Patent Publication Number: US-2011057606-A1

Title: Safety feature for wireless charger

Description:
FIELD 
     The technical field relates to wireless charging of batteries in portable devices. More particularly, the technical field relates to techniques for reducing exposure to users of the electromagnetic charging fields used in wireless chargers. 
     BACKGROUND 
     Rechargeable batteries in cellular phones and other portable communication devices, such as NiCd, nickel-metal hydride (NiMH), Lithium-ion, and Lithium-Polymer batteries and Super Capacitors, can be recharged with household alternating current (AC) power coupled through a voltage reduction transformer, an alternating-to-direct current converter, and appropriate battery monitoring and charging circuits. They can also be recharged with a 12-volt cigarette lighter socket provided in an automobile coupled through a DC voltage reduction circuit and appropriate battery monitoring and charging circuits. However, in both cases, the portable communication device must be plugged into the household AC power source or into the automobile power source, limiting the mobility of the communication device. 
     Recently, wireless charging has become available for rechargeable batteries in cellular phones and other portable communication devices, using contact-less electromagnetic induction. A power source circuit in a wireless charging device drives a resonant frequency circuit that produces a source alternating current in a frequency range for example between 50 kHz and 20 MHz, which is driven through a transmitting coil in the charging device. The alternating magnetic field produced by the transmitting coil inductively couples with a corresponding receiving coil in the cellular phone or other portable communication device, thereby producing a corresponding induced alternating current that drives a circuit at its resonant frequency in the range for example between 50 kHz and 20 MHz to produce an output AC voltage. A conversion circuit in the cellular phone or other portable communication device, uses a transformer to adjust the output AC voltage, an alternating-to-direct current converter, and appropriate battery monitoring and charging circuits to produce an appropriate DC charging voltage for the rechargeable battery. 
     Large sized wireless charging pads have become available to charge rechargeable batteries in multiple portable communication devices, high powered hand tools, domestic appliances, or garden tools using contact-less electromagnetic induction. Wireless charging pads are generally shaped as a flat plate and typically have an active charging surface approximately the size of a sheet of typing paper. Other shapes for the charging pad may not be flat, but instead shaped to conform to particularly shaped user devices to be charged, for example a charger shaped as a wall-mounted holder for a garden tool. Wireless charging pads use multiple transmitting coils or a single large transmitting coil to distribute their magnetic flux over the active charging surface. Higher power levels greater than one watt may be required to drive the transmitting coils in a wireless charging pad in order to provide sufficient power to charge rechargeable batteries in multiple portable communication devices or other hand tools or appliances. This may be a cause for concern for the safety of users nearby. 
     The International Commission on Non-Ionizing Radiation Protection (ICNIRP) has published guidelines for limiting exposure to electromagnetic fields, in an article entitled “Limiting Exposure to Time-Varying Electric, Magnetic, and Electromagnetic Fields (up to 300 GHz)”,  Health Physics  74 (4): 494-522; 1998. The high power levels required in wireless charging pads may produce electromagnetic fields whose intensity near to the active charging surface may exceed the ICNIRP guidelines. 
     SUMMARY 
     Example embodiments are disclosed for detecting the proximity of a user to a wireless charger and switching off or gradually reducing the power applied to the transmitting coils as long as the user is closer than a threshold distance. In embodiments, a power source circuit in a wireless charging device is configured to produce a source alternating current. A transmitting coil is configured to magnetically couple with a proximately located receiving coil in a user&#39;s device, using contact-less electromagnetic induction, to wirelessly provide power to the receiving coil. A power control circuit is coupled between the power source and the transmitting coil, having a control input configured to control power delivered from the power source to the transmitting coil. The controlled power can be a simple binary on/off control or it may be a graduated step-wise control, or it may be a continuous control between a minimum and maximum output power. A proximity detector is positioned near the transmitting coil and coupled to the control input of the power control circuit, to detect proximity of the user to the detector and provide a control signal to the power control circuit to cause the power control circuit to reduce power delivered from the power source to the transmitting coil. Power control circuit may optionally be integrated with circuits of the power source. In this manner, the exposure of the user is minimized to the intense electromagnetic fields required in wireless chargers. 
