Patent Publication Number: US-2019181693-A1

Title: Non-contact power receiving device and non-contact power transmitting device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is based upon and claims the benefit of priorities from Japanese Patent Application No. P2017-235692 filed on Dec. 8, 2017 and Japanese Patent Application No. P2018-120094 filed on Jun. 25, 2018, the entire contents of which are hereby incorporated by reference. 
     FIELD 
     Embodiments of the present invention relate to a non-contact power receiving device and a non-contact power transmitting device. 
     BACKGROUND 
     Non-contact electric power transmitting apparatuses that transmit electric power in a non-contact manner are becoming widespread. The non-contact electric power transmitting apparatus includes a non-contact power transmitting device for supplying electric power (transmitting power) and a non-contact power receiving device for receiving electric power supplied from the non-contact power transmitting device. The non-contact power transmitting device supplies the electric power to the non-contact power receiving device by using electromagnetic coupling, such as electromagnetic induction or magnetic field resonance. The non-contact power transmitting device has a power transmission table provided with a power transmission coil and supplies the electric power to the non-contact power receiving device placed on the power transmission table by generating a magnetic field from the power transmission coil. The non-contact power receiving device generally includes a secondary battery and performs charging for storing the electric power supplied from the non-contact power transmitting device with respect to the secondary battery. 
     In such a non-contact electric power transmitting apparatus, it is assumed that some foreign object is inserted between the power transmission table of the non-contact power transmitting device and the non-contact power receiving device. For example, when the foreign object is a conductor, such as a metal, when the electric power is supplied from the non-contact power transmitting device to the non-contact power receiving device placed on the power transmission table, an eddy-current is generated in the conductor and heat is generated. Here, there is a non-contact power transmitting device which compares a difference between the temperature in the vicinity of the coil and the temperature at a position away from the coil with a preset threshold value and detects and reports the foreign object. 
     When it takes time to detect the foreign object, there is a possibility that a user may be away from the non-contact power transmitting device and the user cannot recognize the existence of the foreign object. Therefore, there is a problem that it is necessary to detect the foreign object at an early stage. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view for describing a configuration example of a non-contact electric power transmitting apparatus according to an embodiment; 
         FIG. 2  is a view for describing a configuration example of a non-contact power transmitting device and a non-contact power receiving device according to a first embodiment; 
         FIG. 3  is an explanatory view for describing a disposition position of a temperature sensor of the non-contact power receiving device according to the first embodiment; 
         FIG. 4  is a view for describing an example of an operation of the non-contact power transmitting device according to the first embodiment; 
         FIG. 5  is a view for describing an example of an operation of the non-contact power receiving device according to the first embodiment; 
         FIG. 6  is a view for describing an example of the operation of the non-contact power receiving device according to the first embodiment; 
         FIG. 7  is an explanatory view for describing a temperature difference of a plurality of temperature sensors of the non-contact power receiving device according to the first embodiment; 
         FIG. 8  is an explanatory view for describing a gradient of the temperature difference of the plurality of temperature sensors of the non-contact power receiving device according to the first embodiment; 
         FIG. 9  is a view for describing a configuration example of a non-contact power transmitting device and a non-contact power receiving device according to a second embodiment; 
         FIG. 10  is an explanatory view for describing a disposition position of a temperature sensor of the non-contact power transmitting device according to the second embodiment; 
         FIG. 11  is a view for describing an example of an operation of the non-contact power transmitting device according to the second embodiment; 
         FIG. 12  is a view for describing an example of an operation of a non-contact power transmitting device according to a third embodiment; 
         FIG. 13  is an explanatory view for describing a temperature difference of a plurality of temperature sensors of the non-contact power transmitting device according to the third embodiment; 
         FIG. 14  is an explanatory view for describing a gradient of a temperature difference of the plurality of temperature sensors of the non-contact power transmitting device according to the third embodiment; and 
         FIG. 15  is a view for describing another configuration example of the non-contact power transmitting device and the non-contact power receiving device according to the second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     An exemplary embodiment provides a non-contact power receiving device and a non-contact power transmitting device which are capable of detecting foreign object at an early stage. 
     In general, according to one embodiment, a non-contact power receiving device, which receives electric power wirelessly supplied from a non-contact power transmitting device, includes a power receiving coil, a load circuit, a first temperature sensor, a second temperature sensor, and a control circuit. The control circuit calculates a gradient indicating a change in a temperature difference between a temperature detected by a first temperature sensor and a temperature detected by a second temperature sensor, and outputs information for stopping power transmission to the non-contact power transmitting device when the gradient is equal to or greater than a preset threshold value. 
     Hereinafter, a non-contact power transmitting device, a non-contact power receiving device, and a non-contact electric power transmitting apparatus according to an embodiment will be described with reference to the drawings. 
       FIG. 1  is an explanatory view illustrating a configuration example of a non-contact electric power transmitting apparatus  1  according to the embodiment. 
     The non-contact electric power transmitting apparatus  1  includes a non-contact power transmitting device  2  for supplying electric power (transmitting power) and a non-contact power receiving device  3  for receiving electric power supplied from the non-contact power transmitting device  2 . 
     The non-contact power transmitting device  2  supplies the electric power to the non-contact power receiving device  3  by using magnetic coupling, such as electromagnetic induction or magnetic field resonance. In other words, the non-contact power transmitting device  2  supplies the electric power to the non-contact power receiving device  3  in a state not electrically connected to the non-contact power receiving device  3 . As illustrated in  FIG. 1 , the non-contact power transmitting device  2  includes a power transmission table  11 , a display unit  12 , and a power transmission coil  13 . 
     The power transmission table  11  is a part in which a part of a housing of the non-contact power transmitting device  2  is formed in a flat plate shape, and the power transmission coil  13  is provided on the inside of the housing. 
     The display unit  12  is an indicator (for example, an LED or a display) indicating a state of the non-contact power transmitting device  2 . 
     The power transmission coil  13  is connected to a power transmission circuit that generates a magnetic field by AC power. The power transmission coil  13  is configured to be disposed in parallel with a surface (placement surface) on which the non-contact power receiving device  3  of the power transmission table  11  is placed. 
     The non-contact power receiving device  3  is a device that receives the electric power transmitted from the non-contact power transmitting device  2 . The non-contact power receiving device  3  is configured as a portable information terminal, such as a smartphone or a tablet PC. In addition, the non-contact power receiving device  3  may be configured to be connected to a power source terminal of the portable information terminal, such as a smartphone or a tablet PC, and to supply the electric power transmitted from the non-contact power transmitting device  2  to the portable information terminal. In addition, as illustrated in  FIG. 1 , the non-contact power receiving device  3  includes a power receiving coil  21 , a display unit  22 , and a secondary battery  23 . 
     The power receiving coil  21  is an element that generates a current based on a change in the magnetic field, and is configured to be disposed in parallel with any surface of the housing of the non-contact power receiving device  3 . The power receiving coil  21  may be configured as a winding structure in which an insulated wire is wound or may be configured such that a coil pattern is formed on a printed board. In a state where a surface on which the power receiving coil  21  of the housing of the non-contact power receiving device  3  is provided is oriented toward the placement surface of the power transmission table  11 , when the non-contact power receiving device  3  is placed on the power transmission table  11 , the power receiving coil  21  is electromagnetically coupled to the power transmission coil  13  of the non-contact power transmitting device  2 . 
     The secondary battery  23  is a battery that is charged with electric power generated in the power receiving coil  21  and supplies the electric power to each part of the non-contact power receiving device  3 . 
     The display unit  22  is a display device for displaying various pieces of information. 
     The non-contact power transmitting device  2  generates the magnetic field from the power transmission coil  13  by supplying the AC power (transmission power) to the power transmission coil  13 . The non-contact power transmitting device  2  supplies the electric power to the non-contact power receiving device  3  via the power receiving coil  21  electromagnetically coupled to the power transmission coil  13  by generating the magnetic field from the power transmission coil  13 . 
     The power receiving coil  21  of the non-contact power receiving device  3  generates an induced current by the magnetic field output from the power transmission coil  13  of the non-contact power transmitting device  2 . The non-contact power receiving device  3  performs charging for storing the electric power generated in the power receiving coil  21  in the secondary battery  23 . 
     In addition, the efficiency of the power transmission between the non-contact power transmitting device  2  and the non-contact power receiving device  3  deteriorates in accordance with the magnitude of shift (positional shift) of the center C 1  of the power transmission coil  13  and the center C 2  of the power receiving coil  21 . Further, there is a case where foreign object is inserted between the non-contact power transmitting device  2  and the non-contact power receiving device  3 . When the foreign object is a conductor, such as a metal, for example, the electric power transmitted from the non-contact power transmitting device  2  is absorbed by the foreign object, and thus, the foreign object generates heat and at the same time the efficiency of the power transmission deteriorates. 
     First Embodiment 
       FIG. 2  is an explanatory view for describing a configuration example of the non-contact power transmitting device  2  and the non-contact power receiving device  3  of the non-contact electric power transmitting apparatus  1  according to the first embodiment. 
