Physically-Configurable External Charger for an Implantable Medical Device with Receptacle in Coil Housing for Electronics Module

A physically-configurable external charger device for an implantable medical device is disclosed, which facilitates the generation of different powers of a magnetic field with reduced heating concerns. A housing which includes an internal charging coil includes a receptacle for holding an electronics module for energizing the charging coil. A cable coupled to the charging coil spans around the edges of the housing and connects to the electronics module when it is retained by the receptacle. In this first configuration, a low-power magnetic field can be produced, as the electronics module is still relatively near the charging coil, and thus may heat to some degree. In a second configuration, the electronics module is removed from the receptacle and extendable from the housing by the length of the cable, and thus a higher-power magnetic field can be produced with reduced heating concerns.

FIELD OF THE INVENTION

The present invention relates to a wireless charger for an implantable medical device such as an implantable pulse generator.

BACKGROUND

Implantable stimulation devices are devices that generate and deliver electrical stimuli to 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.

As shown inFIGS. 1A and 1B, a SCS system typically includes an Implantable Pulse Generator (IPG)10, referred to more generically as an Implantable Medical Device (IMD)10. IMD10includes a biocompatible device case12formed of a metallic material such as titanium for example. The case12typically holds the circuitry and battery14necessary for the IMD10to function, although IMDs can also be powered via external RF energy and without a battery, as described further below. The IMD10is coupled to electrodes16via one or more electrode leads (two such leads18are shown), such that the electrodes16form an electrode array20. The electrodes16are carried on a flexible body22, which also houses the individual signal wires24coupled to each electrode. In the illustrated embodiment, there are eight electrodes on each lead, although the number of leads and electrodes is application specific and therefore can vary. The leads18couple to the IMD10using lead connectors26, which are fixed in a header28comprising epoxy for example, which header is affixed to the case12. In a SCS application, distal ends of electrode leads18with the electrodes16are typically implanted on the right and left side of the dura within the patient's spinal cord. The proximal ends of leads18are then tunneled through the patient's tissue to a distant location such as the buttocks where the IMD10is implanted, where the proximal leads ends are then connected to the lead connectors26.

As shown in cross section inFIG. 2B, the IMD10typically includes a printed circuit board (PCB)30containing various electronic components32necessary for operation of the IMD10. Two coils are present in the IMD10as illustrated: a telemetry coil34used to transmit/receive data to/from an external controller (not shown); and a charging coil36for receiving power from an external charger40(FIG. 2A). These coils34and36are also shown in the perspective view of the IMD10inFIG. 1B, which omits the case12for easier viewing. Although shown as inside in the case12in the Figures, the telemetry coil34can alternatively be fixed in header28. Coils34and36may alternative be combined into a single telemetry/charging coil.

FIG. 2Ashows a plan view of the external charger40, andFIG. 2Bshows it in cross section and in relation to the IMD10as it provides power—either continuously if the IMD10lacks a battery14, or intermittently if the charger is used during particular charging sessions to recharge the battery. In the depicted example, external charger40includes two PCBs42aand42b;various electronic components44for implementing charging functionality; a charging coil46; and a battery48for providing operational power for the external charger40and for the production of a magnetic field60from the charging coil46. These components are typically housed within a housing50, which may be made of hard plastic such as polycarbonate for example.

The external charger40has a user interface54, which typically comprises an on/off switch56to activate the production of the magnetic field60; an LED58to indicate the status of the on/off switch56and possibly also the status of the battery48; and a speaker (not shown). The speaker emits a “beep” for example if the external charger40detects that its charging coil46is not in good alignment with the charging coil36in the IMD10. More complicated user interfaces54can be used as well, such as those involving displays or touch screens, or involving realistic audio output (e.g., speech or music) beyond a mere beep, etc.

The external charger's housing50is sized such that the external charger40is hand-holdable and portable. In an SCS application in which the IMD10is implanted behind the patient, the external charger40may be placed in a pouch (not shown) around a patient's waist to position the external charger in alignment with the IMD10. Typically, the external charger40is touching the patient's tissue70as shown (FIG. 2B), although the patient's clothing or the material of the pouch may intervene.