     In example embodiments, the transmitting coil in the charger may be part of a self-resonant circuit and the receiving coil in the user&#39;s device may be part of a self-resonant circuit and each self-resonant circuit may be tuned to resonate at the same frequency so as to operate as magnetically coupled resonators. The transmitting coil and the receiving coil are then strongly coupled when the power source circuit in the charging device drives the transmitting coil at the resonant frequency common to both coils, even when the distance between the two coils is several times larger than the geometric sizes of the coils. This resonant magnetic coupling enables efficient power transfer from the wireless charger to the wirelessly charged device. 
     In example embodiments, the proximity detector may be an infrared body heat detector configured to detect a threshold level of infrared body heat radiating from the user and to cause the power control circuit to reduce power delivered from the power source to the transmitting coil. In embodiments, the proximity detector may be an infrared pulse detector configured to transmit a primary infrared pulse signal and to detect a threshold level of reflected infrared pulse signal from the user and to cause the power control circuit to reduce power delivered from the power source to the transmitting coil. The proximity detector may be an ultrasonic detector configured to transmit a primary ultrasound signal and to detect a threshold level of reflected ultrasound signal from the user and to cause the power control circuit to reduce power delivered from the power source to the transmitting coil. The proximity detector may be an optical detector configured to transmit a primary light signal and to detect a threshold level of reflected light signal from the user and to cause the power control circuit to reduce power delivered from the power source to the transmitting coil. The proximity detector may be an acoustic detector configured to transmit a primary acoustic signal and to detect a threshold level of reflected acoustic signal from the user and to cause the power control circuit to reduce power delivered from the power source to the transmitting coil. The proximity detector may be a microwave detector configured to transmit a primary microwave signal and to detect a threshold level of reflected microwave signal from the user and to cause the power control circuit to reduce power delivered from the power source to the transmitting coil. The proximity detector may be a combination of two or more detectors taken from the group consisting of an infrared detector, an ultrasonic detector, an optical detector, an acoustic detector, and a microwave detector, the combination of detectors configured to detect proximity of the user and to cause the power control circuit to reduce power delivered from the power source to the transmitting coil. 
     In embodiments, the detector and the transmitting coil may be configured to be positioned in close proximity to one another on a substrate generally shaped as a flat plate. 
     In embodiments, the power control circuit may reduce power to the transmitting coil upon receiving the control signal from the detector, so as to reduce ambient electromagnetic fields near the transmitting coil below a safe exposure level for the user. 
     In embodiments, the transmitting coil being configured to wirelessly charge rechargeable batteries in multiple portable communication devices, high powered hand tools, domestic appliances, or garden tools using contact-less electromagnetic induction. 
     In embodiments, a method includes the steps of generating an alternating current in a wireless charger; driving a transmitting coil with the alternating current to produce an electromagnetic field; magnetically coupling a proximately located receiving coil in a user&#39;s device with the electromagnetic field to wirelessly provide power to the receiving coil; detecting proximity of a user to the transmitting coil; and reducing the alternating current to the transmitting coil in response to detecting the proximity of the user, to reduce exposure of the user to the electromagnetic field. In this manner, the exposure of the user is minimized to the intense electromagnetic fields required in wireless chargers. 
    
    
     
       DESCRIPTION OF THE FIGURES 
         FIG. 1A  illustrates an example embodiment for a wireless charger. 
         FIG. 1B  illustrates another example embodiment for a wireless charger. 
         FIG. 2A  illustrates an example embodiment for a wirelessly charged user device, such as a portable communication device. 
         FIG. 2B  illustrates an example embodiment wherein the transmitting coil in the charger may be part of a resonant circuit and the receiving coil in the user&#39;s device may be part of a resonant circuit and each resonant circuit may be tuned to resonate at the same frequency “F” so as to operate as magnetically coupled resonators. 