     First, the non-contact power transmitting device  2  will be described. 
     DC power is supplied to the non-contact power transmitting device  2  from a commercial power source via a DC power source, such as an AC adapter  4 . The non-contact power transmitting device  2  is operated by the DC power source in either a power transmission state of supplying the electric power to the non-contact power receiving device  3  or a standby state of not supplying the electric power to the non-contact power receiving device  3 . 
     The non-contact power transmitting device  2  includes a power source circuit  14 , a power transmission circuit  15 , a power transmission coil  13 , a display unit  12 , a wireless communication circuit  16 , a control circuit  17 , and the like. The power source circuit  14  converts a voltage of an external DC power source into a voltage appropriate for the operation of each circuit. Accordingly, the power source circuit  14  generates the electric power for causing the power transmission circuit  15  to perform the power transmission, and supplies the electric power to the power transmission circuit  15 . In addition, the power source circuit  14  generates the electric power for operating the control circuit  17 , and supplies the electric power to the control circuit  17 . 
     Under the control of the control circuit  17 , the power transmission circuit  15  generates the AC power (transmission power) by switching the DC power supplied from the power source circuit  14 . The power transmission circuit  15  generates the magnetic field in the power transmission coil  13  by supplying the AC power to the power transmission coil  13 . 
     As the power transmission coil  13  is connected to a capacitor for resonance (not illustrated) in series or in parallel, a resonance circuit may be configured. The power transmission coil  13  generates the magnetic field by the electric power supplied from the power transmission circuit  15 . 
     The display unit  12  is an indicator illustrating the state of the non-contact power transmitting device  2 . The display unit  12  switches the display according to the control of the control circuit  17 . For example, the display unit  12  switches the display color according to the operation state of the non-contact power transmitting device  2 . Further, for example, the display unit  12  may switch the display color according to the result of the foreign object detecting. Otherwise, the display unit  12  may display the operation state as a message. 
     The wireless communication circuit  16  is an interface for performing wireless communication with the non-contact power receiving device  3 . The wireless communication circuit  16  is a circuit that performs the wireless communication at a frequency different from the frequency of the power transmission. The wireless communication circuit  16  is, for example, a wireless LAN using a 2.4 GHz or 5 GHz band, a near field wireless communication device using a 920 MHz band, a communication device using infrared, or the like. Specifically, the wireless communication circuit  16  is a circuit that performs the wireless communication with the non-contact power receiving device  3  according to standards, such as Bluetooth (registered trademark) or Wi-Fi (registered trademark). In addition, the wireless communication circuit  16  may be a circuit that performs signaling for load-modulating a carrier wave of the power transmission and performing communication with the non-contact power receiving device  3 . 
     The control circuit  17  controls the operations of the power transmission circuit  15 , the display unit  12 , and the wireless communication circuit  16 , respectively. The control circuit  17  includes a processor and a memory. The processor executes arithmetic processing. The processor performs various processes based on, for example, a program stored in the memory and data used in the program. The memory stores the program and the data used in the program. In addition, the control circuit  17  may be configured of a microcomputer and/or an oscillation circuit or the like. 
     For example, the control circuit  17  switches the display of the display unit  12  in accordance with the state of the non-contact power transmitting device  2 . In addition, for example, the control circuit  17  controls the communication with the non-contact power receiving device  3  via the wireless communication circuit  16 . 
     Further, for example, the control circuit  17  controls the frequency of the AC power output from the power transmission circuit  15  and controls ON and OFF of the operation of the power transmission circuit  15 . For example, by controlling the power transmission circuit  15 , the control circuit  17  switches the state (power transmission state) of generating the magnetic field in the power transmission coil  13  and the state (standby state) of not generating the magnetic field in the power transmission coil  13 . Further, the control circuit  17  may perform a control for detecting the state where the non-contact power receiving device  3  is placed on the non-contact power transmitting device  2  by intermittently generating the magnetic field in the power transmission coil  13 , or may perform a control by generating the magnetic field smaller than that of the general power transmission state. 
     When an electromagnetic induction method is used for the power transmission, the control circuit  17  controls the power transmission circuit  15  so as to supply the AC power of approximately 100 kHz to 200 kHz to the power transmission coil  13 . Further, when a magnetic field resonance method is used for the power transmission, the control circuit  17  supplies the AC power having a MHz band, such as 6.78 MHz or 13.56 MHz, to the power transmission coil  13 . In addition, the frequency of the AC power supplied from the power transmission circuit  15  to the power transmission coil  13  is not limited to the description above, and may be changed in accordance with the specification of the non-contact power receiving device  3 . 
     Next, the non-contact power receiving device  3  will be described. 
     The non-contact power receiving device  3  includes a power receiving coil  21 , a power receiving circuit  24 , a charging circuit  25 , a secondary battery  23 , a display unit  22 , a wireless communication circuit  26 , a first temperature sensor  27 , a second temperature sensor  28 , and a control circuit  29 . In addition, the non-contact power receiving device  3  may be configured to include an output terminal for supplying the electric power to the load instead of the charging circuit  25  and the secondary battery  23 . 
     As the power receiving coil  21  is connected to the capacitor (not illustrated) in series or in parallel, the resonance circuit may be configured. When the non-contact power receiving device  3  is placed on the power transmission table  11  of the non-contact power transmitting device  2 , the power receiving coil  21  is electromagnetically coupled to the power transmission coil  13  of the non-contact power transmitting device  2 . The power receiving coil  21  generates the induced current by the magnetic field output from the power transmission coil  13  of the non-contact power transmitting device  2 . In other words, the power receiving resonance circuit configured with the power receiving coil  21  and the capacitor (not illustrated) functions as the AC power source for supplying the AC power (received power) to the power receiving circuit  24  connected to the power receiving resonance circuit. 
     For example, when using the magnetic field resonance method for the power transmission, a configuration in which a self-resonance frequency of the power receiving resonance circuit is the same as or substantially the same as the frequency of the power transmission of the non-contact power transmitting device  2  is desirable. Accordingly, the power transmission efficiency when the power receiving coil  21  and the power transmission coil  13  are electromagnetically coupled to each other. 
     The power receiving circuit  24  rectifies the received power supplied from the power receiving resonance circuit and converts the received power into a direct current. The power receiving circuit  24  includes, for example, a rectifying bridge configured with a plurality of diodes. A pair of input terminals of the rectifying bridge is connected to the power receiving resonance circuit. The power receiving circuit  24  outputs the DC power from the pair of output terminals by full-wave rectifying the received power supplied from the power receiving resonance circuit. The charging circuit  25  is connected to the pair of output terminals of the power receiving circuit  24 . The power receiving circuit  24  supplies the DC power to the charging circuit  25 . 
     The charging circuit  25  converts the DC power supplied from the power receiving circuit  24  into the DC power (charging power) used for the charging. In other words, the charging circuit  25  outputs the charging power for charging the secondary battery  23  with the received power output from the power receiving circuit  24 . For example, when charging the secondary battery  23 , the charging circuit  25  supplies the power having a predetermined current value or a voltage value to the secondary battery  23 . 
     The secondary battery  23  is charged with the charging power generated by the charging circuit  25  and is used for various configuration operations of the non-contact power receiving device  3 . 
     For example, the secondary battery  23  supplies the electric power to the control circuit  29  that executes various processes of the non-contact power receiving device  3 . Further, the secondary battery  23  is connected with the display unit  22 , the wireless communication circuit  26 , a camera (not illustrated), a speaker, and the like. 
     The display unit  22  is a display device for displaying various pieces of information. The display unit  22  displays a screen under the control of the control circuit  29  or a graphic controller (not illustrated). 
     The wireless communication circuit  26  is an interface for performing the wireless communication with the non-contact power transmitting device  2 . The wireless communication circuit  26  is a circuit that performs the wireless communication at a frequency different from the frequency of the power transmission. The wireless communication circuit  26  is, for example, a wireless LAN using a 2.4 GHz or 5 GHz band, a near field wireless communication device using a 920 MHz band, a communication device using infrared, or the like. Specifically, the wireless communication circuit  26  is a circuit that performs the wireless communication with the non-contact power transmitting device  2  according to standards, such as Bluetooth (registered trademark) or Wi-Fi (registered trademark). In addition, the wireless communication circuit  26  may be a circuit that performs signaling for load-modulating a carrier wave of the power transmission and performing communication with the non-contact power transmitting device  2 . 
     The first temperature sensor  27  and the second temperature sensor  28  are sensors for detecting the temperature, respectively. The first temperature sensor  27  and the second temperature sensor  28  respectively supply detection signals indicating the detected temperatures to the control circuit  29 . 
       FIG. 3  is an explanatory view for describing an example of an installation position of the first temperature sensor  27  and the second temperature sensor  28 . As illustrated in  FIG. 3 , the first temperature sensor  27  detects the temperature in the vicinity of the center C 2  of the power receiving coil  21 . The vicinity of the center C 2  is a position where the magnetic field received by the power receiving coil  21  is strong (magnetic flux density is high). As the first temperature sensor  27  is configured to detect the temperature at a position where the magnetic flux density is high in this manner, it becomes easy to detect a change in temperature due to the foreign object. 
