Patent Publication Number: US-2022231546-A1

Title: Dynamic control of wireless power transfer efficiency

Description:
TECHNICAL FIELD 
     Embodiments described herein generally relate to powering of electronic devices and, more particularly, to wireless power transfer. 
     BACKGROUND 
     Wireless power transfer (WPT) is a practical and convenient solution for powering and recharging the batteries of portable electronic devices. WPT uses a non-contact technique to deliver energy from a base station to a power-recipient device. Most typically, WPT applications utilize inductive coupling arrangements in which coil antennas at the base station and power-recipient device are placed into close proximity to form a transformer through which power may be transferred. The electronics industry has developed WPT standards, such as Qi by the Wireless Power Consortium, and Rezence, promoted by The Alliance for Wireless Power and the Power Matters Alliance. A variety of proprietary WPT technologies also exist. 
     Advantageously, the transmitter coil in the base station and the receiver coil in the power-recipient device may be separated from one another by the respective housings of the base station and device. Thus, a user of a device employing a WPT system can avoid having to plug and un-plug the device from the power source using an electrical connector, and likewise need not worry about the degradation of exposed electrical contacts due to mechanical wear or corrosion. WPT systems are particularly advantageous in wet or dusty environments, and in environments where the device is frequently coupled and decoupled from a power source. 
     A common application for WPT systems is in charging cradles or pads for handheld devices that are powered by rechargeable batteries. Recent advances in battery technology have increased the energy density of the battery cells, and have developed electrode and electrolyte materials and geometries that enable rapid charging of battery cells using high currents. Whereas traditional battery-chargers have utilized relatively low power, such as  5  V and  500  mA for charging cycles that lasted many hours, it is not unusual to see modern chargers using higher voltages and currents, such as 12 V and 1.5 A. For instance, the Qi standard includes a baseline power profile (BPP) up to 5 W, and extended power profile (EPP) up to 15 W, and a Class 0 EPP up to 30 W. It is expected that energy demands for battery charging and other uses in the future will call for greater power levels. A proprietary power-delivery extension (PPDE) certification program has been introduced by the Wireless Power Consortium to facilitate the use of higher power levels, subject to additional safety testing. 
     In many applications, there remains a need for charging systems to be backwards compatible with traditional low-power supplies, such as universal serial bus (USB) versions 1.0 or 2.0, which are commonly found in personal computers, portable chargers, ubiquitous power adapters, and the like. Conventionally, the amount of power that can be transferred between the base station and the power-recipient device is arranged during a negotiation protocol that occurs during communication phases occurring before power transfer is initiated. The power-recipient device requests a certain amount of power, and the base station delivers the requested amount. This communication assures interoperability between lower-power and higher-power base stations/devices. 
     Typically, the WPT systems of power-receiving devices and corresponding base stations have WPT system properties that are optimized for an expected power level. For instance, the WPT system may have a particular coil antenna geometry, WPT frequency, signal-conditioning circuitry, such as power regulation or conversion circuitry, or the like, that works most efficiently at a certain wattage. 
     Whether a device uses lower-power or higher-power WPT, various challenges must be dealt with. For example, higher-power WPT devices must contend with heating caused by the WPT system. There is a practical temperature range in which batteries may be safely and reliably charged, and the high end of this range tends to be not much warmer than certain high-temperature environments, such as factories, kitchens, warm outdoor locations, and the like. The temperature rise attributed to inefficiencies of the WPT system may prevent charging to be performed at the maximum rate. In lower-power WPT devices, the inefficiencies of the WPT system may limit the available power from the power source for charging the batteries at an acceptable rate, or such inefficiencies may prevent a device from being powered on and used while also receiving power for charging of its battery. 
     Solutions are needed to address these, and other challenges in WPT systems. 
     SUMMARY 
     Aspects of the embodiments are generally directed to wireless power transfer (WPT) efficiency optimizations. In some embodiments, an apparatus for a WPT transmitter includes a power input arranged to receive electrical power from a power source, first transmitter (TX) path circuitry arranged to be coupled to the power input and to a first set of at least one transmission coil, second TX path circuitry arranged to be coupled to the power input and to a second set of at least one transmission coil, transmitter mode selector circuitry arranged to selectively couple one of the first TX path circuitry and the second TX path circuitry between the power input and the first or the second set of at least one transmission coil in response to a path selection signal, and controller circuitry communicatively coupled to the power input and to the transmitter mode selector circuitry. The controller circuitry is operative to determine a power level of the power source from among a first power level and a second power level, and to generate the path selection signal based on the determined power level of the power source. The first TX path circuitry is operative to provide WPT transmission at a greater efficiency to a WPT receiver when the power source is at the first power level than when the power source is at the second power level. The second TX path circuitry is operative to provide WPT transmission at a greater efficiency to a WPT receiver when the power source is at the second power level than when the power source is at the first power level. 