         FIG. 3A  illustrates an example embodiment for a wireless charger with the power transmitting coil being a printed wiring coil on a printed wiring board and the proximity detector mounted on the board. 
         FIG. 3B  illustrates an example embodiment for the wireless charger with the power transmitting coil being a printed wiring coil on a printed wiring board and the wirelessly charged user device with the power receiving coil being a printed wiring coil on a printed wiring board. 
         FIG. 3C  illustrates a side-view of a single turn of the power transmitting coil and the resulting pattern of magnetic flux encircling the single turn of the coil. 
         FIG. 4A  illustrates an example of the infrared pulse proximity detector providing a control signal to the power control circuit to cause the power control circuit to reduce power delivered from the power source to the transmitting coil. 
         FIG. 4B  illustrates an example of the ultrasonic proximity detector providing a control signal to the power control circuit to cause the power control circuit to reduce power delivered from the power source to the transmitting coil. 
         FIG. 4C  illustrates an example of the optical proximity detector providing a control signal to the power control circuit to cause the power control circuit to reduce power delivered from the power source to the transmitting coil. 
         FIG. 4D  illustrates an example of the acoustic proximity detector providing a control signal to the power control circuit to cause the power control circuit to reduce power delivered from the power source to the transmitting coil. 
         FIG. 4E  illustrates an example of the microwave proximity detector providing a control signal to the power control circuit to cause the power control circuit to reduce power delivered from the power source to the transmitting coil. 
         FIG. 4F  illustrates an example of the proximity detector being a combination of an infrared detector, an ultrasonic detector, an optical detector, an acoustic detector, and a microwave detector, the combination of detectors configured to detect proximity of the user and to cause the power control circuit to reduce power delivered from the power source to the transmitting coil. 
         FIG. 5  is an example set of graphs in the time domain, illustrating the relationship of the measured proximity distance from the proximity detector using a time of flight measurement to the user and the resulting power output from the power control circuit to the transmitting coil. 
         FIG. 6  illustrates an example of the infrared body heat proximity detector providing a control signal to the power control circuit to cause the power control circuit to reduce power delivered from the power source to the transmitting coil. 
         FIG. 7  is an example set of graphs in the time domain, illustrating the relationship of the measured proximity distance from the proximity detector using infrared user&#39;s body heat proximity detector of  FIG. 6A  and the resulting power output from the power control circuit to the transmitting coil. 
         FIG. 8  illustrates an example of the proximity detector being a combination of the infrared user&#39;s body heat proximity detector of  FIG. 6A , an ultrasonic detector, an optical detector, an acoustic detector, and a microwave detector, the combination of detectors configured to detect proximity of the user and to cause the power control circuit to reduce power delivered from the power source to the transmitting coil. 
         FIG. 9  is an example flow diagram of an example operation for a wireless charger. 
     
    
    
     DISCUSSION OF EXAMPLE EMBODIMENTS OF THE INVENTION 
       FIG. 1A  illustrates an example embodiment for a wireless charger  100 , also known as a wireless charging pad  100 . The wireless charger  100  includes a proximity detector  106  that detects the proximity of a user to the wireless charger  100  and switches off or gradually reduces the power applied to the transmitting coils  120  as long as the user is closer than a threshold distance. In embodiments, a power source circuit  102  in the wireless charger  100  is configured to produce a source alternating current, for example in a frequency range for example between 50 kHz and 20 MHz. A transmitting coil  120  is configured to magnetically couple with a proximately located receiving coil  220  in a user&#39;s device  200  of  FIG. 2 , using contact-less electromagnetic induction, to wirelessly provide power to the receiving coil  220 . A power control circuit  105  is coupled between the power source  102  and the transmitting coil  120 , having a control input configured to control power delivered from the power source  102  to the transmitting coil  120 . The controlled power can be a simple binary on/off control or it may be a graduated step-wise control, or it may be a continuous control between a minimum and maximum output power. The proximity detector  106  is positioned near the transmitting coil  120  and coupled to the control input of the power control circuit  105 , to detect proximity of the user to the detector  106  and provide a control signal to the power control circuit  105  to cause the power control circuit to reduce power delivered from the power source  102  to the transmitting coil  120 . In this manner, the exposure of the user is minimized near the active charging surface of the wireless charger  100 , to the intense electromagnetic fields. The detector(s) can also detect the proximity of pets or domestic animals, in addition to a human user. 