Wireless power transfer from the external charger40to the IMD10occurs by near-field magnetic inductive coupling between coils46and36. When the external charger40is activated (e.g., on/off switch56is pressed), charging coil46is driven with an AC current to create the magnetic field60. The frequency of the magnetic field60may be on the order of80kHz for example, and may generally be set by the inductance of the coil46and the capacitance of a tuning capacitor (not shown) in the external charger40. The magnetic field60transcutaneously induces an alternating current in the IMD10's charging coil36, which current is rectified to DC levels and used to power circuitry in the IMD10directly and/or to recharge the battery14if present.

The IMD10can communicate relevant data back to the external charger40, such as the capacity of the battery using Load Shift Keying, as explained for example in U.S. Pat. Application Publication 2015/0077050, or by any other means. For example, either or both of the charging coil36or the telemetry coil34can be used to transmit data, or other separate data antennas (e.g., short-range far-field RF antennas, communicating by Bluetooth, WiFi, Zigbee, MICS, or other protocols) can be used in either or both of the IMD10and the external charger40.

Referring again toFIG. 2B, the depicted example of the external charger40includes two PCBs42aand42b,which are generally orthogonal. The bulk of the electronic components44are carried on the vertical PCB42b.Horizontal PCB42aby contrast is generally free of components, and carries only the charging coil46. Further, the battery48is placed outside of the area extent of the charging coil46. As explained in U.S. Pat. No. 9,002,445, such design of the external charger40is useful to reduce heating, in particular heating of conductive components resulting from Eddy currents caused by the alternating magnetic field60. The design moves conductive materials (the PCB42bwith its electronic components44; the battery48with its conductive housing) away from where the magnetic field60is most intense in the center of the charging coil46, as illustrated by the concentration of magnetic field flux lines, shown in dotted lines inFIG. 2C. Further, placing the electronic components44on a vertical PCB42btends to orient the major planes of the PCB42band components44parallel to the highest-intensity portions of the magnetic field60in the center of the coil46, rendering such components that much less susceptible to Eddy current heating. The design of the external charger40is thus able to remain compact within its hand-holdable housing50without significant heating concerns.

Even if heating of the external charger40is mitigated by these design choices, it is still prudent to monitor temperature to ensure that a patient will not be injured while charging his IMD10. In this regard, external charger40preferably includes at least one temperature sensor, such as a thermistor52(FIG. 2B), to monitor the external charger40's temperature while charging. Thermistor52is preferably placed on the inside surface of the housing50that faces (and potentially touches) the patient when the external charger40is producing the magnetic field60.

The thermistor52can communicate temperature to control circuitry (part of electronic components44) within the external charger70, to ensure that a maximum safe temperature for the patient, Tmax (e.g., 41° C.), is not exceeded. If the thermistor52reports this maximum temperature, and particularly in the circumstance where the external charger40is used to recharge an IMD10's battery14, charging may be suspended by ceasing current through the charging coil46to allow the external charger40to cool. Once cool enough, for example once the temperature drops to a lower minimum temperature, Tmin (e.g., 39° C.), charging may again be enabled by reinitiating the current through the charging coil46, until Tmax is again reached and charging suspended, etc. This is illustrated inFIG. 3, and borrowed from U.S. Pat. No. 8,321,029. The patient may not be aware that the external charger40is actually duty cycling between enabled and suspended states to maintain a safe temperature during a battery charging session. Other means of temperature control beyond duty cycling exist, such as adjusting the magnitude of the current through the charging coil46, detuning the frequency of the magnetic field60, etc.