     In a related embodiment, a WPT receiver includes a power output arranged to provide WPT-received electrical power to a load, first receiver (RX) path circuitry arranged to be coupled to the power output and to a first set of at least one reception coil, second RX path circuitry arranged to be coupled to the power output and to a second set of at least one reception coil, receiver mode selector circuitry arranged to selectively couple one of the first RX path circuitry and the second RX path circuitry between the power output and the first or the second set of at least one reception coil in response to a path selection signal, and controller circuitry communicatively coupled to the receiver mode selector circuitry, and operative to generate the path selection signal based on a power requirement of the load. One of the first RX path circuitry and the second RX path circuitry is operative to selectively receive WPT transmission from a WPT transmitter based on the path selection signal. The first RX path circuitry is operative to receive the WPT transmission at a greater efficiency when the received WPT is at a first power level than when the received WPT is at a second power level, and the second RX path circuitry is operative to receive the WPT transmission at a greater efficiency when the received WPT is at the second power level than when the received WPT is at the first power level. 
     In another embodiment, a WPT includes a power output arranged to provide WPT-received electrical power to a load, RX path circuitry coupled to the power output and to a set of at least one reception coil, and to receive WPT via the set of at least one reception coil, and controller circuitry arranged to read measurements of voltage at the load and of current being drawn by the load, and to dynamically adjust a characteristic of the WPT in response to the measurements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, which are not necessarily drawn to scale, like numerals may describe similar components in different views. Like numerals having different letter suffixes may represent different instances of similar components. Some embodiments are illustrated by way of example, and not limitation, in the figures of the accompanying drawings. 
         FIG. 1A  is a perspective-view diagram illustrating a wireless power transfer (WPT) charging station for a power-receiving device (PRD) that is a gun-style handheld information device (HID) having a grip, according to some embodiments. 
         FIG. 1B  is a diagram illustrating a WPT charging station that is a cradle-style charging station according to some embodiments. 
         FIG. 1C  is a diagram illustrating a pad-style WPT charging station for HIDs according to some embodiments. 
         FIG. 2  is a high-level block diagram illustrating a WPT system according to an embodiment. 
         FIG. 3  is a block diagram illustrating a wireless power (WP) transmitter and control circuit of the WPT system of  FIG. 2  according to some embodiments. 
         FIG. 4  is a block diagram illustrating a WP receiver and control circuit of the WPT system of  FIG. 2  according to some embodiments. 
         FIG. 5  is a sequence diagram illustrating startup, power adjustment, and optimization operations of a WPT system according to various embodiments. 
         FIG. 6  is a flow diagram of another WPT-optimization process that may be carried out by WP receiver and control circuit according to a related embodiment. 
         FIG. 7  is a simplified circuit diagram illustrating an example WPT system in which the WP transmitter and receiver&#39;s magnetic (or, in some embodiments, electromagnetic) coupling is dynamically adjustable to optimize performance for the selected WPT mode. 
         FIG. 8  is a simplified schematic diagram illustrating an electronic switch for inserting a shunt capacitance in the RX side of the WPT system depicted in  FIG. 7  according to an embodiment. 
         FIG. 9  is a diagram showing two plots that illustrate the effect of the addition of a shunt capacitance into the RX side of the WPT circuit of  FIG. 7  according to an example. 
     
    
    
     DETAILED DESCRIPTION 
     The following description and the drawings sufficiently illustrate specific embodiments to enable those skilled in the art to practice them. Other embodiments may incorporate structural, logical, electrical, process, and other changes. Portions and features of some embodiments may be included in, or substituted for, those of other embodiments. Embodiments set forth in the claims encompass all available equivalents of those claims. 
       FIGS. 1A-1C  are perspective-view diagrams illustrating various styles of charging stations for handheld information devices (HIDs).  FIG. 1A  illustrates a gun-style HID  100  that has grip  101 . Station  102  in this example is a charging station having a form that is made to correspond with the exterior shape of HID  100 . In particular, station  102  has a body that includes a receiving surface designed conform with grip  101  and portions of the underside of HID  100 . 
     Charging stations and HIDs according to various embodiments may take a variety of other form factors.  FIG. 1B  is a diagram illustrating a partially-enclosed charging cradle-style HID station  122  for HID  120 . HID station  122  has a body that accommodates HID  120 , which has a more compact form factor than HID  100 . When HID  120  is engaged with charging cradle  122 , a portion of HID  120  sits inside the partial enclosure.  FIG. 1C  is a diagram illustrating a pad-style wireless charging station  142  for use with HID  140 . Charging station  142  has a top surface on which HID  140  may be placed, and this surface may include printed graphics or other visual indicia to assist the operator to properly place HID  140 , but it otherwise lacks any structural alignment feature to enforce the proper placement. 
     Each pair of HIDs  100 ,  120 , and  140 , and charging stations  102 ,  122 , and  142 , respectively, utilize a wireless power transfer (WPT) system according to aspects of this disclosure that can provide power over a wide range of power levels.  FIG. 2  is a high-level block diagram illustrating WPT system  200  according to an embodiment. WPT system  200  includes a WP transmitter and control circuit  212 , which is in base station  210 , and WP receiver and control circuit  222 , which is in HID  220 . Base station  210  in this example may represent base station  102 ,  122 , or  142  ( FIG. 1 ), and HID  220  may represent HIDs  100 ,  120 , or  140  ( FIG. 1 ). 
