Abstract:
Wireless power transfer between a power transmitter and a power receiver may include a power transfer established by a detector of the wireless power transmitter detecting a magnetic field from a wireless power receiver in proximity to the wireless power transmitter and activating the wireless power transmitter to transfer power to the wireless power receiver.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 12/883,457 filed Sep. 16, 2010, which is a continuation of PCT application Serial No. PCT/IL2008/001641 filed Dec. 18, 2008, which claims the benefit of U.S. provisional application Ser. Nos. 61/064,618 filed Mar. 17, 2008, 61/071,151 filed Apr. 15, 2008, 61/129,526 filed Jul. 2, 2008, 61/129,859 filed Jul. 24, 2008 and 61/129,970 filed Aug. 4, 2008, the disclosures of which are hereby incorporated in their entirety by reference herein. 
    
    
     TECHNICAL FIELD 
     The present invention is directed to inductive electrical power transfer. More specifically, the present invention relates to providing a transmission guard for preventing an inductive power outlets from transmitting power in the absence of an inductive power receiver. 
     BACKGROUND 
     Inductive power coupling, as known in the art, allows energy to be transferred from a power supply to an electric load without connecting wires. A power supply is wired to a primary coil and an oscillating electric potential is applied across the primary coil, thereby inducing an oscillating magnetic field. The oscillating magnetic field may induce an oscillating electrical current in a secondary coil placed close to the primary coil. In this way, electrical energy may be transmitted from the primary coil to the secondary coil by electromagnetic induction without the two coils being conductively connected. When electrical energy is transferred from a primary coil to a secondary coil the coil pair are said to be inductively coupled. An electric load wired in series with such a secondary coil may draw energy from the power source wired to the primary coil when the secondary coil is inductively coupled thereto. 
     Induction type power outlets may be preferred to the more common conductive power sockets because they provide seamless power transmission and minimize the need for trailing wires. 
     Low power inductive electrical power transmission systems have been proposed. One such example is described in U.S. Pat. No. 7,164,255 to Hui. In Hui&#39;s system a planar inductive battery charging arrangement enables electronic devices to be charged. The system includes a planar charging module having a charging surface on which a device to be charged is placed. Within the charging module, and parallel to the charging surface, at least one, and preferably an array of primary windings are provided. The primary windings inductively couple with secondary windings within the device to be charged. 
     Such systems provide inductive coupling at relatively low power adequate for charging batteries. It will be appreciated however, that base units such as Hui&#39;s charging surface which transmit energy continuously, in a largely uniform manner over an extended area, are not suitable for use with high energy systems, such as those required to power computers, light bulbs, televisions and the like. 
     Energy losses associated with high power inductive transfer systems are typically larger than those in low power systems such as Hui&#39;s charging surface. In addition whereas in low power systems excess heat may be readily dissipated, an uncoupled high power primary coil or its surroundings may become dangerously hot. 
     Moreover, the oscillating voltage in a high power primary coil produces a oscillating magnetic field. Where a secondary coil is inductively coupled to the primary coil, the resulting flux linkage causes power to be drawn by the secondary coil. Where there is no secondary coil to draw the power, the oscillating magnetic field causes high energy electromagnetic waves to be radiated in all directions which may have undesired side affects, such as erasing data from credit cards and may be harmful to bystanders particularly to those with pacemakers. 
     U.S. Pat. No. 6,803,744, to Sabo, titled “Alignment independent and self aligning inductive power transfer system” describes an inductive power transfer device for recharging cordless appliances. Sabo&#39;s device includes a plurality of inductors which serve as the primary coil of a transformer. The secondary coil of the transformer is arranged within the appliance. When the appliance is positioned proximate to the power transfer device with the respective coils in alignment, power is inductively transferred from the device to the appliance via the transformer. 
     The inductors of Sabo&#39;s system are arranged in an array and connected to a power supply via switches which are selectively operable to activate the respective inductors. These selectively operable switches are provided to conserve power and to eliminate objectionable electromagnetic fields. &#39;744 thus indicates the problem of electromagnetic leakage as well as the need for each primary coil to be energized from the power supply only when a secondary coil is within effective range. Furthermore the power receiving units described in &#39;744 are bulky and impractical for use with small electrical devices. 
     The need remains therefore for a practical inductive power transfer system for safely and conveniently delivering power wirelessly from inductive power outlets to inductive power receivers in an energy efficient manner. The present invention addresses this need. 
     SUMMARY 
     It is an aim of the invention to provide a transmission system comprising at least one inductive power outlet for transferring power inductively to at least one inductive power receiver, at least one alignment mechanism for aligning the power receiver to the power outlet and at least one signal transfer system for passing control signals from the power receiver to the power outlet. Typically, the inductive power outlet comprises at least one primary inductive coil wired to a power source via a driver, the driver for providing an oscillating voltage across the primary inductive coil at high frequency, and a primary ferromagnetic core. The inductive power receiver comprises at least one secondary inductive coil for coupling with the primary inductive coil, the secondary inductive coil being wired to a power regulator. The signal transfer system comprises a signal emitter associated with the power receiver and a signal detector associated with the power outlet. 
     Optionally, the alignment mechanism comprises a first element associated with the power outlet and a second magnetic element associated with the power receiver. Typically, the signal emitter comprises an optical emitter and the signal detector comprises an optical detector. Optionally, again, the power regulator comprises a rectifier for converting an AC input from the inductive power outlet into a DC output for powering the electrical device. Another aim of the invention is to present an inductive power receiver for receiving power inductively from an inductive power outlet. Optionally, the system comprises a power converter selected from the group comprising: a transformer, a DC-to-DC converter, an AC-to-DC converter, an AC-to-AC converter, a flyback transformer, a flyback converter, a full-bridge converter, a half-bridge converter and a forward converter. 
     A further aim of the invention is to present an inductive power transfer system comprising at least one inductive power outlet for transferring power inductively to at least one inductive power receiver, the inductive power outlet comprising at least one primary inductor wired to a power source via a driver, and the inductive power receiver comprising at least one secondary inductor for coupling with the primary inductor, the secondary inductor for providing power to an electric load; the inductive power transfer system comprising a low heat-loss full wave rectifier comprising: a first half-wave rectifier having one anode wired to a first output terminal and one cathode wired to a first input terminal; a second half-wave rectifier having one anode wired to the first output terminal and one cathode wired to a second input terminal; a third half-wave rectifier having one anode wired to the first input terminal and one cathode wired to a second output terminal, and a fourth half-wave rectifier having one anode wired to the second input terminal and one cathode wired to the second output terminal; the full wave rectifier for providing an output of constant polarity from an input of variable polarity, wherein at least one half-wave rectifier comprises an electronic switch configured to be in its ON state when the current flowing through the cathode of the switch exceeds a predetermined threshold. 
     Optionally, the first half-wave rectifiers comprises a first electronic switch configured to be in its ON setting when the current flowing through its cathode exceeds a first predetermined threshold, and the second half-wave rectifiers comprises a second electronic switch configured to be in its ON setting when the current flowing through its cathode exceeds a second predetermined threshold. Preferably, at least one half-wave rectifier comprises an electronic switch configured to be switched between it&#39;s ON and OFF states in synchrony with the frequency of the input signal. 
     Preferably, the first half-wave rectifier comprises a first electronic switch configured to be in its ON state when the current flowing through its cathode exceeds a predetermined threshold; the second half-wave rectifiers comprises a second electronic switch configured to be in its ON state when the current flowing through its cathode exceeds a predetermined threshold; the third half-wave rectifiers comprises a third electronic switch configured to be switched between its ON and OFF states in phase with the voltage signal at the second input terminal, and the fourth half-wave rectifiers comprises a third electronic switch configured to be switched between its ON and OFF states in phase with the voltage signal at the first input terminal. 
     Typically the electronic switch comprises a transistor, in particular, a MOSFET device. Preferably, the electronic switch comprises: a MOSFET device comprising a source terminal, a drain terminal and a gate terminal; a half-wave rectifier wired to the source terminal and the drain terminal in parallel with the MOSFET device, and a current monitor configured to monitor a drain-current flowing through the drain terminal and to send a gate signal to the gate terminal such that the MOSFET is switched to its ON state when the drain-current exceeds a first threshold current and the MOSFET is switched to its OFF state when the drain-current falls below a second threshold current. Optionally, the current monitor comprises a current transformer. 
     It is another aim of the invention to present an inductive power transfer system comprising at least one inductive power outlet comprising at least one primary inductive coil wired to a power supply via a driver; the primary inductive coil for forming an inductive couple with at least one secondary inductive coil wired to an electric load, the secondary inductive coil associated with an inductive power receiver wherein the driver is configured to provide a driving voltage across the primary inductive coil, the driving voltage oscillating at a transmission frequency significantly different from the resonant frequency of the inductive couple. Optionally, the driver comprises a switching unit for intermittently connecting the primary inductive coil to the power supply. 
