Implant location detection and adaptive temperature control

Devices, systems, and techniques are described to detect when a power transmitting and receiving system is in an inefficient position, which may cause a thermal response that less desirable than a more efficient position. The system may power transmitting device configured to wirelessly transfer electromagnetic energy to a power receiving device. Processing circuitry of the system may compute a target output power deliverable by the power transmitting device for a first duration and control the power transmitting device to output the target output power based in part on a heat limit. The processing circuitry may further calculate an energy transfer efficiency to the power receiving unit, update an adjustment factor based on the calculated energy transfer efficiency, and apply the adjustment factor to the heat limit for a subsequent duration.

TECHNICAL FIELD

The disclosure relates to rechargeable implantable medical devices.

BACKGROUND

Medical devices may be external or implanted and may be used to monitor patient signals such as cardiac activity, biological impedance and to deliver electrical stimulation therapy to patients via various tissue sites to treat a variety of symptoms or conditions such as chronic pain, tremor, Parkinson's disease, epilepsy, urinary or fecal incontinence, sexual dysfunction, obesity, or gastroparesis and other conditions. In some examples, medical devices may include a rechargeable electrical power source, or may be powered directly by transmitting energy through tissue. In some examples, transmitting the energy through the tissue may result in an applied thermal dose, which may be caused by the energy heating the tissue, or heating the implanted device that in turn heats the surrounding tissue.

SUMMARY

In general, the disclosure describes devices, systems, and techniques to determine an energy transfer factor associated with recharging a device and adjust energy delivery based on the energy transfer factor. For example, the system may determine when a power transmitting and receiving system is in an inefficient position, which may cause a thermal response in the device and/or adjacent tissue that less desirable than a more efficient position. In response to detecting the efficiency of the system, the system may also control the levels of energy delivered to the implant based on the detected position. For example, the system may reduce the energy delivered to lower levels when the transmitter and receiver are in an inefficient position relative to a more efficient position to manage the thermal response and the energy transfer.

In one example, this disclosure describes a device comprising a power transmitting unit configured to wirelessly transfer electromagnetic energy to a power receiving unit; and processing circuitry configured to: compute a target output power deliverable by the power transmitting unit for a first duration; control the power transmitting unit to output the target output power based in part on a heat limit; calculate an energy transfer efficiency to the power receiving unit; update an adjustment factor based on the calculated energy transfer efficiency; and apply the adjustment factor to the heat limit for a subsequent duration.

In another example, this disclosure describes a system comprising a power receiving unit; a power transmitting unit configured to wirelessly transfer electromagnetic energy to the power receiving unit; and comprising processing circuitry configured to: compute a target output power deliverable by the power transmitting unit for a first duration; control the power transmitting unit to output the target output power based in part on a heat limit; calculate an energy transfer efficiency to the power receiving unit; update an adjustment factor based on the calculated energy transfer efficiency; and apply the adjustment factor to the heat limit for a subsequent duration.

In another example, this disclosure describes a method comprising computing, by processing circuitry, a target output power deliverable by a wireless power transmitting unit for a first duration; controlling, by the processing circuitry, circuitry to output the target output power based in part on a heat limit; calculating, by the processing circuitry, an energy transfer efficiency to a power receiving unit; and updating, by the processing circuitry, an adjustment factor based on the calculated energy transfer efficiency applying the adjustment factor to the heat limit for a subsequent duration.

DETAILED DESCRIPTION

Devices, systems, and techniques are described for determining when a power transmitting and receiving system is in an inefficient position, which may cause a thermal response in a device or adjacent tissue of a patient that is less desirable than a more efficient position. In long term or chronic uses, implantable medical devices may include a rechargeable power source (e.g., one or more capacitors or batteries) that extends the operational life of the medical device to weeks, months, or even years over a non-rechargeable device. When the energy stored in the rechargeable power source has been depleted, the patient may use an external charging device to recharge the power source. In the example of an implantable medical device, the rechargeable power source is implanted in the patient in the power receiving unit (PRU) and the charging device is external of the patient, this charging process may be referred to as transcutaneous charging. In some examples, transcutaneous charging may be performed via inductive coupling between a primary coil in the charging device and a secondary coil in the implantable medical device.

An electrical current applied to the primary coil generates a magnetic field, and when the primary coil is aligned to the secondary coil, the magnetic field induces an electrical current in the secondary coil within the patient. A charging circuit within the implantable medical device then applies current from the secondary coil to charge the rechargeable power source within the implantable medical device. With transcutaneous transfer via inductive coils, the external charging device does not need to physically connect with the rechargeable power source for charging to occur. However, the power transfer efficiency between the two or more devices may change based on the physical orientation of the secondary coil to the primary coil. Inefficient power transfer can lead to transferred power causing eddy currents in metal components that result in heat and/or direct heating of tissue instead of electrical current that charges the rechargeable power source in the implantable medical device. Increased heat can cause patient discomfort, tissue damage, and/or system operational issues.

An example system described herein may adjust power settings and/or provide feedback to a user to readjust one or more components of the system into to a more efficient position. The system described herein may include a power transmitting unit and a power receiving unit. In some examples, the power receiving unit (PRU) is in or coupled to an implantable medical device (IMD). The PRU heat limit and time spent receiving transmitting power may have a direct impact on the thermal response as measured by an applied thermal dose (CEM43) on the PRU, e.g., the implant during a charging session. In the example of a rechargeable PRU, when the system efficiency increases, then the time to recharge may decrease, thereby reducing the applied thermal dose. However, when operating at low efficiency the applied thermal dose may increase, in some examples, because of the increased duration of the recharge time. In some examples, an exacerbating factor is that some poor coupling positions cause a rise to higher temperatures for the same amount of energy input, which may have an exponential effect on the thermal dose. For example, instead of the charging energy being transferred to the battery of the IMD from a target coupling position, the energy may cause increased eddy currents in one or more components of the IMD or cause direct heating of tissue. The thermal dose may be measured based on CEM43, which is a normalizing method to convert the various time—temperature exposures applied into an equivalent exposure time expressed as minutes at the reference temperature of 43° C.

In response to detecting the efficiency of the system, the system may also control the levels of energy delivered to the implant based on the detected relative position. For example, the system may control the energy delivered to lower levels when the transmitter is in an inefficient relative position to the receiver. To optimize the energy transfer, which may reduce recharge time and applied thermal dose, the techniques of this disclosure may include an adaptive recharging algorithm that applies a monotonic transfer function. The transfer function may increase with transfer efficiency to dynamically adjust the heat limit during the charging session. In some examples, for coupling positions in which transfer efficiency is poor, the system may apply the algorithm to reduce the applied heat limit to its lowest allowable value to minimize the applied thermal dose such that the applied thermal dose does not exceed safety limits during the duration of the charging session. The “coupling position” may refer to the relative location, orientation and angle between the power transmitting unit and the PRU. In other examples, for coupling positions where the IMD efficiency is high, the system may increase the applied heat limit to a maximum allowable value such that the applied thermal dose does not exceed safety limits. In other examples, for coupling positions in the range between poor and high coupling efficiency, the system may adjust the applied heat limit to between the minimum and maximum allowable heat limit. In some examples, the coupling efficiency may be based on efficiency as measured by the PRU and communicated, e.g., wirelessly, to the power transmitting unit, or some other communication receiving device. The heat limit may also be referred to as a “heat control limit” in this disclosure.

FIG.1is a conceptual diagram illustrating example system10that includes an implantable medical device (IMD)14and an external charging device22that charges a rechargeable power source of the IMD14via an energy transfer coil26. Although the techniques described in this disclosure are generally applicable to a variety of medical devices including medical devices such as patient monitors, electrical stimulators, or drug delivery devices, application of such techniques to implantable neurostimulators will be described for purposes of illustration. More particularly, the disclosure will refer to an implantable neurostimulation system for use in spinal cord stimulation therapy, but without limitation as to other types of medical devices. An implantable neural stimulator (INS) is an example of an IMD such that one type of IMD is an INS.

As shown inFIG.1, system10includes an IMD14and external charging device22shown in conjunction with a patient12, who is ordinarily a human patient. In the example ofFIG.1, IMD14is an implantable electrical stimulator that delivers neurostimulation therapy to patient12, e.g., for relief of chronic pain or other symptoms. Generally, IMD14may be a chronic electrical stimulator that remains implanted within patient12for weeks, months, or even years. In the example ofFIG.1, IMD14and lead18may be directed to delivering spinal cord stimulation therapy. In other examples, IMD14may be a temporary, or trial, stimulator used to screen or evaluate the efficacy of electrical stimulation for chronic therapy. IMD14may be implanted in a subcutaneous tissue pocket, within one or more layers of muscle, or other internal location. IMD14includes a rechargeable power source (not shown) and IMD14is coupled to lead18.

Electrical stimulation energy, which may be constant current or constant voltage based pulses, for example, is delivered from IMD14to one or more targeted locations within patient12via one or more electrodes (not shown) of lead18. The parameters for a program that controls delivery of stimulation energy by IMD14may include information identifying which electrodes have been selected for delivery of stimulation according to a stimulation program, the polarities of the selected electrodes, i.e., the electrode configuration for the program, and voltage or current amplitude, pulse rate, pulse shape, and pulse width of stimulation delivered by the electrodes. Electrical stimulation may be delivered in the form of stimulation pulses or continuous waveforms, for example. In other examples, IMD14may be configured to monitor patient biological signals, such as biological impedance, cardiac signals, temperature, activity, and so on. In some examples IMD14may not deliver stimulation therapy.

In the example ofFIG.1, lead18is disposed within patient12, e.g., implanted within patient12. Lead18tunnels through tissue of patient12from along spinal cord20to a subcutaneous tissue pocket or other internal location where IMD14is disposed. Although lead18may be a single lead, lead18may include a lead extension or other segments that may aid in implantation or positioning of lead18. In addition, a proximal end of lead18may include a connector (not shown) that electrically couples to a header of IMD14. Although only one lead18is shown inFIG.1, system10may include two or more leads, each coupled to IMD14and directed to similar or different target tissue sites. For example, multiple leads may be disposed along spinal cord20or leads may be directed to spinal cord20and/or other locations within patient12. Lead18may carry one or more electrodes that are placed adjacent to the target tissue, e.g., spinal cord20for spinal cord stimulation (SCS) therapy.