     Base station  210  may be powered from low-power source  202 , or from high-power source  204 . In the present context, low-power source is a supply that is less than or equal to about 5 W (e.g., ≤5W+5%). Examples of such a low-power source include universal serial bus (USB) ports according to USB 1.0 or 2.0 specifications for 5 V (≤500 mA), or USB 3.0 specifications for 5 V (≤900 mA). High-power source  204  in the present context is a supply that is greater than about 5 W, such as a USB power delivery (USB-PD) port that can support such higher wattages at 5 V and at various other voltages, such as 9 V, 15 V, and 20V, with available current supplies of up to 5 A. Other power supplies, such as proprietary power supplies at other voltages may likewise be supported. 
     According to one aspect of this disclosure, the WPT system is automatically and dynamically reconfigurable for efficiency optimization in a WPT mode (e.g., low-power mode or a high-power mode). Efficiency optimization in the present context means effecting a change in the WPT circuitry based on the present power source to improve the power-transfer efficiency along a path from the power supply  202  or  204  to load circuit  224 , which path includes WPT from WP transmitter and control circuit  212  to WP receiver and control circuit  222 . The improvement in power-transfer efficiency achieved by a certain optimized WPT mode for the present power source, power demands, and other prevailing conditions is in comparison to the WPT operational efficiency that would have been present in the absence of the optimized WPT mode. 
       FIG. 3  is a block diagram illustrating WP transmitter and control circuit  212  in greater detail according to some embodiments. WP transmitter receives supply power via power input  302  from a variety of sources, which may include low-power and high-power supplies. As part of efficiency optimization, WP transmitter and control circuit  212  provides WPT mode selection as represented by mode selector  304 , and separate transmitter (TX) paths  306 A and  306 B, each TX path being optimized for a different power level. 
     Each TX path  306 A,  306 B includes circuitry that converts the supply power received via power input  302  to a high-frequency WPT carrier waveform to be wirelessly transmitted to a WPT receiver via coil antenna  308 A or  308 B. In the present context, the term coil antenna refers to an inductive coil (for magnetic coupling in near-field configurations) or a radiative coil (for electromagnetic coupling in far-field configurations). For instance, each TX path  306 A,  306 B main include power conditioning circuitry such as a voltage regulator, voltage converter (e.g., buck converter, boost converter, charge pump, etc.), or a combination of these. In addition, each TX path  306 A,  306 B includes circuitry for generating the WPT carrier waveform, which is coupled to a coil antenna  308 A,  308 B. 
     In the example shown, low-power TX path  306 A is optimized for operation at low power. For example, low-power TX path  306 A may be specifically designed for operation with a 5 VDC input at a defined limited current, such as 1 A. Accordingly, low-power TX path  306 A may omit power-conditioning circuitry. In other examples, low-power TX path  306 A may include simple and efficient power-conditioning circuitry, such as a single ended primary inductive converter (SEPIC) implemented using relatively small switching transistor with small gate capacitance and fast switching performance. 
     High-power TX path  306 B is optimized for versatility over a range of input voltages. Accordingly, high-power TX path  306 B may include a switching regulator (e.g. SEPIC, Cuk, buck-boost), with larger switching transistor size to accommodate higher current. A switching regulator such as this may work to convert a wide range of input voltages to one or more specific voltages to be passed to downstream circuitry for conversion to high-frequency AC signaling and wireless power transmission. 
     In a related embodiment, low-power TX path  306 A and high-power TX path  306 B may share certain components, as represented by overlap  307  of the block diagram. 
     Other optimizations as between TX paths  306 A and  306 B may include different types of switching transistors and other components needed for generation of the WPT carrier waveform, different carrier frequencies and, optionally, different coil-antenna geometries in embodiments where multiple coil antennas are provided. As illustrated, both TX paths  306 A,  306 B may use the same coil antenna  308 A, or they may use different coil antennas  308 A,  308 B, respectively. 
     The WPT mode selection is implemented by WP transmitter controller  310 . In some implementations, WP transmitter controller  310  is an embedded microcontroller or other instruction processor and non-volatile memory containing program instructions that, when executed by the instruction processor cause the processor to determine the input supply power and select the appropriate TX path  306 A,  306 B. In other implementations, WP transmitter controller  310  is implemented as a digital logic circuit (e.g., as an application-specific integrated circuit (ASIC), field-programmable gate array (FPGA), Complex Programmable Logic Device (CPLD), programmable logic array (PLA), programmable array logic (PAL), or the like). 
     WP transmitter controller  310  may communicate with the power supply via source communication circuitry  314 , for example, to negotiate an extended range of supply power via a supported protocol (e.g., USB). Also, WP transmitter controller  310  may communicate with WP receiver and control circuit  222  via WP communication circuit  312 . WP communication circuit  312  may support bi-directional communications (half-duplex or full-duplex) over the WPT channel. 
       FIG. 4  is a block diagram illustrating WP receiver and control circuit  222  in greater detail according to some embodiments. WP receiver and control circuit  222  includes one or more coil antennas  408 A,  408 B arranged to be paired with one or more coil antennas  308 A,  308 B of WP transmitter and control circuit  212  ( FIG. 3 ). In a single-antenna arrangement, coil antenna  408 A is coupled to low-power receiver (RX) path  406 A and high-power RX path  406 B; in a multi-antenna arrangement, coil antenna  408 A is coupled to low-power RX path  406 A, and coil antenna  408 B is coupled to high-power RX path  406 B. 
