Abstract:
Preferred orientations and placements of an inductor relative to a communication coil in an Implantable Medical Device (IMD) are disclosed. The inductor can comprise part of a boost converter used to generate a power supply voltage in the IMD, which inductor may interfere with the coil. The inductor may have a length defined by its windings around an axis, which axis may be in a plane of the coil or in a plane parallel to the coil. The inductor can be included within the area extent of the coil, and is preferably oriented such that its axis is parallel to a maximum dimension of the coil. Ends of the inductor are further preferably equidistant from the coil. So oriented and placed, the inductor is less prone to interfering with the coil, thus improving communications with the IMD.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a non-provisional of U.S. Provisional Patent Application Ser. No. 61/902,089, filed Nov. 8, 2013, to which priority is claimed, and which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to improving wireless communications in an implantable medical device such as an implantable pulse generator. 
     BACKGROUND 
     Implantable stimulation devices 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 with any implantable medical device or in any implantable medical device system. 
     An SCS system typically includes an Implantable Pulse Generator (IPG), such as that described in U.S. Provisional Patent Application Ser. No. 61/874,194, entitled “Construction for an Implantable Medical Device Employing an Internal Support Structure,” filed Sep. 5, 2013, which is incorporated herein by reference. The IPG  10  of the &#39;194 Application is shown in  FIG. 1  in plan and cross sectional views, and includes a biocompatible device case  30  that holds the circuitry  27  and battery  34  necessary for the IPG to function. The IPG  10  is coupled to electrodes  16  via one or more electrode leads  14  that form an electrode array  12 . The electrodes  16  are carried on a flexible body  18 , which also houses the individual signal wires  20  coupled to each electrode  16 . The signal wires  20  are also coupled to proximal contacts  22 , which are insertable into lead connectors  24  fixed in a header  28  on the IPG  10 , which the header can comprise an epoxy for example. Once inserted, the proximal contacts  22  connect to header contacts  26  in the lead connectors  24 , which header contacts  26  are in turn coupled by feedthrough pins  48  ( FIG. 2 ) to circuitry within the case  30 . In the illustrated embodiment, there are sixteen electrodes  16  split between two leads  14 , although the number of leads and electrodes is application specific and can vary. In an SCS application, electrode leads  14  are typically implanted on the right and left side of the dura within a patient&#39;s spinal cord. The proximal contacts  22  are then tunneled through the patient&#39;s tissue to a distant location where the IPG case  30  is implanted, at which point they are coupled to the lead connectors  24 . 
       FIG. 2  shows perspective views of the bottom and top sides of the IPG  10  with the case  30  removed so that internal components can be seen, including the battery  34 , a communication coil (antenna)  40 , and a printed circuit board (PCB)  42 . As explained in the &#39;194 Application, these components are affixed to and integrated using a rigid (e.g., plastic) support structure  38 . Battery  34  in this example is a permanent, non-wirelessly-rechargeable battery. (Battery  34  could also be rechargeable, in which case either communication coil  40  or another recharging coil would be used to wirelessly receive a charging field that is rectified to charge the battery  34 ). The communication coil  40  enables bi-directional communication between the IPG  10  and a device external to the patient ( FIG. 3 ) via magnetic induction. The ends of communication coil  40  are soldered to coil pins  44  molded into the support structure  38  to facilitate the communication coil  40 &#39;s eventual connection to the PCB  42 . PCB  42  integrates the various circuitry  27  needed for operation of the IPG  10 . Communication coil  40  is proximate to the bottom side of the IPG  10  in plane  40   p , while the PCB  42  is proximate to the top side in plane  42   p , as shown in the cross section of  FIG. 1 . 
       FIG. 3  shows an external controller  100  with a coil  108  for communicating with the IPG  10 &#39;s communication coil  40  via a magnetic induction link  90 . External controller  100  is preferably hand-holdable and portable, and includes a user interface (a display, buttons, etc.) to allow a user to adjust the therapeutic current that the IPG  10  is providing (e.g., to increase or decrease the stimulation being provided, to change which electrodes are providing the stimulation, etc.), and to review status information reported by the IPG  10 . 
