-layer shield constructed from the material of layer  80 A has the highest saturation point HsatA, but its flux B is lower than the other shields over the entire range of magnetic intensities. Conversely, the single-layer shield constructed from the material of layer  80 B has the lowest saturation point HsatB, but its flux B is higher than the other shields over the entire range of magnetic intensities. The multi-layer shield  80 ′ provides a balance between the two single layer shields. Because some of the flux through the multi-layer shield passes through the layer  80 B, which has a higher magnetic permeability than layer  80 A, the flux through the shield  80 ′ is increased as compared to the single-layer shield constructed of the material of layer  80 A. Moreover, because layer  80 A shields layer  80 B in the multi-layer shield  80 ′, the saturation point HsatAB is higher than the saturation point of the single-layer shield constructed of the material of layer  80 B. Therefore, the multi-layer shield  80 ′ can obtain some of the advantages of the higher magnetic permeability of the layer  80 B at a fraction of the thickness of a single-layer shield constructed from the material of layer  80 B. 
       FIG. 12  shows example plots of the inductance of the coil  52  Lcoil over a range of charging currents Icharge for the shields of  FIG. 11  and assuming a common thickness of all shields and a common orientation of the coils  52  and  36 . As shown, the single-layer shield constructed from the material of layer  80 A results in the lowest inductance but the inductance stays stable over the largest range of charging currents. The single-layer shield constructed from the material of layer  80 B results in the highest inductance, but the inductance is stable over only a small range of lower values of the charging current. The multi-layer shield  80 ′ again strikes a balance between the two single-layer shields as it provides an inductance value between those provided by the single-layer shields and is stable over a larger range of charging currents than is the single-layer shield constructed of the material of layer  80 B. 
     While the external charger has to this point been described as a device contained within a single housing,  FIG. 13  illustrates a charging system  100  in which an electronics module  104  is separated from a charging coil assembly  102 . 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, but as illustrated cable  106  is permanently affixed to the charging coil assembly  102 . The other end of the cable  106  includes a connector  108  that can attach to and detach from a port  122  of the electronics module  104 . 
     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. 14B  can include an on/off switch 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. LED  118   b  may operate to provide an indication of alignment of the charging coil assembly  102  with the IMD  10 . 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 . 
     Charging coil assembly  102  preferably contains only passive electronic components that are stimulated or read by active circuitry  112  within the electronics module  104 . Such components include the primary charging coil  126 , which is mounted above a circuit board  124  that is used to integrate the electronic components within the charging coil assembly  102 . The charging coil  126  is energized by charging circuitry  64  ( FIG. 14 ) in the electronics module  104  to create the magnetic charging field  66 . The magnetic field  66  generated through energization of the charging coil  126  is directed towards the IMD  10  by the multi-layer shield  80 ′, which is positioned so that it is located opposite the coil  126  from the patient&#39;s tissue when the charging system  100  is in use. In the illustrated embodiment, the shield  80 ′ is circular and concentric with the coil  126  and has a slightly larger diameter than the coil  126 , although the shield  80 ′ may have a different shape or size than illustrated. In one embodiment, the shield  80 ′ is suspended above the coil  126  such as by a mechanical support or clamp that affixes the shield  80 ′ to the housing  125  or other component of the coil assembly  102 . Alternatively, the shield may be positioned in a different location. For example, the shield  80 ′ may be adhered to the coil  126  or to the inside top surface of the housing  125 . 
     Further included within the charging coil assembly  102  are one or more sense coils  128 , which as shown in the cross section of  FIG. 14B , are preferably formed using one or more traces in the PCB  124 . While it is preferred that charging coil  126  comprise a wound conductor, and that the one or more sense 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 sense coils can comprise one or more wound conductors. Note that the charging coil  126  and the one or more sense coils  128  are formed in planes that are parallel, and can also be formed in the same plane. Additional description regarding the one or more sense coils  128  and their use with respect to alignment circuitry  140  can be found in U.S. Patent Application No. 62/350,451, filed Jun. 15, 2016, which is incorporated herein by reference in its entirety. 
     Further passive components preferably included within the charging coil assembly  102  include one or more tuning capacitors  131 , which are utilized to tune the charging coil  126  to its resonant frequency (fres). Each of the one or more sense coils  128  may also be coupled to a tuning capacitor  131 , although this is not necessary and is not shown in further circuit diagrams. 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 . Such temperature data can in turn control production of the magnetic field 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. 
     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. 
     Like the external chargers  50  and  50 ′ described earlier ( FIG. 3 ), the electronics module  104  may include (as part of circuitry  112 ;  FIG. 14 ) control circuitry  72  that controls charging circuitry  64  to generate the charging current Icharge. This current is passed via connector/port  108 / 122  through a wire  134  in cable  106  to energize the charging coil  126  to produce the magnetic field  66 . The resulting voltage across the charging coil  126 , Vcoil, perhaps as dropped in voltage using a voltage divider, can be monitored for LSK communication from the IMD  10  with the assistance of LSK demodulator  68 . And again, one or more indications of temperature (Vtherm) can be reported from the one or more thermistors  136  in the charging coil assembly  102  to allow the control circuitry  72  to control production of the magnetic field  66  as mentioned previously. While it is preferable to place control circuitry  72  and other circuitry  112  aspects in the electronics module  104 , this is not strictly necessary, and instead such components can reside in the charging coil assembly  102 , for example, on its circuit board  124 . Thus, electronics module  104  may retain only battery  110  and user interface aspects. 
     As described above, the multi-layer shield  80 ′ provides a balance between magnetic permeability and saturation. As such, the multi-layer shield  80 ′ increases the amount of flux that can be generated through the IMD&#39;s charging coil  36  in a way that limits the thickness and associated weight of the shield. 
     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.