     Each RX path  406 A,  406 B includes power-conditioning circuitry that is operative to extract received power via the WPT and convert the received high-frequency waveform into DC power for use by the load. Such circuitry may include a rectifier, filter, and voltage regulator. Each RX path  406 A,  406 B is optimized for a different power level, in similar principle to optimization of TX paths  306 A and  306 B ( FIG. 3 ). Accordingly, in one embodiment, the circuitry of low-power RX path  406 A may have smaller and more efficient components, optimized for operation at relatively lower received power levels. In a related embodiment, circuitry of high-power RX path  406 B may have more robust components that are able to safely withstand higher voltages and currents. 
     As represented by overlap  407 , RX paths  406 A and  406 B may share some portion of the RX-path circuitry in common. 
     In one type of embodiment, as described in greater detail below, one or both RX paths  406 A,  406 B have a reactance circuit to optimize the WP transmission efficiency using the principle of resonance. 
     Selection of a RX path  406 A,  406 B is represented in this diagram by mode selector  404 , which is controlled by WP receiver controller  410 . In some implementations, WP receiver controller  410  is an embedded microcontroller or other instruction processor and non-volatile memory containing program instructions that, when executed by the instruction processor cause the processor to determine the input supply power and select the appropriate RX path  406 A,  406 B. In other implementations, WP receiver controller  410  is implemented as a digital logic circuit (e.g., as an application-specific integrated circuit (ASIC), field-programmable gate array (FPGA), Complex Programmable Logic Device (CPLD), programmable logic array (PLA), programmable array logic (PAL), or the like). 
     WP receiver controller  410  may communicate with WP transmitter and control circuit  212  via WP communication circuit  412 . WP communication circuit  412  may support bi-directional communications (half-duplex or full-duplex) over the WPT channel. These communications may be used to negotiate power-level settings between WP transmitter and control circuit  212  and WP receiver and control circuit  222 , as well as to communicate other relevant WPT messaging. 
     In some embodiments, WP receiver controller  410  is operative to control the received power based on voltage or current measurements, or both, at the load. Accordingly, in some embodiments, as shown, electrical measurement circuitry  416  at the output of the WP receiver circuitry  402  provides voltage, current, or voltage and current measurements to WP receiver controller  410 . 
       FIG. 5  is a sequence diagram illustrating startup, power adjustment, and optimization operations according to various embodiments. Notably,  FIG. 5  illustrates a full-featured example with a number of operations, which may or may not all be present in a given implementation. In addition, the illustrated operations are presented in an exemplary order, which may or may not be followed in the exact sequence according to various implementations. Thus, it should be understood that various other embodiments may have fewer operations, operations in a different order, or both variations from the embodiment depicted in  FIG. 5 . 
     The sequence diagram shows operations carried out by WP transmitter and control circuit  212  in the left-hand column, and operations carried out by WP receiver and control circuit  222  in the right-hand column. Certain operations are interactive between WP transmitter and control circuit  212  and WP receiver and control circuit  222 , as shown. 
     Operations  502 - 512  and  522 - 532  exemplify initial startup and power mode selection by WP transmitter and control circuit  212  and WP receiver and control circuit  222 , respectively. At  502 , WP transmitter and control circuit  212  is powered on. At  504 , WP transmitter and control circuit  212  determines the available supply power. This operation may be involve conducting communications with a USB host device in accordance with USB connectivity protocol, for instance, where WP transmitter and control circuit  212  may negotiate a maximum available power level from its power source. In non-USB embodiments, WP transmitter and control circuit  212  may measure the input voltage and conduct a message exchange with the power source to determine if the power source is a compatible power source. 
     At  506 , regardless of the type of power supply, WP transmitter and control circuit  212  powers up the WPT TX path circuitry in the low-power mode, and initiates WPT when a HID  220  is placed in WPT proximity and when the WPT proximity placement of HID  220  is detected. Accordingly, at  522  WP receiver and control circuit  222  receives wireless power transmitted in the low-power mode. At  524 , if the WP receiver and control circuit  222  is not already powered from the HID&#39;s battery, it powers up using its low-power mode to receive the WPT initiated by WP transmitter and control circuit  212  at  506 . 
     At  526 , WP receiver and control circuit  222  sends a power-request message. In one type of embodiment, the power-request message is sent using signaling over the WPT channel. For instance, the power-request message may be a part of a standard Qi startup protocol for power recipient devices. In another embodiment, the power-request message may be sent over another link, such as over a personal-area network (PAN) such as Bluetooth Low Energy, for instance, as standardized under IEEE 802.15.4, under a WiFi standard such as IEEE 802.11, near-field communications (NFC) as standardized under ECMA-340 or ISO/IEC 18092, or other suitable communication channel. The power-request message may include an indicator corresponding to the maximum power level that HID  220  can accept via WPT. 