     In traditional SCS systems, data is bi-directionally transmitted along link  90  using a Frequency Shift Keying (FSK) protocol, in which a serial string of bits is wirelessly transmitted at different frequencies around a center frequency (e.g., fc=125 kHz). For example, if a ‘0’ bit is to be transmitted to the IPG  10 , control circuitry  102  in the external controller  100  (e.g., a microcontroller) provides that bit digitally to modulator/transmitter circuitry  104  in the external controller  100 . The modulator/transmitter  104  tunes coil  108  to resonate at 121 kHz for example for a bit duration (e.g., 250 microseconds). This frequency is transmitted via link  90  to the communication coil  40  in the IPG  10 , whose demodulator/receiver circuitry  49  decodes it per its frequency as a digital ‘0’, and reports it to the IPG&#39;s control circuitry  50  (e.g., a microcontroller) for interpretation. A ‘1’ bit would be transmitted similarly, but at a different frequency, for example 129 kHz. Transmission of data from the IPG  10  to the external controller  100  occurs similarly via modulator/transmitter circuitry  47  in the IPG  10  and demodulator/receiver circuitry  106  in the external controller  100 . 
     Wireless communications between the external controller  100  and IPG  10  can occur in different manners, and external controller can be differently configured, as explained in U.S. Patent Application Ser. No. 61/874,863, filed Sep. 6, 2013. 
       FIG. 4A  shows an architecture for IPG  10 , which is described in U.S. Patent Application Publication 2013/0331910. Shown with particular emphasis are the various power supplies in the IPG  10 , which are shown with thicker lines. Primary battery  34  provides the main power supply voltage, Vbat, from which all other power supply voltages in the IPG  10  are derived. Because Vbat is relatively small (e.g., around 3 Volts, but dropping as it depletes over the IPG  10 &#39;s lifetime), and because certain circuits in the IPG  10  require higher power supply voltages than Vbat may be able to provide, the IPG  10  includes boost circuitry. In particular, IPG  10  includes a first boost converter  52  and a second boost converter  70 , both of which comprise DC-DC converters for converting Vbat to different power supply voltages, i.e., to Vup and V+, as explained further below. 
     The first boost converter  52  generates power supply Vup, which comprises the power supply for most of the circuitry in the IPG  10 , including analog circuitry  62 , digital circuitry  64  (including microcontroller  50 ), and memory  60 . Vup may be regulated (per regulators  54 ,  56 , and  58 ) to derive separate power supply voltages Va, Vd, and Vf dedicated to each of these circuits. In one example, Vup can equal approximately 3.2 Volts, with low-drop-out regulators  54 ,  56 , and  58  producing power supplies Va, Vd, and Vf of approximately 2.8 Volts. As the particulars of analog circuitry  62 , digital circuit  64 , and memory  64  are described in the above-cited &#39;510 Application, they are not elaborated upon here. Vup is monitored via a monitor and adjust block  53 , which compares Vup to a reference voltage, Vref, to determine whether Vup is too low. If so, this block  53  via control signal boost 1  instructs the first boost converter  53  to operate, as explained further below. 
     The second boost converter  70  is used to generate a different power supply voltage, V+, called the compliance voltage, for powering the current generation circuitry  74  that produces the therapeutic current pulses (Iout) at one or more of the electrodes  16 . In  FIG. 4A , such current generation circuitry comprises one or more Digital-to-Analog converters (DAC(s)  74 ) that provide current pulses of the prescribed magnitude, frequency, and duration in accordance with digital control signals (CNTR). Because the prescribed current pulses can differ from time to time for a given patient, or from patient to patient, V+ is not fixed, but is instead set at an optimal level that is not too low to provide the prescribed current pulses, nor too high as to waste battery  34  power. Specifically, V+ monitor and adjust circuit  76  monitors a voltage drop across the DAC(s)  74 , which it uses to control the second boost converter  70  to generate a power supply voltage V+ of an appropriate magnitude in accordance with control signal boost 2 . Again, further details regarding compliance voltage generation can be found in the above-cited &#39;510 Application. 