     Further, the second temperature sensor  28  detects the temperature at a position away from the center C 2  of the power receiving coil  21 , for example, the temperature at a position further on the outside than the power receiving coil  21 . The position away from the center C 2  is a position where the magnetic field received by the power receiving coil  21  is weak (magnetic flux density is low) or where it is unlikely to be influenced by the magnetic field. As the second temperature sensor  28  is configured to detect the temperature at a position where the magnetic flux density is low in this manner, it becomes difficult to detect the change in temperature due to the foreign object compared to the first temperature sensor  27 . 
     As described above, the first temperature sensor  27  is disposed at a position close to the center C 2  of the power receiving coil  21 , and the second temperature sensor  28  is disposed at a position far from the center C 2  of the power receiving coil  21 . In other words, the first temperature sensor  27  detects the temperature at a position closer to the center C 2  of the power receiving coil  21  than the second temperature sensor  28 . As the positions for detecting the temperatures of the first temperature sensor  27  and the second temperature sensor  28  is set in this manner, a difference between the detection results of the change in temperature due to the foreign object by the first temperature sensor  27  and the second temperature sensor  28  arises. 
     The control circuit  29  controls operations of the power receiving circuit  24 , the charging circuit  25 , the display unit  22 , and the wireless communication circuit  16 , respectively. The control circuit  29  includes a processor and a memory. The processor executes arithmetic processing. The processor performs various processes based on, for example, a program stored in the memory and data used in the program. The memory stores the program and the data used in the program. 
     For example, the control circuit  29  causes the display unit  22  to display various pieces of information. In addition, for example, the control circuit  29  controls the communication with the non-contact power transmitting device  2  via the wireless communication circuit  26 . 
     The control circuit  29  controls the operation of the charging circuit  25 . For example, as the control circuit  29  controls the charging circuit  25 , a state of charging the secondary battery  23  (performing the charging) and a state where the charging is not performed are switched to each other. 
     Next, the operation of the non-contact power transmitting device  2  having the above-described configuration will be described. 
       FIG. 4  is a flowchart for describing an example of the operation of the non-contact power transmitting device  2 . When being activated, the non-contact power transmitting device  2  operates in a standby state (ACT  11 ). At this time, the control circuit  17  performs a control such that the power transmission circuit  15  operates at regular time intervals. Accordingly, the power transmission circuit  15  intermittently supplies the transmission power to the power transmission coil  13 . 
     The control circuit  17  performs placement detecting while intermittently performing the power transmission (ACT  12 ). The placement detecting is a process for determining whether or not the non-contact power receiving device  3  to which the electric power is supplied is placed on the power transmission table  11 . The control circuit  17  of the non-contact power transmitting device  2  determines whether or not the non-contact power receiving device  3  to which the electric power is supplied is placed on the power transmission table  11  based on the detection result of a current detection circuit (not illustrated) that detects the current supplied from the power transmission circuit  15  to the power transmission coil  13 . Otherwise, the control circuit  17  determines whether or not the non-contact power receiving device  3  to which the electric power is supplied is placed on the power transmission table  11  based on the detection result of a current detection circuit (not illustrated) that detects the current supplied from the power source circuit  14  to the power transmission circuit  15 . 
     For example, in the standby state, the control circuit  17  intermittently supplies the transmission power from the power transmission circuit  15  to the power transmission coil  13 . When the detection result of the current detection circuit increases while the transmission power is being supplied from the power transmission circuit  15  to the power transmission coil  13 , the control circuit  17  determines that the non-contact power receiving device  3  is placed on the power transmission table  11 . 
     When it is determined that the non-contact power receiving device  3  is placed on the power transmission table  11 , the control circuit  17  performs authenticating (ACT  13 ). The authenticating is a process for determining whether or not a counterpart device that performs the power transmission is a correct device between the non-contact power transmitting device  2  and the non-contact power receiving device  3 . The non-contact power transmitting device  2  and the non-contact power receiving device  3  perform the authenticating by transmitting and receiving predetermined information to and from each other. 
     For example, the authenticating is a process in which the non-contact power transmitting device  2  determines whether or not the non-contact power receiving device  3  placed on the power transmission table  11  is a correct device. For example, the control circuit  17  of the non-contact power transmitting device  2  acquires authentication information from the non-contact power receiving device  3  via the wireless communication circuit  16 . The authentication information is information indicating identification information of the non-contact power receiving device  3 , model number, corresponding power transmission method, corresponding frequency, and the like. By comparing the acquired authentication information with the information recorded in the memory, the control circuit  17  determines that the non-contact power receiving device  3  placed on the power transmission table  11  is a correct device that can supply the electric power by the non-contact power transmitting device  2 . The authentication information may be only the identification information of the non-contact power transmitting device  2  or the non-contact power receiving device  3 , or may be other information. 
     In addition, the authenticating may be a process in which the non-contact power receiving device  3  determines whether or not the non-contact power transmitting device  2  having the power transmission table  11  on which the non-contact power receiving device  3  is placed is a correct device. In this case, the control circuit  29  of the non-contact power receiving device  3  acquires authentication information from the non-contact power transmitting device  2  via the wireless communication circuit  26 . The authentication information is information indicating the identification information of the non-contact power transmitting device  2 , model number, corresponding power transmission method, corresponding frequency, and the like. By comparing the acquired authentication information with the information recorded in the memory, the control circuit  29  determines whether or not the non-contact power transmitting device  2  is a correct device that corresponds to the control circuit  29 . 
     The control circuit  17  determines whether or not the result of the authenticating is normal (ACT  14 ). When it is determined that the authenticating is not performed normally (the authentication result is NG) (ACT  14 , NO), the control circuit  17  moves to the process of ACT  11 . 
     When it is determined that the authenticating is performed normally (ACT  14 , YES), the control circuit  17  starts the power transmission to the non-contact power receiving device  3  by supplying the transmission power from the power transmission circuit  15  to the power transmission coil  13  (ACT  15 ). 
     The control circuit  17  determines whether to stop the power transmission to the non-contact power receiving device  3  (ACT  16 ). For example, the control circuit  17  sequentially determines whether or not the non-contact power receiving device  3  has been removed from the power transmission table  11  during the power transmission, based on the value of the current supplied from the power transmission circuit  15  to the power transmission coil  13 . When it is determined that the non-contact power receiving device  3  has been removed from the power transmission table  11 , the control circuit  17  determines to stop the power transmission. In addition, when information for instructing to stop the power transmission is supplied from the non-contact power receiving device  3 , the control circuit  17  determines to stop the power transmission. 
     When it is determined not to stop the power transmission to the non-contact power receiving device  3  (ACT  16 , NO), the control circuit  17  moves to the process of ACT  15  and continues the power transmission. 
     In addition, when it is determined to stop the power transmission to the non-contact power receiving device  3  (ACT  16 , YES), the control circuit  17  stops the power transmission to the non-contact power receiving device  3  (ACT  17 ) and ends the process. 
     In addition, when information for instructing to restart the power transmission is supplied from the non-contact power receiving device  3  after the power transmission is stopped, the control circuit  17  operates the power transmission circuit  15  and restarts the power transmission. 
     Next, the operation of the non-contact power receiving device  3  having the above-described configuration will be described. 
       FIG. 5  is a flowchart for describing an example of the operation of the non-contact power receiving device  3 . 
     When being placed on the power transmission table  11  of the non-contact power transmitting device  2 , the non-contact power receiving device  3  is activated by the electric power supplied from the non-contact power transmitting device  2  (ACT  21 ). 
     When being activated, the control circuit  29  of the non-contact power receiving device  3  executes the charging for converting the DC power supplied from the power receiving circuit  24  to the charging circuit  25  into the charging power used for the charging, and supplying the charging power to the secondary battery  23  (ACT  22 ). 
     In addition, the control circuit  29  acquires each detected value from the first temperature sensor  27  and the second temperature sensor  28 , performs foreign object detecting based on the acquired detected value (ACT  23 ), and determines the presence or absence of the foreign object (ACT  24 ). The foreign object detecting is a process for determining whether or not the foreign object is inserted between the non-contact power receiving device  3  and the non-contact power transmitting device  2 . The foreign object detecting will be described later. 
     When it is determined that there is no foreign object in the foreign object detecting (ACT  24 , NO), the control circuit  29  executes the charging for supplying the charging power to the secondary battery  23  (ACT  25 ). In other words, the control circuit  29  continues the charging for charging the secondary battery  23 . 
     The control circuit  29  determines whether to end the charging (ACT  26 ). For example, the control circuit  29  monitors the charging state of the secondary battery  23  and determines whether or not the secondary battery  23  has been sufficiently charged. When it is determined that the secondary battery  23  has been sufficiently charged, the control circuit  29  determines to end the charging. 