     At  508 , WP transmitter and control circuit  212  receives the power-request message, and responds at  510 . The response may likewise be transmitted over the WPT channel or over any other suitable channel such as those listed above, and the response may include a message indicating a power level that the base station is able to provide to HID  220 . The available power-level indicator may state or represent the voltage and maximum current that is available over the WPT channel. The WP receiver and control circuit  222  receives the response message at  528 . Based on the limits of the available power, WP receiver and control circuit  222  sets a limit for the WP receiver on the amount of current that it may draw so that the available power is not exceeded. 
     At  512 , WP transmitter and control circuit  212  initiates WPT at the available high power in accordance with the determination made at operation  504  and the negotiation at operations  508 - 510 . The WP receiver and control circuit  222  receives this higher power at  532  via the WPT. 
     Operations  534 - 544  by WP receiver and control circuit  222  and operations  514 - 518  by WP transmitter and control circuit  212  correspond to a dynamic WPT optimization technique that may be performed interactively between WP transmitter and control circuit  212  and WP receiver and control circuit  222  according to an example implementation. In one such embodiment, WP transmitter and control circuit  212  and WP receiver and control circuit  222  negotiate a new operating point for the WPT. This new operating point may depend on the power needs of HID  220 , the currently available power level from WP transmitter and control circuit  212 , and on optimization criteria that is stored by WP transmitter and control circuit  212 . Accordingly, at  534 , WP receiver and control circuit  222  reads measurements of the load voltage and current. Based on these measurements, at  536 , WP receiver and control circuit  222  determines if there is an opportunity to increase the WPT efficiency. In general, the opportunity to increase the WPT efficiency for a given level of power transfer is based on the ability of the base station to source higher current at a lower voltage. 
     In one approach, WP receiver and control circuit  222  determines whether an optimization opportunity exists by comparing the measured voltage at the load with a reference voltage. As an example, the reference voltage is a voltage value corresponding to the negotiated power level, for instance, as received by WP receiver and control circuit  222  at  528 . In another example, the reference voltage is a measured voltage in the RX path  406 A,  406 B ( FIG. 4 ) that is being used to receive the WPT power. Such a measured voltage in the RX path  406 A,  406 B may be at the coil antenna terminals, at an output of a rectifier, or at an input of a power-conditioning circuit. 
     The comparing of the measured voltage at the load with the reference voltage may involve addition or subtraction of a voltage offset value to or from one of the compared quantities. The comparing may instead or also involve the use of a scaling factor applied to one of the compared quantities. In general, the aim of the comparison is to determine if the measured load voltage is less than the reference voltage by some significant amount, which is indicative of inefficiencies, such as the operation of a voltage regulator in the RX path, to significantly drop the voltage of the WPT-received power in order to provide the appropriate amount of current to the load. 
     Thus, for example, efficiency may be gained by reducing the WPT-received voltage to be at a level that matches or slightly exceeds the voltage corresponding to the required current draw at the load. A further mechanism by which efficiency may be gained is by transmitting a set power level via WPT at a lower voltage and higher current, with the higher current tending to improve the magnetic or electromagnetic coupling of the coil antennas. 
     Upon determining that optimizations are available, WP transmitter and control circuit  212  sends a power optimization request to WP receiver and control circuit  222  at  538 , which, in turn, is received at  514 . The power optimization request may utilize the same or similar messaging as the power request at  526  described above, or it may utilize different messaging. For example, in one embodiment, WP receiver and control circuit  222  specifies a particular voltage to be transmitted as part of the optimization request at  538 . In another embodiment, WP receiver and control circuit  222  specifies increment voltage amounts by which to increase or reduce the transmitted voltage of the WPT as part of the optimization request at  538 . At  516 , WP transmitter and control circuit  212  responds to the optimization request by sending a message confirming the requested transmission voltage setting, rejecting it, or proposing an alternative power transmission setting at  516 , which WP receiver and control circuit  222  receives at  540 . At  542 , WP receiver and control circuit  222  verifies that the confirmed or modified WPT settings are within predefined safety limits. If necessary, a new power optimization request at  538  may be generated by WP receiver and control circuit  222  in response to a determined failure to meet safety limits. 
     At  518 , WP transmitter and control circuit  212  initiates WTP at the optimal power settings, which in this example are the power settings agreed to in the optimization negotiation protocol carried out in operations  514 - 516  of WP transmitter and control circuit  212  and operations  538 - 540  of WP receiver and control circuit  222 . At  544 , assuming the safety limits are not violated, WP receiver and control circuit  222  receives the WPT at the optimized settings. 
     Operations  534 - 544  and  514 - 518  of the sequence may be performed iteratively by WP receiver and control circuit  222  and WP transmitter and control circuit  212 , respectively, as the loading conditions of HID  220  may vary over time, causing the optimal WPT operating point to shift. 
       FIG. 6  is a flow diagram of another WPT-optimization process that may be carried out by WP receiver and control circuit  222  according to a related embodiment. The process of  FIG. 6  aims to dynamically determine an operating point for WP receiver and control circuit  222  in which the voltage at the load is minimized while the current at the output of WP receiver and control circuit  222  is maximized within practical constraints. This process utilizes monitoring of both, current and voltage, at the output of WP receiver and control circuit  222 . A control system is arranged to vary the voltage at the output of WP receiver and control circuit  222  while monitoring the output current to the load. 