     In an example embodiment, a power source circuit  102  in the wireless charging device  100  drives a power frequency driver and interface  104  through the power control circuit  105 , which produces a source alternating current in a frequency range, for example, between 50 kHz and 20 MHz, which will provide energy to recharge the rechargeable batteries  216  in the user&#39;s charged device  200  of  FIG. 2 . 
     The controlled power can be a simple binary on/off control or it may be a graduated step-wise control, or it may be a continuous control between a minimum and maximum output power.  FIG. 1B  illustrates another example embodiment for a wireless charger, wherein an AC mains or DC battery  101  provides power to the AC power source  102  to output alternating current in a range, for example, from 50 kHz to 20 MHz. Control circuits  103  monitor the output from the AC mains or DC battery  101  and the control signals from the proximity detector  105  to control the level of power output by the power source  102  through the power control circuit  105  to the power transmitting coil  120 . For example, the graduated power steps output by the power source  102  through the power control circuits  105  to the power transmitting coil  120 , may be controlled by the control circuits  103  based on the distance measured by the proximity detector  105  between the user and the transmitting coil  120 . For example, for a relative Max power=5, and a low power=1, the controlled graduations in power vs. proximity (in centimeters) may be:
         User touch to device or distance 10 cm-&gt;Power off or smallest power step 1   User to charger distance 20 cm-&gt;Power level 2   User to charger distance 30 cm-&gt;Power level 3   User to charger distance 40 cm-&gt;Power level 4   User to charger distance 50 cm-&gt;Power level 5.       

     In the example embodiments, the source alternating current may be passed through an optional radio frequency blocking filter  110  to limit the radio frequency noise that would otherwise reach the communication circuits and RF antenna  18  of the user&#39;s communication device  200  of  FIG. 2A . The optional radio frequency blocking filter  110  and the radio frequency blocking filter  210  in the user&#39;s charged device  200  of  FIG. 2A  are described in greater detail in the copending US patent application entitled “Wireless Charging Coil Filtering” by Esa Ilmari Saunamäki, application Ser. No. 12/498,872, filed Jul. 7, 2009, which is incorporated herein by reference. 
     The alternating magnetic field  300  shown in  FIG. 3A , which is produced by the power transmitting coil  120 , magnetically couples with a proximately located receiving coil  220  in the user&#39;s charged device  200 , using contact-less electromagnetic induction. The two coils  120  and  220  may be planar coils that are positioned proximate to each other in a coplanar mutual orientation, as shown in  FIG. 3B , where the close proximity of the coplanar coils  120  and  220  improves the inductive coupling between them.  FIG. 3C  illustrates a side-view of a single turn of the power transmitting coil  120  and the resulting pattern of magnetic flux  300  encircling the single turn of the coil  120 . 
     The user&#39;s charged device  200  may be a mobile communications device, FM radio, two-way radio, PDA, cell phone, laptop or palmtop computer, or the like. The device  200  may also be a high powered hand tool, a domestic appliance, or a garden tool using contact-less electromagnetic induction to charge its rechargeable batteries. The alternating magnetic field  300  produces a corresponding induced alternating current in the power receiving coil  220 . The induced alternating current may be passed through a radio frequency blocking filter  210 . 
     The filtered induced alternating current drives the rectifier and interface  212  in a range for example between 50 kHz and 20 MHz to produce an appropriate DC charging voltage for the rechargeable battery  216 . A battery control circuit  214  adjusts the DC voltage and current. Optionally, charging identification circuits (not shown) may identify the target current and voltage to be applied to each type of rechargeable battery. 