     Both the first boost converter  52  (producing Vup) and the second boost converter  70  (producing V+) can comprise the same basic circuitry as shown in  FIG. 4B , which comprises a well-known inductor-based boost converter. When enabled via control signal boost 1  or boost 2 , a pulse width modulator  80  modulates a pulse width (PW) of a clock signal (CLK), which is sent to the gate of a transistor  84 . When the transistor  84  is on, current (I) passes through an inductor  82 . When the transistor  84  is turned off, the current in the inductor  82  discharges through a diode  86  to a capacitor  88 , whose top plate comprises Vup in the first boost converter  52 , or the compliance voltage V+ in the second boost converter  70 . Because the capacitor  88  was already charged to the battery voltage, Vbat, the additional charge from the inductor  82  boosts Vup or V+ to a value higher than Vbat, with diode  86  preventing this excess charge from dissipating backwards into the circuit. Capacitor  88 , in addition to storing the charge, also filters and stabilizes Vup and V+. Thus, as the gate of transistor  84  oscillates between on and off, Vup or V+ continues to boost at a rate determined by the duty cycle of the gate pulse train. When control signal boost 1  or boost  2  is disabled, oscillations at the gate of the transistor  84  are halted, which causes Vup or V+ to fall as charge is consumed by the circuitry to which these power supplies are connected. Of course, the particulars of the circuitry values used in the first and second boost converters  52  and  70  will differ in accordance with their different functions and the voltages they must produce. Control signals boost 1  and boost 2  may be digital or analog, and may comprise a digital or analog value indicating how “hard” the boost converter must work to produce the desired power supply voltage. 
     It is known that a boost converter has the potential to interfere with the telemetry circuitry operable in an IPG. See U.S. Patent Application Publication 2010/0211132, discussing this issue in the context of the second boost converter  70  that produces the compliance voltage, V+. This is because the boost converter, via the current I through its inductor  82 , will produce a magnetic field  85  when it operates, which magnetic field  85  may couple to the communication coil  40  in the IPG. Even if the communication coil  40  has a high quality factor and good out-of-band noise rejection, the magnetic field  85  produced by inductor  82  may still have frequency components generally within the band of the communication coil (e.g., from 100 kHz to 150 kHz). Moreover, the frequency components present in magnetic field  85  can be difficult to control because they are dependent on the power supply voltage being produced by the boost converter at any given time. If interference by the inductor  82  is severe, telemetry may not be reliable. Interference by the inductor  82  during reception of data at the communication coil  40  is especially problematic, as the telemetry signal received by the communication coil  40  may be quite small in magnitude. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an Implantable Pulse Generator (IPG) in accordance with the prior art. 
         FIG. 2  shows the IPG with its case removed in accordance with the prior art. 
         FIG. 3  shows the IPG in communication with an external controller, and the circuitry involved in each, in accordance with the prior art. 
         FIG. 4A  shows the power supply architecture of the IPG, and  FIG. 4B  shows boost converter circuitry used to generate higher power supply voltages, including an inductor that can interfere with a communication coil in the IPG. 
         FIGS. 5A and 5B  show preferred and non-preferred orientations and placements of the boost converter&#39;s inductor within the IPG&#39;s communication coil, in accordance with an aspect of the invention, while  FIG. 5C  shows the preferred orientation and placement in cross section. 
         FIGS. 6A and 6B  show various inductor designs useable for the boost converter, each having a length relevant to the orientations of  FIGS. 5A and 5B . 
         FIGS. 7A and 7B  show inductive coupling between an IPG coil and an external controller coil, in accordance with the prior art. 
         FIGS. 8A and 8B  show application of the disclosed technique to communication coils of different shapes. 
         FIGS. 9A and 9B  show application of the disclosed technique to inductors having no lengths along their axes. 
     
    
    
     DETAILED DESCRIPTION 
     The design of IPG  10  is especially concerning with regarding to coupling between the communication coil  40  and the inductors  82  in either of the boost converters  52  or  70 . Because of the size of the battery  34  within the case  30 , the PCB  42  and communication coil  40  are relegated to a relatively small volume V in the case  30  between the battery  34  and the header  28 , as shown in  FIG. 2 . Both the communication coil  40  and the PCB  42  are preferably made as large as possible within this volume V to increase the area of the communication coil  40  (which improves external communications) and to maximize the PCB  42 &#39;s area for IPG electronics. 