     At  602 , the voltage at the output of WP receiver and control circuit  222 , V OUT RX , is read. At decision  604 , WP receiver and control circuit  222  determines if V OUT RX  is greater than the current minimum voltage required to operate HID  220  (e.g., whether in battery-charging-only mode or in active operation), plus a reliability margin, ΔV. The reliability margin ΔV is an empirically-determined and predefined quantity. In the affirmative case as a result of decision  604 , the process advances to  608 , where the output current I OUT RX , that is, the current being drawn by the load, is read. 
     Decision  610 , determines if the output current I OUT RX  falls in an acceptable range within which the current I OUT RX  can vary without calling for any change in operating point. If the result of decision  610  is negative (i.e., the current is outside of the acceptable range), the output current I OUT RX  is compared against a predefined maximum allowed current value I MAX  at  612 , which may be set, for instance, by WP transmitter and control circuit  212  based on its available power source. In the negative case, the process branches to  614 , which reduces V OUT RX  in RX path  406 A,  406 B ( FIG. 4 ) in order to cause an increase in I OUT RX  due to the drawing of more current by a power regulator, for instance. Otherwise, if I OUT RX  is greater than or equal to I MAX , the process branches to  616  to increase V OUT RX  in RX path  406 A,  406 B and cause a reduction in I OUT RX . The process then iterates to account for any changing operating conditions. 
       FIG. 7  is a simplified circuit diagram illustrating an example WPT system in which the WP transmitter and receiver&#39;s magnetic or, in some embodiments, electromagnetic coupling is dynamically adjustable to optimize performance for the selected WPT mode (e.g., low-power mode and high-power mode). As depicted, simplified WP transmitter includes an AC power source P 1 , which may have an operating power-transfer frequency in the tens to hundreds of kHz. On the transmitter side, a first series RLC circuit is provided including coil antenna L 1 , series capacitor C 1 , and resistor R 1  representing the small parasitic resistance of the components and interconnects. 
     On the receiver side, a second series RLC circuit is provided including coil antenna L 2 , series capacitor C 2 , and R 2  representing a small parasitic resistance of the components and interconnects. Power conditioner U 1 , which converts the received AC signal into a DC power signal, supplies power to the load, which may be a battery charging circuit. Notably, a shunt capacitor C SHUNT  is switchably added or removed from the circuit by switch SW 1 , which is controlled by control input CTRL. 
     Switch SW 1  may be implemented as a controllable insertion circuit as depicted in  FIG. 8 . Control input CTRL switches drive transistor Q 3 , which activates optoisolator OPT 1 . In turn, optoisolator OPT 1  activates switching transistors Q 1  and Q 2 , which form a shunt across the output terminals. Switching transistors Q 1  and Q 2  are preferably low-on-resistance devices, such as trench MOSFET transistors. Balancing capacitors C BAL1  and C BAL2  are small-value capacitors that balance the voltage across each open channel of Q 1  and Q 2  when these switching transistors are deactivated. 
     Referring again to  FIG. 7 , in operation, the shunt capacitor C SHUNT  is inserted into the circuit only when the selected WPT mode is the low-power mode. The shunt capacitance reduces the no-load losses of the WPT system by introduction of a reactance to create a resonance characteristic. The resonance characteristic is present regardless of the amount of loading, even at small or zero loads. As a result, the introduced resonance characteristic increases the TX to RX gain of the system as a whole, with a lower magnetization current on the TX side, hence improving overall WPT efficiency for low-power operations. 
     In a related embodiment, the shunt capacitance is switched out of the circuit in the high-power mode of operation. In a high-loading mode of operation, the addition of the shunt capacitance is negligible and tends to undesirably affect the AC response of the system. 
     Referring again to  FIG. 4 , the switching in and out of the shunt capacitance is an example of mode selection the RX path from among path  406 A (which includes the shunt capacitance), and path  406 B (excluding the shunt capacitance) with much of the rest of the circuitry on the RX side constituting overlapping RX path components  407 . 
       FIG. 9  shows two plots illustrating the effect of the addition of the shunt capacitance into the RX side of the WPT circuit of  FIG. 7 . Trace  900  shows the current in the TX coil antenna L 1 , and trace  910  shows the current at the input to power conditioner U 1 . The traces initially represent the respective currents without the shunt capacitance in the circuit. At the introduction of the shunt capacitance, as indicated at  902  and  912 , the current in the TX coil antenna is reduced in amplitude, while the current at the input to U 1  increases in amplitude. Notably, this change occurs with no change in the input voltage on the TX side. Hence, the change represents an increase in WPT efficiency. 