In alternative examples, lead18may be configured to deliver stimulation energy generated by IMD14to stimulate one or more sacral nerves of patient12, e.g., sacral nerve stimulation (SNS). SNS may be used to treat patients suffering from any number of pelvic floor disorders such as pain, urinary incontinence, fecal incontinence, sexual dysfunction, or other disorders treatable by targeting one or more sacral nerves. Lead18and IMD14may also be configured to provide other types of electrical stimulation or drug therapy (e.g., with lead18configured as a catheter). For example, lead18may be configured to provide deep brain stimulation (DBS), peripheral nerve stimulation (PNS), or other deep tissue or superficial types of electrical stimulation. In other examples, lead18may provide one or more sensors configured to allow IMD14to monitor one or more parameters of patient12. The one or more sensors may be provided in addition to, or in place of, therapy delivery by lead18.

IMD14delivers electrical stimulation therapy to patient12via selected combinations of electrodes carried by lead18. The target tissue for the electrical stimulation therapy may be any tissue affected by electrical stimulation energy, which may be in the form of electrical stimulation pulses or waveforms. In some examples, the target tissue includes nerves, smooth muscle, and skeletal muscle. In the example illustrated byFIG.1, the target tissue for electrical stimulation delivered via lead18is tissue proximate spinal cord20(e.g., one or more target locations of the dorsal columns or one or more dorsal roots that branch form spinal cord20. Lead18may be introduced into spinal cord20via any suitable region, such as the thoracic, cervical, or lumbar regions. Stimulation of dorsal columns, dorsal roots, and/or peripheral nerves may, for example, prevent pain signals from traveling through spinal cord20and to the brain of the patient. Patient12may perceive the interruption of pain signals as a reduction in pain and, therefore, efficacious therapy results. For treatment of other disorders, lead18may be introduced at any exterior location of patient12.

Although lead18is described as generally delivering or transmitting electrical stimulation signals, lead18may additionally or alternatively transmit electrical signals from patient12to IMD14for monitoring. For example, IMD14may utilize detected nerve impulses to diagnose the condition of patient12or adjust the delivered stimulation therapy. Lead18may thus transmit electrical signals to and from patient12.

A user, such as a clinician or patient12, may interact with a user interface of an external computing device25to communicate with and in some examples, to program IMD14. Programming of IMD14may refer generally to the generation and transfer of commands, programs, or other information to control the operation of IMD14. For example, the external programmer may transmit programs, parameter adjustments, program selections, group selections, or other information to control the operation of IMD14, e.g., by wireless telemetry or wired connection.

In some cases, external computing device25may be characterized as a physician or clinician programmer if it is primarily intended for use by a physician or clinician. In other cases, external computing device25may be characterized as a patient programmer if it is primarily intended for use by a patient. A patient programmer is generally accessible to patient12and, in many cases, may be a portable device that may accompany the patient throughout the patient's daily routine. In general, a physician or clinician programmer may support selection and generation of programs by a clinician for use by the stimulator, e.g., IMD14, whereas a patient programmer may support adjustment and selection of such programs by a patient during ordinary use. In other examples, external charging device25may be included, or part of, an external programmer. In this manner, a user may program and charge IMD14using one device, or multiple devices.

IMD14may be constructed of any polymer, metal, or composite material sufficient to house the components of IMD14within patient12. In this example, IMD14may be constructed with a biocompatible housing, such as titanium or stainless steel, or a polymeric material such as silicone or polyurethane, and surgically implanted at a site in patient12near the pelvis, abdomen, or buttocks. The housing of IMD14may be configured to provide a hermetic seal for components, such as a rechargeable power source. In addition, the housing of IMD14may be selected of a material that facilitates receiving energy to charge a rechargeable power source.

As described herein, secondary coil16may be included within IMD14. However, in other examples, secondary coil16could be located external to a housing of IMD14, separately protected from fluids of patient12, and electrically coupled to electrical components of IMD14. This type of configuration of IMD14and secondary coil16may provide implant location flexibility when anatomical space available for implantable devices is minimal and/or improved inductive coupling between secondary coil16and primary coil26. In any case, an electrical current may be induced within secondary coil16to charge the battery of IMD14when energy transfer coil26(e.g., a primary coil) produces a magnetic field that is aligned with secondary coil16. The induced electrical current may first be conditioned and converted by a charging module (e.g., a charging circuit) to an electrical signal that can be applied to the battery with an appropriate charging current. For example, the inductive current may be an alternating current that is rectified to produce a direct current suitable for charging the battery. In some examples, primary coil26may comprise multiple separate coils that are displaced in location from each other.

The rechargeable power source of IMD14may include one or more capacitors, batteries, or other components (e.g., chemical, or electrical energy storage devices). Example batteries may include lithium-based batteries, nickel metal-hydride batteries, or other materials. The rechargeable power source may be replenished, refilled, or otherwise capable of increasing the amount of energy stored after energy has been depleted. The energy received from secondary coil16may be conditioned and/or transformed by a charging circuit. The charging circuit may then send an electrical signal used to charge the rechargeable power source when the power source is fully depleted or only partially depleted.

Charging device22may be used to recharge the rechargeable power source within IMD14implanted in patient12. Charging device22may be a hand-held device, a portable device, or a stationary charging system. In any case, charging device22may include components necessary to charge IMD14through tissue of patient12. Charging device22may include housing24and energy transfer coil26, also referred to as primary coil26. In addition, heat sink device28may be removably attached to energy transfer coil26to manage the temperature of then energy transfer coil during charging sessions. Housing24may enclose operational components such as a processor, memory, user interface, telemetry module, power source, and charging circuit configured to transmit energy to secondary coil16via energy transfer coil26. Although a user may control the recharging process with a user interface of charging device22, charging device22may alternatively be controlled by another device (e.g., an external programmer such as external computing device25). In other examples, charging device22may be integrated with an external programmer, such as a patient programmer carried by patient12.

Charging device22and IMD14may utilize any wireless power transfer techniques that are capable of recharging the power source of IMD14when IMD14is implanted within patient14. In one example, system10may utilize inductive coupling between primary coils (e.g., energy transfer coil26) and secondary coils (e.g., secondary coil16) of charging device22and IMD14. In inductive coupling, energy transfer coil26is placed near implanted IMD14such that energy transfer coil26is aligned with secondary coil16of IMD14. Charging device22may then generate an electrical current in energy transfer coil26based on a selected power level for charging the rechargeable power source of IMD14. When the primary and secondary coils are aligned, the electrical current in the primary coil may magnetically induce an electrical current in the secondary coil within IMD14. Since the secondary coil is associated with and electrically coupled to the rechargeable power source, the induced electrical current may be used to increase the voltage, or charge level, of the rechargeable power source. Although inductive coupling is generally described herein, any type of wireless energy transfer may be used to transfer energy between charging device22and IMD14.

Energy transfer coil26may include a wound wire (e.g., a coil) (not shown inFIG.1). The coil may be constructed of a wire wound in an in-plane spiral (e.g., a disk-shaped coil). In some examples, this single or even multi-layers spiral of wire may be considered a flexible coil capable of deforming to conform with a non-planar skin surface. The coil may include wires that electrically couple the flexible coil to a power source and a charging module configured to generate an electrical current within the coil. Energy transfer coil26may also include a housing that encases the coil. The housing may be constructed of a flexible material such that the housing promotes, or does not inhibit, flexibility of the coil. Energy transfer coil26may be external of housing24such that energy transfer coil26can be placed on the skin of patient12proximal to IMD14. In this manner, energy transfer coil26may be tethered to housing24using cable27or other connector that may be between approximately a few inches and several feet in length. In other examples, energy transfer coil26may be disposed on the outside of housing24or even within housing24. Energy transfer coil26may thus not be tethered to housing22in other examples.

Heat sink device28may be removably attached to energy transfer coil26. In examples where energy transfer coil26is disposed on or within housing24, heat sink device28may be configured to be removably attached to housing24.

Together, system10may include energy transfer coil26and heat sink device28. Energy transfer coil26may be configured to recharge a rechargeable power source of IMD14. In the example of system10, charging device22is the power transmitting unit and IMD14is the power receiving unit. IMD14may be in a flipped or non-flipped position.

Heat sink device28may include a housing that contains a phase change material. The housing may be configured to be removably attached to energy transfer coil26. In this manner, the system may operate such that energy transfer coil26generates heat during a recharge session and the phase change material of heat sink device28absorbs at least a portion of the generated heat. When the phase change material is at the melting temperature, the heat may contribute to the heat of fusion of the phase change material and not to increasing the temperature of energy transfer coil26.

A flexible coil of energy transfer coil26may be formed by one or more coils of wire. In one example the coil is formed by a wire wound into a spiral within a single plane (e.g., an in-plane spiral). This in-plane spiral may be constructed with a thickness equal to the thickness of the wire, and the in-plane spiral may be capable of transferring energy with another coil. In other examples, the coil may be formed by winding a coil into a spiral bent into a circle. However, this type of coil may not be as thin as the in-plane spiral.

A variety of system metrics are available to external charging device22from its own computations of power and heat and from metrics communicated to the recharger from IMD14. In this disclosure charging device22may also be referred to as recharger22. These metrics may include but are not limited to: battery current (Iimd_batt)— for fixed power levels or speeds, power transfer efficiency (Pimd_batt/Ptank), IMD Efficiency (Pimd_batt/QIMD) or (Pimd_batt/Pimd). Analysis of system characterization data that the IMD efficiency, which may be measured by IMD14and communicated to external charging device22, may be a good indicator of when the recharger primary coil26is concentric with secondary coil16. A concentric relative position of primary coil26and secondary coil16may be in positions with the lowest overall transient thermal response (increase in temp for the same heat). In some examples, the energy transfer in concentric positions (e.g., near 0, −20 in X and Y) may be higher and the battery of IMD14may charge faster. Therefore, there may be an exponential relationship between the IMD efficiency, which may also be referred to as INS efficiency in this disclosure, and the overall thermal dose in units of CEM43.