While external charger40works fine to provide power to a patient's IMD10, the inventor sees room for improvement in external charger design. For example, the inventor notes that while the design of external charger40reduces Eddy-current-related heating by moving and orienting components as described above, Eddy current heating will still exist to some degree. AsFIG. 2Cshows, while the amount of magnetic flux impinging upon the vertically-oriented electronic components44and the battery48may be lessened, such components are still relatively close to the charging coil46, and hence still receive magnetic field60and will heat to some degree.

The propensity of external charger40to heat ultimately impedes its ability to provide significant power to the IMD10, or to quickly charge the IMD10's battery14. This is because Tmax effectively limits the strength of the magnetic field60that can be produced, and hence limits the rate at which the battery14can be charged.

Further, the inventor considers it unfortunate that the external charger40is formed as a single integrated unit. If just one portion of the external charger is malfunctioning (e.g., the charging coil46, some electronic components44, the battery48, etc.), the entire external charger40will likely need to be replaced even though other portions may be working suitably. Likewise, the integrated design of the external charger40impedes the ability to upgrade its various portions with improved technology, even if such portions are otherwise working normally.

In recognition of these concerns, the inventor proposes a new external charger design that includes separable portions and is also physically configurable. A first physical configuration allows for low-power charging as described to this point, while a second physical configuration allows for high-powered charging, and hence faster IMD battery charging.

DETAILED DESCRIPTION

A physically-configurable external charger device for an Implantable Medical Device (IMD) is disclosed, which facilitates the generation of different powers of a magnetic field but with reduced heating concerns at higher powers. A housing which includes an internal charging coil includes a receptacle for holding an electronics module for energizing the charging coil. A cable coupled to the charging coil spans around the edges of the housing and connects to the electronics module when it is retained by the receptacle, preferably by a connector/port arrangement. In this first physical configuration, a relatively low-power magnetic field can be produced, as the electronics module is still relatively near the charging coil (although outside of its area), and thus may heat to some degree. In a second physical configuration, the electronics module is removed from the receptacle and extended from the housing preferably by the length of the cable, and thus a higher-power magnetic field can be produced with reduced heating concerns. Thus, in this second configuration, the charging rate of the IMD can be increased. The design of the external charger is also modular, as the electronics module can be separable from the housing, and because circuitry and battery modules in the electronics module can be separable. This allows for easy replacement of portions of the external charger should one portion fail or need to be upgraded.

An example of an improved, physically-configurable external charger100is shown first inFIGS. 4A and 4B, which respectively show the charger from the top and side. The external charger100includes a housing104which as shown comprises three portions. A flat charging coil housing104aincludes a charging coil102, which like the prior art charger is energized to produce a magnetic field60to power and/or charge the IMD10. Further housing portions104band104care used to retain an electronics module106, as explained below. The electronics module106is preferably split into two portions, namely a circuitry module106aand a battery module106b,as also explained below. In the depicted example, the electronics module106is cylindrical in shape (seeFIG. 7), although this isn't necessary. For example, the electronics module106(i.e., either or both of106aand106b) could also be rectangular, triangular, elliptical, etc.

Housing portions104band104care configured to retain and release the electronics module106and are preferably formed to match the electronics module106's shape. In particular, housing portion104ccomprises a cup into which an end of the electronics module106(e.g., battery module106b) can be pressed (seeFIG. 6D). Housing portion104bis largely open to allow insertion and removal of the electronics module106, and only partially surrounds the module. Housing portion104bin the depicted example comprises a curved wall shaped to mate with part of the electronics module106's curved outer surface. As shown inFIG. 6C, the curved wall104bmay span less than 180 degrees (θ) of the electronics module106's curved surface. As such, curved wall104bgenerally stabilizes the electronics module106when it is retained within the cup104c,but still allows the electronics module106to be easily removed from the cup104c.However, curved wall104bmay be greater than 180 degrees, allowing the electronics module106to be further secured, such as by snapping it into place within the curved wall104bwhen the electronic module is retained, and snapping it back out when the electronics module106is removed. Together, either or both of housing portions104band104ccan be referred to generically as a “receptacle”105, and in this regard receptacle105may include any means for releasably retaining the electronics module106, and need not have cups or curved walls. For example, receptacle105could include clips, a groove, etc.