       FIG. 2A  shows a functional block diagram of an example embodiment of the wireless user&#39;s device  200 , which is shown as a communications device, for example. The wireless device  200  may be for example a mobile communications device, FM-radio, two-way radio, PDA, cell phone, laptop or palmtop computer, or the like. The wireless device  200  includes a control module  20 , which includes a central processing unit (CPU)  60 , a random access memory (RAM)  62 , a read only memory (ROM)  64 , and interface circuits  66  to interface with the transceiver  12 , battery and other energy sources, key pad, touch screen, display, microphone, speakers, ear pieces, camera or other imaging devices, etc. The RAM  62  and ROM  64  can be removable memory devices such as smart cards, SIMs, WIMs, semiconductor memories such as RAM, ROM, PROMS, flash memory devices, etc. The application and MAC layer may be embodied as program logic stored in the RAM  62  and/or ROM  64  in the form of sequences of programmed instructions which, when executed in the CPU  60 , carry out the functions of the disclosed embodiments. The program logic can be delivered to the writeable RAM, PROMS, flash memory devices, etc.  62  of the wireless device  200  from a computer program product or article of manufacture in the form of computer-usable media such as resident memory devices, smart cards or other removable memory devices. Alternately, the MAC layer and application program can be embodied as integrated circuit logic in the form of programmed logic arrays or custom designed application specific integrated circuits (ASIC). 
       FIG. 2B  illustrates an example embodiment wherein the transmitting coil  120  in the charger  100  may be part of a self-resonant circuit  125  and the receiving coil  220  in the user&#39;s device  200  may be part of a self-resonant circuit  225 . The resonant circuit  125  is self-resonant at the frequency “F” and the resonant circuit  225  is self-resonant at the frequency “F”. Each resonant circuit  125  and  225  is tuned to resonate at the same frequency “F” so that they operate as magnetically coupled resonators. The transmitting coil  120  and the receiving coil  220  are strongly coupled by the resonant magnetic flux  300  oscillating at the frequency “F” when the power source circuit  102  in the charging device  100  drives the transmitting coil  120  at the resonant frequency “F” common to both coils  120  and  220 . The resonant magnetic coupling is strong even when the distance between the two coils is several times larger than the geometric sizes of the coils. The resonant frequency “F” can be in the MHz range, for example from 1 MHz to over 27 MHz. This enables efficient power transfer from the wireless charger  100  to the wirelessly charged device  200 . 
       FIG. 3A  illustrates an example embodiment for a wireless charger  100  with the power transmitting coil  120  being a printed wiring coil on a printed wiring board  122  and the proximity detector  106  mounted on the board  122 . The large sized wireless charger  100  has the capacity to charge rechargeable batteries in multiple portable user devices such as cell phones, high powered hand tools, domestic appliances, or garden tools using contact-less electromagnetic induction. The wireless charger  100  of  FIGS. 3A and 3B  is generally shaped as a flat plate and typically has an active charging surface approximately the size of a sheet of typing paper. However, the size of the charging surface may be considerably larger or smaller (than a sheet of typing paper) depending on the number and/or size of transmitting coils. The wireless charger  100  may use multiple transmitting coils  120  or a single large transmitting coil  120  to distribute its magnetic flux  300  over the active charging surface. Higher power levels greater than one watt may be required to drive the transmitting coil  120  in the wireless charger  100  in order to provide sufficient power to charge rechargeable batteries in multiple portable user devices such as cell phones or other hand tools or appliances. 