The power transfer efficiency on the other hand may be more skewed towards the geometrical center of the IMD (near 0, 0 in X and Y). In some examples both power transfer efficiency and IMD efficiency metrics may be lower when primary coil26is positioned over the header of IMD14, which may lead to decreased efficiency and a less desirable thermal profile (e.g., an increase in temperature for the same heat) caused by the thermal response in the device and/or adjacent tissue. Furthermore, at such positions the time to charge may be longer so the overall thermal dose may be worse than for shorter and quicker charging periods that result from a more efficient coupling.

The system of this disclosure may use a variety of techniques to adapt an algorithm and heat doses to reduce the thermal dose for less desirable, e.g., less efficient, relative positions between primary coil26and secondary coil16. System10may include techniques to increase the charging speed or charging rate for more desirable relative positions of energy transfer coil26and secondary coil16, and thereby reduce charging time and thermal dose.

Processing circuitry of charging device22, or other processing circuitry in system10, may calculate the heat at IMD14. Charging device22may calculate PTANK, which is the power sent to primary coil26and may include the inductance and capacitance between power generation circuitry and primary coil26. Charging device22may measure the heat lost by primary coil26(QPRIM), for example by receiving signals from a temperature sensor within primary coil26. The heat lost by primary coil26(QPRIM) may also be described as the amount of energy sent to primary coil26is not wirelessly transmitted to IMD14, but is instead lost to heating primary coil26and associated structure around charging coil26. Charging device22may receive wireless communication from IMD14that includes an indication of the amount of power delivered to the electrical energy storage device of IMD14(Pimd_batt). Therefore, charging device22may calculate the heat at IMD14based on:
QIMD=PTANK−QPRIM−Pimd_batt
Therefore QIMD, as defined above may include heat generated by IMD14, as well as generated in tissue surrounding IMD14. QIMDmay provide a conservative estimate, e.g., leave a safety margin, because QIMDassumes any energy output by the primary coil and not converted to power delivered to the device (Pimd_batt), or lost to heat at the primary coil (QPRIM) becomes heat at IMD14.

In some examples, processing circuitry of system10may multiply the heat limits for IMD14in different power transfer modes (or in the highest speed mode) by an adjustment factor (AF). The AF may be configured as a function of the IMD efficiency and the range of possible values on AF may be set as from 0 to 1. For example, for the dynamically set heat limit for IMD14:
Heat Limit=AF*maximum Heat limit, where
IMDefficiency=Pimd_batt/QIMD, and
Total efficiency=Pimd_batt/Ptank

In this disclosure, AF may be a function of IMD efficiency. In some examples, AF may be linear or non-linear. A linear example may include AF=m*IMD efficiency+b. In other examples, AF may be bounded so that a predetermined zone of measured IMD efficiency, as transmitted to charging system22, is set to a specific value, such as:
minimum(m*IMDEfficiency+b,maximumIMDheat limit)
OR
maximum[minimum(m*IMDEfficiency+b,maximumIMDheat limit),minimum(IMDheat limit)].

In other examples, instead of a piecewise linear function, the adjustment factor may be set by another form of a piecewise function, such as an S-curve using a sigmoid function, logistic function, or similar function. For example, with a logistic function: AF=maximum IMD heat/(1+e{circumflex over ( )}(k*(IMD Efficiency−IMD Efficiency Center Point))).

In this way, the adjustment factor may include a smooth transition of recharge heat levels allowed from the good positions to the less desirable positions. Additional options for adjustment factor may include more binary and conditional logic, for example:

if IMD Efficiency<X,then use heat control limit A,otherwise use heat control limit B.

In other examples, if efficiency persists to be less than a certain value, e.g., for longer than a predetermined duration, then exit the high power phase and transmit energy at lower power settings. In other examples, system10may implement the adjustment factor as a look up table between IMD efficiency and AF. In some examples, it may be desirable to have the adjustment factor adjusted at a slower rate, e.g., less often, than the algorithm controlling the QIMDlimit to avoid oscillations in the control algorithm.

In other words, system10may measure efficiency, such as IMD efficiency, to determine whether the relative position of primary coil26and secondary coil16may be in a less desirable relative position. Processing circuitry of system10, e.g., processing circuitry of charging system22, processing circuitry of external computing device25, and/or processing circuitry of IMD14, may calculate a new limit, such as using one of the techniques described above based on the measured efficiency. System10may then adjust power transmitted based on the newly calculated limit.

In other examples, system10may also detect hot spots based on multiple thermistors. In some examples, IMD14may include temperature sensors at a plurality of locations within the implant, for example within the header or near the header. One of the temperature sensors may be located near the center of the device or on the can in the center of the recharge coil. When another sensor is significantly hotter than the central sensor, e.g., a temperature differential that satisfies a temperature threshold, the algorithm may scale the energy transfer and charge slower or faster as needed. In some examples, the primary coil may also include one or more temperature sensors.

In some examples, different portions of IMD14may increase in temperature more than other portions. The differences may be caused by the type of material, the arrangement of components in relation to secondary coil16, or for other reasons. In some examples, the temperature of portions of IMD14may change based on the relative position of primary coil26to the IMD14. In some examples, the header of IMD14may be sensitive to the relative position of primary coil26, which in some examples may be caused by eddy currents.

In other examples, IMD14may include multiple secondary coils. Each of the coils may be located in a different position on IMD14, for example in a first side and on a second side opposite the first side. In some examples, IMD14may determine that a first coil is receiving more electromagnetic energy than other coils of the multiple secondary coils and therefore determine an approximate relative location of the primary coil.

Processing circuitry of system10may determine if primary coil26is over the header of IMD14by using system metrics for, e.g., IMD charging efficiency (Pimd_batt/QIMD). In some examples, processing circuitry of system10may decide how to behave differently when in one of these relative positions. The processing circuitry may execute other techniques to determine charge rate, such as a proportional-integral-derivative controller (PID controller or three-term controller) as a control loop mechanism. In some examples processing circuitry may employ a digital low-pass filter (LPF), such as a 2-pole, or similar LPF.

The recharger, e.g., charging device22, may compute IMD_efficiency during closed loop recharge according to the equation: IMD_efficiency=Pimd_batt/QIMD. If the Pimd_batt is zero, the efficiency may be returned as zero. The processing circuitry of the recharger may adjust the current QIMDlimit by an adjustment factor according to a look up table, or some other technique for determining the adjustment factor described above. The adjustment factor may be bounded between 0 and 1 and be updated periodically. An adaptive algorithm, also called an adaptive recharge algorithm, executed by processing circuitry of system10may use a variety of system metrics. Examples of system metric options may include various energy transfer efficiency measurements, including:

IMB Battery Current (mA) in a given phase or power level

IMD Current inefficiency QIMD/Iimd_batt(V) or efficiency Iimd_batt/QIMD(1/V)

FIG.2is a block diagram illustrating example components of IMD14ofFIG.1. In the example illustrated inFIG.2, IMD14includes temperature sensor39, coil40, processing circuitry30, therapy module34, recharge module38, memory32, telemetry module36, and rechargeable power source40. In other examples, IMD14may include a greater or a fewer number of components. In general, IMD14may comprise any suitable arrangement of hardware, alone or in combination with software and/or firmware, to perform the various techniques described herein attributed to IMD14and processing circuitry30, and any equivalents thereof.

Processing circuitry30of IMD14may include one or more processors, such as one or more microprocessors, digital signal processors (DSPs), application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), or any other equivalent integrated or discrete logic circuitry, as well as any combinations of such components. IMD14may include a memory32, such as random access memory (RAM), read only memory (ROM), programmable read only memory (PROM), erasable programmable read only memory (EPROM), electronically erasable programmable read only memory (EEPROM), flash memory, comprising executable instructions for causing the processing circuitry30to perform the actions attributed to this circuitry. Moreover, although processing circuitry30, therapy module34, recharge module38, telemetry module36, and temperature sensor39are described as separate modules, in some examples, some combination of processing circuitry30, therapy module34, recharge module38, telemetry module36and temperature sensor39are functionally integrated. In some examples, processing circuitry30, therapy module34, recharge module38, telemetry module36, and temperature sensor39correspond to individual hardware units, such as ASICs, DSPs, FPGAs, or other hardware units. As described above in relation toFIG.1, temperature sensor39may comprise one or more temperature sensors in different locations on IMD14.

For example, memory32may store one or more formulas, as further described below, that may be used to determine the temperature of the housing19and/or exterior surface(s) of housing19based on temperature(s) sensed by the temperature sensor39. Memory32may store values for one or more determined constants used by these formulas. Memory32may store instructions that, when executed by processing circuitry such as processing circuitry30, perform an algorithm, including using the formulas, to determine a current temperature, or temperatures over time, for the housing19and/or exterior surface(s) of the housing19of IMD14during a charging session and/or for some time after a charging session performed on IMD14. In some examples, memory32may store instructions that, when executed by processing circuitry such as processing circuitry30, perform an algorithm, including using one or more formulas, to determine a value to be assigned to one or more of the constants used in the algorithm to determine a temperature for the housing19and/or exterior surface(s) of the housing19of IMD14during a charging session and/or for some time after a charging session performed on IMD14.

Generally, therapy module34may generate and deliver electrical stimulation under the control of processing circuitry30. In some examples, processing circuitry30controls therapy module34by accessing memory32to selectively access and load at least one of the stimulation programs to therapy module34. For example, in operation, processing circuitry30may access memory32to load one of the stimulation programs to therapy module34. In such examples, relevant stimulation parameters may include a voltage amplitude, a current amplitude, a pulse rate, a pulse width, a duty cycle, or the combination of electrodes17A,17B,17C, and17D (collectively “electrodes17”) that therapy module34uses to deliver the electrical stimulation signal. Therapy module34may be configured to generate and deliver electrical stimulation therapy via one or more of electrodes17A,17B,17C, and17D of lead18. Alternatively, or additionally, therapy module34may be configured to provide different therapy to patient12. For example, therapy module34may be configured to deliver drug delivery therapy via a catheter. These and other therapies may be provided by IMD14.

IMD14also includes components to receive power from external charging device22to recharge rechargeable power source40when rechargeable power source40has been at least partially depleted. As shown inFIG.2, IMD14includes secondary coil40and recharge module38coupled to rechargeable power source40. Recharge module38may be configured to charge rechargeable power source40with the selected power level determined by either processing circuitry30or external charging device22. Recharge module38may include any of a variety of charging and/or control circuitry configured to process or convert current induced in coil40into charging current to charge power source40. Although processing circuitry30may provide some commands to recharge module38, in some examples, processing circuitry30may not need to control any aspect of recharging.