Housing portions104a,104b,and104cmay comprise a hard rubberized material or a polyurethane which are mold injected and hence formed as an integral piece. Note that because the coil housing104acontains only minimal electronics, as described later, it can be made relatively thin compared to the thickness of the electronics module106, as best shown in the side view ofFIG. 4B. The thinness of the coil housing104ais beneficial because its low profile is less conspicuous when used by a patient to charge his IMD10, as explained further later with reference toFIGS. 11A and 11B. However, housings104a,104b,and104ccan be formed in other ways, such as of separate parts or of different materials.

As noted, electronics module106is preferably formed as two separate modules: a circuitry module106aand a battery module106b.Circuitry module106aincludes electronics components124(FIG. 6D) necessary for external charger operation, while battery module106bincludes a battery126(FIG. 6D) to power such electronics. Modules106aand106bare preferably attachable to and detachable from each other, and in this regard a connector/port arrangement may be used to secure them together. For example, and as shown inFIG. 7, battery connectors (terminals)130on the battery module106bmay be secured at ports132on the circuitry module106ato allow the battery module106bto provide power to the circuitry module106a.

Battery126within the battery module106bis depicted in the cross-section ofFIG. 6Das having its own housing128. Housing128may comprise the battery's prefabricated housing, which is typically conductive. Alternatively, housing128may comprise an additional housing into which an otherwise completed battery is placed, in which case housing128would preferably be insulating, similar to housing120of the circuitry module106a.Battery126may be either non-rechargeable (primary) or rechargeable (e.g., a Li-ion polymer battery). If battery126is rechargeable, it may be recharged via port112of the circuitry module106a,and in this regard electronic components124within the circuitry module106acan include battery recharging circuitry, such as is disclosed in U.S. Patent Application Serial No. 2016/0126771.

Having separable circuitry106aand battery106bmodules is preferable as it allows one or the other to be replaced. For example, battery module106bcan be replaced if battery126is either depleted (if non-rechargeable) or will no longer hold an adequate charge (if rechargeable). Likewise, circuitry module106acan be replaced if it is malfunctioning. Replacements for either module106aor106bcan include more advanced technology, for example, improved circuitry or a higher capacity battery126. This being said, it is not required that circuitry and battery modules106aand106bbe separable. Instead, they can be combined into a single electronics module106with a common housing120, as shown inFIG. 10, which is explained later.

Referring again toFIG. 4A, a cable108connects electronics in the coil housing104asuch as the coil102to the circuitry module106a(or electronics module106more generally). To assist in this connection, the end of cable108includes a connector110attachable to a port112(see alsoFIG. 7) on the flat face of the circuitry module106a.Notice that the cable108spans around a portion of the edge of the coil housing104awhen the electronic module106is retained within the receptacle105. The cable108preferably spans around an edge of the housing104(e.g., the coil housing104a) which is different from the edge where the electronics module106/receptacle105is located. In this regard, the cable108preferably spans approximately 270 degrees (φ) around the housing104. Given possible differences in which external charger100can be fabricated, “approximately 270 degrees” should be understood as ranging from 180 degrees to 360 degrees. Further, an edge of the housing104need not be linear, but can comprise curved edges as well.

Spanning the cable108around the housing104is preferred both because it renders an organized and compact design when the electronics module106is retained in the receptacle105, and because it yields a cable108of sufficient length X (FIG. 5) to position the electronics module106sufficiently far away from the charging coil102when it is removed from the receptacle105, such as during high-power charging, and as illustrated inFIG. 5, explained further later. That being said, it is not strictly necessary that cable108proceed around the coil housing104aor in a counter clockwise direction as shown. Instead, the cable108can alternatively proceed around the housing104in a clockwise direction, as shown by a dotted line inFIG. 4A.