     ADDITIONAL NOTES AND EXAMPLES 
     Example 1 is apparatus for a wireless power transfer (WPT) transmitter, the apparatus comprising: a power input arranged to receive electrical power from a power source; first transmitter (TX) path circuitry arranged to be coupled to the power input and to a first set of at least one transmission coil; second TX path circuitry arranged to be coupled to the power input and to a second set of at least one transmission coil; transmitter mode selector circuitry arranged to selectively couple one of the first TX path circuitry and the second TX path circuitry between the power input and the first or the second set of at least one transmission coil in response to a path selection signal; and controller circuitry communicatively coupled to the power input and to the transmitter mode selector circuitry, the controller circuitry operative to determine a power level of the power source from among a first power level and a second power level, and to generate the path selection signal based on the determined power level of the power source; wherein the first TX path circuitry is operative to provide WPT transmission at a greater efficiency to a WPT receiver when the power source is at the first power level than when the power source is at the second power level; and wherein the second TX path circuitry is operative to provide WPT transmission at a greater efficiency to a WPT receiver when the power source is at the second power level than when the power source is at the first power level. 
     In Example 2, the subject matter of Example 1 includes, wherein the first power level is about 5 W or less, and wherein the second power level is greater than about 5 W. 
     In Example 3, the subject matter of Example 2 includes, wherein the power input is a universal serial bus (USB) power input operative to accept power via a USB connection at the first power level and at the second power level. 
     In Example 4, the subject matter of Examples 1-3 includes, wherein the first TX path circuitry and the second TX path circuitry include at least one shared portion of circuitry. 
     In Example 5, the subject matter of Examples 1-4 includes, wherein the first set of at least one transmission coil and the second set of at least one transmission coil include a common transmission coil. 
     In Example 6, the subject matter of Examples 1-4 includes, wherein the first set of at least one transmission coil and the second set of at least one transmission coil include a plurality of different transmission coils. 
     In Example 7, the subject matter of Examples 1-6 includes, wherein the second TX path circuitry includes a voltage level converter circuit, and wherein the second TX path circuitry does not include a voltage level converter circuit. 
     In Example 8, the subject matter of Examples 1-7 includes, wherein the first TX path circuitry includes a first power carrier signal generator circuit operative to generate a power carrier signal at a first frequency, and wherein the second TX path circuitry includes a second power carrier signal generator circuit operative to generate a power carrier signal at a second frequency that is different from the first frequency. 
     In Example 9, the subject matter of Examples 1-8 includes, wherein the controller circuitry is operative to: determine that the power level of the power source is the second power level which is greater than the first power level; detect a presence of the WPT receiver within WPT proximity; in response to detection of the presence of the WPT receiver within the WPT proximity, cause the first transmitter path circuitry to initiate WPT transmission in a low-power mode to the WPT receiver; receive a power request from the WPT receiver that requests a change in power to be wirelessly transmitted to the WPT receiver; and in response to the power request, cause a change the WPT power level to be wirelessly transmitted to the WPT receiver, including changing the path selection signal. 
     In Example 10, the subject matter of Example 9 includes, wherein the controller circuitry is further operative to: in response to initiation of WPT transmission in the low-power mode to the WPT receiver, transmit information identifying the determined power level of the power source to the WPT receiver. 
     In Example 11, the subject matter of Examples 9-10 includes, wherein the controller circuitry is further operative to: in response to the change of the WPT power level, receive a power optimization request from the WPT receiver that requests a second change in WPT power to be wirelessly transmitted to the WPT receiver, wherein the second change is a reduction of WPT carrier voltage; and in response to the power optimization request, cause the second change in WPT power. 
     Example 12 is apparatus for a wireless power transfer (WPT) receiver, the apparatus comprising: a power output arranged to provide WPT-received electrical power to a load; first receiver (RX) path circuitry arranged to be coupled to the power output and to a first set of at least one reception coil; second RX path circuitry arranged to be coupled to the power output and to a second set of at least one reception coil; receiver mode selector circuitry arranged to selectively couple one of the first RX path circuitry and the second RX path circuitry between the power output and the first or the second set of at least one reception coil in response to a path selection signal; and controller circuitry communicatively coupled to the receiver mode selector circuitry, and operative to generate the path selection signal based on a power requirement of the load; wherein one of the first RX path circuitry and the second RX path circuitry is operative to selectively receive WPT transmission from a WPT transmitter based on the path selection signal; wherein the first RX path circuitry is operative to receive the WPT transmission at a greater efficiency when the received WPT is at a first power level than when the received WPT is at a second power level; and wherein the second RX path circuitry is operative to receive the WPT transmission at a greater efficiency when the received WPT is at the second power level than when the received WPT is at the first power level. 
     In Example 13, the subject matter of Example 12 includes, wherein the first power level is about 5 W or less, and wherein the second power level is greater than about 5 W. 
     In Example 14, the subject matter of Examples 12-13 includes, wherein the first TX path circuitry and the second TX path circuitry include at least one shared portion of circuitry. 
     In Example 15, the subject matter of Examples 12-14 includes, wherein the first RX path circuitry includes a shunt capacitance across the first set of at least one reception coil that provides a resonance characteristic, and wherein the second RX path circuitry omits the shunt capacitance from the second set of at least one reception coil. 
     In Example 16, the subject matter of Examples 12-15 includes, wherein the first set of at least one reception coil and the second set of at least one reception coil include a common reception coil. 
     In Example 17, the subject matter of Examples 12-16 includes, wherein the controller circuitry is arranged to read a measurement of voltage at the load and of current being drawn by the load. 