     Preferably, the transmission frequency lies within a range in which induced voltage varies approximately linearly with frequency. Optionally, the driver is configured to adjust the transmission frequency in response to the feedback signals. 
     Optionally, the inductive power outlet comprising a signal detector adapted to detect a first signal and a second signal, and the driver is configured to: increase the transmission frequency when the first signal is detected by the detector, and decrease the transmission frequency when the second signal is detected by the detector. The feedback signals generally carry data pertaining to the operational parameters of the electric load. Operational parameters are selected from the group comprising: required operating voltage for the electric load; required operating current for the electric load; required operating temperature for the electric load; required operating power for the electric load; measured operating voltage for the electric load; measured operating current for the electric load; measured operating temperature for the electric load; measured operating power for the electric load; power delivered to the primary inductive coil; power received by the secondary inductive coil, and a user identification code. Optionally, the detector is selected from the list comprising optical detectors, radio receivers, audio detectors and voltage peak detectors. 
     Preferably, the driver further comprises a voltage monitor for monitoring the amplitude of a primary voltage across the primary coil. Optionally, the voltage monitor is configured to detect significant increases in primary voltage. 
     In preferred embodiments, the driving voltage oscillating at a transmission frequency higher than the resonant frequency of the inductive couple, wherein the primary inductive coil is further wired to a reception circuit comprising a voltage monitor for monitoring the amplitude of a primary voltage across the primary coil, and the secondary inductive coil is further wired to a transmission circuit for connecting at least one electric element to the secondary inductive coil thereby increasing the resonant frequency such that a control signal may be transferred from the transmission circuit to the reception circuit. Optionally, the secondary inductive coil is wired to two inputs of a bridge rectifier and the electric load is wired to two outputs of the bridge rectifier wherein the transmission circuit is wired to one input of the bridge rectifier and one output of the bridge rectifier. Typically, the transmission circuit further comprises a modulator for modulating a bit-rate signal with an input signal to create a modulated signal and a switch for intermittently connecting the electrical element to the secondary inductive coil according to the modulated signal. Optionally, the voltage monitor further comprises a correlator for cross-correlating the amplitude of the primary voltage with the bit-rate signal for producing an output signal. 
     In certain embodiments, the control signal is for transferring a feedback signal from the secondary inductive coil to the primary inductive coil for regulating power transfer across an inductive power coupling. The driver may be configured to adjust the transmission frequency in response to the feedback signals. Typically, the system is adapted to transfer a first signal and a second signal, and the driver is configured to: increase the transmission frequency when the first signal is received by the receiver, and decrease the transmission frequency when the second signal is received by the receiver. 
     It is still a further aim of the invention to present an inductive power adaptor comprising: at least one inductive power receiver for receiving power from an inductive power outlet; at least one power connector for conductively connecting the power receiver to at least one electrical device, and a grip for handling the adapter, the grip being thermally isolated from the power receiver such that when the power receiver is in operation a user may handle the adapter without injury. Optionally, the adapter further comprises a printed circuit board. 
     Typically, the power receiver is for providing power at a rate above 50 W. Preferably, the adapter comprises a cooling system for dissipating heat generated therein. Optionally, the cooling system comprises at least one air outlet, situated above the power receiver, and at least one air inlet, situated below the power receiver such that hot air heated by the power receiver flows out of the adapter through the air outlet and cool air from outside is drawn into the adapter through the air inlets. 
     Preferably, the adapter further comprises at least one heat sink for dissipating heat generated by the power receiver. Optionally, the heat sink is a metallic disk. Typically, the heat sink is smaller than the internal diameter of a casing of the adapter thereby allowing air to circulate between the heat sink and the casing. 
     It is a further aim of the invention to present a transmission-guard for preventing an inductive power outlet from transmitting power in the absence of an electric load inductively coupled thereto, the inductive power outlet comprising at least one primary coil connectable to a power supply, for inductively coupling with a secondary coil wired to the electric load, the transmission-guard comprising at least one transmission-lock for preventing the primary coil from connecting to the power supply in the absence of a transmission-key. 
     Optionally, the transmission-lock comprises at least one magnetic switch and the transmission-key comprises at least one magnetic element associated with the secondary coil. Typically, the transmission-lock comprises an array of magnetic switches configured to connect the primary coil to the power supply only when activated by a matching configuration of magnetic elements. Optionally, the magnetic switch comprises a magnetic sensor. 
     Alternatively, the transmission-guard comprises: at least one emitter for emitting a release-signal, and at least one detector for detecting the release signal; the transmission-key comprises at least one bridge associated with the secondary coil for bridging between the at least one emitter and the at least one detector, such that when the secondary coil is brought into alignment with the primary coil the release signal is guided from the emitter to the detector. Optionally, the release-signal is an optical signal and the bridge comprises at least one optical wave-guide. Alternatively, the release-signal is a magnetic signal and the bridge comprises a magnetic flux guide. In other embodiments the transmission-key comprises a release-signal emitted by an emitter associated with the secondary coil. The release-signal may be an optical signal and the optical signal may be an infra-red pulse received by an optical detector configured to release the transmission-lock. Alternatively the release-signal is a magnetic signal. In some embodiments of the transmission guard the emitter is the secondary coil. 
     Optionally, the transmission-guard comprises a low-power power pulse transmitted by the primary coil, such that when the secondary coil is aligned to the primary coil the power pulse is transferred to the secondary coil and the transmission-key is triggered by the power pulse. Preferably, a first transmission-lock is released by a first transmission-key indicating the probable presence of a secondary coil and a second transmission-lock is released by a second transmission-key confirming the presence of the secondary coil. The first transmission-lock may initiate a power pulse transmitted from the primary coil, and the second transmission key being triggered by the power pulse being received by the secondary coil. The magnetic element may comprise a ferrite flux guidance core. Variously, the release-signal is selected from the group comprising: mechanical signals, audio signals, ultra-sonic signals and microwaves. It is also an aim to present separately: a transmission-key for use in the transmission-guard, a transmission-lock for use in the transmission-guard, and an inductive power outlet protected by the transmission-guard. 
     It is yet another aim of the invention to present an inductive power transfer system comprising at least one inductive power receiver for receiving power from an inductive power outlet, the inductive power outlet comprising at least one primary inductor wired to a power source via a driver, and the inductive power receiver comprising at least one secondary inductor for coupling with the primary inductor, the secondary inductor for providing power to an electric load; the inductive power transfer system comprising at least one magnetic flux guide for directing magnetic flux from the primary inductor to the secondary inductor wherein the magnetic flux guide comprises an amorphous ferromagnetic material. Optionally, the amorphous ferromagnetic material has a thickness of less than 30 microns. Typically, the amorphous ferromagnetic material is sandwiched between two polymer layers. Preferably, the magnetic flux guide comprises a plurality of layers of amorphous ferromagnetic material separated by electrically insulating material. 
     In some embodiments, the magnetic flux guide comprises a wafer of the amorphous ferromagnetic material. Preferably, the wafer is at least partially split so as to reduce the build up of eddy currents. Optionally, the wafer is circular and having a split extending along at least one diameter. In certain embodiments, the wafer is cut from a sheet of the amorphous ferromagnetic material. Alternatively, the magnetic flux guide comprises microwires of the amorphous ferromagnetic material. Optionally, the microwires form a cloth. 
     It is a particular aim of the invention to present an inductive power transfer system comprising at least one inductive power outlet for transferring power inductively to at least one inductive power receiver, the inductive power outlet comprising at least one primary inductor wired to a power source via a driver, and the inductive power receiver comprising at least one secondary inductor for coupling with the primary inductor, the secondary inductor for providing power to an electric load; the inductive power transfer system comprising at least one of the optimization components selected from the group comprising:
         a low heat-loss full wave rectifier comprising at least one electronic switch configured to be in its ON state when the current flowing through the cathode of the switch exceeds a predetermined threshold;   a driver connected between the power source and the primary inductor, the driver being is configured to provide a driving voltage across the primary inductive coil, the driving voltage oscillating at a transmission frequency significantly different from the resonant frequency of the inductive couple formed by the primary inductor and the secondary inductor;   an inductive power adaptor comprising a grip for handling the power receiver, the grip being thermally isolated from the power receiver such that when the power receiver is in operation a user may handle the adapter without injury;   a transmission-guard for preventing the inductive power outlet from transmitting power in the absence of an electric load inductively coupled thereto, the transmission-guard comprising at least one transmission-lock for preventing the primary coil from connecting to the power supply in the absence of a transmission-key, and   a magnetic flux guide for directing magnetic flux from the primary inductor to the secondary inductor wherein the magnetic flux guide comprises an amorphous ferromagnetic material.       