       FIG. 3B  illustrates an example embodiment for the wireless charger  100  of  FIG. 3A  with the power transmitting coil  120  being a printed wiring coil on a printed wiring board  122  shown in the side view. In alternate embodiments, a separate printed wiring board  122  may be omitted and the coil  120  may incorporated into the body of the printed wiring board or it may be glued to a plastic substrate.  FIG. 3B  also illustrates an example embodiment for the wirelessly charged user device  200  with the power receiving coil  220  being a printed wiring coil on a printed wiring board  222  shown in the side view. In alternate embodiments, a separate printed wiring board  222  may be omitted and the coil  220  may incorporated into the body of the printed wiring board or it may be glued to a plastic substrate. Coils  120  and  220  are planar coils printed on their respective circuit boards  122  and  222 . Coils  120  and  220  are shown juxtaposed, coplanar, and in close proximity to enable efficient inductive coupling by the magnetic field  300 . The two coils  120  and  220  are positioned proximate to each other in a coplanar mutual orientation, so that the close proximity of the coplanar coils  120  and  220  improves the magnetic coupling between them. In embodiments, an additional ferromagnetic foil may be affixed to the backside of the coils  120  and  220  to shield any stray magnetic flux. 
       FIG. 4A  illustrates an example of the infrared pulse proximity detector  106 A positioned near the transmitting coil  120  on the printed wiring board  122  and coupled to the control input of the power control circuit  105 , to detect proximity of the user to the detector  106 A and provide a control signal to the power control circuit  105  to cause the power control circuit to reduce power delivered from the power source  102  to the transmitting coil  120 . The measured proximity distance L(A) between the user and the proximity detector  106 A is determined by a round trip time of flight measurement to the user. 
       FIG. 5  is an example set of graphs in the time domain, illustrating the relationship of the measured proximity distance from the proximity detector  106  using a round trip time of flight measurement to the user, such as infrared pulse proximity detector  106 A, and the resulting power output from the power control circuit  105  to the transmitting coil  120 . 
     Graph A of  FIG. 5  illustrates an example of the proximity distance “L” between the detector  106  and the user, plotted versus time. Graph B of  FIG. 5  illustrates an example of the round trip time of flight “TL”, plotted versus time, for a pulse of infrared light emitted by the detector  106  and reflected back from the user to the detector  106 . As the user&#39;s body approaches the detector  106 , the round trip time of flight “TL” becomes smaller. In one example embodiment, the round trip time of flight “TL” may be used as a measure of the proximity of the user to the transmitting coil  120  and the proximity detector  106  will output a control signal to the power control circuit  105  when the value of “TL” indicates that the user is closer than the threshold distance. 
     Graph C of  FIG. 5  illustrates an example of the time derivative “dTL/dt” of the round trip time of flight “TL” in Graph B, plotted versus time. The maximum negative value of the time derivative “dTL/dt” may be used as a trigger event to begin signaling the power control circuit  105  reduce the power from a full power value to a low power value for power delivered to the transmitting coil  120 , in order to minimize exposure of the user to the high magnetic flux  300 . Other values of the time derivative “dTL/dt” may be used as the triggering event to begin reducing power. The advantage of using the time derivative of “TL” instead of the measured round trip time of flight “TL” is that the time derivative of “TL” enables the detector  106  to distinguish a moving object, such as the user&#39;s body, from stationary objects in the vicinity of the detector  106 . Graph D of  FIG. 5  illustrates an example of the power output by the power control circuit  105  to the transmitting coil  120 , showing that the output power begins to decrease from a full power value to a low power value for power delivered to the transmitting coil  120 , when the trigger event occurs of the maximum negative value of the time derivative “dTL/dt”. 
     As the user&#39;s body moves away from the detector  106 , the round trip time of flight “TL” becomes larger and the chance of exposure to the user is reduced. The maximum positive value of the time derivative “dTL/dt” may then be used as a trigger event, for example, to signal the power control circuit  105  to begin increasing the power to full power delivered to the transmitting coil  120 . Graph D of  FIG. 5  illustrates an example of the output power beginning to increase from a low power value to a full power value for power delivered to the transmitting coil  120 , when the trigger event occurs of the maximum positive value of the time derivative “dTL/dt”. Other values of the time derivative “dTL/dt” may be used as the triggering event to begin increasing power. 
       FIG. 4B  illustrates an example of the ultrasonic proximity detector  106 B providing a control signal to the power control circuit  105  to cause the power control circuit to reduce power delivered from the power source  102  to the transmitting coil  120 . The relationship of the measured proximity distance L(B) from the proximity detector  106 B using a round trip time of flight measurement to the user and the resulting power output from the power control circuit  105  to the transmitting coil  120 , is similar to that described in  FIG. 5 . 