Secondary coil40may include a coil of wire or other device capable of inductive coupling with a primary coil disposed external to patient12. Although secondary coil40is illustrated as a simple loop of inFIG.2, secondary coil40may include multiple turns of conductive wire. Secondary coil40may include a winding of wire configured such that an electrical current can be induced within secondary coil40from a magnetic field. The induced electrical current may then be used to recharge rechargeable power source40. In this manner, the electrical current may be induced in secondary coil40associated with rechargeable power source40. The induction may be caused by electrical current generated in the primary coil of external charging device22, where the level of the current may be based on the selected power level. The coupling between secondary coil40and the primary coil of external charging device22may be dependent upon the alignment of the two coils. Generally, the coupling efficiency may increase when the two coils share a common axis and are in close proximity to each other. External charging device22and/or IMD14may provide one or more audible tones or visual indications of the alignment.

Although inductive coupling is generally described as the method for recharging rechargeable power source40, other wireless energy transfer techniques may alternatively be used (such as RF energy transfer). Any of these techniques may generate heat in IMD14such that the charging process may need to be controlled by matching the determined temperature to one or more thresholds, modeling tissue temperatures based on the determined temperature, or using a calculated cumulative thermal dose as feedback.

Recharge module38may include one or more circuits that process, filter, convert and/or transform the electrical signal induced in the secondary coil to an electrical signal capable of recharging rechargeable power source40. For example, in alternating current induction, recharge module38may include a half-wave rectifier circuit and/or a full-wave rectifier circuit configured to convert alternating current from the induction to a direct current for rechargeable power source40. The full-wave rectifier circuit may be more efficient at converting the induced energy for rechargeable power source40. However, a half-wave rectifier circuit may be used to store energy in rechargeable power source40at a slower rate. In some examples, recharge module38may include both a full-wave rectifier circuit and a half-wave rectifier circuit such that recharge module38may switch between each circuit to control the charging rate of rechargeable power source40and temperature of IMD14.

Rechargeable power source40may include one or more capacitors, batteries, and/or other energy storage devices. Rechargeable power source40may deliver operating power to the components of IMD14. In some examples, rechargeable power source40may include a power generation circuit to produce the operating power. Rechargeable power source40may be configured to operate through many discharge and recharge cycles. Rechargeable power source40may also be configured to provide operational power to IMD14during the recharge process. In some examples, rechargeable power source40may be constructed with materials to reduce the amount of heat generated during charging. In other examples, IMD14may be constructed of materials and/or using structures that may help dissipate generated heat at rechargeable power source40, recharge module38, and/or secondary coil40over a larger surface area of the housing of IMD14.

Although rechargeable power source40, recharge module38, and secondary coil40are shown as contained within the housing of IMD14, in alternative implementations, at least one of these components may be disposed outside of the housing. For example, in some implementations, secondary coil40may be disposed outside of the housing of IMD14to facilitate better coupling between secondary coil40and the primary coil of external charging device22. These different configurations of IMD14components may allow IMD14to be implanted in different anatomical spaces or facilitate better inductive coupling alignment between the primary and secondary coils.

IMD14may also include temperature sensor39. Temperature sensor39may include one or more temperature sensors configured to measure the temperature of respective portions of IMD14. As described herein, these temperature sensor(s) may not be thermally coupled to, and may not be directly attached to, the portion of the device for which a temperature is to be determined based on the sensed temperature measured by temperature sensor39. In one instance, the temperature sensor is not directly attached to the housing19or to the exterior surface(s) of housing19of the device. In other words, temperature measurement is not performed through direct contact or physical contact between the temperature sensor and the target portion to be measured. Although the temperature sensor may be physically attached to the target portion or target surface through one or more structures, thermal conduction that may occur between the target portion and the sensor is not directly used to measure the temperature of the target portion.

Temperature sensor39may be arranged to measure the temperature of a component, surface, or structure, e.g., secondary coil40, power source40, recharge module38, and other circuitry housed within IMD14. Temperature sensor39may be disposed internal of the housing of IMD14or otherwise disposed relative to the external portion of housing (e.g., tethered to an external surface of housing via an appendage cord, light pipe, heat pipe, or some other structure). As described herein, temperature sensor39may be used to make temperature measurements of internal portions of the IMD14, the temperature measurements used as a basis for determining the temperature of the housing and/or external surface of IMD14. For example, processing circuitry30or processing circuitry of external charging device22may use these temperature measurements to determine the housing/external surface temperatures of IMD14.

In other examples, temperature measurements may be used to determine temperatures of a specific portion of housing19or a component coupled thereto, such as header block15, or another module that is coupled to IMD14. For instance, IMD14may comprise an additional housing that is separate from, but affixed to, housing19that contains some components of IMD14. As one specific example, a secondary coil such as secondary coil40may reside within an additional housing that is external to, but affixed to, main housing19.

Temperature measurements may be used to determine a temperature of a surface or portion of this additional housing or a structure within this housing such as the secondary coil itself. As another example, IMD14may carry an appendage protruding from housing19carrying one or more electrodes that serves as a stub lead for delivering electrical stimulation therapy. Temperature sensor39may be used to make temperature measurements that may be used as a basis for determining the temperature of a portion of this structure. The determined temperatures are then further used as feedback to control the power levels or charge times (e.g., cycle times) used during the charging session of rechargeable power source40. In some examples, temperature sensor39may be used to obtain temperature measurements of a header block15, or another module that is coupled to IMD14. For instance, IMD14may comprise an additional housing that is separate from, but affixed to, housing19that contains some components of IMD14. As one specific example, a secondary coil may reside within an additional housing. As another example, IMD14may carry an appendage protruding from housing19carrying one or more electrodes that serves as a stub lead for delivering electrical stimulation therapy. Temperature sensor39may be used to make temperature measurements that may be used as a basis for determining the temperature of a surface, or another portion, of these and other structures.

Although a single temperature sensor may be adequate, multiple temperature sensors may provide more specific temperature readings of separate components or of different portions of the IMD. Although processing circuitry30may continuously measure temperature using temperature sensor39, processing circuitry30may conserve energy by only measuring temperatures during recharge sessions. Further, temperatures may be sampled at a rate necessary to effectively control the charging session, but the sampling rate may be reduced to conserve power as appropriate. Processing circuitry30may be configured to access memory, such as memory32, to retrieve information comprising instructions, formulas, determined values, and/or one or more constants, and to use this information to execute an algorithm to determine a current temperature, and/or a series of temperatures over time, for the housing19and/or exterior surface(s) of housing19of IMD14based on the measured temperature(s) provided by temperature sensor39.

Processing circuitry30may also control the exchange of information with external charging device22and/or an external programmer using telemetry module36. Telemetry module36may be configured for wireless communication using radio frequency protocols, such as BLUETOOTH, or similar RF protocols, as well as using inductive communication protocols. Telemetry module36may include one or more antennas37configured to communicate with external charging device22, for example. Processing circuitry30may transmit operational information and receive therapy programs or therapy parameter adjustments via telemetry module36. Also, in some examples, IMD14may communicate with other implanted devices, such as stimulators, control devices, or sensors, via telemetry module36. In addition, telemetry module36may be configured to control the exchange of information related to sensed and/or determined temperature data, for example temperatures sensed by and/or determined from temperatures sensed using temperature sensor39. In some examples, telemetry module36may communicate using inductive communication, and in other examples, telemetry module36may communicate using RF frequencies separate from the frequencies used for inductive charging.

In some examples, processing circuitry30may transmit additional information to external charging device22related to the operation of rechargeable power source40. For example, processing circuitry30may use telemetry module36to transmit indications that rechargeable power source40is completely charged, rechargeable power source40is fully discharged, or any other charge status of rechargeable power source40. In some examples, processing circuitry30may use telemetry module36to transmit instructions to external charging device22, including instructions regarding further control of the charging session, for example instructions to lower the power level or to terminate the charging session, based on the determined temperature of the housing/external surface19of the IMD.

Processing circuitry30may also transmit information to external charging device22that indicates any problems or errors with rechargeable power source40that may prevent rechargeable power source40from providing operational power to the components of IMD14. In various examples, processing circuitry30may receive, through telemetry module36, instructions for algorithms, including formulas and/or values for constants to be used in the formulas, that may be used to determine the temperature of the housing19and/or exterior surface(s) of housing19of IMD14based on temperatures sensed by temperature sensor39located within IMD14during and after a recharging session performed on rechargeable power source40.

FIG.3is a block diagram of an example external charging device22ofFIG.1and may also be referred to as recharger22. While external charging device22may generally be described as a hand-held device, external charging device22may be a larger portable device or a more stationary device. In addition, in other examples external charging device22may be included as part of an external programmer or include functionality of an external programmer. External charging device22may also be configured to communicate with an external programmer. As shown inFIG.3, external charging device22includes two separate components. Housing24encloses components such as a processing circuitry50, memory52, user interface54, telemetry module56, and power source60. Charging head26may include charging module58, temperature sensor59, and coil48. As shown inFIG.2, housing24is electrically coupled to charging head26via charging cable62. Charging head26is an example of energy coil26, e.g., primary coil26, described above in relation toFIG.1.

A separate charging head26may facilitate optimal positioning of coil48over coil40of IMD14. However, charging module58and/or coil48may be integrated within housing24in other examples. Memory52may store instructions that, when executed by processing circuitry50, causes processing circuitry50and external charging device22to provide the functionality ascribed to external charging device22throughout this disclosure, and/or any equivalents thereof.

External charging device22may also include one or more temperature sensors, illustrated as temperature sensor59, similar to temperature sensor39ofFIG.2. As shown inFIG.3, temperature sensor59may be disposed within charging head26. In other examples, one or more temperature sensors of temperature sensor59may be disposed within housing24. For example, charging head26may include one or more temperature sensors positioned and configured to sense the temperature of coil48and/or a surface of the housing of charging head26. In some examples, external charging device22may not include temperature sensor59.