     In further embodiments the inductive power transfer system comprises at least two of the optimization components listed above. Preferably, the inductive power transfer system comprises at least three of the optimization components. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the invention and to show how it may be carried into effect, reference will now be made, purely by way of example, to the accompanying drawings. 
       With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention; the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice. In the accompanying drawings: 
         FIG. 1 a    is a schematic diagram representing an inductive power transfer system according to an exemplary embodiment of the present invention; 
         FIG. 1 b    is a schematic diagram representing an inductive power receiver for use in the inductive power transfer system of  FIG. 1   a;    
         FIG. 1 c    is a block diagram representation of the main components of the inductive power transfer system according to the exemplary embodiment of the present invention; 
         FIG. 2 a    is a block diagram representing the main components of a transmission-guard for an inductive power outlet according to another embodiment of the present invention; 
         FIG. 2 b    is a schematic representation of an inductive power outlet protected by an exemplary transmission-guard according to a further embodiment of the present invention wherein a transmission-lock is released by a magnetic key; 
         FIGS. 2 c - e    are schematic representations of a transmission-guard according to another embodiment of the invention in which a transmission-lock is releasable by a passive optical transmission-key; 
         FIG. 2 f    is a schematic representation of a transmission-guard according to a further embodiment of the invention in which a transmission-lock is releasable by an active optical transmission-key; 
         FIG. 3 a    is a circuit diagram of a full-wave diode bridge rectifier of the prior art; 
         FIG. 3 b    is a diagram of a Power MOSFET of the prior art; 
         FIG. 4 a    is a block diagram of a first synchronous full-wave rectifier in which two of the diodes of the diode bridge of  FIG. 3  have been replaced by electronic switches; 
         FIG. 4 b    is a block diagram of a second synchronous full-wave rectifier according to an exemplary embodiment of the invention in which all four diodes of the diode bridge of  FIG. 3  have been replaced by electronic switches; 
         FIG. 4 c    is a schematic diagram showing a current triggered Power MOSFET which draws a gate signal from the current flowing through its drain terminal; 
         FIG. 4 d    is a graphical representation of the variations in drain-current and state of the MOSFET of  FIG. 4 c   , over a single cycle of a sinusoidal input voltage; 
         FIG. 4 e    is a circuit diagram representing a synchronous full-wave MOSFET bridge rectifier according to another embodiment of the invention; 
         FIG. 5 a    shows schematic diagram of a computer being powered by an inductive power outlet via an inductive power adapter according to a further embodiment of the present invention; 
         FIG. 5 b    is an isometric projection of an inductive power adapter according to an exemplary embodiment of the invention; 
         FIG. 5 c    is an exploded view showing the internal components of the power receiver of the exemplary embodiment; 
         FIG. 5 d    is a side view cross section of the power receiver of the exemplary embodiment; 
         FIG. 5 e    is an exploded view of an inductive power receiver having a magnetic flux guide according to another embodiment of the invention; 
         FIG. 5 f    is an isometric view of the inductive power receiver of  FIG. 5   e;    
         FIG. 6 a    is a block diagram showing the main elements of an inductive power transfer system with a feedback signal path; 
         FIG. 6 b    is a graph showing how the amplitude of operational voltage varies according to frequency; 
         FIG. 6 c    is a schematic diagram representing a laptop computer drawing power from an inductive power outlet; 
         FIG. 6 d    is a flowchart showing a method for regulating power transfer by varying the power transmission frequency in an inductive power transfer system; 
         FIG. 6 e    is a circuit diagram of an inductive power transfer system including a peak detector for detecting large increases in transmission voltage; 
         FIG. 7 a    is a block diagram showing the main elements of an inductive power transfer system with an inductive feedback channel according to another embodiment of the present invention; 
         FIG. 7 b    is a graph showing how the amplitude of operational voltage of an inductive power transfer system varies according to the voltage transmission frequency and the resonant frequency of the system; 
         FIG. 7 c    is a circuit diagram of an inductive power transfer system including an inductive feedback channel for providing coil-to-coil signal transfer concurrently with uninterrupted inductive power transfer between the coils in accordance with another embodiment of the invention, and 
         FIG. 7 d    is a flowchart showing a method for transferring a signal from the secondary inductive coil to a primary inductive coil of an inductive power transfer system according to still a further embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. 
     Reference is now made to  FIGS. 1 a  and 1 b    showing an inductive power outlet  200  and an inductive power receiver  300  for use in an exemplary inductive power transfer system  100  according to an exemplary embodiment of the invention. 
     The inductive power outlet  200  consists of four primary inductors  220   a - d  incorporated within a platform  202 . The inductive power receiver  300  includes a secondary inductor  320  incorporated within a case  302  for accommodating a mobile telephone  342 . When a mobile telephone  342  is placed within the case  302  a power connector  304  electrically connects the secondary inductor  320  with the mobile telephone  342 . As shown in  FIG. 1 a   , the inductive power receiver  300  may be placed upon the platform  202  in alignment with one of the primary inductors  220   b  so that the secondary inductor  320  inductively couples with the primary inductor  220   b.    
     Referring now to  FIG. 1 c   , which shows a block diagram representing the main components of the inductive transfer system  100 , various features are included to improve power transfer across the inductive couple. 
     The inductive power outlet  200  includes a primary inductor  220 , wired to a power supply  240  via a driver  230 . The driver  230  typically includes electronic components, such as a switching unit for example, for providing an oscillating electrical potential to the primary inductor  220 . The oscillating electrical potential across the primary inductor  220  produces an oscillating magnetic field in its vicinity. 
     The inductive power receiver  300  includes a secondary inductor  320  wired to an electric load  340 , typically via a rectifier  330 . The secondary inductor  320  is configured such that, when placed in the oscillating magnetic field of an active primary inductor  220 , a secondary voltage is induced across the secondary inductor  320 . The secondary voltage may be used to power the electric load  340 . It is noted that an induced secondary voltage across the secondary inductor  320  produces an alternating current (AC). Where the electric load  340  requires direct current (DC), such as for charging electrochemical cells, the rectifier  330  is provided to convert AC to DC. 
     In contradistinction to prior art inductive power transfer systems, which have proved impractical or commercially unviable, embodiments of the current invention include further elements for improving the efficiency of power transfer from the inductive power outlet  200  to the inductive power receiver  300 . For example, preferred embodiments of the invention include a signal transfer system  400 , an alignment mechanism  500  and a magnetic flux guide  600 . 
     The signal transfer system  400  provides a channel for passing signals between the inductive power receiver  300  and the inductive power outlet  200 . The signal transfer system  400  includes a signal emitter  420 , associated with the inductive power receiver  300  and a signal detector  440 , associated with the inductive power outlet  200 . Signals may perform a variety of functions such as inter alia, confirming the presence of a power receiver  300 , regulating power transfer or for communicating required power transmission parameters. The latter being particularly useful in systems adapted to work at multiple power levels. Various signal transfer systems may be used such as optical, inductive, ultrasonic signal emitters or the like in combination with appropriate detectors. 
     The alignment mechanism  500  is provided to facilitate the alignment of the secondary inductor  320  with the primary inductor  220  thereby improving the efficiency of the inductive transfer system  100 . Where the user is able to see the primary inductor  220  directly, the secondary inductor  320  may be aligned by direct visual observation. However, where the primary inductor  220  is concealed behind an opaque surface, alternative alignment mechanisms  500  may be necessary. Such alignment mechanisms  500  may include tactile, visual and/or audible indications, for example. 
     The magnetic flux guide  600  is provided to guide magnetic flux from the primary inductor  220  to the secondary inductor  320  and to prevent flux leakage out of the inductive power transfer system  100 , particularly into metallic or other conductive materials in the vicinity. 
     Prior art inductive power transfer systems have typically been either inefficient or impractical for powering electrical devices wirelessly. As a result, in spite of the long felt need to reduce trailing wires, the use of inductive power transfer has been generally limited to low power applications such as the charging of batteries. In order to be practical, an inductive power transfer system must be efficient, safe and unobtrusive, preferably having small dimensions and being lightweight. As will be described herein below, embodiments of the present invention are directed towards providing an inductive power transfer system which answers these requirements. 
     Particular aspects of the current invention include:
         A transmission-guard for preventing the inductive power outlet  200  from transmitting power in the absence of an inductive power receiver  300 .   An AC-DC rectifier  330  which uses electronic switches for reducing heat loss from diodes.   An inductive power receiver  300  having a heat dissipation system such that a user may comfortably and safely handle the inductive power receiver  300 .   A magnetic flux guide  600  constructed from thin materials and which is adapted to improve flux linkage between the primary inductor  220  and the secondary inductor  320  as well as to prevent flux leakage into the surroundings.   A driver  230  configured and operable to generate a driving voltage which oscillates at a transmission frequency which is substantially different from the resonant frequency of the inductive couple.       