     When it is determined not to end the charging (ACT  26 , NO), the control circuit  29  moves to the process of ACT  23  and performs the foreign object detecting again. When the foreign object is not detected, the charging is continued. 
     In addition, when it is determined to end the charging (ACT  26 , YES), the control circuit  29  ends the charging (ACT  27 ) and ends the process of  FIG. 5 . 
     Further, when it is determined that there is the foreign object in the ACT  24  (ACT  24 , YES), the control circuit  29  transmits information for instructing to stop the power transmission or information that the foreign object is detected (ACT  28 ) to the non-contact power transmitting device  2  via the wireless communication circuit  26 , moves to the process of ACT  27 , and ends the charging. When the information for instructing to stop the power transmission performed in ACT  28  is received, the non-contact power transmitting device  2  stops the power transmission. Here, when the power source supply for the operation of the control circuit  29  is stopped immediately before moving to the process of the ACT  27 , the control circuit  29  stops the operation without performing the process of the ACT  27 . 
     Accordingly, the non-contact power transmitting device  2  is switched from the power transmission state of supplying the electric power to the non-contact power receiving device  3  to the standby state of not supplying the electric power to the non-contact power receiving device  3 . In addition, when there is a remaining amount of the secondary battery  23  and the non-contact power receiving device  3  can be operated without receiving the electric power from the non-contact power transmitting device  2 , the control circuit  29  displays that the foreign object exists on display unit  22 . Furthermore, the control circuit  17  of the non-contact power transmitting device  2  may display that the foreign object exists on the display unit  12  of the non-contact power transmitting device  2 . 
     Next, the foreign object detecting performed in the non-contact power receiving device  3  will be described. 
     As a result of detecting the temperature of the first temperature sensor  27 , the control circuit  29  of the non-contact power receiving device  3  performs the foreign object detecting for detecting the presence or absence of the foreign object based on the detection result of the temperature of the first temperature sensor  27  and the detection result of the temperature of the second temperature sensor  28 . The foreign object detecting is a process for determining whether or not an foreign object having some electric conductor to which the electric power is not supplied exists between the power transmission table  11  and the non-contact power receiving device  3 . The foreign object may be foreign object generated by peeling off the conductor, such as a clip, or may be foreign object generated as the conductor is accommodated in a housing of resin or the like, such as the non-contact IC card. 
     When the power transmission from the non-contact power transmitting device  2  to the non-contact power receiving device  3  is started, each component, such as the power transmission circuit  15 , the power transmission coil  13 , the power receiving coil  21 , and the power receiving circuit  24  generates heat. When the foreign object does not exist between the non-contact power transmitting device  2  and the non-contact power receiving device  3 , the temperature of the center C 1  of the power transmission coil  13  having a high magnetic flux density and the temperature of the center C 2  of the power receiving coil  21  are higher than the temperature at other positions. In particular, the temperature at the center C 1  of the power transmission coil  13  is higher than the temperature around the power transmission coil  13  having a low magnetic flux density, and the temperature at the center C 2  of the power receiving coil  21  is higher than the temperature around the power receiving coil  21  having a low magnetic flux density. In other words, a temperature difference arises between the position close to the center C 1  of the power transmission coil  13  and the center C 2  of the power receiving coil  21  and a position far from the center C 1  of the power transmission coil  13  and the center C 2  of the power receiving coil  21 . 
     Furthermore, when the foreign object, such as a metal, exists between the non-contact power transmitting device  2  and the non-contact power receiving device  3 , a part of the electric power output from the power transmission coil  13  is absorbed by the foreign object. The electric power absorbed by the foreign object causes the eddy-current in the foreign object. Accordingly, the heat is generated in the foreign object. For example, when the foreign object, such as a metal, exists on the power transmission table  11 , it is presumed that a difference in temperature arises between the position where the foreign object exists and another position. 
     For the above-described reasons, the temperature difference caused by the presence or absence of the foreign substance is added to the temperature difference caused by the difference in position where the temperature is detected. In other words, when the foreign object exists between the non-contact power transmitting device  2  and the non-contact power receiving device  3 , compared a case where the foreign object does not exist between the non-contact power transmitting device  2  and the non-contact power receiving device  3 , the difference in temperature detected by the temperature sensors provided at different positions increases. Therefore, based on the difference between the temperatures detected by the first temperature sensor  27  and the second temperature sensor  28  which are installed at different positions, the control circuit  29  determines the presence or absence of the foreign object. 
       FIG. 6  is a flowchart for describing an example of the foreign object detecting in the non-contact power receiving device  3 . 
     The control circuit  29  of the non-contact power receiving device  3  acquires a temperature T 1  from the first temperature sensor  27  (ACT  31 ). As described above, the first temperature sensor  27  supplies a detection signal indicating the temperature in the vicinity of the center C 2  of the power receiving coil  21  to the control circuit  29 . The control circuit  29  A/D converts the detection signal supplied from the first temperature sensor  27  and acquires the temperature T 1  which is a value indicating the temperature. 
     The control circuit  29  of the non-contact power receiving device  3  acquires a temperature T 2  from the second temperature sensor  28  (ACT  32 ). As described above, the second temperature sensor  28  supplies a detection signal indicating the temperature at the position away from the center C 2  of the power receiving coil  21  to the control circuit  29 . The control circuit  29  A/D converts the detection signal supplied from the second temperature sensor  28  and acquires the temperature T 2  which is a value indicating the temperature. 
     The control circuit  29  calculates a temperature difference G between the temperature T 1  and the temperature T 2  (ACT  33 ). The control circuit  29  calculates the temperature difference G by subtracting a lower value from a higher value at the temperature T 1  and the temperature T 2 . In other words, the control circuit  29  calculates an absolute value of the temperature T 1 −the temperature T 2  as the temperature difference G (G=|T 1 −T 2 |). 
     The control circuit  29  calculates a gradient S of the temperature difference based on the change of the temperature difference G according to the time (ACT  34 ). For example, the control circuit  29  stores the temperature difference G calculated every predetermined time in the memory. Based on the stored temperature difference G, the control circuit  29  calculates a change amount of the temperature difference G at predetermined time intervals as the gradient S of the temperature difference. 
     The control circuit  29  compares the gradient S of the calculated temperature difference with a preset threshold value (first detection threshold value) Th 1  (ACT  35 ). The first detection threshold value Th 1  is stored in the memory of the control circuit  29 , for example. The first detection threshold value Th 1  is a value which is lower than the maximum value (referred to as gradient S 1 ) of the gradient S of the temperature difference estimated when the foreign object exists between the non-contact power transmitting device  2  and the non-contact power receiving device  3 , and higher than the maximum value (referred to as gradient S 2 ) of the gradient S of the temperature difference estimated when the foreign object does not exist between the non-contact power transmitting device  2  and the non-contact power receiving device  3 . 
     The first detection threshold value Th 1  may be stored for each type of the non-contact power transmitting device  2  that transmits the electric power. In this case, the control circuit  29  recognizes the type of the non-contact power transmitting device  2  in the above-described authenticating, reads the first detection threshold value Th 1  that corresponds to the recognized type from the memory, and compares the first detection threshold value Th 1  with the gradient S of the temperature difference. 
     Further, when the non-contact power receiving device  3  is configured to be capable of performing process, such as rapid charging with a processing load higher than that of ordinary charging, the first detecting threshold value Th 1  may further be stored for each type of the charging. In this case, the control circuit  29  reads the first detection threshold value Th 1  that corresponds to the type of the charging, and compares the first detection threshold value Th 1  with the gradient S of the temperature difference. 
     When the gradient S of the calculated temperature difference is less than the first detection threshold value Th 1  (ACT  35 , NO), the control circuit  29  determines that the foreign object does not exist between the non-contact power transmitting device  2  and the non-contact power receiving device  3  (ACT  36 ) and ends the foreign object detecting. In this case, as illustrated in  FIG. 5 , the control circuit  29  continues the charging. In addition, it is not indispensable to determine that the foreign object by the control circuit  29  does not exist, and the charging may be continued based on the determination result of the ACT  35 . 
     In addition, when the gradient S of the calculated temperature difference is equal to or greater than the first detection threshold value Th 1  (ACT  35 , YES), the control circuit  29  determines that the foreign object does not exist between the non-contact power transmitting device  2  and the non-contact power receiving device  3  (ACT  37 ) and ends the foreign object detecting. In this case, as illustrated in  FIG. 5 , the control circuit  29  stops the charging and outputs information for stopping the power transmission to the non-contact power transmitting device  2 . In addition, it is not indispensable to determine that the foreign object exists by the control circuit  29 , the charging may be stopped based on the determination result of ACT  35 , and the information for stopping the power transmission may be output to the non-contact power transmitting device  2 . 
     Next, a change in the temperature difference G when the foreign object detecting is performed as described above will be described. 
       FIG. 7  is an explanatory view for describing a change in the temperature difference G after the charging is started. The vertical axis of  FIG. 7  indicates the temperature difference G, and the horizontal axis indicates time. 