       FIG. 4C  illustrates an example of the optical proximity detector  106 C providing a control signal to the power control circuit  105  to cause the power control circuit to reduce power delivered from the power source  102  to the transmitting coil  120 . The relationship of the measured proximity distance L(C) from the proximity detector  106 C using a round trip time of flight measurement to the user and the resulting power output from the power control circuit  105  to the transmitting coil  120 , is similar to that described in  FIG. 5 . 
       FIG. 4D  illustrates an example of the acoustic proximity detector  106 D providing a control signal to the power control circuit  105  to cause the power control circuit to reduce power delivered from the power source  102  to the transmitting coil  120 . The relationship of the measured proximity distance L(D) from the proximity detector  106 D using a round trip time of flight measurement to the user and the resulting power output from the power control circuit  105  to the transmitting coil  120 , is similar to that described in  FIG. 5 . 
       FIG. 4E  illustrates an example of the microwave proximity detector  106 E providing a control signal to the power control circuit  105  to cause the power control circuit to reduce power delivered from the power source  102  to the transmitting coil  120 . The relationship of the measured proximity distance L(E) from the proximity detector  106 E using a round trip time of flight measurement to the user and the resulting power output from the power control circuit  105  to the transmitting coil  120 , is similar to that described in  FIG. 5 . 
       FIG. 4F  illustrates an example of the proximity detector  106  being a combination of an infrared detector  106 A, an ultrasonic detector  106 B, an optical detector  106 C, an acoustic detector  106 D, and a microwave detector  106 E. The output of these detectors is integrated in the integrator  107  to obtain an aggregate proximity distance value L′ using an empirical relationship=F[L(A),L(B),L(C),L(D),L(E)]. The output L′ of the integrator  107  is applied to the control input of the power control circuit  105  to cause the power control circuit  105  to reduce power delivered from the power source  102  to the transmitting coil  120 . 
       FIG. 6  illustrates an example of the infrared body heat proximity detector  106 ′ providing a control signal to the power control circuit  105  to cause the power control circuit to reduce power delivered from the power source  102  to the transmitting coil  120 .  FIG. 7  is an example set of graphs in the time domain, illustrating the relationship of the measured proximity distance L(BT) from the proximity detector  106 ′ using the user&#39;s measured infrared body heat and the resulting power output from the power control circuit  105  to the transmitting coil  120 . 
     Graph A of  FIG. 7  illustrates an example of the proximity distance “L” between the detector  106  and the user, plotted versus time. Graph B of  FIG. 7  illustrates an example of the user&#39;s measured infrared body heat “BT”, plotted versus time. As the user&#39;s body approaches the detector  106 , user&#39;s measured infrared body heat becomes larger. In one example embodiment, the user&#39;s measured infrared body heat “BT” may be used as a measure of the proximity of the user to the transmitting coil  120  and the proximity detector  106  will output a control signal to the power control circuit  105  when the value of “BT” indicates that the user is closer than the threshold distance. 
     Graph C of  FIG. 7  illustrates an example of the time derivative “dBT/dt” of the user&#39;s measured infrared body heat in Graph B, plotted versus time. The maximum positive value of the time derivative “dBT/dt” may be used as a trigger event to begin signaling the power control circuit  105  reduce the power from a full power value to a low power value for power delivered to the transmitting coil  120 , in order to minimize exposure of the user to the high magnetic flux  300 . Other values of the time derivative “dBT/dt” may be used as the triggering event to begin reducing power. The advantage of using the time derivative of “BT” instead of the user&#39;s measured infrared body heat “BT” is that the time derivative of “BT” enables the detector  106 ′ to distinguish a moving object, such as the user&#39;s body, from stationary objects in the vicinity of the detector  106 . Graph D of  FIG. 7  illustrates an example of the power output by the power control circuit  105  to the transmitting coil  120 , showing that the output power begins to decrease from a full power value to a low power value for power delivered to the transmitting coil  120 , when the trigger event occurs of the maximum positive value of the time derivative “dBT/dt”. 