     Any one of the above described aspects by itself represents a significant improvement to the prior art. However, it is particularly noted that for any inductive power transfer system  100  to be practical for powering electrical devices, it needs to incorporate at least two or more of the above described features in combination. More detailed descriptions of embodiments of the invention which incorporate these features are given below. 
     Transmission-Guard 
     Reference is now made to  FIG. 2 a    which shows a block diagram representing a transmission-guard  2100  for preventing an inductive power outlet  2200  from transmitting power in the absence of a secondary unit  2300  connected to an electric load  2340 , according to another embodiment of the invention. 
     The inductive power outlet  2200  consists of a primary coil  2220 , wired to a power supply  2240 , for inductively coupling with a secondary coil  2320  wired to an electric load  2340 . The primary coil  2220  is wired to the power supply  2240  via a driver  2230  which provides the electronics necessary to drive the primary coil  2220 . Driving electronics may include a switching unit providing a high frequency oscillating voltage supply, for example. Where the power outlet  2200  consists of more than one primary coil  2220 , the driver  2230  may additionally consist of a selector for selecting which primary coil  2220  is to be driven. 
     It is a particular feature of this embodiment of the invention that a transmission-guard  2100  is provided consisting of a transmission-lock  2120  connected in series between the power supply  2240  and the primary coil  2220 . The transmission-lock  2120  is configured to prevent the primary coil  2220  from connecting to the power supply  2240  unless it is released by a transmission-key  2140 . The transmission-key  2140  is associated with the secondary unit  2300  and serves to indicate that the secondary coil  2320  is aligned to the primary coil  2220 . 
     With reference to  FIG. 2 b   , a schematic representation is shown of an inductive power outlet  2200  protected by an exemplary magnetic transmission-guard  2100  according to another embodiment of the present invention. Power may only be provided by the protected power outlet  2200  when an authenticated secondary unit  2300  is aligned thereto. 
     The protected power outlet  2200  includes a magnetic transmission-lock  2120  consisting of an array of magnetic switches  2122  electrically connected in series between the primary coil  2220  and the driver  2230 . A magnetic transmission-key  2140  consisting of an array of magnetic elements  2142  is provided within the authenticated secondary unit  2300 . 
     The configuration of magnetic elements  2142  in the transmission-key  2140  is selected to match the configuration of magnetic switches  2122  in the transmission-lock  2120 . The authenticated secondary unit  2300  may be aligned with the protected induction outlet  2200  by aligning both the transmission-key  2140  with the transmission-lock  2120  and the secondary coil  2320  with the primary coil  2220 . Once correctly aligned, all the magnetic switches  2122  in the transmission-lock  2120  are closed and the driver  2230  is thereby connected to the primary coil  2220 . 
     Various examples of magnetic switches  2122  are known in the art including for example reed switches, Hall-effect sensors or such like. Such magnetic switches  2122  may be sensitive to any magnetic elements  2142  such as either North or South poles of permanent magnets or electromagnetic coils for example. It is further noted that Hall-effect sensors may be configured to sense magnetic fields of predetermined strength. 
     According to certain embodiments, the magnetic transmission-key  2140  may consist of a permanent magnet and a ferromagnetic element incorporated with in the secondary unit  2300 . The characteristics of the magnetic field produced by a transmission-key of this type depend upon the strength and position of the permanent magnetic as well as the dimensions and characteristics of the ferromagnetic element. The magnetic transmission-lock  2120  may consist of an array of magnetic switches, such as unipolar Hall switches for example, which are strategically placed and orientated such that they connect the primary coil  2220  to the driver  2230  only when triggered by a particular combination of a permanent magnet and ferromagnetic element. 
     It is noted that permanent magnets may commonly be provided to assist with alignment of the secondary coil  2320  to the primary coil  2220 . Ferromagnetic elements may also be commonly included in secondary units  2300  for providing flux guidance from the primary coil  2220  to the secondary coil  2320 . The magnetic transmission-lock  2120  may therefore be made sensitive to these components. Indeed a single magnetic transmission-lock  2120  may be provided which is configured to detect various secondary units and to selectively connect more than one primary coil  2220  depending on the secondary unit detected. 
     Referring back to  FIG. 2 a   , according to other embodiments of the transmission-guard  2100 , a power outlet  2200  may be protected by a transmission-lock  2120  which may be released when a release signal S R  is received by a detector  2124 . The release signal S R  may be actively emitted by the transmission-key  2140  or alternatively the transmission-key may passively direct the release signal towards the detector  2124 . 
     One example of a passive transmission-key  2140  is shown in  FIGS. 2 c - e    which represent an optical transmission-guard  2100  according to a further embodiment of the invention. 
     The transmission-guard  2100  consists of an active optical transmission-lock  2120 ′ incorporated within an inductive power outlet  2200 ′ and a passive optical transmission-key  2140 ′ incorporated within the secondary unit  2300 . 
     With particular reference to  FIG. 2 c   , the optical transmission-lock  2120 ′ includes a switch  2122 ′, an optical detector  2124 ′, such as a photodiode, a phototransistor, a light dependent resistor or the like, and an optical emitter  2126 ′ such as light emitting diode (LED). The switch  2122 ′ is normally open but is configured to close when a release signal S R  is received by the optical detector  2124 ′, thereby connecting a primary coil  2220  to a driver  2230 . The optical emitter  2126 ′ is configured to emit the optical release-signal S R  which is not directly detectable by the optical detector  2124 ′. 
     Referring now to  FIG. 2 d   , the optical transmission-key  2140 ′ includes a bridging element  2142 ′ such as an optical wave-guide, optical fiber, reflector or the like. The bridging element  2142 ′ is configured to direct the optical release-signal S R  from the optical emitter  2124 ′ towards the optical detector  2126 ′, when a secondary coil  2320  is aligned with the primary coil  2220 . 
     When the secondary unit  2300  is correctly aligned with the inductive power outlet  2200 , as shown in  FIG. 2 e   , the secondary coil  2320  aligns with the primary coil  2220 ′ and the passive optical transmission-key  2140 ′ aligns with the optical transmission-lock  2120 ′. The optical release-signal S R  is thus detected by the optical detector  2126 ′ and the switch  2122 ′ is closed connecting the primary coil  2220  to the driver  2230 . 
     It is noted that many materials are partially translucent to infra-red light. It has been found that relatively low intensity infra red signals from LEDs and the like, penetrate several hundred microns of common materials such as plastic, cardboard, Formica or paper sheet, to a sufficient degree that an optical detector  2124 ′, such as a photodiode, a phototransistor, a light dependent resistor or the like, behind a sheet of from 0.1 mm to 2 mm of such materials, can receive and process the signal. For example a signal from an Avago HSDL-4420 LED transmitting at 850 nm over 24 degrees, may be detected by an Everlight PD 15-22C-TR8 NPN photodiode, from behind a 0.8 mm Formica sheet. For signaling purposes, a high degree of attenuation may be tolerated, and penetration of only a small fraction, say 0.1% of the transmitted signal intensity may be sufficient. 
     Although an optical transmission-key  2140 ′ is described above, it will be appreciated that other passive transmission-keys may incorporate bridging elements configured to guide release-signals of other types. For example, a ferromagnetic bridge may be incorporated for transmitting magnetic release-signal from a magnetic element to a magnetic detector such as a Hall-effect sensor or the like. The magnetic emitter in such a case may be the primary coil itself. 
     Alternatively, audio signals may be guided through dense elements, or low power microwaves along microwave wave guides for example. 
     An example of an active optical transmission-key  2140 ″ is shown in  FIG. 2 f    representing a transmission-guard  2100 ″ according to another embodiment of the invention. 
     The transmission-guard  2100 ″ of this embodiment includes a transmission-lock  2120 ″ incorporated within an inductive power outlet  2200  and an active optical transmission-key  2140 ″ incorporated within secondary unit  2300 . 
     The active optical transmission-key  2140 ″ includes an optical emitter  2142 ″, configured to emit an optical release-signal S R , and the transmission-lock  2120 ″ includes a switch  2122 ″ and an optical detector  2124 ″. The transmission-lock  2120 ″ is configured to close the switch  2122 ″ thereby connecting a primary coil  2220  to a driver  2230  when the optical detector  2124 ″ receives the release-signal S R . 
     When the secondary unit  2300  is aligned with the inductive power outlet  2200 , the transmission-key  2140 ″ emits an optical release-signal S R  which is received by the optical detector  2124 ″ of the transmission-lock  2120 ″ and this closes the switch  2122 ″. Thus the inductive power outlet  2200 ″ is enabled to transfer power to the secondary coil  2320 . 
     It will be appreciated that a release signal S R  may be coded to provide a unique identifier. Coding may be by modulation of frequency, pulse frequency, amplitude or the like. The code may be used, for example, to identify the type or identity of the secondary unit for authentication. Other data may additionally be encoded into the release-signal. This data may include required power transmission parameters, billing information or other information associated with the use of the power outlet. 
     Although an optical active transmission-key  2140 ″ is described above, it will be appreciated that other active transmission-keys may emit other types of release-signals. For example, the secondary coil  2320  may be used to transmit a magnetic release-signal to a magnetic detector incorporated in the transmission-lock. This could be a Hall-effect sensor or the like or even the primary coil  2220  itself. 
     To actively emit a release-signal transmission-keys typically require a power source. In some cases, particularly where the secondary unit is incorporated into a portable electrical device, power may be provided by internal power cells with the secondary unit. Alternatively, power may be drawn from a power pulse transferred from the primary coil to the secondary coil. 
     In certain embodiments of the invention, the inductive power outlet transfers a periodic low energy power pulse, for example a pulse of a few milliseconds duration may be transmitted by the primary coil at a frequency of 1 hertz or so. When a secondary coil is brought into the vicinity of the primary coil the power may be transferred to the secondary coil and may be used to power an active transmission-key. 