Cable108includes inner wires114(FIGS. 6A & 6B) as necessary to connect electronics in the coil housing104ato electronics in the circuitry module106a.In this regard, coil housing104apreferably includes a printed circuit board116, as seen inFIGS. 6A-6Cto support the charging coil102and any other electronics in the coil housing104a.For example, the coil housing104amay include at least one thermistor118(FIG. 6A) to report temperature to electronic components124in the circuitry module106a.As shown, the thermistor118is preferably centered with respect to the charging coil102. Printed circuit board116can be rigid (FR4), or of a flexible type such as Kapton™. Coil housing104amay include other circuitry as well, such as driver circuitry for the charging coil102, and thus while cable108may be coupled to the charging coil102via such other circuitry or connections, cable108is not necessarily connected directly to the charging coil102.

The connector110type used with cable108should be chosen in light of how many wires114are required to adequately communicate between the various electronics in the coil housing104aand the circuitry module106a.In this regard, the connector110/port112can comprise a mini HDMI port, a mini USB port, and the like, or may be customized.

Cable108and its connector110are attachable to and detachable from the electronics module106, preferably the circuitry module106a.This is preferred because (like the separability of circuitry module106aand battery module106b) it allows defective or out-of-date components in the external charger100to be replaced. For example, if the charging coil102in coil housing104acontinues to function appropriately, it may be retained while either or both of circuitry module106aor battery module106bare replaced. Similarly, either or both of circuitry module106aor battery module106bcan be retained while coil housing104ais replaced, which might occur either because coil102is defective (e.g., open circuited), or simply to provide a newer coil102/housing104athat might be of a different size and/or a more efficient design. This being said, connector110and port112in the electronics module106(circuitry module106a) may alternatively be hardwired and not separable.

Cable108is preferably bendable to allow the electronics module106to be both retained within (FIG. 4A) and extended from (FIG. 5) the housing104. In one example, the covering of the cable108may comprise a rubberized material, which along with its connector110can be mold injected along with one or more of the housing portions104a-c. If housings104a-care made of harder materials, cable108may have a more softer covering similar to charging cables used with mobile devices generally. Although not shown, cable108can further include a stiffening member throughout its length, such as a bendable metal material that allows the cable to retain its shape when bent. This would allow the electronics module106when extended (FIG. 5) to independently retain its position relative to the housing104. Although not shown, cable108may comprise at the opposite end from connector110a discrete attachment109(FIG. 6A) to the coil housing104a.This attachment109may be hardwired, or may comprise a connector/port arrangement that allows cable108to be attached to and detached from the coil housing104a.

If cable108is softer and “floppy,” it may be desirable to retain it against the edge of the coil housing104awhen the electronics module106is retained (FIG. 4A). In this regard, the edge of the coil housing104acan include a cable-holding mechanism140, as shown inFIGS. 8A and 8B. In this example, cable-holding mechanism140comprises a deformable rubberized material including a groove142into which the cable108can be press fit when the electronics module106is retained within receptacle105(FIG. 8A), and from which the cable108can be “peeled” when the electronics module106is removed from the receptacle105and extended from the housing104(FIG. 5). Although cable-holding mechanism140is shown inFIGS. 8A and 8Bas comprising a material separate from the coil housing104a,in other examples it could simply comprise the edge of the coil housing104aas it is formed. Also, cable-holding mechanism140could comprise other well-known structures such as clips, clasps, Velcro™, etc. Further, cable-holding mechanism140can retain the cable108at discrete locations around the edge of the coil housing104a,rather than retaining the cable along the continuum of the edge of the coil housing104aas illustrated inFIG. 8A.

As best shown in the cross-section ofFIG. 6D, circuitry module106apreferably includes a printed circuit board122for integrating electronic components124to enable external charger100to operate to provide power/charging to an IMD10. In this regard, electronic components124can be identical or similar to electronic components44otherwise generally included in traditional external chargers, such as external charger40of the prior art (FIGS. 2A-2C), and general functionality and control of external charger100can be the same, except as further described herein. As further shown inFIG. 6D, circuitry module106acan receive power from the battery126(connectors132/ports130) and coil-housing related signals from the coil housing104a(connector110/port112) via connections131that connect to the PCB122.

