Abstract:
A communication apparatus includes: a communication antenna, a communication unit which can transmit and receive signals, a switch connected between the communication antenna and the communication unit and composed of a semiconductor switch, a switch control unit, and a high-voltage output means. The switch, when receiving a connection command signal causes the communication unit to be electrically connected with the communication antenna, and when not receiving the signal cuts them off. The switch control unit outputs the signal to the switch under prescribed conditions, and stops the signal when overvoltage applied to the communication unit is detected. The high-voltage output means, connected between the switch control unit and the switch, sets voltage of the signal received from the switch control unit to a voltage at which the communication unit in a transmitting mode would not be cut off from the communication antenna, and outputs the voltage to the switch.

Description:
TECHNICAL FIELD 
       [0001]    This invention relates to a communication device comprising a communication antenna and a communication section connected to the communication antenna. 
       BACKGROUND ART 
       [0002]    Recently, non-contact electric power transmission to a communication device is practically used. For example, when a communication device receives electric power via a communication antenna, the communication antenna during reception of the electric power might generate an overvoltage, or a voltage which exceeds an endurable voltage of a communication section. In such a case, the communication section might be damaged by the overvoltage. Similar problem may also be caused when a communication device without a non-contact electric power transmission function is placed in the vicinity of a device during transmission of the electric power. In order to avoid such problems, a communication device needs to include structure for protecting its communication section from the overvoltage. 
         [0003]    For example, each of Patent Document 1 and Patent Document 2 discloses a communication device which is capable of receiving the electric power in a non-contact manner and which includes structure for protecting its communication section from the overvoltage. 
         [0004]    The reception device (communication device) of Patent Document 1 comprises a coil (communication antenna) and a communication control integrated circuit (communication section), wherein the communication antenna is used for communication with a transmission device, and the communication section is connected to the communication antenna. The communication antenna is also used for the reception of the electric power from the transmission device. The communication device further comprises an input connection circuit (protection circuit). The protection circuit is provided between the communication antenna and the communication section. When a voltage in the communication antenna is elevated because of the reception of the electric power, the protection circuit works to lower a voltage applied to the communication section. As a result, the communication section is protected from an overvoltage generated because of the reception of the electric power. 
         [0005]    The protection circuit of Patent Document 1 lowers the voltage applied to the communication section by leaking a part of electric current to the ground, wherein the electric current is generated because of the non-contact electric power transmission. Accordingly, a part of the transmitted electric power is lost. 
         [0006]    The module (communication device) of Patent Document 2 comprises an antenna (communication antenna) and a communication section, wherein the communication antenna is used for communication with an external device, and the communication section is connected to the communication antenna. The communication antenna is also used for the reception of the electric power from a primary device. The communication device further comprises a switch circuit (switch) and a switch control circuit (switch control section). The switch is provided between the communication antenna and the communication section. When the communication antenna has high electric power, the switch control section turns the switch into an OFF-state to electrically disconnect the communication section from the communication antenna. The switch under the OFF-state basically consumes no electric power. Accordingly, the communication section is prevented from the overvoltage while suppressing the consumption of the transmitted electric power. 
       PRIOR ART DOCUMENTS 
     Patent Document(s) 
       [0000]    
       