       FIG. 8  is an explanatory view for describing a change in the gradient S of the temperature difference after the charging is started. The vertical axis of  FIG. 8  indicates the gradient S of the temperature difference, and the horizontal axis indicates time. 
     In addition, it is assumed that, at timing t 0 , the power transmission from the non-contact power transmitting device  2  to the non-contact power receiving device  3  is not started, and at timing t 1 , the power transmission from the non-contact power transmitting device  2  to the non-contact power receiving device  3  is started. 
     A first graph  31  in  FIG. 7  is a graph illustrating a change in the temperature difference G when the foreign object does not exist. The first graph  31  illustrates that the temperature difference G is 0 between timing t 0  and timing t 1 . Further, the first graph  31  illustrates that the temperature difference G is increasing after timing t 1 . 
     A first graph  41  in  FIG. 8  is a graph illustrating a change in the gradient S of the temperature difference when the foreign object does not exist. The first graph  41  illustrates that the gradient S of the temperature difference is 0 between timing t 0  and timing t 1 . Further, the first graph  41  illustrates that the gradient S of the temperature difference increases to the gradient S 2  after timing t 1 , and thereafter, the gradient S of the temperature difference gradually decreases. In other words, the first graph  31  and the first graph  41  illustrate that, when the foreign object does not exist, the temperature difference increases from the timing when the power transmission is started, the temperature approaches saturation as time elapses, and an increase ratio of the temperature difference decreases. 
     A second graph  32  of  FIG. 7  is a graph illustrating the change in the temperature difference G when the foreign object, such as a metal, exists and the foreign object detecting is not performed. The second graph  32  illustrates that the temperature difference G is 0 between timing t 0  and timing t 1 . Further, the second graph  32  illustrates that the temperature difference G is increasing after timing t 1 . 
     A second graph  42  of  FIG. 8  is a graph illustrating the change in the gradient S of the temperature difference when the foreign object exists and the foreign object detecting is not performed. The second graph  42  illustrates that the gradient S of the temperature difference is 0 between timing t 0  and timing t 1 . Further, the second graph  42  illustrates that the gradient S of the temperature difference increases to the gradient S 1  after timing t 1 , and thereafter, the gradient S of the temperature difference gradually decreases. 
     The second graph  32  and the second graph  42  illustrate that, when the foreign object exists, the temperature difference sharply increases from the timing when the power transmission is started, the temperature approaches saturation as time elapses, and the increase ratio of the temperature difference decreases. In addition, the second graph  32  and the second graph  42  illustrates that the temperature difference G and the gradient S are greater than those when the foreign object does not exist. 
     A third graph  33  of  FIG. 7  is a graph illustrating the change in the temperature difference G when the foreign object exists and the foreign object detecting is performed. The third graph  33  illustrates that the temperature difference G is 0 between timing t 0  and timing t 1 . Further, in the third graph  33 , the temperature difference G increases from timing t 1  to timing t 2 , the temperature difference G decreases from timing t 2  to timing t 4 , and after timing t 4 , the temperature difference G increases again. 
     A third graph  43  of  FIG. 8  is a graph illustrating the change in the gradient of the temperature difference when the foreign object exists and the foreign object detecting is performed. The third graph  43  illustrates that the gradient S of the temperature difference is 0 between timing t 0  and timing t 1 . In addition, the third graph  43  illustrates that the gradient S of the temperature difference increases from timing t 1  to timing t 2  and the gradient S of the temperature difference becomes equal to or higher than the first detection threshold value Th 1  at timing t 2 . In this case, the control circuit  29  determines that the foreign object exists, and outputs the information for stopping the power transmission of the non-contact power transmitting device  2 . At the same time, the operation of the charging circuit  25  is also stopped. The third graph  43  illustrates that the gradient S changes from a positive value to a negative value immediately after timing t 2  at which the power transmission is stopped. In addition, the third graph  43  illustrates that the gradient S gradually returns to 0 from the negative value from timing t 3  to timing t 4 . Furthermore, the foreign object is removed and the power transmission from the non-contact power receiving device  3  is restarted at timing t 4 . Therefore, the third graph  43  illustrates that the gradient S increases from timing t 4  to timing t 5 , and the gradient S gradually decreases after timing t 5 . 
     The third graph  33  and the third graph  43  illustrate that, when the foreign object exists and the power transmission is stopped at the timing when the gradient S becomes equal to or greater than the first detection threshold value Th 1 , and when the temperature difference G has returned, the power transmission is restarted. Further, the third graph  33  and the third graph  43  illustrate that the foreign object is removed from the time when the power transmission is stopped until the power transmission is restarted, and the gradient S after the restart of the power transmission becomes gentle similar to that at the time when there is no foreign object. 
     In addition, in the above-described example, it is described that the temperature difference G in the third graph  33  returns to 0 at timing t 4 , but the configuration is not limited to the configuration. At timing t 4  at which the power transmission is restarted, the temperature difference G may remain. The time until the temperature difference G in the third graph  33  returns to 0 changes depending on the space around each temperature sensor and the material of the structure. For example, when the time period from timing t 2  to timing t 4  is short (for example, several ten seconds to several minutes), the temperature difference G in the third graph  33  does not decrease to 0. 
     As described above, the non-contact power receiving device  3  includes the first temperature sensor  27  provided at the position close to the center C 2  of the power receiving coil  21  and the second temperature sensor  28  provided at the position away from the center C 2  of the power receiving coil  21 . The control circuit  29  of the non-contact power receiving device  3  calculates the temperature difference G between the temperature T 1  detected by the first temperature sensor  27  and the temperature T 2  detected by the second temperature sensor  28 , and calculates the gradient S indicating the change ratio of the temperature difference G. When the gradient S becomes equal to or greater than the preset first detection threshold value Th 1 , the control circuit  29  determines that the foreign object exists between the non-contact power transmitting device  2  and the non-contact power receiving device  3 , and outputs the information for stopping the power transmission by the non-contact power transmitting device  2 . The gradient S sharply changes compared to the temperature difference G and reaches the maximum value in a short time period. Therefore, the control circuit  29  can determine whether or not the foreign object exists more quickly than in the configuration in which the foreign object is detected in accordance with the temperature difference G. 
     Second Embodiment 
       FIG. 9  is an explanatory view for describing a configuration example of a non-contact power transmitting device  2 A and a non-contact power receiving device  3 A of a non-contact electric power transmitting apparatus  1 A according to a second embodiment. In addition, the second embodiment is different from the first embodiment in that the non-contact power transmitting device  2 A performs the foreign object detecting instead of the non-contact power receiving device  3 A. 
     The non-contact power transmitting device  2 A includes the power source circuit  14 , the power transmission circuit  15 , the power transmission coil  13 , the display unit  12 , the wireless communication circuit  16 , a first temperature sensor  18 A, a second temperature sensor  19 A, a control circuit  17 A, and the like. 
     The first temperature sensor  18 A and the second temperature sensor  19 A are sensors for detecting the temperature, respectively. The first temperature sensor  18 A and the second temperature sensor  19 A respectively supply detection signals indicating the detected temperatures to the control circuit  17 A. 
       FIG. 10  is an explanatory view for describing an example of an installation position of the first temperature sensor  18 A and the second temperature sensor  19 A. As illustrated in  FIG. 10 , the first temperature sensor  18 A detects the temperature in the vicinity of the center C 1  of the power transmission coil  13 . Further, the second temperature sensor  19 A detects the temperature at a position away from the center C 1  of the power transmission coil  13 , for example, the temperature at a position further on the outside than the power transmission coil  13 . 
     The control circuit  17 A has the same configuration as the control circuit  17 , and the operation at the time of the power transmission is different from that of the control circuit  17 . 
     The non-contact power receiving device  3 A includes the power receiving coil  21 , the power receiving circuit  24 , the charging circuit  25 , the secondary battery  23 , the display unit  22 , the wireless communication circuit  26 , and the control circuit  29 . In other words, the embodiment is different from the first embodiment in that the non-contact power receiving device  3 A does not include the first temperature sensor  27  and the second temperature sensor  28 . 
     Next, the operation of the non-contact power transmitting device  2 A having the above-described configuration will be described. 
       FIG. 11  is a flowchart for describing an example of the operation of the non-contact power transmitting device  2 A. When being activated, the non-contact power transmitting device  2 A operates in a standby state (ACT  41 ). At this time, the control circuit  17 A controls such that the power transmission circuit  15  operates at regular time intervals. Accordingly, the power transmission circuit  15  intermittently supplies the transmission power to the power transmission coil  13 . 
     The control circuit  17 A performs the placement detecting while intermittently performing the power transmission (ACT  42 ). 
     When it is determined that the non-contact power receiving device  3 A is placed on the power transmission table  11 , the control circuit  17 A performs the authenticating (ACT  43 ). 
     The control circuit  17 A determines whether or not the result of the authenticating is normal (whether or not the authentication result is OK) (ACT  44 ). When it is determined that the authenticating is not performed normally (the authentication result is NG) (ACT  44 , NO), the control circuit  17 A moves to the process of ACT  41 . 