In general, external charging device22comprises any suitable arrangement of hardware, alone or in combination with software and/or firmware, to perform the techniques ascribed to external charging device22, and processing circuitry50, user interface54, telemetry module56, and charging module58of external charging device22, and/or any equivalents thereof. In various examples, external charging device22may include one or more processors, such as one or more microprocessors, DSPs, ASICs, FPGAs, or any other equivalent integrated or discrete logic circuitry, as well as any combinations of such components. External charging device22also, in various examples, may include a memory52, such as RAM, ROM, PROM, EPROM, EEPROM, flash memory, a hard disk, a CD-ROM, comprising executable instructions for causing the one or more processors to perform the actions attributed to them. Moreover, although processing circuitry50, telemetry module56, charging module58, and temperature sensor59are described as separate modules, in some examples, processing circuitry50, telemetry module56, charging module58, and/or temperature sensor59are functionally integrated. In some examples, processing circuitry50, telemetry module56, charging module58, and/or temperature sensor59correspond to individual hardware units, such as ASICs, DSPs, FPGAs, or other hardware units.

Memory52may store instructions that, when executed by processing circuitry50, cause processing circuitry50and external charging device22to provide the functionality ascribed to external charging device22throughout this disclosure, and/or any equivalents thereof. For example, memory52may include instructions that cause processing circuitry50to control the power level used to charge IMD14in response to the determined temperatures for the housing/external surface(s) of IMD14, as communicated from IMD14, or instructions for any other functionality. In addition, memory52may include a record of selected power levels, sensed temperatures, determined temperatures, or any other data related to charging rechargeable power source40. Processing circuitry50may, when requested, transmit any of this stored data in memory52to another computing device for review or further processing. Processing circuitry50may be configured to access memory, such as memory32of IMD14and/or memory52of external charging device22, to retrieve information comprising instructions, formulas, and determined values for one or more constants, and to use this information to perform an algorithm to determine a current temperature, and/or a series of temperatures over time, for the housing19and/or exterior surface(s) of housing19of IMD14based on the measured temperature(s) provided by temperature sensors39of IMD14.

Memory52may be configured to store instructions for communication with and/or control of one or more temperature sensors39of IMD14. In various examples, memory52stores information related to determining the temperature of the housing19and/or exterior surface(s) of housing19of IMD14based on temperatures sensed by one or more temperature sensors, such as temperature sensors39, located within IMD14. For example, memory52may store one or more formulas, as further described below, that may be used to determine the temperature of the housing19and/or exterior surface(s) of housing19based on temperature(s) sensed by the temperature sensors39. Memory52may store values for one or more determined constants used by these formulas. Memory52may store instructions that, when executed by processing circuitry such as processing circuitry50, performs an algorithm, including using the formulas, to determine a current temperature, or a series of temperatures over time, for the housing19and/or exterior surface(s) of housing19of IMD14during a charging session and/or for some time after a charging session performed on IMD14. In some examples, memory52may store instructions that, when executed by processing circuitry such as processing circuitry50, perform an algorithm, including using one or more formulas, to determine a value to be assigned to one or more of the constants used in the algorithm used to determine the temperature(s) associated with the housing19and/or exterior surface(s) of housing19of IMD14during a charging session and/or for some time after a charging session performed on IMD14.

User interface54may include a button or keypad, lights, a speaker for voice commands, a display, such as a liquid crystal (LCD), light-emitting diode (LED), or cathode ray tube (CRT). In some examples, the display may be a touch screen. As discussed in this disclosure, processing circuitry50may present and receive information relating to the charging of rechargeable power source40via user interface54. For example, user interface54may indicate when charging is occurring, quality of the alignment between coils40and48, the selected power level, current charge level of rechargeable power source40, duration of the current recharge session, anticipated remaining time of the charging session, sensed temperatures, or any other information. Processing circuitry50may receive some of the information displayed on user interface54from IMD14in some examples. In some examples, user interface54may provide an indication to the user regarding the quality of alignment between coils40, depicted inFIG.2and coil48, based on the charge current to the battery.

User interface54may also receive user input via user interface54. The input may be, for example, in the form of pressing a button on a keypad or selecting an icon from a touch screen. The input may request starting or stopping a recharge session, a desired level of charging, or one or more statistics related to charging rechargeable power source40(e.g., the cumulative thermal dose). User input may also include inputs related to temperature thresholds for the IMD that may be used to regulate for example a maximum housing/surface temperature the patient is willing to experience during a charging session of the IMD. The inputs related to threshold values may be store in memory52, and/or transmitted through telemetry module56to IMD14for storage in a memory, such as memory32, located within IMD14. In this manner, user interface54may allow the user to view information related to the charging of rechargeable power source40and/or receive charging commands, and to provide inputs related to the charging process. In various examples, user interface54as shown and described with respect toFIG.1is arranged to perform and to provide the features and/or functions ascribed to user interface54as illustrated and described with respect toFIG.3.

External charging device22also includes components to transmit power to recharge rechargeable power source40associated with IMD14. As shown inFIG.3, external charging device22includes primary coil48and charging module58coupled to power source60. Charging module58may be configured to generate an electrical current in primary coil48from electrical energy stored in or provided by power source60. Although primary coil48is illustrated as a simple loop inFIG.3, primary coil48may include multiple turns of wire. Charging module58may generate the electrical current according to a power level selected by processing circuitry50based on the sensed and/or determined temperature or temperatures received from IMD14and/or a temperature sensor within external charging device22. As described herein, processing circuitry50may select a “high” power level, a “low” power level, or a variety of different power levels to control the rate of recharge in rechargeable power source40and the temperature of IMD14. In some examples, processing circuitry50may control charging module58based on a power level selected by processing circuitry30of IMD14. The determined temperature of the housing19and/or exterior surface(s) of housing19of IMD14used as feedback for control of the recharge power level may be derived from a temperature sensed by a temperature sensor within IMD14. Although processing circuitry50may control the power level used for charging rechargeable power source40, charging module58may include processing circuitry including one or more processors configured to partially or fully control the power level based on the determined temperatures.

Primary coil48may include a coil of wire, e.g., having multiple turns, or other devices capable of inductive coupling with a secondary coil40disposed within patient12. Primary coil48may include a winding of wire configured such that an electrical current generated within primary coil48can produce a magnetic field configured to induce an electrical current within secondary coil40depicted inFIG.2. The induced electrical current may then be used to recharge rechargeable power source40. In this manner, the electrical current may be induced in secondary coil40associated with rechargeable power source40. The coupling efficiency between secondary coil40and primary coil48of external charging device22may be dependent upon the alignment of the two coils. Generally, the coupling efficiency increases when the two coils share a common axis and are in close proximity to each other. User interface54of external charging device22may provide one or more audible tones or visual indications of the alignment.

Charging module58may include one or more circuits that generate an electrical signal, and an electrical current, within primary coil48. Charging module58may generate an alternating current of specified amplitude and frequency in some examples. In other examples, charging module58may generate a direct current. In any case, charging module58may be capable of generating electrical signals, and subsequent magnetic fields, to transmit various levels of power to IMD14. In this manner, charging module58may be configured to charge rechargeable power source40of IMD14with the selected power level. Processing circuitry50may calculate PTANK, which is the power sent to primary coil48and may include the inductance and capacitance between the power generation circuitry of charging module58and primary coil48.

The power level that charging module58selects for charging may be used to vary one or more parameters of the electrical signal generated for coil48. For example, the selected power level may specify wattage, electrical current of primary coil48or secondary coil40, current amplitude, voltage amplitude, pulse rate, pulse width, a cycling rate, or a duty cycle that determines when the primary coil is driven, or any other parameter that may be used to modulate the power transmitted from coil48. In this manner, each power level may include a specific parameter set that specifies the signal for each power level. Changing from one power level to another power level (e.g., a “high” power level to a lower power level) may include adjusting one or more parameters. For instance, at a “high” power level, the primary coil may be substantially continuously driven, whereas at a lower power level, the primary coil may be intermittently driven such that periodically the coil is not driven for a predetermined time to control heat generation. The parameters of each power level may be selected based on hardware characteristics of external charging device22and/or IMD14.

Power source60may deliver operating power to the components of external charging device22. Power source60may also deliver the operating power to charging module58to drive primary coil48during the charging process. Power source60may include a battery and a power generation circuit to produce the operating power. In some examples, a battery of power source60may be rechargeable to allow extended portable operation. In other examples, power source60may draw power from a wired voltage source such as a consumer or commercial power outlet.

External charging device22may include one or more temperature sensors shown as temperature sensor59(e.g., similar to temperature sensor39of IMD14) for sensing the temperature of a portion of the device. For example, temperature sensor59may be disposed within charging head26and oriented to sense the temperature of the housing of charging head26. In another example, temperature sensor59may be disposed within charging head26and oriented to sense the temperature of charging module58and/or coil48. In other examples, external charging device22may include multiple temperature sensors59each oriented to any of these portions of device to manage the temperature of the device during charging sessions.

Telemetry module56supports wireless communication between IMD14and external charging device22under the control of processing circuitry50. Telemetry module56may also be configured to communicate with another computing device via wireless communication techniques, or direct communication through a wired connection, such as external computing device25depicted inFIG.1. In some examples, telemetry module56may be substantially similar to telemetry module36of IMD14described herein, providing wireless communication via an RF or proximal inductive medium. In some examples, telemetry module56may include an antenna57, which may take on a variety of forms, such as an internal or external antenna. Although telemetry modules56and36may each include dedicated antennas for communications between these devices, telemetry modules56and36may instead, or additionally, be configured to utilize inductive coupling from coils40and48to transfer data.

Examples of local wireless communication techniques that may be employed to facilitate communication between external charging device22and IMD14include radio frequency and/or inductive communication according to any of a variety of standard or proprietary telemetry protocols, or according to other telemetry protocols such as the IEEE 802.11x or Bluetooth specification sets. In this manner, other external devices may be capable of communicating with external charging device22without needing to establish a secure wireless connection. As described herein, telemetry module56may be configured to receive a signal or data representative of a sensed temperature from IMD14or a determined temperature of the housing19and/or exterior surface(s) of housing19of the IMD based on the sensed temperature. The determined temperature may be determined using an algorithm, including use of formula(s) as further described below, based on measuring the temperature of the internal portion(s) of the IMD, such as circuitry mounted to a circuit board located within IMD14. In some examples, multiple temperature readings by IMD14may be averaged or otherwise used to produce a single temperature value that is transmitted to external charging device22. The sensed and/or determined temperature may be sampled and/or transmitted by IMD14(and received by external charging device22) at different rates, e.g., on the order of microseconds, milliseconds, seconds, minutes, or even hours. Processing circuitry50may then use the received temperature information to control charging of rechargeable power source40(e.g., control the charging level used to recharge power source40).