     In Example 18, the subject matter of Example 17 includes, wherein the controller circuitry is operative to further generate the path selection signal based on the measurement of voltage and current. 
     In Example 19, the subject matter of Examples 17-18 includes, wherein the controller circuitry is arranged to be communicatively coupled to a WPT transmitter, and to generate a power optimization request to the WPT transmitter that requests a change in power to be wirelessly transmitted by the WPT transmitter, the power request being based on the measurement of voltage at the load and of current being drawn by the load. 
     In Example 20, the subject matter of Example 19 includes, wherein the power optimization request requests the change in power which is to reduce a voltage of the WPT to be wirelessly transmitted while substantially maintaining the same power level as was received via the WPT prior to the power optimization request such that greater current is transmitted via the WPT. 
     In Example 21, the subject matter of Examples 19-20 includes, wherein the controller circuitry is arranged to determine whether the voltage measurement at the load is less than a reference voltage level by a predefined margin, and wherein the power optimization request is based on the determination of whether the voltage measurement at the load is less than the reference voltage level by a predefined margin. 
     In Example 22, the subject matter of Examples 19-21 includes, wherein the power optimization request is further based on a determination of whether the current draw is within a defined range. 
     In Example 23, the subject matter of Examples 12-22 includes, wherein the controller circuitry arranged to be communicatively coupled to a WPT transmitter, to receive information indicating available power from the WPT transmitter, and to generate the path selection signal based further on the available power from the WPT transmitter. 
     Example 24 is apparatus for a wireless power transfer (WPT) receiver, the apparatus comprising: a power output arranged to provide WPT-received electrical power to a load; receiver (RX) path circuitry coupled to the power output and to a set of at least one reception coil, and to receive WPT via the set of at least one reception coil; controller circuitry arranged to read measurements of voltage at the load and of current being drawn by the load, and to dynamically adjust a characteristic of the WPT in response to the measurements. 
     In Example 25, the subject matter of Example 24 includes, wherein the controller circuitry is arranged to be communicatively coupled to a WPT transmitter, and to generate a power optimization request to the WPT transmitter that requests a change in power to be wirelessly transmitted by the WPT transmitter, the power request being based on the measurement of voltage at the load and of current being drawn by the load. 
     In Example 26, the subject matter of Example 25 includes, wherein the power optimization request requests the change in power which is to reduce a voltage of the WPT to be wirelessly transmitted while substantially maintaining the same power level as was received via the WPT prior to the power optimization request such that greater current is transmitted via the WPT. 
     In Example 27, the subject matter of Examples 25-26 includes, wherein the controller circuitry is arranged to determine whether the voltage measurement at the load is less than a reference voltage level by a predefined margin, and wherein the power optimization request is based on the determination of whether the voltage measurement at the load is less than the reference voltage level by a predefined margin. 
     In Example 28, the subject matter of Examples 25-27 includes, wherein the power optimization request is further based on a determination of whether the current draw is within a defined range. 
     Example 30 is an apparatus comprising means to implement of any of Examples 1-28. 
     Example 31 is a system to implement of any of Examples 1-28. 
     Example 32 is a method to implement of any of Examples 1-28. 
     The above detailed description includes references to the accompanying drawings, which form a part of the detailed description. The drawings show, by way of illustration, specific embodiments that may be practiced. These embodiments are also referred to herein as “examples.” Such examples may include elements in addition to those shown or described. However, also contemplated are examples that include the elements shown or described. Moreover, also contemplated are examples using any combination or permutation of those elements shown or described (or one or more aspects thereof), either with respect to a particular example (or one or more aspects thereof), or with respect to other examples (or one or more aspects thereof) shown or described herein. 
     Publications, patents, and patent documents referred to in this document are incorporated by reference herein in their entirety, as though individually incorporated by reference. In the event of inconsistent usages between this document and those documents so incorporated by reference, the usage in the incorporated reference(s) are supplementary to that of this document; for irreconcilable inconsistencies, the usage in this document controls. 
     In this document, the terms “a” or “an” are used, as is common in patent documents, to include one or more than one, independent of any other instances or usages of “at least one” or “one or more.” In this document, the term “or” is used to refer to a nonexclusive or, such that “A or B” includes “A but not B,” “B but not A,” and “A and B,” unless otherwise indicated. In the appended claims, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein.” Also, in the following claims, the terms “including” and “comprising” are open-ended, that is, a system, device, article, or process that includes elements in addition to those listed after such a term in a claim are still deemed to fall within the scope of that claim. Moreover, in the following claims, the terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to suggest a numerical order for their objects. 
     The above description is intended to be illustrative, and not restrictive. For example, the above-described examples (or one or more aspects thereof) may be used in combination with others. Other embodiments may be used, such as by one of ordinary skill in the art upon reviewing the above description. The Abstract is to allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. Also, in the above Detailed Description, various features may be grouped together to streamline the disclosure. However, the claims may not set forth every feature disclosed herein as embodiments may feature a subset of said features. Further, embodiments may include fewer features than those disclosed in a particular example. Thus, the following claims are hereby incorporated into the Detailed Description, with a claim standing on its own as a separate embodiment. The scope of the embodiments disclosed herein is to be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.