     When it is determined that the authenticating is performed normally (ACT  44 , YES), the control circuit  17 A performs a control for supplying the transmission power from the power transmission circuit  15  to the power transmission coil  13  and starting the power transmission to the non-contact power receiving device  3 A (ACT  45 ). 
     In addition, the control circuit  17 A acquires each detected value from the first temperature sensor  18 A and the second temperature sensor  19 A, performs the foreign object detecting based on the acquired detected value (ACT  46 ), and determines the presence or absence of the foreign object (ACT  47 ). The foreign object detecting is a process for determining whether or not the foreign object, such as a metal, is inserted between the non-contact power receiving device  3 A and the non-contact power transmitting device  2 A. Foreign object detecting executed by the control circuit  17 A in ACT  46  is the same process as the foreign object detecting executed by the control circuit  29  of the non-contact power receiving device  3  in the first embodiment. 
     When it is determined that there is no foreign object in the foreign object detecting (ACT  47 , NO), the control circuit  17 A executes the power transmission (ACT  48 ). In other words, the control circuit  17  performs a control so as to supply the transmission power to the power transmission coil  13  from the power transmission circuit  15 , and continues the power transmission. 
     The control circuit  17 A determines whether to stop the power transmission to the non-contact power receiving device  3 A (ACT  49 ). For example, the control circuit  17 A sequentially determines whether or not the non-contact power receiving device  3 A has been removed from the power transmission table  11  during the power transmission, based on the value of the current supplied from the power transmission circuit  15  to the power transmission coil  13  or the value of the current supplied from the power source circuit  14  to the power transmission circuit  15 . When it is determined that the non-contact power receiving device  3 A has been removed from the power transmission table  11 , the control circuit  17 A performs the control so as to stop the power transmission. Further, the control circuit  17 A may be configured to perform the control to stop the power transmission when receiving the information for instructing to stop the power transmission from the non-contact power receiving device  3 A, due to, for example, full charge of the secondary battery  23 . 
     When it is determined that the power transmission to the non-contact power receiving device  3 A is not stopped (ACT  49 , NO), the control circuit  17 A moves to the process of ACT  46 . In other words, the control circuit  17 A continues the power transmission while repeatedly executing the foreign object detecting. 
     In addition, when it is determined to stop the power transmission to the non-contact power receiving device  3 A (ACT  49 , YES), the control circuit  17 A stops the power transmission to the non-contact power receiving device  3 A (ACT  50 ) and ends the process. 
     In addition, when the control circuit  17 A determines that there is the foreign object in the ACT  47  (ACT  47 , YES), the control circuit  17 A moves to the process of the ACT  50 , stops the power transmission, and ends the process. Thereafter, when the foreign object is removed and the non-contact power receiving device  3 A is removed from the power transmission table  11 , the control circuit  17 A restarts the operation from the standby state (ACT  41 ). 
     As described above, the non-contact power transmitting device  2 A may include the first temperature sensor  18 A provided at the position close to the center C 1  of the power transmission coil  13  and the second temperature sensor  19 A provided at the position away from the center C 1  of the power receiving coil  13 , and may be configured to perform the foreign object detecting. 
     Third Embodiment 
     The third embodiment is different from the other embodiments in that the non-contact power transmitting device  2 A does not determine the presence or absence of the foreign object based on the comparison result between the gradient S of the temperature difference G in the first temperature sensor  18 A and the second temperature sensor  19 A and the preset first detection threshold value Th 1 , and determines the presence or absence of the foreign object based on the comparison result of between the temperature difference G and the variable second detection threshold value Th 2 . In addition, the non-contact power receiving device  3  in the first embodiment or the non-contact power transmitting device  2 A in the second embodiment may execute the foreign object detecting. In the description of the embodiment, it is assumed that the non-contact power transmitting device  2 A executes the foreign object detecting based on the comparison result between the temperature difference G and the variable second detection threshold value Th 2 . 
       FIG. 12  is a flowchart for describing an example of the foreign object detecting in the non-contact power transmitting device  2 A. 
     The control circuit  17 A of the non-contact power transmitting device  2 A acquires the temperature T 1  from the first temperature sensor  18 A (ACT  61 ). As described above, the first temperature sensor  18 A supplies the detection signal indicating the temperature in the vicinity of the center C 1  of the power receiving coil  13  to the control circuit  17 A. By A/D converting the detection signal supplied from the first temperature sensor  18 A, the control circuit  17 A acquires the temperature T 1  which is a value indicating the temperature. 
     The control circuit  17 A of the non-contact power transmitting device  2 A acquires the temperature T 2  from the second temperature sensor  19 A (ACT  62 ). As described above, the second temperature sensor  19 A supplies the detection signal indicating the temperature at the position away from the center C 1  of the power transmission coil  13  to the control circuit  17 A. By A/D converting the detection signal supplied from the second temperature sensor  19 A, the control circuit  17 A acquires the temperature T 2  which is a value indicating the temperature. 
     The control circuit  17 A calculates the temperature difference G between the temperature T 1  and the temperature T 2  (ACT  63 ). The control circuit  17 A calculates the temperature difference G by subtracting a lower value from a higher value at the temperature T 1  and the temperature T 2 . In other words, the control circuit  17 A calculates the absolute value of the temperature T 1 −the temperature T 2  as the temperature difference G (G=|T 1 −T 2 |). 
     The control circuit  17 A calculates the gradient S of the temperature difference based on the change of the temperature difference G according to the time (ACT  64 ). For example, the control circuit  17 A stores the temperature difference G calculated every predetermined time in the memory. Based on the stored temperature difference G, the control circuit  17 A calculates a change amount of the temperature difference G at predetermined time intervals as the gradient S of the temperature difference. 
     Further, the control circuit  17 A calculates the second detection threshold value Th 2  (ACT  65 ). The second detection threshold value Th 2  is a value that changes according to the elapsed time from the start of the power transmission. Specifically, the control circuit  17 A calculates the second detection threshold value Th 2  by adding the value (increase estimated value) according to the elapsed time from the start of the power transmission to the preset initial value. The increase estimated value is a value that increases according to the elapsed time from the start of the power transmission. For example, the increase estimated value corresponds to the temperature difference G according to the elapsed time from the start of the power transmission when the foreign object does not exist. The initial value and the increase estimated value for calculating the second detection threshold value Th 2  are stored in the memory of the control circuit  17 A, for example. 
     In addition, the initial value and the increase estimated value for calculating the second detection threshold value Th 2  may be stored for each type of the non-contact power receiving device  3 A that receives the electric power. In this case, the control circuit  17 A recognizes the type of the non-contact power receiving device  3 A in the above-described authenticating, reads the initial value and the increase estimated value that correspond to the recognized type from the memory, and uses the values in calculation of the second detection threshold value Th 2 . 
     In addition, when the non-contact power transmitting device  2 A is configured to be capable of transmitting a larger amount of power than that of the ordinary power transmission, the initial value and the increase estimated value for calculating the second detection threshold value Th 2  for each amount to be transmitted may be stored. In this case, the control circuit  17 A reads the initial value and the increase estimated value that corresponds to the amount to be transmitted from the memory, and uses the values in calculation of the second detection threshold value Th 2 . 
     The control circuit  17 A determines whether or not the calculated gradient S of the temperature difference is a negative value (ACT  66 ). When it is determined that the gradient S of the calculated temperature difference is not a negative value (ACT  66 , NO), the control circuit  17 A determines whether or not the temperature difference G is equal to or greater than the second detection threshold value Th 2  (ACT  67 ). 
     When the calculated temperature difference G is less than the second detection threshold value Th 2  (ACT  67 , NO), the control circuit  17 A determines that the foreign object does not exist between the non-contact power transmitting device  2 A and the non-contact power receiving device  3 A (ACT  68 ) and ends the foreign object detecting. In this case, the control circuit  17 A continues the charging. In addition, it is not indispensable to determine that the foreign object does not exist by the control circuit  17 A, and the charging may be continued based on the determination result of the ACT  67 . 
     In addition, when the calculated temperature difference G is equal to or greater than the second detection threshold value Th 2  (ACT  67 , YES), the control circuit  17 A determines that the foreign object does not exist between the non-contact power transmitting device  2 A and the non-contact power receiving device  3 A (ACT  69 ) and ends the foreign object detecting. In this case, the control circuit  17 A stops the power transmission to the non-contact power receiving device  3 A. In addition, it is not indispensable to determine that the foreign object exists by the control circuit  17 A, the charging may be stopped based on the determination result of ACT  67 , and the power transmission to the non-contact power transmitting device  3 A may be stopped. 