The external charger100is advantageous as regards heating, in that the electronics module106—more particularly battery126in the battery module106band PCB122/electronic components124in the circuitry module106a—are outside of the area extent of the charging coil102. This is true regardless whether the electronics module106is retained within (FIG. 4A) or extended from (FIG. 5) the receptacle105of the housing104. As discussed in the Background, it can be advantageous to orient the major planes of charger electronics, including the plane of the PCB122and the planes of electronic components124, parallel to highest-intensity portions of the magnetic field60present in the center of the charging coil102, that is, perpendicular to the plane of the coil102. This is shown inFIG. 9B. Notice that to match the orientation of PCB122, the major plane of connector110of cable108can also be made parallel to assist in connection of signals131(FIG. 6D) from the connector110to the PCB122. While the orientations of the PCB122, electronic components124, and connector110inFIG. 9Bare preferred, these components are also suitably far away from high-intensity portions of the magnetic field60even when the electronics module106is retained within receptacle105, and thus may be placed perpendicular to the field, as shown inFIG. 9A.

Like the prior art external charger40described earlier, external charger100preferably includes a user interface, which could be implemented in different manners. For example, and as shown inFIG. 4A, electronics module106, more specifically circuitry module106a,can include an LED144and an on/off switch146. Circuitry module106amay also include a speaker, although not shown. Such user interface aspects may perform as described earlier in conjunction with external charge40—to begin and indicate generation of the magnetic field60; to indicate alignment, etc. As shown, the LED144and on/off switch146are carried on the cylindrical side of the circuitry module106a's housing120. Having user interface components proximate to the circuitry module106ais logical as such components would communicate with the control circuitry on the PCB122within that module.

Alternatively, user interface aspects may also be carried on the circular faces of the circuitry module106a,the battery module106b,or both. This is illustrated inFIG. 10, which also shows circuitry module106aand battery module106bunified into a single (non-separable) electronics module106with a single housing120. As shown, LED144is provided on the same face that includes port112for the cable connector110, while the other face includes on/off switch146. As shown, on/off switch146protrudes through the housing120of the electronics module106, and may be depressible through the material at the circular face of the cup104cif it is suitably flexible. If the material of the cup104cis rigid, a hole148may be cut through the face to allow user access to the on/off switch146, as shown in dotted lines.FIG. 10is merely an example in which use interface aspects can be carried by the electronics module106. One skilled will recognize that other examples are possible. User interface aspects can also be present on the coil housing104aas well, which aspects can communicate with control circuitry in the electronics module via cable108.

With the structure of the external charger100explained, attention now turns to use of the external charger100, and particularly use of the external charger in different power modes. An advantage to the design of external charger100is that its physical configurability—in which electronics module106can either be retained within (FIG. 4A) or extended from (FIG. 5) the housing104—facilitates different power levels to be used to produce the magnetic field60for the IMD10.

Specifically, the first configuration ofFIG. 4Ain which the electronics module106is retained in the receptacle105allows for the external charger100to produce a magnetic field60of a normal power level, comparable to the external charger40of the prior art. Such a normal power level is referred to as “low” for comparative purposes. By contrast, the second configuration ofFIG. 5in which the electronics module106is removed from the receptacle105and extended from the housing104allows the external charger100to produce a higher-power magnetic field. This is because the extended configuration moves the majority of conductive structures of the external charger100—including significantly the battery126and PCB122/components124—significantly far away from the influence of the magnetic field60that Eddy current heating is mitigated. Magnetic field60may thus be of higher power while at the same time being less likely to exceed a safe operating temperature (Tmax) for the external charger100. This is beneficial to the IMD powering process as a whole, because the IMD10can receive and use higher amounts of power (should it lack a battery14), and/or because the battery14in the IMD10can be charged at a faster rate.