FIGS.4A-4Care conceptual diagrams illustrating changes in IMD efficiency based on relative location of the primary coil and secondary coil.FIGS.4A-4Cdepict contour plots at different separation distances for an example primary coil of a power transmitting unit and secondary coil of a power receiving unit.FIG.4Aillustrates an X-axis and Y-axis of a power receiving device and the changes in IMD efficiency, as the primary coil moves relative to the power receiving device at approximately 10 mm separation. As described above in relation toFIG.1, IMD efficiency may be defined as (Pimd_batt/QIMD) or (Pimd_batt/Pimd). The center area shows a region in which IMD efficiency is greater than 25%, centered on approximately (0, −20) in the example ofFIG.4A, which may be considered a concentric position, as noted above in relation toFIG.1. The power transfer efficiency may decrease as the primary coil moves to other regions. Similarly, though not shown inFIGS.4A-4C, the charge time may increase as the primary coil moves away from the central area. A charge time contour plot may have a similar shape as the efficiency plot shown inFIG.4A, with the lowest charge times, and lowest heat buildup in the device and surrounding tissue, near regions with the highest IMD efficiency. As noted above in relation toFIG.1, the term “implantable medical device efficiency” is term used for convenience in this disclosure. The efficiency plots ofFIGS.4A-4Cmay apply to any power receiving unit, such as a mobile device, mobile phone, remote sensor and so on.

FIG.4Bdepicts a contour plot that illustrates changes in IMD efficiency as the primary coil moves relative to the power receiving device at approximately 20 mm separation. Similarly,FIG.4Cdepicts a contour plot that illustrates changes in IMD efficiency as the primary coil moves relative to the power receiving device at approximately 30 mm separation. The contour plots change somewhat as the separation distance changes.

FIG.4Dis a conceptual diagram illustrating an example efficiency map by relative location of the power transmitting unit and the power receiving unit. Areas with the same color indicate zones with approximately equivalent power transfer efficiency, e.g.,402,404,406and408. In some examples, other measurable factors may more accurately determine the relative location. For example, using an additional factor, such as QPRIM, in an array, e.g., a 3D look up table, along with IMD efficiency may determine an adjustment factor that may more accurately reflect the actual power transfer efficiency. In other examples, a third, fourth or more additional factors may result in a multi-dimensional look-up table, or a multi-dimensional relationship, which the processing circuitry of system10, described above in relation toFIG.1, may use to determine the adjustment factor.

In other examples, the additional factor may divide the equivalent zones into smaller zones. For example, the processing circuitry may limit the circle of values indicated by402and408to just the region indicated by408based on the additional factor, such as when QPRIMis in a first range, use zone408and when QPRIMis outside the first range, use a different zone, within the circle indicated by402.

In some examples, the feedback to the user at the user interface, e.g., user interface54described above in relation toFIG.3, may include coupling feedback. The coupling feedback displayed by the user interface may spatially map to where the system is delivering more charge current. In this manner the system may provide an indication of the relative location of the primary and secondary coils that may give consistent charge current, consistent energy delivery and consistent charge times.

FIGS.5A and5Bare conceptual diagrams illustrating relative positioning of the power transmitting unit and the power receiving unit.FIG.5Aillustrates wireless recharger526at skin surface542positioned to recharge either of IMD A510or IMD B512. Wireless recharger526is an example of charging head26described above in relation toFIG.3. As described above in relation toFIGS.1and3, in some examples, wireless recharger526may include a power source, charging module, processing circuitry and a primary coil, such as charging device22. In other examples, wireless recharger526may include fewer components, e.g., only a primary coil and coil housing, or a primary coil along with a charging module, temperature sensor, heat sink device and other components.

Wireless recharge526may be offset in the Z-direction by implant depth 1540from IMD A510, which may correspond to the separation distances described above in relation toFIGS.4A-4C. Wireless recharger526may be offset from IMD A510in the X-direction and/or Y-direction by mispositioning distance502. As described above in relation toFIGS.1-4D, changes in relative position between wireless recharger526and IMD A510may affect charging efficiency, charge times and heat generated by the power transfer process, caused for example by device heating or tissue heating.

In addition to offset in the X, Y and Z direction, differences in the relative angle, e.g., theta X1/theta Y1548, between wireless recharger526and IMD510is another component of the relative position and may also impact charging efficiency. In some examples, processing circuitry, e.g.,50, may cause user interface54, to output an indication of relative position, as described above in relation toFIGS.1-3. The indication on the user interface may allow a user to adjust the relative position of wireless recharger526to a more efficient location for power transfer.

In some examples, the processing circuitry controlling wireless recharger526may multiply the heat limits for IMD14in different power transfer modes by an adjustment factor. The AF may be configured as a function of the power transfer efficiency. In other words, processing circuitry for wireless recharger526may measure power transfer efficiency, such as IMD efficiency, to determine whether the relative position of wireless recharger526and the secondary coil of IMD A510may be in a less desirable relative position. The processing circuitry may calculate a new limit, such as using one of the techniques described above based on the measured efficiency and may then adjust power transmitted by wireless recharger526based on the newly calculated limit.

In the example ofFIG.5A, IMD B512is located at implant depth2544and separated from IND A510by separation distance546. IMD B512may also be angled, e.g., relative to skin surface542by angle theta X2/theta Y2552. In some examples, separation distance546may be close enough such that IMD B512may receive at least some power transferred from wireless recharger526, while wireless recharger526delivers power to IMD A510. In other examples, a user may reposition wireless recharger526to transfer power to IMD B512.

As described above in relation toFIG.2, IMD A510, and/or IMD B512, may include one or more temperature sensors, such as temperature sensor(s)39depicted inFIG.2. Temperature sensors may be configured to determine the temperature of any of the internal portions of IMD A510, the housing and/or exterior surface(s) of the housing. In some examples, one of the temperature sensors may be near the center of IMD A510, or on the housing at the center of the secondary coil of IMD A510. When another sensor measures a temperature hotter than this central sensor, e.g., the measured temperature exceeds a threshold amount higher than the central sensor, IMD A510may signal wireless recharger526to change the power output, for example, to charge slower.

In other examples, processing circuitry, e.g., of the power transmitting unit, wireless recharger526, or processing circuitry of power receiving unit, IMD A510, may calculate the approximate amount of heat within IMD A510, (QIMD), such as according to the equations as described above in relation toFIG.1:
QIMD=PTANK−QPRIM−Pimd_batt

At the same time a 2nd order dynamic transfer function to derive temperature output from calculated heat input, QIMD, may keep track of how much the temperature of IMD A510may be rising or falling. The 2nd order dynamic transfer function may predict the temperature of IMD A510, for example based on:
Tins(t)=F(IMDEfficiency,QIMD,Tins(t−1))
In other examples, for systems with temperature sensors, the function for surface temperature of the housing of IMD A510may be based on the additional temperature sensor inputs:
Tins(t)=F(IMD_efficiency,Tsense1,Tsense2,QIMD,Tins(t−1))
Then, the processing circuitry, e.g., of the power transmitting unit and/or power receiving unit554may execute a proportional-integrated-derivative (PID) algorithm to control the maximum temperature on the surface contacting portion of the power receiving unit554.

As described above in relation toFIG.1, a variety of system metrics may be available to wireless recharger526, from computations of power and heat by processing circuitry (not shown in inFIG.5A) that control wireless recharger526, as well as and the metrics communicated to wireless recharger526from IMD A510, and/or IMD B512. Some examples of system metrics may include battery current (Iimd_batt) measured by IMD A510, for fixed power levels or speeds, system power transfer efficiency (Pimd_batt/Ptank), IMD Efficiency e.g., (Pimd_batt/QIMD) or (Pimd_batt/Pimd).

As described above in relation toFIGS.1-4D, IMD efficiency may be a good indicator of when the primary coil of wireless recharger526is concentric with the secondary coil of IMD A510. A concentric relative location may be a relative position with a high IMD efficiency and a low overall transient thermal response, defined as an increase in temperature for the same calculated heat input (QIMD). In some examples, as described above in relation toFIGS.4A-4C, positions in which the primary coil is near (0, −20) in X and Y, may charge faster. Therefore, there may exist an exponential relationship between the IMD efficiency and the overall thermal dose in units of CEM43.

In other examples, the system power transfer efficiency (Pimd_batt/Ptank) may be more skewed towards the geometrical center of the IMD A510, e.g., near (0, 0) in X and Y. In some relative positions of the primary coil and secondary coil, both the IMD efficiency and the system power transfer efficiency metrics may be lower compared to other relative positions. For example, for an implantable medical device that includes a header, the metrics may be lower when the primary coil is positioned over the header of the device, which may lead to decreased power transfer efficiency and a less desirable thermal profile (e.g., a less desirable increase in temp for the same calculated heat input). Furthermore, as described above in relation toFIG.1, at less efficient relative positions the time to charge may be longer so the overall thermal dose may also be less desirable.

Other techniques of determining the temperature on the surface of the IMD A510, may have been based on a first order low pass filter or transfer function from the hybrid. The techniques of this disclosure may provide advantages and improve on the other techniques by extend the utility of the other techniques. In other words, the techniques of this disclosure, that include making a second order dynamic transfer function adapt to the changing relative position of wireless recharger526and the IMD A510. In other words, processing circuitry of the system, which may include any of processing circuitry controlling wireless recharger326, processing circuitry of an external computing device (not shown inFIG.5A), and/or processing circuitry of IMD A510, may detect the relative position of the primary and secondary coils, based on one or more metrics available to the system. The processing circuitry may change the behavior of the system based on the one or more metrics, e.g., energy transfer efficiency based on system efficiency, IMD efficiency and so on. In some examples, the IMD efficiency may provide advantages over other metrics, because IMD efficiency may be insensitive, and therefore useable, across the different power transfer modes, where power transfer modes may correlate to charging rates and each mode may have a separate heat limit. In some examples, the processing circuitry may use an adjustment factor, as described above in relation toFIG.1, which is a monotonically decreasing function for the IMD heat limit that decreases as the system metric becomes less desirable.