     In addition, when it is determined that the gradient S of the temperature difference is a negative value in ACT  66 , the control circuit  17 A resets the second detection threshold value Th 2  (ACT  70 ) and moves to the process of ACT  61 . For example, after switching from the power transmission state of supplying the electric power to the non-contact power receiving device  3 A to the standby state of not supplying the electric power to the non-contact power receiving device  3 A, the temperature difference G which has increased until now starts to decrease. In other words, although the temperature difference G is greater than the second detection threshold value Th 2 , a state where the power transmission is not performed is achieved. In the state, since the temperature difference G continues to decrease, it is not appropriate for the detection of the foreign object. Therefore, the control circuit  17 A does not compare the temperature difference G with the second detection threshold value Th 2  until the gradient S of the temperature difference reaches 0. In other words, the control circuit  17 A calculates the gradient S indicating the change in the temperature difference G between the temperature T 1  detected by the first temperature sensor  18 A and the temperature T 2  detected by the second temperature sensor  19 A, and when the gradient S is a negative value, a state where it is not determined whether or not the foreign object exists and the power transmission is stopped is continued. 
     Next, a change in the temperature difference G when the foreign object detecting is performed as described above will be described. 
       FIG. 13  is an explanatory view for describing the change in the temperature difference G after the charging is started. The vertical axis of  FIG. 13  indicates the temperature difference G, and the horizontal axis indicates time. 
       FIG. 14  is an explanatory view for describing the change in the gradient S after the charging is started. The vertical axis of  FIG. 14  indicates the gradient S of the temperature difference, and the horizontal axis indicates time. 
     In addition, it is assumed that, at timing t 0 , the power transmission from the non-contact power transmitting device  2 A to the non-contact power receiving device  3 A is not started, and at timing t 1 , the power transmission from the non-contact power transmitting device  2 A to the non-contact power receiving device  3 A is started. 
     The second detection threshold value Th 2  in  FIG. 13  is a value obtained by adding the increase estimated value that corresponds to the elapsed time from the start of the power transmission to the initial value. The second detection threshold value Th 2  starts to increase from timing t 1  at which the power transmission is started. Further, the second detection threshold value Th 2  is reset when the gradient S of the temperature difference G becomes a negative value and returns to the initial value. The second detection threshold value Th 2  starts to increase again when the power transmission is started. 
     A first graph  31 A in  FIG. 13  is a graph illustrating a change in the temperature difference G when the foreign object does not exist. The first graph  31 A illustrates that the temperature difference G is 0 between timing t 0  and timing t 1 . Further, the first graph  31 A illustrates that the temperature difference G is increasing after timing t 1 . 
     A first graph  41 A in  FIG. 14  is a graph illustrating a change in the gradient S of the temperature difference when the foreign object does not exist. The first graph  41 A illustrates that the gradient S of the temperature difference is 0 between timing t 0  and timing t 1 . Further, the first graph  41 A illustrates that the gradient S of the temperature difference increases to the gradient S 2  after timing t 1 , and thereafter, the gradient S of the temperature difference gradually decreases. In other words, the first graph  31 A and the first graph  41 A illustrate that, when the foreign object does not exist, the temperature difference increases from the timing when the power transmission is started, the temperature approaches saturation as time elapses, and the increase ratio of the temperature difference decreases. 
     A second graph  32 A of  FIG. 13  is a graph illustrating the change in the temperature difference G when the foreign object exists and the foreign object detecting is not performed. The second graph  32 A illustrates that the temperature difference G is 0 between timing t 0  and timing t 1 . Further, the second graph  32 A illustrates that the temperature difference G is further increasing than that in the graph  31 A after timing t 1 . 
     A second graph  42 A of  FIG. 14  is a graph illustrating the change in the gradient S of the temperature difference when the foreign object exists and the foreign object detecting is not performed. The second graph  42 A illustrates that the gradient S of the temperature difference is 0 between timing t 0  and timing t 1 . Further, the second graph  42 A illustrates that the gradient S of the temperature difference increases to the gradient S 1  after timing t 1 , and thereafter, the gradient S of the temperature difference gradually decreases. 
     The second graph  32 A and the second graph  42 A illustrate that, when the foreign object exists, the temperature difference sharply increases from the timing when the power transmission is started, the temperature approaches saturation as time elapses, and the increase ratio of the temperature difference decreases. In addition, the second graph  32 A and the second graph  42 A illustrates that the temperature difference G and the gradient S are greater than those when the foreign object does not exist. 
     A third graph  33 A of  FIG. 13  is a graph illustrating the change in the temperature difference G when the foreign object exists and the foreign object detecting is performed. The third graph  33 A illustrates that the temperature difference G is 0 between timing t 0  and timing t 1 . In addition, the third graph  33 A illustrates that the temperature difference G increases from timing t 1  to timing t 2  and the temperature difference G becomes equal to or higher than the second detection threshold value Th 2  at timing t 2 . In this case, the control circuit  17 A determines that the foreign object exists and stops the power transmission. Therefore, as illustrated in the third graph  33 A, the temperature difference G decreases from timing t 2  when the power transmission is stopped to timing t 4 , and the temperature difference G increases again after timing t 4  at which the power transmission is restarted. 
     A third graph  43 A of  FIG. 14  is a graph illustrating the change in the gradient S of the temperature difference when the foreign object exists and the foreign object detecting is performed. The third graph  43 A illustrates that the gradient S of the temperature difference is 0 between timing t 0  and timing t 1 . In addition, the third graph  43 A illustrates that the gradient S of the temperature difference increases from timing t 1  to timing t 2  and the gradient S changes from a positive value to a negative value immediately after timing t 2  at which the power transmission is stopped. In addition, the third graph  43 A illustrates that the gradient S gradually returns to 0 thereafter from timing t 3  to timing t 4 . The control circuit  17 A does not perform the foreign object detecting immediately after timing t 2  when the gradient S is negative until timing t 4 . Therefore, the power transmission is not restarted. When the gradient S returns to 0 at timing t 4 , the control circuit  17 A performs the foreign object detecting again, and restarts the power transmission when the temperature difference G is less than the second detection threshold value Th 2 . In addition, the third graph  43 A illustrates that the gradient S increases from timing t 4  to timing t 5 , and the gradient S gradually decreases after timing t 5 . 
     The third graph  33 A and the third graph  43 A illustrate that, when the foreign object exists and the power transmission is stopped at the timing when the temperature difference G becomes equal to or greater than the second detection threshold value Th 2 , and when the temperature difference G has returned, the power transmission is restarted. Further, the third graph  33 A and the third graph  43 A illustrate that the foreign object is removed from the time when the power transmission is stopped until the power transmission is restarted, and the gradient S after the restart of the power transmission becomes gentle. 
     As described above, the non-contact power transmitting device  2 A includes the first temperature sensor  18 A provided at the position close to the center C 1  of the power transmission coil  13  and the second temperature sensor  19 A provided at the position away from the center C 1  of the power receiving coil  21 . The control circuit  17 A of the non-contact power transmitting device  2 A calculates the temperature difference G between the temperature T 1  detected by the first temperature sensor  18 A and the temperature T 2  detected by the second temperature sensor  19 A, and calculates the gradient S indicating the change ratio of the temperature difference G. Further, the control circuit  17 A calculates the second detection threshold value Th 2  that corresponds to the elapsed time after the power transmission is started. When the temperature difference G becomes equal to or greater than the second detection threshold value Th 2 , the control circuit  17 A determines that the foreign object exists between the non-contact power transmitting device  2  and the non-contact power receiving device  3 , and stops the power transmission. 
     When sufficient time has elapsed from the start of the power transmission, the temperature T 1  detected by the first temperature sensor  18 A and the temperature T 2  detected by the second temperature sensor  19 A are saturated. For example, when setting the threshold value to be compared with the temperature difference G to a fixed value, it is necessary to set a value Gmax higher than the saturated value of the temperature difference G when the foreign object does not exist as the threshold value. According to the example of  FIG. 13 , the timing at which the second graph  32 A becomes equal to or greater than the second detection threshold value Th 2  is timing t 2 , and the timing of exceeding the threshold value Gmax is timing t 5 . In this manner, the non-contact power transmitting device  2 A can determine whether or not the foreign object exists earlier than a case where the threshold value compared with the temperature difference G is a fixed value. 
     In addition, in the non-contact power transmitting device  2 A, a threshold value Th 3  for comparison with the temperature T 1  detected by the first temperature sensor  18 A and the temperature T 2  detected by the second temperature sensor  19 A may further be set. The threshold value Th 3  is a preset fixed value. For example, the control circuit  17 A stores the threshold value Th 3  in the memory. The control circuit  17 A may be configured to compare the temperature T 1  detected by the first temperature sensor  18 A and the temperature T 2  detected by the second temperature sensor  19 A with the threshold value Th 3 , and to stop the power transmission when either one of the temperature T 1  and the temperature T 2  is equal to or higher than the threshold value Th 3 . Accordingly, the non-contact power transmitting device  2 A can prevent the power transmission table  11  from generating heat when both the temperature T 1  and the temperature T 2  increase and the temperature difference G does not exceed the second detection threshold value Th 2 . 