     In other embodiments of the transmission-guard, a first transmission-lock (preferably a passive transmission-lock) associated with the secondary unit, releases a first transmission-lock thereby indicating the probable presence of a secondary coil. A low energy power pulse is then emitted by the primary coil to power an active second transmission-key which may release a second transmission-lock thereby connecting the primary coil to a driver. 
     Synchronous Rectifier 
     Reference is now made to  FIG. 3 a    showing a circuit diagram of a typical full-wave rectifier  3100  of the prior art. The rectifier has two input terminals T 1  and T 2  and two output terminals T 3  and T 4 . When an alternating current source AC in  is wired to the two input terminals T 1  and T 2 , a direct current output DC out  may be drawn from the two output terminals T 3  and T 4  of the rectifier  3100 . 
     Four diodes D 1-4  are arranged so that two diodes D 1  and D 2  form a first branch  3110  of a Graetz circuit and the other two diodes D 3  and D 4  form a second branch  3120  of the Graetz circuit. The anodes of two upstream diodes D 1  and D 3  are wired to the first output terminal T 3  and the cathodes of the two downstream diodes D 2  and D 4  are wired to the second output terminal T 4 . The cathode of the first upstream diode D 1  and the anode of first downstream diode D 2  are wired to the first input terminal T 1  and the cathode of the second upstream diode D 3  and the anode of second downstream diode D 4  are wired to the second input terminal T 2 . 
     When the polarity of the first input terminal T 1  is positive relative to the second input terminal T 2 , current flows through the first downstream diode D 2  and through the second upstream diode D 3 . When the polarity of the first input terminal T 1  is negative relative to the second input terminal T 2 , current flows through the second downstream diode D 4  and through the first upstream diode D 1 . 
     Diode bridge rectifiers, such as that shown in  FIG. 3 a   , are used to produce an output with a fixed polarity that is independent of the polarity of the input. Such diode bridge rectifiers may be used in AC-to-DC power converters, for example. Optionally, the output is smoothed by a smoothing capacitor C. 
     It will be appreciated that power is lost from each diode with each reversal of polarity. In high frequency power converters, where the polarity of the input terminals T 1  and T 2  may oscillate at a frequencies of 100 kHz or more, such power losses may result in significant heating of the bridge circuit and its surrounding components, which may result in reduced reliability or failure. 
     Power loss may be reduced by replacing diodes with electronic switches, such as the Power MOSFETs shown in  FIG. 3 a   , which have much lower associated power loss.  FIG. 4 a    is a block diagram of one such synchronous full-wave rectifier  4200  in which the first downstream diode D 2  and the second downstream diode D 4  of the diode bridge of  FIG. 3 a    have been replaced by two electronic switches M 2  and M 4 . 
     The electronic switches M 2  and M 4  are controlled by switching signals G 2  and G 4  which switch them between the ON and OFF states. The switching signal G 2  controlling the electronic switch M 2  must be synchronized to switch to the ON state whenever the polarity of the first input terminal T 1  is positive relative to the second input terminal T 2 . The switching signal G 4  controlling the electronic switch M 4  must be synchronized to switch to the ON state whenever polarity of the first input terminal T 1  is negative relative to the second input terminal T 2 . 
     Typically, this synchronization is achieved by drawing the first switching signal G 2  from the voltage of the second input terminal T 2  and drawing the second switching signal G 4  from the voltage of the first input terminal T 1 . 
     The above described synchronous full-wave rectifier  4200  in which two diodes are replaced by MOSFETs may reduce power loss from the rectifier by up to 50% as compared with the diode bridge rectifier  4100  of the prior art. Where further reduction in power loss is required it would be desirable to replace the remaining two diodes D 1  and D 3  with electronic switches. However, it is much more difficult to synchronize four electronic switches without inadvertently causing short circuits between either the input or output terminals. 
       FIG. 4 b    is a block diagram of a second synchronous full-wave rectifier  4300  in which all four diodes D 1-4  of the diode bridge of  FIG. 3 a    have been replaced by electronic switches M 1-4 . In order to provide an output DC out  of constant polarity, the switching signals G 1-4  need to be carefully controlled. 
     When the polarity of the first input terminal T 1  is positive relative to the polarity of the second input T 2 , the first upstream and second downstream electronic switches M 1  and M 4  must be switched to the OFF state and the first downstream and second upstream electronic switches M 2  and M 3  must be switched to the ON state. When the polarity of the first input terminal T 1  is negative relative to the polarity of the second input terminal T 2 , the first upstream and second downstream electronic switches M 1  and M 4  must be switched to the ON state and the electronic switches first downstream and second upstream electronic M 2  and M 3  must be switched to the OFF state. 
     Synchronization of the switching signals G 1-4 , is complicated by an additional constraint. In order to prevent shorting across the output terminals, the upstream and downstream electronic switches along a common branch  4310 ,  4320  must never be in the ON state at the same time. In practice, when both of the switching signals G 1  and G 2  controlling the two electronic switches M 1  and M 2  along the first branch  4310  are each drawn from one of the input terminals T 1  and T 2 , the two switches M 1  and M 2  are periodically both in their ON states. Because the switches M 1  and M 2  are adjacent along the first branch  4310  of the circuit, a short circuit is formed between the output terminals T 3  and T 4 . Similar shorting may occur along the second branch  4320  when the switching signals G 3  and G 4  which control the other two electronic switches M 3  and M 4  are each drawn from one of the input terminals T 1  and T 2 . 
     According to preferred embodiments of the invention, only the switching signals G 2  and G 4  for the downstream electronic switches M 2  and M 4  are drawn directly from the voltage at the input terminals T 1  and T 2  whilst the switching signals G 1  and G 3  for the upstream switches M 1  and M 3  are controlled independently. Preferably, the switching signals G 1  and G 3  are responsive to changes in the cathode current of switches M 1  and M 3  respectively. 
       FIG. 4 c    shows an exemplary current-triggered synchro-rectifier  4330 , which may serve as an electronic switch M incorporated into a bridge synchro-rectifier  4300 . The current-triggered synchro-rectifier  4330  includes a Power MOSFET  4130 , such as that shown in  FIG. 3 b   , and a current monitor  4332 . The current monitor  4332  is wired to the drain terminal  4136  of the Power MOSFET  4130  and is configured to send a current-based gate signal G i  to the gate terminal  4138  of the Power MOSFET when the drain-current I d  exceeds a predetermined threshold I th . Although in the above example the current-triggered synchro-rectifier  4330  includes an n-channel MOSFET  4130 , it will be appreciated that in other embodiments current-triggered synchro-rectifiers may incorporate p-channel MOSFETs. 
     In order to understand the functioning of the current-triggered synchro-rectifier  4330  consider the case where a sinusoidal alternating voltage is connected across the cathode  4334  and the anode  4336  terminals of the current-triggered synchro-rectifier  4330 .  FIG. 4 d    shows three graphs showing variations in  1 ) the voltage drop V d  from the cathode  4334  to the anode  4336 ,  2 ) the drain-current I d , and  3 ) the MOSFET state during one voltage cycle. 
     For the first half of the sinusoidal cycle the voltage drop V d  between the cathode  4334  and the anode  4336  is negative, thus the polarity of the cathode  4334  is negative relative to the anode  4336 . Consequently, no current flows through the drain-terminal  4136  and the MOSFET remains in the OFF state. 
     At the beginning of the second half of the sinusoidal cycle, the voltage drop V d  between the cathode  4334  and the anode  4336  increases above zero. The polarity of the cathode  4334  becomes positive relative to the anode.  4336  so a small drain-current I d  begins to flow through the diode  4132 . This current is measured by the current monitor  4332 . 
     During the third quarter of the cycle, the voltage drop V d  between the cathode  4334  and the anode  4336  continues to rise. The current monitor  4332  measures an increasing drain-current I d . 
     When the drain-current I d  exceeds the predetermined threshold I th , the current-based gate signal G i  triggers the MOSFET  4130  to switch to the ON state. 
     As long as the MOSFET  4130  is in the ON state, current flows through the ohmic conductive path of the electronic switch  4131 . Consequently, the drain-current I d  varies in proportion to the voltage drop V d . 
     During the last quarter of the cycle, the voltage drop V d  between the cathode  4334  and the anode  4336  decreases. The current monitor  4332  measures a decreasing drain-current I d . 
     When the drain-current falls below the predetermined threshold I th , the current-based gate signal G i  triggers the MOSFET  4130  to switch to the OFF state. 
       FIG. 4 e    is a circuit diagram representing a synchronous full-wave bridge rectifier  4400  incorporated within an inductive power receiver according to a further embodiment of the invention. The electronic switches M 1-4  are all MOSFET transformers having three terminals: a source terminal, a drain terminal and a gate terminal. The upstream MOSFETs M 1  and M 3  are both n-channel MOSFETs and their source terminals are both wired to the first output terminal T 3  of the rectifier. The downstream MOSFETs M 2  and M 4  are both p-channel MOSFETs and their source terminals are both wired to the second output terminal T 4  of the rectifier. The drain terminals of the first upstream MOSFET M 1  and the first downstream MOSFET M 2  are both wired to the first input terminal T 1  of the rectifier and the drain terminals of the second upstream MOSFET M 3  and the second downstream MOSFET M 4  are both wired to the second input terminal T 3  of the rectifier. 
     The input terminals T 1  and T 2  are wired to a secondary coil L 2  of a power transformer which is inductively coupled to a primary coil (not shown). The secondary coil L 2  provides an alternating current input to the two input terminals T 1  and T 2 . 
     The gate terminals of the downstream MOSFETs M 2  and M 4  are wired to the input terminals T 2  and T 1  via smoothing circuits  4420 ,  4440  respectively. The switching signals G 2  and G 4 , are therefore in out of phase with each other. 
     The gate terminals of the upstream MOSFETs M 1  and M 3  receive switching signals G 1  and G 3  driven by their own drain-currents I d1  and I d3 . The drain current I d1  of the first upstream MOSFET M 1  is monitored by a first current transformer  4410 , in which a primary current monitor coil CT 1P  transfers the current signal to a secondary current monitor CT 2S  the output of which is rectified and relayed to a first input IN 1  of a driver  4450  which amplifies the signal before outputting a signal from a first output OUT 1 . This first output signal from the driver is then fed back to the first upstream MOSFET M 1  such that when the drain current I d1  exceeds a threshold value the MOSFET M 1  switches itself to the ON state. This produces a switching signal G 1  at the same frequency as the alternating current input AC in . 