The fact that the thermal dynamic transfer function may change as a function of primary and secondary coil relative position may be an additional input that can be added to temperature sensing controlled and/or heat controlled rechargeable systems. Adjusting the power output based on changes in power transfer efficiency may increase the recharge speed and make the system more accurate and therefore safer for patients.

FIGS.5B and5Care conceptual diagrams illustrating heat distribution changes as the relative position between power transmitting coil and power receiving coil changes. Wireless recharger526inFIGS.5B and5Cis an example of wireless recharger526described above in relation toFIG.5Aand has the same functions and characteristics. IMD514inFIGS.5B and5Cis an example of a power receiving unit, such as IMD14described above in relation toFIGS.1and2, and IMD A510and IMD B512described above in relation toFIG.5Aand may have the same or similar functions and characteristics as IMD14, IMD A510and IMD B512.

FIGS.5B and5Cdepict a top-down view, similar to the side view illustrated byFIG.5A. In addition to a relative X, Y and Z position, IMD514may be angled relative to wireless recharger526, as described above inFIG.5Afor angles theta X1/theta Y1548and theta X2/theta Y2552. Wireless recharger526and IMD514may be separated by one or more layers of material. In some examples, wireless recharger526may be located in the seat of an automobile, a recliner, a mattress, or other piece of furniture and separated from the power receiving unit of IMD514by layers of leather, padding, cloth, or other material. In other examples, IMD514may be implanted in a patient, and separated from wireless recharger526by layers of skin, muscle, fatty tissue, blood vessels, bone and so on.

In the example ofFIG.5B, IMD514and wireless recharger526are in a first relative position, e.g., (0, 30 mm, 10 mm). For this power receiving unit, IMD514with this wireless recharger526, the zone indicated by520A may reach a higher temperature than other areas of IMD514. In the example ofFIG.5C, IMD514and wireless recharger526are in a second relative position, e.g., (35 mm, −10 mm, 10 mm). In the second relative position, the higher temperature zone may move to the area indicated by520B. In some examples, some of the charging energy being transferred from wireless recharger526, instead of coupling to the secondary coil of IMD514the energy may cause increased eddy currents in one or more components of IMD514or may cause direct heating of tissue, in the example of an implantable device. In some examples, the duration of the recharge time, which may be impacted by the energy transfer efficiency, may also contribute to heating certain portions of IMD514.

FIG.5Dis a conceptual diagram illustrating an example heat distribution on an example power receiving unit during power transfer. Power receiving unit554is an example of the power receiving units described above in relation toFIGS.1,2,4A-4D and5A-5C, e.g., IMD14, IMD A510, IMD B512and IMD514. In the example, ofFIG.5D, power receiving unit554may be oriented relative to a power transmitting unit (not shown inFIG.5D) such that a region of highest temperature is region550. The region with the lowest temperature is region560. The regions closer to region550may register a higher temperature compared to region560. For example, regions558,556and554may increase in temperature compared to region560, with region553at a higher temperature than regions556and558. Changing the relative position of power receiving unit554compared to the power transmitting unit may change the relative temperatures of the regions.

FIG.5Eis a conceptual diagram illustrating example temperature sensors for an IMD of this disclosure. The example ofFIG.5Edepicts temperature sensors located at various locations on the housing and the header of IMD551, which is an example of IMD14described above in relation toFIGS.1and2. In some examples, the temperatures sensors inFIG.5Emay be temporary and used for testing IMD temperature in the lab. In other examples, IMD551may include more or fewer temperature sensors, and placed in different locations than the locations shown inFIG.5E. As described above in relation toFIGS.1,2, and5A, the adaptive recharge algorithm of this disclosure may include multiple temperature sensor inputs to the algorithm in addition to the IMD efficiency. Processing circuitry of this disclosure may control the recharge power based on measured temperatures and/or temperature gradients across the device as well as the power transfer efficiency, calculations of the amount of heat, and other system metrics.

As described above in relation toFIGS.1and4D, the adaptive recharge algorithm may use a look-up table, in some examples, in the form of a multi-dimensional array. Temperature vector inputs to the adaptive algorithm may add second, third, or higher order dimensions to the look up table for control set points in the adaptive recharge algorithm.

As one example, the temperature difference may increase with increasing X or Y relative displacement to a point, then the temperature difference may eventually return back to no difference, when the relative position of the charging coil and IMD coil is a large enough distance. In contrast, IMD efficiency may start high, when the coils are nearly concentric. The IMD efficiency may decrease with increasing distance. So, the combination of these two inputs, temperature deltas and IMD efficiency, may provide the system a proxy for relative position. The relative position may be defined using radial or cartesian coordinates and may depend on how many temperature sensors located on the IMD. In some examples, the combined information including the temperature gradients may increase the recharge area, e.g., the size of the area in which the relative position of the system including the power transmitting coil and power receiving coil still effectively transfer power. In some examples, including temperature sensors may improve usability by allowing higher heats at positions which an adaptive recharge algorithm that only considered IMD efficiency may not be able distinguish one position from another, as described above in relation toFIG.4D.

In the example ofFIG.5E, sensors570and574are located on the front side and back side, respectively of header580of IMD551. Sensor572is approximately centered on the front side of housing582of IMD551. In the example ofFIG.5E, sensor572marks the origin of the X-direction and Y-direction. Sensors576and578are separated in the X-direction on the back side of housing582. Though shown adjacent to housing582, in the example ofFIG.5E, in other examples one or more temperature sensors may also be located within housing582, and at some distance from the housing, e.g., mounted as an element on a circuit board. To simplify the description, a temperature gradient in the Y direction may be defined, in the example ofFIG.5Eas the difference between a first temperature (T1) measured at sensor570and a second temperature (T2) measured at sensor572. A temperature gradient in the X-direction may be defined, in the example ofFIG.5Eas a difference between a third temperature (T3) measured at sensor578and a fourth temperature (T4) measured at sensor576. In other examples, not shown inFIG.5E, the adaptive recharge algorithm may include other temperature gradients between other temperature sensors.

In some examples, the adaptive recharge algorithm may estimate the maximum temperature on IMD551based on temperature gradients, efficiency, or some combination of different system metrics as inputs to the algorithm. Processing circuitry executing the adaptive recharge algorithm may vary the temperature control limit according to a look up table based on different vectors including T1−T2(Y direction), T4−T3(X-direction), IMD efficiency (similar to a radius), and possibly other metrics like Qprim, as described above in relation toFIGS.1and4D. Other dimensions may add to the look-up table, such as additional temperature gradients. In other words, the system may generate a representative temperature from a plurality of temperature sensors and use the representative temperature in the adaptive recharge algorithm as described herein.

In some examples, the estimated maximum temperature may be a function of the different temperature readings received by the processing circuitry from the various temperature sensors. In this disclosure, the control function may be based on whatever point in space on IMD551is the hottest, e.g., the “estimated maximum temperature.” In some examples, the temperature sensors may be mounted in a position on IMD551that is not the hottest. The processing circuitry may estimate the hot spot temperature, e.g., estimated maximum temperature, somewhere different than directly near a temperature sensor. For example, processing circuitry may calculate the maximum temperature according to:

In other examples the estimated maximum temperature may include conditional logic, such as:

In some examples, the adaptive recharge algorithm of this disclosure may use the calculated temperatures to adjust a setpoint, such as a PID setpoint, as described above in relation toFIGS.1and5A, according to the below look-up table:

In some examples, the multi-dimensional look up table may have higher temperatures in certain relative locations, set to prevent any harm to the patient and harm to the IMD, which may be caused by heat dissipated from an inefficient relative position. At the same time the processing circuitry may perform calculations to maximize the recharge area and performance.

FIG.5Fis a graph illustrating an example temperature gradient in the Y direction (T1−T2) as described above inFIG.5E.FIG.5Gis a graph illustrating an example temperature gradient in the X direction (T4−T3) as described above inFIG.5E.FIG.5His a graph illustrating an example of IMD efficiency as the relative position of the power transmitting coil and power receiving coil change.

FIG.6is a graph illustrating an example output of an example adjustment factor look-up table. The table below includes one example look-up table for the adjustment factor andFIG.6depicts a sample graph of the adjustment factor as IMD efficiency changes. In other examples, e.g., for different types of power transmitting units and power receiving units, the values may differ from those inFIG.6.

As described above in relation toFIG.1, the recharging system, e.g., system10may measure efficiency, such as IMD efficiency, to determine whether the relative position of the primary coil and secondary coil may be in a less desirable relative position. Processing circuitry of the recharging system may calculate a new heat limit for the power receiving unit, such as a heat limit between the minimum and maximum allowable heat limit. System10may then adjust power transmitted based on the newly calculated limit using the adjustment factor from the table above. As described above in relation toFIG.1, the AF may be configured as a function of the IMD efficiency and the range of possible values on AF may be set as from 0 to 1. In the example above, the adjustment factor is multiplied by 256=2{circumflex over ( )}8.

FIG.6, depicts an example for the adjustment factor for the different modes, e.g., mode 1602, mode 2,604, mode 3606, mode 4b708and boost mode610.FIG.6also include a plot of the adjustment factor600. Each mode may include heat limits for the IMD, e.g., QIMD, heat limits for the primary coil QPRIM, battery current limit Imp BATT, and so on. In other examples, the values may vary based on, for example, a specific wireless recharger, a power receiving unit, size of the primary coil and secondary coil, whether the power receiving unit is an implantable medical device or a different type of device, such as a hearing aid, mobile computing device, rechargeable sensor and so on.

FIG.7is a flowchart illustrating an example operation of the system of this disclosure. The blocks ofFIG.7will be described based onFIGS.1-3, unless otherwise noted. A wireless charging system may include a power transmitting unit and a power receiving unit, such as an implantable medical device, IMD10. In some examples, processing circuitry, such as processing circuitry50, may execute instructions stored at, for example, memory52to execute the blocks ofFIG.7. In other examples, processing circuitry of other components of system10, such as processing circuitry of charging system22, processing circuitry of external computing device25, and/or processing circuitry of IMD14may execute one or more blocks ofFIG.7, or steps within blocks ofFIG.7.