     In addition, in the above-described embodiment, it is described that the control circuit  17 A calculates the second detection threshold value Th 2  that corresponds to the elapsed time from the start of the power transmission, and determines whether or not the foreign object exists based on the comparison result of the temperature difference G between the temperature T 1  detected by the first temperature sensor  18 A and the temperature T 2  detected by the second temperature sensor  19 A and the second detection threshold value Th 2 . The timing when the control circuit  17 A calculates the second detection threshold value Th 2  and the temperature difference G and determines whether or not the foreign object exists may be any timing as long as the second detection threshold value Th 2  exceeds Gmax. For example, the control circuit  17 A may be configured to calculate the second detection threshold value Th 2  and the temperature difference G at the timing when a predetermined time period has elapsed from the start of the power transmission, and to determine whether or not the foreign object exists one time. In addition, for example, the control circuit  17 A may be configured to calculate the second detection threshold value Th 2  and the temperature difference G at a plurality of timings from the start of the power transmission, and to determine whether or not the foreign object exists. 
     In addition, in the above-described embodiment, it is described that the control circuit  17 A resets the second detection threshold value Th 2  when it is determined that the gradient S of the temperature difference is a negative value in the ACT  66  of  FIG. 12 , but the exemplary embodiment is not limited to the configuration. The control circuit  17 A may be configured to reset the second detection threshold value Th 2  when the gradient S of the temperature difference is less than an arbitrary value (−α) that is a negative value, and to move to ACT  61 . With such a configuration, the control circuit  17 A can determine again whether or not the temperature difference G is equal to or greater than the second detection threshold value before the gradient S of the temperature difference reaches 0. Accordingly, it is possible to shorten the time period until restarting the power transmission. 
     In addition, the heat generation due to the foreign object changes in distribution on the power transmission table  11  due to the position of the foreign object, the size of the foreign object, the material of the foreign object, and the like. Therefore, in the above-described configuration, when a region that generates heat over the detection positions of the plurality of temperature sensors exists, there is a possibility that the detection based on the temperature difference becomes difficult. Specifically, when the foreign object exists close to the second sensor  19 A, there is a possibility that the temperature difference does not arise even though the existence of the foreign object. Here, the non-contact power transmitting device  2 A may include more second temperature sensors  19 A. When the non-contact power transmitting device  2 A includes two or more second temperature sensors  19 A, the control circuit  17 A calculates the absolute value of the difference in a round-robin manner with  1 : 1  with respect to the temperatures detected by the first temperature sensor  18 A and the plurality of second temperature sensors  19 , and performs the foreign object detecting with the largest value as the above-described temperature difference G. Accordingly, for example, even when the second temperature sensor  19 A exists in which the temperature difference from the first temperature sensor  18 A is unlikely to arise because the foreign object exists close to the second temperature sensor  19 A, the control circuit  17 A can perform the foreign object detecting based on the temperature difference G between another second temperature sensor  19 A far from the foreign object and the first temperature sensor  18 A or the second temperature sensor  19 A close to the foreign object. Accordingly, it is possible to prevent the accuracy of the foreign object detecting from deteriorating due to the position of the foreign object. 
     In addition, the example in which the non-contact power transmitting device  2 A includes the plurality of second temperature sensors  19 A has been described, but the non-contact power receiving device  3  may have a configuration including the plurality of second temperature sensors  28 . In this case, the control circuit  29  calculates the absolute value of the difference in a round-robin manner with  1 : 1  with respect to the temperatures detected by the first temperature sensor  27  and the plurality of second temperature sensors  28 , and performs the foreign object detecting with the largest value as the above-described temperature difference G. Accordingly, the control circuit  29  can perform the foreign object detecting based on the temperature difference G between the second temperature sensor  28  far from the foreign object and the first temperature sensor  27  or the second temperature sensor  28  close to the foreign object. 
     Further, it has been described that the second temperature sensor  28  is disposed at a position where the magnetic field received by the power receiving coil  21  is weak, but more ideally, it is desirable that the second temperature sensor  28  is disposed so as to be capable of detecting the temperature (ambient temperature) at a position that is not influenced by the temperature rise caused by the magnetic field. In this case, since the temperature detected by the second temperature sensor  28  does not change according to the position of the foreign object, the non-contact power receiving device  3  can perform the foreign object detecting with high accuracy by using one first temperature sensor  27  and one second temperature sensor  28 . In addition, the same applies to the second temperature sensor  19 A of the non-contact power transmitting device  2 A. 
     In addition, when the non-contact power receiving device  3 A to which the electric power is supplied by the non-contact power transmitting device  2 A is large, there is a case where the power transmission coil  13  also becomes large. 
       FIG. 15  is a view for describing a non-contact power transmitting device  2 B and a non-contact power receiving device  3 B which are another configuration examples of the non-contact power transmitting device  2 A and the non-contact power receiving device  3 A according to the second embodiment. 
     The non-contact power receiving device  3 B is a device that is incorporated in a large-sized apparatus, such as an electric car, and receives the electric power from the non-contact power transmitting device  2 B by using magnetic coupling, such as electromagnetic induction or magnetic field resonance. 
     The non-contact power receiving device  3 B includes a power receiving coil  21 B, the secondary battery  23 , the power receiving circuit  24 , the charging circuit  25 , the wireless communication circuit  26 , the control circuit  29 , and the like. The power receiving coil  21 B is an element that generates a current based on a change in the magnetic field, and is provided in a vehicle body (chassis) of the non-contact power receiving device  3 B. The power receiving circuit  24  and the charging circuit  25  charge the secondary battery  23  with the power generated in the power receiving coil  21 B. 
     The non-contact power transmitting device  2 B is a device that supplies the electric power to the non-contact power receiving device  3 B incorporated in a large-sized apparatus, such as an electric vehicle, by using magnetic field coupling, such as electromagnetic induction or magnetic field resonance. 
     The non-contact power transmitting device  2 B includes a power transmission table  11 B, a power transmission coil  13 B, the power source circuit  14 , the power transmission circuit  15 , the wireless communication circuit  16 , a control circuit  17 B, a plurality of first temperature sensors  18 B, a plurality of second temperature sensors  19 B, and the like. 
     On the power transmission table  11 B, a housing (chassis) of the non-contact power receiving device  3 B is disposed. 
     The power transmission coil  13 B is an element that generates the magnetic field by current. The power transmission coil  13 B is disposed on the power transmission table  11 B. When the non-contact power receiving device  3 B is placed on the power transmission table  11 B, the power transmission coil  13 B is electromagnetically coupled to the power receiving coil  21 B of the non-contact power receiving device  3 B. 
     The control circuit  17 B includes a processor and a memory. The control circuit  17 B controls the operation of the non-contact power transmitting device  2 B as the processor executes the program in the memory. By controlling the power transmission circuit  15 , the control circuit  17 B causes an alternating current to flow to the power transmission coil  13 B connected to the power transmission circuit  15 . Accordingly, the magnetic field generated in the power transmission coil  13 B changes. Accordingly, the electric power is generated in the power receiving coil  21 B electromagnetically coupled to the power transmission coil  13 B. As a result, the electric power is supplied from the non-contact power transmitting device  2 B to the non-contact power receiving device  3 B. 
     The first temperature sensor  18 B and the second temperature sensor  19 B are sensors for detecting the temperature, respectively. The first temperature sensor  18 B and the second temperature sensor  19 B respectively supply detection signals indicating the detected temperatures to the control circuit  17 B. 
     As illustrated in  FIG. 15 , the plurality of first temperature sensors  18 B are provided on the inside of the power transmission coil  13 B, that is, on the side closer to the center C 1  than the power transmission coil  13 B. In addition, the first temperature sensor  18 B may not be provided on the inside of the power transmission coil  13 B and may be provided between the power transmission coil  13 B and the power transmission table  11 B. In other words, the first temperature sensor  18 B may be provided on the power transmission coil  13 B. The first temperature sensor  18 B may be provided at least on the side closer to the center C 1  than the outer circumference of the power transmission coil  13 B. The second temperature sensors  19 B are provided on the outside of the power transmission coil  13 B, that is, on the side further from the center C 1  than the power transmission coil  13 B. 
     As described above, when the non-contact power receiving device  3 B and the non-contact power transmitting device  2 B are large, the power transmission coil  13  of the non-contact power transmitting device  2 B becomes large. In such a configuration, when there is only one first temperature sensor, there is a possibility that the influence of the temperature rise due to foreign object M does not reach the detection position of the first temperature sensor, and the foreign object M cannot be detected. 
     Here, the control circuit  17 B calculates the absolute value of the difference in a round-robin manner with  1 : 1  with respect to the detection result of the plurality of first temperature sensors  17 B and the detection result of the second temperature sensors  18 B. Furthermore, the control circuit  17 B performs the foreign object detecting with the largest value among the calculation results as the above-described temperature difference G. Accordingly, the control circuit  17 B can perform the foreign object detecting based on the temperature difference G between the first temperature sensor  18 B close to the foreign object M and the second temperature sensor  19 B. 
     In addition, the function described in each of the above-described embodiments is not limited to the configuration using hardware, and can be realized by causing a computer to read a program that describes each function therein by using software. Further, each function may be configured by selecting either software or hardware as appropriate. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.