     Similarly the drain current I d3  of the second upstream MOSFET M 2  is monitored by a second current transformer  4430 , in which a primary current monitor coil CT 2P  transfers the current signal to a secondary current monitor CT 2S  the output of which is rectified and relayed to a second input IN 2  of the driver  4450  which amplifies the signal before outputting a signal from a second output OUT 2 . The second output signal from the driver is then fed back to the second upstream MOSFET M 3  such that when the drain current I d2  exceeds a threshold value the MOSFET M 3  switches itself to the ON state. This produces a switching signal G 3  at the same frequency as the alternating current input AC in . 
     Although in the example here above, current transformers  4410 ,  4430  are used to monitor the drain-currents I d1 , I d2 , in alternative embodiments other current monitors such as ammeters, galvanometers, Hall effect sensors or the like may be preferred. 
     Heat Dissipation within Inductive Power Receivers 
     Reference is now made to  FIG. 5 a    showing a laptop computer  5300  drawing power from an inductive power outlet  5200  via an inductive power adapter  5100 , according to a further embodiment of the present invention. The adaptor is configured such that it can be safely handled by a user while it is operation. 
     The power adapter  5100  includes an inductive receiver  5120 , housed in a casing  5160  and a power connector  5140  for connecting to an electrical device, such as the computer  5300 . The inductive receiver  5120  includes a secondary inductor  5122  configured to couple with a primary inductor  5220  in the power outlet  5200 . Typically, the primary inductor  5220  is wired to a power source  5240  via a driver  5230 . The driver  5230  provides an oscillating driving voltage to the primary inductive coil  5220 . 
     Preferably, an alignment mechanism (not shown) is provided for aligning the secondary inductor  5122  to the primary core  5220 . The alignment mechanism may consist of a primary magnetic element in the inductive outlet configured to snag and/or engage a secondary magnetic element in the power adaptor  5100 . 
     It will be appreciated that electrical components of power converters generate heat. There are a number of problems associated with the heat generated in an inductive receiver  5120 , particularly in systems running at high power above say 50 W or 100 W. Heat produces high temperatures which can reduce overall efficiency and may also reduce the reliability of components. Much design effort is typically required to overcome this problem, and other factors such as the dimensions of the system may be compromised as a result. 
     In practice, electrical components of the power adapter  5100  are selected which function at high temperatures. However, the maximum temperature of the casing  5160  is further constrained by the requirement that it is to be handled by the user. If the casing  5160  reaches high temperatures, above 50 degrees Celsius or so, a user may find handling the adapter to be unpleasant and may even be at risk of injury. In order to allow a user to comfortably and safely handle the adaptor  5100 , it is a particular feature of the present invention that a heat dissipation system for directing heat away from the hand grip  5162 . 
     The heat dissipation system may be better understood with reference to  FIGS. 5 b - d    showing an exemplary inductive power adapter  5100  according to another embodiment of the invention.  FIG. 5 b    shows an isometric projection,  FIG. 5 c    shows an exploded view and  FIG. 5 d    shows a cross-section through the same embodiment of the power adaptor  5100 . 
     The exemplary power adapter  5100  includes an inductive receiver  5120 , and a heat sink  5130  housed between a lower casing  5160 L, and an upper casing  5160 U and a power connector  5140  which can be wound around a hand grip  5162  for storage. 
     The inductive power receiver  5120  consists of a secondary inductive coil  5122  a ferromagnetic disk  5124  and a printed circuit board (PCB)  5126 . The heat sink  5130  of the exemplary embodiment consists of a metallic disk sandwiched between the inductive receiver  5120  and the upper casing  5160 U. The ferromagnetic disk  5124  may serve as a flux guiding core to improve inductive coupling between the secondary inductive coil  5122  and a primary inductive coil  5220  ( FIG. 1 ) of an inductive power outlet  5200 . 
     When the power adapter  5100  is in operation, heat is generated by a number of components of the inductive receiver  5120 . An alternating current is induced in the secondary inductive coil  5122  therefore causing the coil wire to heat up. Furthermore hot spots are typically generated around certain electrical components typically provided on the PCB  5126 , such as rectifiers, diodes, MOSFETS, power regulators, LDOs, feedback transmitters or the like. 
     The heat sink  5130  is typically a thermal conductive material such as aluminum, copper or the like which serves to distribute heat more evenly around the inductive receiver  5120 . Preferably, thermal vias are provided through the PCB  5126  and thermal grease or a similar agent is used to improve thermal contact between the heat sink  5130 , PCB  5126 , ferromagnetic disk  5124  and secondary coil  5122 . 
     Air outlets  5132  are provided in the top  5161  of the upper casing  5160 U allowing hot air from inside the power adaptor to escape into the atmosphere. Air inlets  5134  are provided in the bottom  5165  and sides  5167  of the lower casing  5160 L and on the sides  5163  of the upper casing  5160 U allowing cool air to enter into the power adaptor from below. It is a particular feature of the exemplary embodiment that the outer diameter d of the heat sink is smaller the inner diameter D of the casing  5160  thus allowing air to circulate around the inductive receiver  5120 . Thus hot air heated by the inductive power receiver  5120  flows out of the adapter  5100  through the outlets  5132  and cool air from outside is drawn into the adapter  5100  through said air inlets  5134 . The hand grip  5162  may be additionally protected from heat by a barrier of thermal insulating material. 
     It is noted that the air outlets  5132  may allow dust to enter the power adapter  5100 . In some embodiments therefore a dust-guard is provided to prevent dust from entering the outlets  5132 . In the exemplary embodiment, the grip  5162  overhangs the outlets  5132  serving as a dust-guard to prevent dust from entering the adapter  5100  whilst in operation. When not in operation, the power connector  5140  may be wound around the hand grip  5162 , thereby providing further protection against dust. 
     In certain embodiments, the PCB  5126  includes a light emitting diode (not shown) used as a feedback transmitter for sending signals to an optical detector in the power outlet  5200  ( FIG. 5 a   ). It will be appreciated that in such embodiments, it is necessary that a clear line-of-sight is maintained between the optical emitter and detector. To this end, in preferred embodiments an optical window, transparent to the wavelength of the wavelength of the optical transmission, is provided through the secondary inductive coil  5122 , ferrite disk  5124 , lower casing  5160 L and other layers between the PCB  5126  and the primary coil  5220  ( FIG. 5 a   ). 
     Magnetic Flux Guidance 
     Referring now to  FIGS. 5 e  and 5 f   , an inductive power receiver  5200  is shown including a secondary inductor  5220 , a magnetic flux guide  5260  and a PCB  5270 , according to a further embodiment of the invention. The secondary inductor  5220  is configured to receive power inductively from a primary inductor of an inductive power outlet (not shown). The magnetic flux guide  5260  is provided to direct magnetic flux from the primary inductor to the secondary inductor  5220  and to reduce flux leakage to the surroundings. The magnetic flux guide  5260  consists of a ferromagnetic core  5262  and a magnetic shield  5264 . The ferromagnetic core  5262  is provided to guide magnetic flux from an active primary inductor to the secondary inductor  5220 . 
     In preferred embodiments, the ferromagnetic core  5262  is constructed from amorphous ferromagnetic material, typically cut into wafers from a sheet approximately 20 microns thick or so. In one exemplary embodiment, the ferromagnetic core consists of two amorphous ferromagnetic wafers  5262   a ,  5262   b . A first wafer  5262   a  is adhered to the primary inductor  5220  by a first adhesive insulating layer  5265   a . A second wafer  5262   b  is adhered to the first wafer  5262   a  by a second adhesive insulating layer  5265   b . The two wafers  5262   a ,  5262   b  serve as a ferromagnetic core guiding magnetic flux from a primary inductor to the secondary inductor  5220 . It is a particular feature of preferred embodiments that the ferromagnetic wafers  5262   a ,  5262   b  each have a radial slit  5265   a ,  5265   b  to prevent the build up of eddy currents within the wafer due to the oscillating magnetic field produced by the primary inductor. Where the wafer has a circular cross section, the slit may extend inwardly diametrically from the circumference. 
     The magnetic shield  5264  is provided to prevent flux leakage into the surroundings. Preferably, the magnetic shield  5264  is also fabricated from a sheet of thin amorphous ferromagnetic material and may be adhered to the PCB by a third adhesive insulating layer  5265   c.    
     It will be appreciated that a magnetic shield is of particular importance when the inductive receiver  5200  is mounted upon a conductive surface or a device containing conductive components. Thus, for example, when such an inductive power receiver  5200  is mounted upon an electrical device, such as a computer, mobile telephone or the like, the magnetic shield  5264  prevents magnetic flux from leaking into the metallic components of the electrical device and causing them to heat up. 
     Amorphous ferromagnetic sheets may have a thickness of around 20 microns. When laminated by a polymer laminate on both sides the overall thickness of the sheet is around 60 microns. Thus, in contradistinction to other ferrite elements used to guide magnetic flux in inductive systems, amorphous ferromagnetic materials may be used to fabricate an extremely thin magnetic guide  5260 . A thin magnetic guide  5260  in turn allows the inductive power receiver  5200  to be flexible and unobtrusive. It will be appreciated that these considerations are very important in the design and manufacture of device mounted inductive receivers. Various methods of fabricating magnetic guiding elements from amorphous ferromagnetic material include, inter alia: printing, stamping, cutting, amorphous ferromagnetic microwire cloth and the like. 
     Power Transmission at a Non-Resonant Frequency 
     The strength of an induced voltage in the secondary inductor of an inductive couple varies according to the oscillating frequency of the electrical potential provided to the primary inductor. The induced voltage is strongest when the oscillating frequency equals the resonant frequency of the system. The resonant frequency f R  depends upon the inductance L and the capacitance C of the system according to the equation 
     