Based on the power transfer mode, the processing circuitry may determine the IMD heat limit, QIMDlimit, (702). The IMD heat limit may be stored in a memory, such as memory52and be based on the power transfer mode. For example, a wireless power transfer system using the values fromFIG.6above in the boost mode (mode 4a), may determine the IMD heat limit is less than or equal to 1500 mW.

The processing circuitry executing the recharging algorithm of this disclosure may compute target output power from the primary coil, PTANK(704). In some examples, processing circuitry50of charging device22, depicted inFIG.3, may compute the target output power. In other examples, processing circuitry30of IMD14, depicted inFIG.2, or processing circuitry of external computing device25, depicted inFIG.1, may compute the target output power based on communication with charging device22.

The processing circuitry may further control the output circuitry, e.g., charging module58, to output the target power from primary coil48(706). As described above in relation toFIG.1, the processing circuitry may compute IMD_Efficiency during closed loop power transfer according to the equation:
IMD_efficiency=Pimd_batt/QIMD.
As noted above in relation toFIG.1, Qins and QIMDas well as INS_efficiency and IMD_efficiency may be used interchangeably in this disclosure.

IMD14may measure the power, Pimd_batt sent to the battery or other power source, from recharge module38. In some examples, processing circuitry of IMD14may output via telemetry module36, the measured power, e.g., Pimd_batt. In some examples, processing circuitry may measure Iimd_batt and compute Pimd_batt based on the voltage of power source40. If the Pimd_batt is zero, then IMD_efficiency may be returned as zero. Processing circuitry of system10may calculate the heat, QIMD, and compare the calculated QIMDto the heat limit based on the below equation, as described above in relation toFIG.1:
QIMD=PTANK−QPRIM−Pimd_batt
In some examples, the recharging algorithm may include timers, and other limits. Based on exceeding one or more limits, or other factors in the recharging algorithm, the processing circuitry may change to a different power transfer mode, or IMD_efficiency may change, e.g., based on a change in relative position between the primary coil26and secondary coil16, depicted inFIG.1, which may drive an update to the heat control limit (708).

In some examples, the recharging algorithm may stay in the same power transfer mode and/or the IMD_efficiency stays approximately the same and therefore make no change to the heat control (NO branch of708). For example, though the relative position of primary coil26and secondary coil16may move, the IMD_efficiency may remain approximately equivalent, such as within region404or406described above in relation toFIG.4D.

In other examples, the processing circuitry may change to a different power transfer mode, or IMD_efficiency may change, which may drive a change in heat control (YES branch of708). For example, primary coil26may move from region404to402as shown inFIG.4D, or between regions of higher IMD_efficiency to a region of lower IMD_efficiency, as shown inFIGS.4A-4C.

The processing circuitry may update the adjustment factor (710) based on a table similar to adjustment factor above or based on one or more other linear or non-linear relationships described above in relation toFIG.1. The processing circuitry of the recharger may adjust the current QIMDlimit for the current power transfer mode based on the adjustment factor (702). In some examples, it may be desirable to have the adjustment factor adjusted at a slower rate, e.g., less often, than the algorithm controlling the QIMDlimit to avoid oscillations in the control algorithm. For example, the adjustment factor may be updated at a rate that is 5 to 10 times slower than the main control algorithm.

By way of example, and not limitation, such computer-readable storage media, may include random access memory (RAM), read only memory (ROM), programmable read only memory (PROM), erasable programmable read only memory (EPROM), electronically erasable programmable read only memory (EEPROM), flash memory, a hard disk, a compact disc ROM (CD-ROM), a floppy disk, a cassette, magnetic media, optical media, or other computer readable media. In some examples, an article of manufacture may include one or more computer-readable storage media.

Also, any connection is properly termed a computer-readable medium. For example, if instructions are transmitted from a website, server, or other remote source using a coaxial cable, fiber optic cable, twisted pair, digital subscriber line (DSL), or wireless technologies such as infrared, radio, and microwave, then the coaxial cable, fiber optic cable, twisted pair, DSL, or wireless technologies such as infrared, radio, and microwave are included in the definition of medium. It should be understood, however, that computer-readable storage media and data storage media do not include connections, carrier waves, signals, or other transient media, but are instead directed to non-transient, tangible storage media. Combinations of the above should also be included within the scope of computer-readable media.

The devices, systems and techniques may also be shown in the following examples.

Example 1: A device comprising a power transmitting unit configured to wirelessly transfer electromagnetic energy to a power receiving unit; and processing circuitry configured to: compute a target output power deliverable by the power transmitting unit for a first duration; control the power transmitting unit to output the target output power based in part on a heat limit; calculate an energy transfer efficiency to the power receiving unit; update an adjustment factor based on the calculated energy transfer efficiency; and apply the adjustment factor to the heat limit for a subsequent duration.

Example 2: The device of example 1, wherein the processing circuitry is further configured to: compute an amount of heat received at the power receiving unit based on: the target output power; and an amount of heat lost by a primary coil of the power transmitting unit; and calculate the energy transfer efficiency to the power receiving unit based on the computed amount of heat received by the power receiving unit.

Example 3: The device of any of examples 1 and 2, wherein the processing circuitry is configured to receive an indication of an amount of power delivered to an electrical energy storage device of the power receiving unit.

Example 4: The device of example 3, wherein the processing circuitry is configured to calculate the energy transfer efficiency based on an amount of heat received by the power receiving unit and the amount of power delivered to the electrical energy storage device of the power receiving unit.

Example 5: The device of any of examples 3 and 4, wherein the processing circuitry is configured to calculate the energy transfer efficiency based on an amount of heat received by the power receiving unit and the amount of power delivered to the electrical energy storage device of the power receiving unit.

Example 6: The device of example 5, wherein the processing circuitry is further configured to: control the power transmitting unit to operate in a first power transfer mode; compute an amount of heat received by the power receiving unit; compare the computed amount of heat received by the power receiving unit to the heat limit for the subsequent duration; and determine whether to operate in a first power transfer mode or a second power transfer mode for the subsequent duration based on the comparison.

Example 7: The device of example 6, wherein while operating in the second power transfer mode, the power transmitting unit outputs less electromagnetic energy than while operating in the first power transfer mode.

Example 8: The device of any of examples 5 through 7, wherein the processing circuitry is configured to avoid oscillations in the control algorithm, wherein to avoid oscillations in the control algorithm comprises to update the heat limit more often than updating the adjustment factor.

Example 9: The device of any of examples 1 through 8, further comprising a temperature sensor configured to measure a temperature of a primary coil of the power transmitting unit.

Example 10: The device of any of examples 1 through 9, wherein the processing circuitry is configured to: receive an indication of a first temperature from a first temperature sensor at a first location on the power receiving unit; receive an indication of a second temperature from a second temperature sensor at a second location on the power receiving unit; compare the first temperature to the second temperature; and compute the target output power based on the comparison.

Example 11: The device of any of examples 1 through 10, wherein a relationship between the adjustment factor and the energy transfer efficiency is a non-linear transfer function.

Example 12: The device of any of examples 1 through 11, wherein a relationship between the adjustment factor and the energy transfer efficiency is a monotonic transfer function.

Example 13: The device of any of examples 1 through 12, wherein the processing circuitry determines the adjustment factor based on a look-up table that includes the energy transfer efficiency.

Example 14: A system comprising a power receiving unit; a power transmitting unit configured to wirelessly transfer electromagnetic energy to the power receiving unit; and comprising processing circuitry configured to: compute a target output power deliverable by the power transmitting unit for a first duration; control the power transmitting unit to output the target output power based in part on a heat limit; calculate an energy transfer efficiency to the power receiving unit; and update an adjustment factor based on the calculated energy transfer efficiency; and apply the adjustment factor to the heat limit for a subsequent duration.

Example 15: The system of example 14, wherein the processing circuitry is further configured to: compute an amount of heat received at the power receiving unit based on: the target output power; and an amount of heat lost by a primary coil of the power transmitting unit; and calculate the energy transfer efficiency to the power receiving unit based on the computed amount of heat received by the power receiving unit.

Example 16: The system of any of examples 14 and 15, wherein the processing circuitry is configured to receive, from the power receiving unit, an indication of an amount of power delivered to an electrical energy storage device of the power receiving unit.

Example 17: The system of example 16, wherein the processing circuitry is configured to: receive an indication of an amount of heat received by the power receiving unit; calculate the energy transfer efficiency based on: the amount of heat received by the power receiving unit; and the amount of power delivered to the electrical energy storage device of the power receiving unit.

Example 18: The system of any of examples 15 through 17, wherein the processing circuitry is further configured to: operate the power transmitting unit in a first power transfer mode; compare the computed amount of heat received at the power receiving unit to the heat limit for the subsequent duration; and determine whether to operate in a first power transfer mode or a second power transfer mode for the subsequent duration based on the comparison.

Example 19: The system of example 18, wherein while operating in the second power transfer mode, the power transmitting unit is configured to output less electromagnetic energy than while operating in the first power transfer mode.

Example 20: The system of any of examples 14 through 19, wherein the power receiving unit is an implantable medical device.

Example 21: The system of any of examples 14 through 20, further comprising a temperature sensor.

Example 22: The system of example 21, wherein the temperature sensor is configured to measure a temperature of a primary coil of the power transmitting unit, and wherein the processing circuitry is configured to calculate the energy transfer efficiency based in part on the temperature of the primary coil.

Example 23: The system of any of examples 21 and 22, wherein the temperature sensor is configured to measure a temperature of the power receiving unit, and wherein the processing circuitry is configured to calculate the energy transfer efficiency based in part on the temperature of the primary coil.

Example 24: The system of example 23, wherein the temperature sensor is a first temperature sensor at a first location on the power receiving unit, the system further includes receive an indication of a first temperature from a first temperature sensor at a first location on the power receiving unit; receive an indication of a second temperature from a second temperature sensor at a second location on the power receiving unit; compare the first temperature to the second temperature; and compute the target output power based on the comparison.

Example 25: A method comprising computing, by processing circuitry, a target output power deliverable by a wireless power transmitting unit for a first duration; controlling, by the processing circuitry, circuitry to output the target output power based in part on a heat limit; calculating, by the processing circuitry, an energy transfer efficiency to a power receiving unit; and updating, by the processing circuitry, an adjustment factor based on the calculated energy transfer efficiency applying the adjustment factor to the heat limit for a subsequent duration.