     As a result, and referring to  FIG. 5A , the communication coil  40  proceeds around the periphery of the PCB  42 , although in different parallel planes  40   p  and  42   p  as noted earlier. Because the inductors  82  of the boost converters  52  or  70  would be mounted to the PCB  42 , they would also be within the area extent A of the communication coil  40  ( FIG. 5A ) as projected onto plane  82   p  in which the inductors  82  reside ( FIG. 5C ). Because inductors  82  are within the area extent A of the communication coil  40 , they are closer and more prone to interfering with the communication coil  40  than they would be if placed outside of the communication coil  40 . In particular, because the communication coil  40  of IPG  10  has minimum and maximum perpendicular dimensions X and Y, the inductors  82  must be at least somewhat close to the maximum dimensions Y. As shown, the communication coil  40 &#39;s area extent A and dimensions X and Y are determined with respect to the coil&#39;s center circumference  40   c , but could also be determined with respect to the coil&#39;s inner or outer circumferences as well. 
     The inventors have noticed that the orientation and placement of the boost converters&#39; inductors  82  relative to the communication coil  40  affects the formers&#39; interference with the latter in IPG  10 , particularly where the inductor  82  has a length L.  FIGS. 6A and 6B  show shows different configurations for the inductors  82  useable in the boost converters  52  or  70 . A helical inductor  82 A is shown in  FIG. 6A , which is wound around an axis  82 Aa in the plane  82   p  of the inductor  82 A ( FIG. 5C ). Inductor  82 B in  FIG. 6B  comprises for example the 8300 Series of power inductors manufactured by Murata Manufacturing Co., Ltd., or the Micro-Pac Plus power inductor manufactured by Cooper Electronic Technologies. In this inductor design, the inductance is primarily governed by a toroid winding  92  within the inductor&#39;s  82 B&#39;s package  94 . However, the ends  96   a  and  96   b  are terminated by being wound around an axis  82 Ba which is again in plane  82   p . In either case, the winding around axes  82   a  define a length L for the inductor  82 . (Dimension Z, which may comprise the diameter of the windings of inductors  82 A and  82 B, is also shown, although such dimension Z is ignored for the time being. Dimension Z is discussed later with respect to  FIGS. 9A and 9B ). 
     Also shown in dotted lines in  FIGS. 6A and 6B  are the magnetic flux lines produced by the inductors  82 A and  82 B as the boost converters operate. A largest magnetic field strength,  82 max, can be identified, which as one skilled in the art will understand are at locations where the magnetic flux density is largest. For the inductor  82 A of  FIG. 6A , the largest magnetic field strength,  82 Amax, occurs in a line along axis  82 Aa inside of the inductor  82 A. For the inductor  82 B of  FIG. 6B , the largest magnetic field strength,  82 Bmax, again occurs along the axis  82 Ba of the termination windings  96   a  and  96   b , although occurring in two separate lines. (In reality, the largest magnetic field strength  82 Bmax would also proceed in a circular shape through the toroid  92 , but as the magnetic field is contained within and doesn&#39;t extend outside of the toroid, it can be ignored). The magnetic field strength is also relatively strong (although not maximal) extending from the ends of the lengths L of the inductors  82  along their axes  82   a , where the magnetic flux density is still relatively high. 
     The inventors have determined that inductors  82  such as those shown in  FIGS. 6A and 6B  interfere with the communication coil  40  depending on their orientation, i.e., depending whether their lengths L, or their axes  82   a , are parallel or perpendicular to the minimum X and maximum Y dimensions of the communication coil  40 . The inventors in particular have determined that the inductor  82  in the first boost converter  52 —that used to produce Vup—is especially problematic, although the inductor  82  in the second boost converter  70 —that used to produce V+—is still of concern, as the above-cited &#39;132 Publication discusses. 
     A preferred and lowest-interference orientation for the inductors  82  relative to the communication coil  40  is shown in  FIG. 5A , in which the inductor&#39;s length L (axis  82   a ) is parallel with the maximum dimension Y, and perpendicular to the minimum dimension X. By contrast, the orientation of the inductor  82  in  FIG. 5B , in which the inductor&#39;s length L (axis  82   a ) is perpendicular with the maximum dimension Y, and parallel to the minimum dimension X, induces more noise to the communication coil  40 . For simplicity, only one inductor  82  of one of the boost converters  52  or  70  is shown, although the disclosed preferred orientations and placements are preferably applied to both inductors  82 . 