       
         
           
             
               f 
               R 
             
             = 
             
               
                 1 
                 
                   2 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   π 
                   ⁢ 
                   
                     
                       L 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       C 
                     
                   
                 
               
               . 
             
           
         
       
     
     Known inductive power transfer systems typically transmit power at the resonant frequency of the inductive coupling. This can be difficult to maintain as the resonant frequency of the system may fluctuate during power transmission, for example in response to changing environmental conditions or variations in alignment between primary and secondary coils. 
     Inductive transfer systems designed to transmit at resonance therefore require tuning mechanisms for maintaining transmission at the resonant frequency of the system. Tuning may be achieved by adjusting the driving frequency to seek resonance. For example, U.S. Pat. No. 6,825,620, titled “Inductively coupled ballast circuit” to Kuennen et al. describes a resonance seeking ballast circuit for inductively providing power to a load. The ballast circuit includes an oscillator, a driver, a switching circuit, a resonant tank circuit and a current sensing circuit. The current sensing circuit provides a current feedback signal to the oscillator that is representative of the current in the resonant tank circuit. The current feedback signal drives the frequency of the ballast circuit causing the ballast circuit to seek resonance. The ballast circuit preferably includes a current limit circuit that is inductively coupled to the resonant tank circuit. The current limit circuit disables the ballast circuit when the current in the ballast circuit exceeds a predetermined threshold or falls outside a predetermined range. 
     Alternatively, tuning may be achieved by adjusting the characteristics of the inductive system. For example, U.S. Pat. No. 7,212,414, titled “Adaptive inductive power supply” to Baarman describes a contactless power supply which has a dynamically configurable tank circuit powered by an inverter. The contactless power supply is inductively coupled to one or more loads. The inverter is connected to a DC power source. When loads are added or removed from the system, the contactless power supply is capable of modifying the resonant frequency of the tank circuit, the inverter frequency, the inverter duty cycle or the rail voltage of the DC power source. 
     Tuning mechanisms such as those described above are necessary in order to maintain transmission at resonance because resonant transmission is highly sensitive. At resonance small variations to the system result in large changes to the power transferred. A further problem associated with resonant transmission is the high transmission voltages involved. At high operating voltages, the capacitors and transistors in the circuit need to be relatively large. 
     Reference is now made to  FIG. 6 a    showing a block diagram of the main elements of an inductive power transfer system  6100  adapted to transmit power at a non-resonant frequency. The inductive power transfer system  6100  consists of an inductive power outlet  6200  configured to provide power to a remote secondary unit  6300  according to another embodiment of the invention. The inductive power outlet  6200  includes a primary inductive coil  6220  wired to a power source  6240  via a driver  6230 . The driver  6230  is configured to provide an oscillating driving voltage to the primary inductive coil  6220 . 
     The secondary unit  6300  includes a secondary inductive coil  6320 , wired to an electric load  6340 , which is inductively coupled to the primary inductive coil  6220 . The electric load  6340  draws power from the power source  6240 . A communication channel  6120  may be provided between a transmitter  6122  associated with the secondary unit  6300  and a receiver  6124  associated with the inductive power outlet  6200 . The communication channel  6120  may provide feedback signals S and the like to the driver  6230 . 
     In some embodiments, a voltage peak detector  6140  is provided to detect large increases in the transmission voltage. As will be descried below the peak detector  6140  may be used to detect the removal of the secondary unit  6200 , the introduction of power drains, short circuits or the like. 
       FIG. 6 b    is a graph showing how the amplitude of the operational voltage varies according to the transmission frequency. It is noted that the voltage is at its highest when the transmission frequency is equal to the resonant frequency f R  of the system, this maximum amplitude is known as the resonance peak  2 . It is further noted that the slope of the graph is steepest in the regions  4   a ,  4   b  to either side of the resonance peak  2 . Thus in inductive transfer systems, which operate at or around resonance, a small variation in frequency results in a large change in induced voltage. Similarly, a small change in the resonant frequency of the system results in a large change in the induced voltage. For this reason prior art inductive transfer systems are typically very sensitive to small fluctuations in environmental conditions or variations in alignment between the induction coils. 
     It is a particular feature of embodiments of the current invention that the driver  6230  ( FIG. 6 a   ) is configured and operable to transmit a driving voltage which oscillates at a transmission frequency which is substantially different from the resonant frequency of the inductive couple. Preferably the transmission frequency is selected to lie within one of the near-linear regions  6 ,  8  where the slope of the frequency-amplitude graph is less steep. 
     One advantage of this embodiment of the present invention may be demonstrated with reference now to  FIG. 6 c   . A schematic diagram is shown representing a laptop computer  6340  drawing power from an inductive power outlet  6200  via a secondary power receiving unit  6300 . The power receiving unit  6300  includes a secondary inductive coil  6320  which is aligned to a primary inductive coil  6220  in the inductive power outlet  6200 . Any lateral displacement of the secondary power receiving unit  6300  changes the alignment between the secondary inductive coil  6320  to the primary inductive coil  6220 . As a result of the changing alignment, the combined inductance of the coil pair changes which in turn changes the resonant frequency of the system. 
     If the inductive power outlet  6200  transmits power at the resonant frequency of the system, even a small lateral movement would reduce significantly the amplitude of the induced voltage. In contradistinction, according to embodiments of the present invention, the inductive power outlet  6200  transmits power at a frequency in one of the regions  6 ,  8  to either side of the resonance peak  2  ( FIG. 6 b   ) where the slope of the resonance graph is much shallower. Consequently, the system has a much larger tolerance of variations such as lateral movement. 
     Another advantage of non-resonant transmission is that the transmission frequency may be used to regulate power transfer. In known inductive power transfer systems, power is typically regulated by altering the duty cycle of the transmission voltage provided by the driver. Thus, it will be appreciated that when the transmission frequency is not equal to the resonance frequency of the system, the driver  6230  may be configured to adjust the transmission frequency in order to regulate the power transfer. 
     Referring back to  FIG. 6 b   , the frequency of transmission is selected to be in the approximately linear region  8  of the curve between a lower frequency value of f L  and an upper frequency value of f U . A transmission frequency f t , higher than the resonant frequency f R  of the system, produces an induced voltage of V t . The induced voltage can be increased by reducing the transmission frequency and can be reduced by increasing the transmission frequency. For example, an increase in transmission frequency of δf produces a decrease in induced voltage of δV. 
     In some embodiments, a communication channel  6120  ( FIG. 6 a   ) is provided between the secondary unit  6300  and the inductive power outlet  6200 . Such a communication channel  6120 , may be used to communicate required operating parameters which, for example, may indicate the transmission frequency required by the electric load  6340  to the driver  6230 . 
     Various transmitters  6122  and receivers  6124  may be used with the communication channel  6120 . Where, as is often the case for inductive systems, the primary and secondary coils  6220 ,  6320  are galvanically isolated for example, optocouplers may have a light emitting diode serving as a transmitter which sends encoded optical signals over short distances to a photo-transistor which serves as a receiver. Optocouplers typically need to be aligned such that there is a line-of-sight between transmitter and receiver. In systems where alignment between the transmitter and receiver may be difficult to achieve, optocoupling may be inappropriate and alternative systems may be preferred such as ultrasonic signals transmitted by piezoelectric elements or radio signals such as Bluetooth, WiFi and the like. Alternatively the primary and secondary coils  6220 ,  6320  may themselves serve as the transmitter  6122  and receiver  6124 . 
     In certain embodiments, an optical transmitter, such as a light emitting diode (LED) for example, is incorporated within the secondary unit  6300  and is configured and operable to transmit electromagnetic radiation of a type and intensity capable of penetrating the casings of both the secondary unit  6300 , and the power outlet  6200 . An optical receiver, such as a photodiode, a phototransistor, a light dependent resistors of the like, is incorporated within the power outlet  6200  for receiving the electromagnetic radiation. 
     The communication channel  6120  may further provide a feedback signal during power transmission. The feedback transmission may communicate required or monitored operating parameters of the electric load  6240  such as:
         required operating voltage, current, temperature or power for the electric load  6240 ,   the measured voltage, current, temperature or power supplied to the electric load  6240  during operation,   the measured voltage, current, temperature or power received by the electric load  6240  during operation and the like.       

     In some embodiments, a microcontroller in the driver  6230  may use such feedback parameters to calculate the required transmission frequency and to adjust the driver accordingly. Alternatively, simple feedback signals may be provided indicating whether more or less power is required. 
     One example of a power regulation method using simple feedback signals is shown in the flowchart of  FIG. 6 d   . The method involves the following steps:
         (a) The driver  6230  provides an oscillating voltage at a transmission frequency f t  which is higher than the resonant frequency f R  of the system.   (b) A secondary voltage is induced in the secondary coil  6320 .   (c) A power monitor in the secondary unit  6300 , monitors the power received by the electric load  6340 .   (d) If the power received by the electric load  6340  lies within a predetermined range then no action is taken. If the power received by the electric load E 340  is below the predetermined range, then a feedback signal of a first type S a  is sent to the driver. If the power received by the electric load  6340  is above the predetermined range, then a feedback signal of a second type S b  is sent to the driver.   (e) A feedback signal is received by the driver  6230 .   (f) If the received feedback signal is of the first type S a , then the transmission frequency is increased by an incremental value +δf 1 . If the received feedback signal is of the second type S b , then the transmission frequency is decreased by an incremental value −δf 2 .       

     It is noted that by using the power regulation method described above, when the power received by the load is too high, a series of feedback signals of the first type S a  will be transmitted until the power is reduced into the acceptable range. Likewise when the power received by the load is too low, a series of feedback signals of the second type S b  will be transmitted until the power is increased into the acceptable range. It is noted that the positive incremental value δf 1  may be greater than, less than or equal to the negative incremental value δf 2 . 
     Alternatively, other power regulation methods using frequency adjustment may be used. For example, in alternative embodiments, the operating parameters of the electric load may be monitored and their values may be transmitted to the power outlet via the communications channel  6120 . A processor in the power outlet may then calculate the required transmission frequency directly. 
     The method described here above, refers to a non-resonant transmission frequency lying within the linear region  8  ( FIG. 6 b   ), higher than the resonant peak  2 . It will be appreciated however that in alternative embodiments frequency controlled power regulation may be achieved when the transmission frequency lies in the lower linear region  6  of the resonance curve. Nevertheless, as explained below, for certain embodiments, the selection of transmission frequencies in the higher linear 8 may be preferred. 
     As described above, the resonant frequency f R  of an inductive couple is given by the formula 
                 f   R     =     1     2   ⁢   π   ⁢     LC           ,         
where L is the inductance of the system and C is the capacitance of the system. Thus any decrease in either the inductance L or the capacitance C of the system thereby increases its resonant frequency.
 
     In inductive power outlets transmitting at frequencies above the normal resonant frequency of the system, an increase in resonant frequency of the system causes a large increase in the transmission voltage. In preferred embodiments, a peak detector  6140  ( FIG. 1 ) is be provided to monitor the transmission voltage of the power outlet  6200  and is configured to detect large increases in the transmission voltage indicating an increase in resonant frequency. Such increases in transmission voltage may be indicative of power drains, short circuits, removal of the secondary unit or the like. 
     As an example of the use of a peak detector reference is again made to  FIG. 6 c   . It will be appreciated that in a desktop environment, conductive bodies such as a paper clip, metal rule, the metal casing a stapler, a hole-punch or any metallic objects may be introduced between the inductive power outlet  6200  and the secondary power receiving unit  6300 . The oscillating magnetic field produced by the primary coil  6220  would then produce eddy currents in the conductive body heating it and thereby draining power from the primary coil  6220 . Such a power drain may be wasteful and/or dangerous. 
     Power drains such as described above reduce the inductance L of the system. The inductance L may also be reduced by the removal of the secondary coil  6220 , a short circuit or the like. A peak detector  6140 , wired to the inductive power outlet, would detect any of these scenarios as a large increase in transmission voltage. Preferably, the power transfer system may be further configured to shut down, issue a warning or otherwise protect the user and the system in the event that the peak detector  6140  detects such an increase in transmission voltage. 
       FIG. 6 e    is a circuit diagram of an inductive power outlet  6200  and a secondary unit  6300 . The secondary unit  6300  comprises a secondary coil  6320  wired to an electric load  6340  via a rectifier  6330 . 
     The inductive power outlet  6200  comprises a primary coil  6220  driven by a half-bridge converter  6230  connected to a power source  6240 . The half-bridge converter  6230  is configured to drive the primary coil  6220  at a frequency higher than the resonant frequency of the system and a peak detector  6140  is configured to detect increases in the transmission voltage. 
     Although only a half-bridge converter is represented in  FIG. E6 , it is noted that other possible driving circuits include: a DC-to-DC converter, an AC-to-DC converter, an AC-to-AC converter, a flyback transformer, a full-bridge converter, a flyback converter or a forward converter for example. 
     Thus, by using a transmission voltage oscillating at a frequency different from the resonant frequency of the system, the inductive transfer system has a higher tolerance to environmental fluctuations and variations in inductive coil alignment than other transfer systems and the frequency may be used to regulate power transfer. Moreover, when the transmission frequency is higher than the resonant frequency of the system, a peak detector may be used to indicate hazards. 
     Inductive Communication Channel 
     U.S. Pat. No. 5,455,466 titled, “Inductive coupling system for power and data transfer” to Terry J. Parks and David S. Register describes a system for inductively coupling power and data to a portable electronic device. The portable device, such as a personal digital assistant (PDA), is powered or recharged via an inductive link between the device and a support unit. The same inductive link is also used to transfer data signals between the device and a second electronic device, such as a conventional desktop computer. The support unit includes a primary winding of a transformer, a power amplifier and a modulator. The portable device includes a secondary winding connected in parallel with the input of a rectifier, the output of which is connected to a battery charging circuit, and to a modem, which is further connected to the device microprocessor. Placement of the device on the support unit effects the inductive coupling when the primary and secondary windings are in proximity to one another. Parks&#39; system is thus directed to providing a data channel for synchronizing two data storage devices for example a PDA and a computer. 
     In Parks&#39; system data transfer from the primary winding to the secondary winding may be provided by modulating the power signal. This requires a separate data signal to be transmitted by the secondary winding which is induced in the primary winding. Power transmission must therefore be interrupted in order to transmit data signals from the secondary winding to the primary winding. As a result, Parks&#39; system does not offer any solution to providing a feedback signal for the regulation of uninterrupted inductive power transfer to an electric load. 
     Reference is now made to  FIG. 7 a    showing a block diagram of the main elements of an inductive power transfer system  7100  consisting of an inductive power outlet  7200  configured to provide power to a remote secondary unit  7300 . The inductive power transfer system  7100  includes an inductive communication channel  7120  according to a further embodiment of the present invention. The communication channel  7120  is configured to produce an output signal S out  in the power outlet  7200  when an input signal S in  is provided by the secondary unit  7300  without interrupting the inductive power transfer from the outlet  7200  to the secondary unit  7300 . 
     The inductive power outlet  7200  includes a primary inductive coil  7220  wired to a power source  7240  via a driver  7230 . The driver  7230  is configured to provide an oscillating driving voltage to the primary inductive coil  7220 , typically at a voltage transmission frequency f t  which is higher than the resonant frequency f R  of the system. 
     The secondary unit  7300  includes a secondary inductive coil  7320 , wired to an electric load  7340 , which is inductively coupled to the primary inductive coil  7220 . The electric load  7340  draws power from the power source  7240 . Where the electric load  7340  requires a direct current supply, for example a charging device for an electrochemical cell or the like, a rectifier  7330  may be provided to rectify the alternating current signal induced in the secondary coil  7320 . 
     An inductive communication channel  7120  is provided for transferring signals from the secondary inductive coil  7320  to the primary inductive coil  7220  concurrently with uninterrupted inductive power transfer from the primary inductive coil  7220  to the secondary inductive coil  7320 . The communication channel  7120  may provide feedback signals to the driver  7230 . 
     The inductive communication channel  7120  includes a transmission circuit  7122  and a receiving circuit  7124 . The transmission circuit  7122  is wired to the secondary coil  7320 , optionally via a rectifier  7330 , and the receiving circuit  7124  is wired to the primary coil  7220 . 
     The signal transmission circuit  7122  includes at least one electrical element  7126 , selected such that when it is connected to the secondary coil  7320 , the resonant frequency f R  of the system increases. The transmission circuit  7122  is configured to selectively connect the electrical element  7126  to the secondary coil  7320 . 
     As known, the resonant frequency f R  of an inductive couple is given by the formula 
                 f   R     =     1     2   ⁢   π   ⁢     LC           ,         
where L is the inductance of the system and C is the capacitance of the system. Thus any decrease in either the inductance L or the capacitance C increases the resonant frequency of the system. The electrical element  7126  may be a low resistance for example, typically the resistance of the electrical element  7126  is under 50 ohms and preferably about 1 ohm.
 