The electronic components124in the electronics module106, in particular its control circuitry, can produce a low- or high-power magnetic field60in a number of ways. For example, a low-power magnetic field can be produced by passing a relatively low AC current through the charging coil102, while a high-power magnetic field can be produced by passing a higher AC current. In another approach, a low-power magnetic field can be produced by passing an AC current through the charging coil102with a relatively low duty cycle—i.e., a low on-to-off ratio. A high-power magnetic field by contrast may use the same magnitude of the coil current, but may increase the duty cycle.

The electronics module106is operable to produce a low- or high-power magnetic field60in different manners. One way, shown inFIGS. 4A and 5, is to include a control mechanism as part of the user interface of the external charger100to allow the user to choose a low- or high-power magnetic field60. Specifically, a power selection switch150is carried by the electronics module106(specifically, circuitry module106a) that allows a user the option to select a low-power (“L”) or high-power (“H”) magnetic field60. Preferably the patient would make these choices with the external charger100in the proper physical configuration as described above, although this isn't required.

Alternatively, whether external charger100produces a low- or high-power magnetic field60can occur automatically depending on the physical configuration of the external charger100. This requires electronic components124in the electronics module106to detect whether the electronics module106is retained in or removed from the receptacle105, and such automatic detection and magnetic field generation can occur in different ways. For example, although not shown, the housings120or128of the electronics module106could include a pressure switch that is engaged when the electronics module106is retained by the receptacle105. In another example, although again not shown, the electronics module106may include a coil whose inductance can be monitored and will be affected by mutual inductance formed with charging coil102when the electronics module106is retained (and hence close to the coil102), but whose inductance will remain unaffected by the charging coil102when the electronics module106is extended (and far away). These are merely examples, and other means of automatically detecting the physical configuration of the external charger100and automatically adjusting the power of the magnetic field60will be recognized by those skilled in the art.

Note that whether the external charger100is producing a low- or high-power magnetic field60, temperature control as described earlier can still be enabled in the external charger100as assisted by temperature data provided by the thermistor(s)118(FIG. 6A). Note further that low- and high-power magnetic fields need not be constant power levels. In other words, the control circuitry in the electronics module106may adjust the magnitude of both the low- or high-power magnetic fields60depending for example on coupling with the IMD10, temperature detection, or for other reasons known in the art.

External charger100is generally sized similarly to the external charger40of the prior art, and is hand-holdable and portable. The manner in which external charger100is used by a patient is also generally similar, although modified depending on the external charger100's physical configuration and/or the power level it is producing.FIG. 11Ashows external charger100used when the electronics module106is retained within receptacle105to produce a low-power magnetic field, whileFIG. 11Bshows use when electronics module106is removed from receptacle105and extended to produce a high-power magnetic field.

In both examples, a charging belt160is used, similar to that described in U.S. Patent Application Publication 2014/0025140. The belt160has a pouch162which in this example is shown at the back of a patient near to where the IMD10(not shown) would be implanted in an SCS application. If a low-power magnetic field is to be used as shown inFIG. 11A, the electronics module106is retained, and the entire external charger100is slipped into pouch162by an opening164in the belt. If a high-power magnetic field is to be used as shown inFIG. 11B, the housing104with its charging coil102(not shown) can remain in the pouch162, while the electronics module106and cable108are removed through opening164and extended away from the housing104. The extended electronics module106as shown inFIG. 11Bmay be placed into a second pouch166on the belt160, which pouch166may be more proximate to the front of the patient. This beneficially reduces heating in the electronics module106, and further beneficially places user interface aspects of the external charger100to where they may be more easily accessed by the patient. However, the extended electronics module106could be placed elsewhere, such as in an opposing pants pocket, etc. It should be understood that while the external charger100is shown as operable in conjunction with a belt160, this is only one example of a usage model, and therefore not the only manner in which the external charger100can be used.

Note that the variations and alternatives shown and described for the external charger100can be used together in any combination, even if such variations and alternatives are not expressly shown in the Figures or discussed in the text.