     It is not evident that the orientations of the inductors  82  in the boost converters should differ significantly with respect to the noise imparted to the communication coil  40 , because the inductors  82  are generally not well coupled to the communication coil  40  in a traditional sense understood for promoting data communications with the communication coil  40 . As is known, good inductive coupling between the communication coil  40  in the IPG  10  and the coil  108  in the external controller  100  ( FIG. 2 ) occurs if the axes  40   a  and  108   a  around which these coils are wound are parallel (and preferably collinear) as shown in  FIG. 7A . See also U.S. patent application Ser. No. 61/887,237, filed Oct. 4, 2013. Coupling decreases as the angle θ between these axes  40   a  and  108   a  increases ( FIG. 7B ), and would be minimized when the axes  40   a  and  108   a  are orthogonal. Because the axes  82   a  of the inductors  82  and axis  40   a  of the communication coil  40  are orthogonal, one might not expect noise coupling from the inductors  82  to be significant, and hence might not expect that that orientation or placement of the inductors  82  relative to the communication coil  40  would matter. 
     Nonetheless, decreased coupling between the inductor  82  and the communication coil  40  in  FIG. 5A  results from minimizing overlap of the magnetic flux from inductor  82  and the communication coil  40 , even if such coupling would seem to be insignificant for the reasons just described. When the inductor  82  is oriented as in  FIG. 5A , a distance D 1  between the ends of the length L of inductor  82  and the communication coil  40  is larger than the distance D 2  in the orientation of  FIG. 5B , which distances are measured with the inductor  82  placed equidistantly from the maximum dimensions Y of the communication coil  40 . Moreover, the strength of the inductor  82 &#39;s magnetic field is strongest along its axis  82   a , making the decreased distance D 2  of  FIG. 5B  that much more problematic. It should be noted that distances D 1  and D 2  may be not be planar as shown in  FIGS. 5A and 5B  if the inductor  82  and communication coil  40  are not in the same planes (as is the case for IPG  10 ). (It is also assumed that dimension Z of the inductor  82  ( FIGS. 6A and 6B ) is significantly less than the minimal dimension X of the communication coil  40 , which may not always be the case in an actual implementation). Such distances D 1  and D 2  may also be determined with respect to other than the center circumference of the communication coil  40 . 
     As well as showing an optimal orientation for the inductor  82 ,  FIG. 5A  additionally shows an optimal placement for the inductor  82  within the area extent A of communication coil  40 , where the inductor  82  is placed at the communication coil&#39;s axis  40   a . With this placement, the ends of the inductor  82 &#39;s length L are equidistant from both the maximum Y and minimum X dimensions of the communication coil  40 . The inventors consider this placement to be more important for the inductor in the first boost converter  52 , which as discussed above is more prone to interfering with the communication coil  40 . 
     Such optimal placement of the inductor  82  may not be possible depending on other circuitry present on the PCB  42 , and is not strictly required. In other examples, the optimally-oriented inductor  82  can be placed within a region R within the area extent A of the communication coil  40 , which region R may be limited by a threshold distance to the Y dimension (Dy) and a threshold distance to the X dimension (Dx). In a preferred embodiment, Dx is larger than Dy, which recognizes that relatively strong magnetic fields emanate from the ends of the length L of the inductor  82  along its axis  82   a , and thus that a larger distance to the communication coil  40  is preferred along this axis  82   a.    
     Region R (and distances Dx and Dy) may be determined with reference to the orientation and placement of the inductor  82  in  FIGS. 5A and 5B . For example, R may be defined as the boundary at which coupling between the inductor  82  and the communication coil  40  increases by a set amount (e.g., 50%) when compared to the orientation and placement of  FIG. 5A , or may defined as the boundary at which coupling decreases by a set amount (e.g., 50%) when compared to the orientation and placement of  FIG. 5B . Other metrics or thresholds may also be used to set the boundary of preferred placement region R. Region R may also be constrained to one dimension. For example, in one example, Dy=D 1 , effectively restraining placement of the optimally-oriented inductor  82  along an axis R′ parallel to and equidistant from the longer Y portions of the coil  40 . Such placement is preferred for the inductors  82  in the boost converters  52  and  70  in IPG  10 , with the inductor of the more-critical first boost converter  52  being placed on axis R′ at or near the axis  40   a  of the coil  40 , and with the inductor  82  of the less-critical second boost converter  70  also placed along axis R′ but closer to the shorter portions X of the coil  40 . 