     As the user&#39;s body moves away from the detector  106 ′, the user&#39;s measured infrared body heat “BT” becomes smaller and the chance of exposure to the user is reduced. The maximum negative value of the time derivative “dBT/dt” may then be used as a trigger event, for example, to signal the power control circuit  105  to begin increasing the power to full power delivered to the transmitting coil  120 . Graph D of  FIG. 7  illustrates an example of the output power beginning to increase from a low power value to a full power value for power delivered to the transmitting coil  120 , when the trigger event occurs of the maximum negative value of the time derivative “dBT/dt”. Other values of the time derivative “dBT/dt” may be used as the triggering event to begin increasing power. 
       FIG. 8  illustrates an example of the proximity detector being a combination of the infrared user&#39;s body heat proximity detector  106 ′ of  FIG. 6 , an infrared pulse proximity detector  106 A of  FIG. 4A , an ultrasonic detector  106 B, an optical detector  106 C, and an acoustic detector  106 D. The output of these detectors is integrated in the integrator  107  to obtain an aggregate proximity distance value L″ using an empirical relationship=F[L(BT),L(A),L(B),L(C),L(D)]. The output L″ of the integrator  107  is applied to the control input of the power control circuit  105  to cause the power control circuit  105  to reduce power delivered from the power source  102  to the transmitting coil  120 . Any combination of detectors shown in  FIGS. 8 and 4F  can be used (e.g.  106 A+ 106 C+ 106 E). Another type of proximity detector is a camera based sensor programmed with software for movement detection. 
     An optional light, buzzer, or other indictor may be coupled to the power control circuit  105  to alert the user when power has been reduced to the transmitting coil  120  because the user has moved closer than a safe distance from the transmitting coil during the charging operation. 
     The method  400  of  FIG. 9  includes the following steps: 
     Step  402 : Generate with power source  102  an alternating current in a wireless charger  100 . 
     Step  404 : Drive a transmitting coil  120  with the alternating current to produce an electromagnetic field  300 . 
     Step  406 : Magnetically couple a proximately located receiving coil  220  in a user&#39;s device  200  with the electromagnetic field  300  to wirelessly provide power to the receiving coil  220 . 
     Step  408 : Detect proximity with proximity detector  106  of a user to the transmitting coil  120 . 
     Step  410 : Reduce with power control circuit  105  the alternating current to the transmitting coil  120  in response to detecting the proximity of the user, to reduce exposure of the user to the electromagnetic field  300 . 
     In this manner, the exposure of the user is minimized near the active charging surface of the wireless charger  100 , to the intense electromagnetic fields. The detector(s) can also detect the proximity of pets or domestic animals, in addition to a human user. 
     Using the description provided herein, the embodiments may be implemented as a machine, process, or article of manufacture by using standard programming and/or engineering techniques to produce programming software, firmware, hardware or any combination thereof. 
     Any resulting program(s), having computer-readable program code, may be embodied on one or more computer-usable media such as resident memory devices, smart cards or other removable memory devices, or transmitting devices, thereby making a computer program product or article of manufacture according to the embodiments. As such, the terms “article of manufacture” and “computer program product” as used herein are intended to encompass a computer program that exists permanently or temporarily on any computer-usable medium. 
     As indicated above, memory/storage devices include, but are not limited to, disks, optical disks, removable memory devices such as smart cards, SIMs, WIMs, semiconductor memories such as RAM, ROM, PROMS, etc. Transmitting mediums include, but are not limited to, transmissions via wireless communication networks, the Internet, intranets, telephone/modem-based network communication, hard-wired/cabled communication network, satellite communication, and other stationary or mobile network systems/communication links. 
     Although specific example embodiments have been disclosed, a person skilled in the art will understand that changes can be made to the specific example embodiments without departing from the spirit and scope of the invention.