     The signal receiving circuit  7124  may include a voltage peak detector  7128  configured to detect large increases in the transmission voltage. In systems where the voltage transmission frequency f t  is higher than the resonant frequency f R  of the system, such large increases in transmission voltage may be caused by an increase in the resonant frequency f R  thereby indicating that the electrical element  7126  has been connected to the secondary coil  7320 . Thus the transmission circuit  7122  may be used to send a signal pulse to the receiving circuit  7124  and a coded signal may be constructed from such pulses. 
     According to some embodiments, the transmission circuit  7122  may also include a modulator (not shown) for modulating a bit-rate signal with the input signal S in . The electrical element  7126  may then be connected to the secondary inductive coil  7320  according to the modulated signal. The receiving circuit  7124  may include a demodulator (not shown) for demodulating the modulated signal. For example the voltage peak detector  7128  may be connected to a correlator for cross-correlating the amplitude of the primary voltage with the bit-rate signal thereby producing the output signal S out . 
     In other embodiments, a plurality of electrical elements  7126  may be provided which may be selectively connected to induce a plurality of voltage peaks of varying sizes in the amplitude of the primary voltage. The size of the voltage peak detected by the peak detector  7128  may be used to transfer multiple signals. 
       FIG. 7 b    is a graph showing how the amplitude of the operational voltage varies according to the transmission frequency. It is noted that the voltage is at its highest when the transmission frequency is equal to the resonant frequency f R  of the system, this maximum amplitude is known as the resonance peak  2 . If the resonant frequency f R  of the system increases, a new resonance peak  2 ′ is produced. 
     According to an exemplary embodiment of the invention, an inductive power transfer system  7100  operates at a given transmission frequency f t  which is higher than the resonant frequency f R  of the system. The normal operating voltage V t  is monitored by the voltage peak detector  7128 . When the electric element  7126  is connected to the secondary inductive coil  7320  the resonant frequency of the system increases. Therefore, the operating voltage increases to a higher value V t ′. This increase is detected by the voltage peak detector  7128 . 
     The present invention allows data signals to be transferred from the secondary coil  7320  to the primary coil  7220  concurrently with inductive transfer of power from the primary coil  7220  to the secondary coil  7320 . Consequently, the signal transfer system may be used to provide feedback signals for real time power regulation. This is in contradistinction to prior art inductive signal transfer systems, such as the system described in U.S. Pat. No. 5,455,466 titled, “Inductive coupling system for power and data transfer” to Terry J. Parks and David S. Register, in which a separate data signal is supplied to the secondary inductive coil such that a voltage is induced in the primary coil. 
       FIG. 7 c    shows an exemplary circuit diagram of an inductive power outlet  7200  and a secondary unit  7300 , according to another embodiment of the invention. An inductive feedback channel  7120  is provided for transferring signals between the coils concurrently with uninterrupted inductive power transfer. 
     The inductive power outlet  7200  comprises a primary coil  7220  driven by a half-bridge converter  7230  connected to a power source  7240 . The half-bridge converter  7230  is configured to drive the primary coil  7220  at a frequency higher than the resonant frequency of the system. The secondary unit  7300  comprises a secondary coil  7320  wired to the input terminals T 1 , T 2  of a rectifier  7330 , and an electric load  7340  wired to the output terminals T 3 , T 4  of the rectifier  7330 . 
     Although only a half-bridge converter  7230  is represented in the inductive power outlet  7200  of  FIG. 7 c   , it is noted that other driving circuits could be used. These include: a DC-to-DC converter, an AC-to-DC converter, an AC-to-AC converter, a flyback transformer, a full-bridge converter, a flyback converter or a forward converter for example. 
     The inductive feedback channel  7120  comprises a transmission circuit  7122 , in the secondary unit  7300  and a receiving circuit  7124  in the inductive power outlet  7200 . The transmission circuit  7122  comprises an electrical resistor  7126  connected to the rectifier  7330  via a power MOSFET switch  7125 . A modulator  7123  may provide an input signal S in  to the power MOSFET  7125 . 
     It is noted that in this embodiment the transmission circuit  7122  is wired to one input terminal T 1  and one output terminal T 3  of the rectifier  7330 . This configuration is particularly advantageous as, even when the transmission circuit  7122  is connected, the resistor  7126  only draws power from the system during one half of the AC cycle, thereby significantly reducing power loss. 
     The receiving circuit  7124  includes a voltage peak detector  7128  that is configured to detect increases in the transmission voltage, and a demodulator  7129  for producing an output signal S out . 
     With reference now to  FIG. 7 d   , a flowchart is presented showing the main steps in a method for transferring a signal from the secondary inductive coil to a primary inductive coil of an inductive power transfer system. The method includes the following steps:
         Step (a)—connecting the primary inductive coil to a voltage monitor for monitoring the amplitude of a primary voltage across the primary coil;   Step (b)—connecting the secondary inductive coil to a transmission circuit for connecting an electric element to the secondary inductive coil thereby increasing the resonant frequency of the inductive power transfer system;   Step (c)—providing an oscillating voltage to the primary inductive coil at an initial transmission frequency higher than the resonant frequency thereby inducing a voltage in the secondary inductive coil;   Step (d)—using the transmission circuit to modulate a bit-rate signal with the input signal to create a modulated signal and connecting the electrical element to the secondary inductive coil intermittently according to the modulated signal;   Step (e)—using the voltage monitor to cross-correlate the amplitude of the primary voltage with the bit-rate signal for producing an output signal.       

     The inductive feedback channel  7120  may be used for transferring data, such as data pertaining to any or all of the following:
         the required operating voltage, current, temperature or power for the electric load  7240     the measured voltage, current, temperature or power supplied to the electric load  7240  during operation   the measured voltage, current, temperature or power received by the electric load  7240  during operation and the like   identification data for the user, electronic device and such like   a release signal of a transmission-key for releasing a transmission-lock.       

     Therefore, the inductive communication channel may be used to transfer a feedback signal from the secondary inductive coil to the primary inductive coil for regulating power transfer across an inductive power coupling. 
     For example the system may be configured to transfer two signals with the driver being configured to decrease the transmission power when a first signal is received, and to increase the transmission power when a second signal is received. 
     Power may be regulated by altering the duty cycle of the transmission voltage provided by the driver. Furthermore, the driver  7230  may be configured to adjust the transmission frequency in order to regulate the power transfer, as described hereinabove. Accordingly, the driver may be configured to adjust the transmission frequency in response to feedback signals. The transmission frequency may be increased when the first signal is received thereby decreasing the operating voltage, and the transmission frequency may be decreased when the second signal is received, thereby increasing the operating voltage. 
     Thus a communication channel is provided for regulating power transfer and/or for transmitting data signals from the secondary coil to the primary coil of an inductive couple while power is being transferred. 
     It will be apparent from the above description that various embodiment of the present invention disclose significant advantages enabling the efficient, safe and unobtrusive inductive transfer of power. It is further noted that, in combination, these advantages allow an inductive power transmission system to become a practical tool suitable for a variety of applications. 
     The scope of the present invention is defined by the appended claims and includes both combinations and sub combinations of the various features described hereinabove as well as variations and modifications thereof, which would occur to persons skilled in the art upon reading the foregoing description. 
     In the claims, the word “comprise”, and variations thereof such as “comprises”, “comprising” and the like indicate that the components listed are included, but not generally to the exclusion of other components. 
     While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the invention.