     As alluded to earlier, the preferred orientation and placement of the boost converters&#39; inductors  82  with respect to the communication coil  40  does not require the inductors  82  to be in the same plane  40   p  of the communication coil  40 , i.e., that the axes  82   a  of the inductors  82  be in plane  40   p  of the communication coil  40 . Instead, the axes  82   a  of the inductors  82  may be located in a different plane  82   p  parallel to plane  40   p  of the communication coil  40 , as shown for IPG  10  in  FIG. 5C . Although the inductors  82  and communication coil  40  are both proximate to the same side of the PCB  42  in IPG  10 , this is not strictly necessary, and inductors  82  may be proximate to the other side of the PCB  42  as part of other circuitry  27  for example. The inductors  82  in the boost converters  52  and  70  may also appear on different sides of the PCB  42 , with one placed as shown in  FIG. 5C , and the other comprising part of circuitry  27 . In other IPG examples, one or more of the inductors  82  and communication coil  40  may reside in the same plane. For example, one or more inductors  82  and communication coil  40  may both be affixed to a same side of the PCB  42 , with the inductor(s)  82  inside of the coil  40 &#39;s windings. 
     The preferred orientation and placement of the inductors  82  of the boost converters may be used with communication coils  40  that do not have readily recognizable maximum and minimum orthogonal dimensions X and Y.  FIG. 8A  for example shows an IPG  10 ′ with a communication coil  40  generally shaped in a semi-circle, as disclosed in U.S. Patent Application Publication 2011/0112610. Although not rectangular in its shape, minimum and maximum orthogonal dimensions X and Y can still be identified by bounding the communication coil  40  (along any circumference) with a rectangle, as shown in dotted lines. A preferred orientation of an inductor  82  is achieved consistently with this disclosure&#39;s teachings by, in one example, orienting axis  82   a  of the inductor  82  parallel to the maximum dimension Y, and equidistantly to the communication coil  40  (D 1 ). Such placement of inductor  82  preferably occurs at a maximum extent of the minimum dimension X, akin to the orientation and placement of  FIG. 5A .  FIG. 8B  illustrates this same principle with respect to an oval-shaped communication coil  40 , and again shows preferred orientation and placement of an inductor  82 . Other modifications to the orientation and placement of the inductors  82  of  FIGS. 8A and 8B  may also be made, as described above. 
     Implementation of the disclosed technique can also be applied to inductors  82  of different shapes than those shown in  FIGS. 6A and 6B . For example, in the cross section of  FIG. 9A , an inductor  82 C is shown comprising a flat coil (akin to communication coil  40 ) wound around an axis  82 Ca and essentially wound in a single plane  82 Cp. This inductor  82 C has no length L along its axis  82 Ca (ignoring the thickness of its windings), although it does have a dimension Z perpendicular to axis  82 Ca between its windings (which dimension Z could comprise inductor  82 C&#39;s diameter, as alluded to earlier). 
     Despite this dimension Z, the preferred orientation for the inductor  82 C can still be one in which the inductor&#39;s axis  82 Ca is parallel to the maximum dimension Y, as shown in  FIG. 9B . This results again from an understanding of the magnetic field strength of inductor  82 C, as shown by the flux lines in  FIG. 9A . The magnetic field strength of inductor  82 C is strongest in the center of the inductor  82 C ( 82 Cmax), but is also relatively strong along its axis  82 Ca, and may be stronger than the magnetic field around the windings at the same distance from the center of the inductor  82 C. As such, it may be preferable to orient the inductor  82 C as shown in  FIG. 9B , even if this does not maximize the distance D 3  between the inductor&#39;s windings and the communication coil  40 . A region R around which the inductor  82 C could be placed for this orientation while still providing suitably low coupling to the communication coil  40  can be determined consistent with earlier explanations. 
     Although focusing on reducing interference with the communication coil  40 , it should be noted that the disclosed teachings regarding orientation and placement of the inductor  82  in the boost converters can assist in preventing interference by inductor  82  with other coils or inductors in the an IPG—for example, the charging coil  41  in  FIG. 8A  used to wirelessly receive power from an external charger. Likewise, the disclosed teachings can also be used to orient and place other inductors, coils, or components with wound terminations in the IPG (not associated with the boost converters) in manners to prevent interference with the communication coil  40 . 
     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 alternatives, modifications, and equivalents that may fall within the spirit and scope of the present invention as defined by the claims.