         Patent Document 1: JP A 2011-172299 
         Patent Document 2: WO2012/090904 
       
     
       SUMMARY OF INVENTION 
     Technical Problem 
       [0009]    The switch of Patent Document 2 is provided between the communication section and the communication antenna. Accordingly, if the switch during communication is turned into the OFF-state in error, the communication is stopped. There is therefore a requirement for a communication device which can reliably maintain its communication state while securely protecting its communication section. 
         [0010]    It is therefore an object of the present invention to provide a communication device which can satisfy this requirement. 
       Solution to Problem 
       [0011]    A switch provided between a communication section and a communication antenna is required to be durable for repeated on/off and not to consume large electric power upon being turned on/off. The switch is therefore preferred to be formed by using a semiconductor switch such as a metal-oxide-semiconductor field-effect transistor (MOSFET). When the MOSFET is used, the source and the drain of the MOSFET may be connected between the communication section and the communication antenna. In this structure, the switch can be turned into an ON-state when a connection command signal, which has a voltage not smaller than a predetermined value, is applied to the gate, and the switch can be turned into the OFF-state when the connection command signal is not applied to the gate. 
         [0012]    However, in some cases, the source and the drain has a large voltage generated not only because of the reception of the electric power but also because of communication by the communication section. In particular, when the communication section transmits a signal, a large voltage might be generated. If electric potential difference between the gate and the source or between the gate and the drain becomes small, the switch is not properly turned into the ON-state. In order to reliably maintain the communication state, or to properly turn the switch into the ON-state, the voltage of the connection command signal needs to be sufficiently larger than the voltage generated because of the signal transmission of the communication section. 
         [0013]    The present invention therefore provides a communication device based on the aforementioned consideration, wherein the communication device can apply the connection command signal of proper voltage to the semiconductor switch while considering the voltage generated during the signal transmission of the communication section. Specifically, the present invention provides a communication device and an electronic apparatus described below. 
         [0014]    First aspect of the present invention provides a communication device comprising a communication antenna, a communication section, a switch, a switch control section and a high voltage output part. The communication section is capable of transmitting and receiving a signal via the communication antenna. The switch is formed of a semiconductor switch. The switch is connected between the communication antenna and the communication section. The switch electrically connects the communication section with the communication antenna when receiving a connection command signal. The switch electrically disconnects the communication section from the communication antenna when not receiving the connection command signal. The switch control section outputs the connection command signal toward the switch under a specific condition. The switch control section stops the connection command signal when detecting in advance that an overvoltage is to be applied to the communication section. The high voltage output part is connected between the switch control section and the switch. The high voltage output part converts a voltage of the connection command signal, which is received from the switch control section and is to be output to the switch, into another voltage that keeps the communication section in a signal transmitting state from being electrically disconnected from the communication antenna. 
         [0015]    Second aspect of the present invention provides an electronic apparatus comprising the communication device according to the first aspect. 
       Advantageous Effects of Invention 
       [0016]    The switch control section according to the present invention stops the connection command signal when detecting in advance that the overvoltage is to be applied to the communication section. The communication section is therefore securely protected. Moreover, the high voltage output part according to the present invention converts the voltage of the connection command signal to be output to the switch into the other voltage that keeps the communication section in the signal transmitting state from being electrically disconnected from the communication antenna. Accordingly, even if the voltage in the communication antenna is raised, for example, by the signal transmission from the communication section, the switch is kept in the ON-state. The signal transmitting state can be more reliably maintained. 
         [0017]    An appreciation of the objectives of the present invention and a more complete understanding of its structure may be had by studying the following description of the preferred embodiment and by referring to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0018]      FIG. 1  is a block diagram schematically showing a communication device according to a first embodiment of the present invention. 
           [0019]      FIG. 2  is a circuit diagram showing an example of a switch of the communication device of  FIG. 1 . 
           [0020]      FIG. 3  is a view showing action of the switch of  FIG. 1 . 
           [0021]      FIG. 4  is a block diagram schematically showing a communication device according to a second embodiment of the present invention. 
           [0022]      FIG. 5  is a circuit diagram showing examples of a switch and an additional switch (the part enclosed by dashed line A) of the communication device of  FIG. 4 . 
           [0023]      FIG. 6  is a view showing action of the switch and the additional switch of  FIG. 4  under a condition where a communication section of the communication device of  FIG. 4  is not in a signal transmitting state. 
           [0024]      FIG. 7  is a view showing action of the switch and the additional switch of  FIG. 4  under a condition where the communication section of the communication device of  FIG. 4  is in the signal transmitting state. 
           [0025]      FIG. 8  is a block diagram schematically showing a communication device according to a third embodiment of the present invention. 
           [0026]      FIG. 9  is a view showing action of a switch of the communication device of  FIG. 8 . 
           [0027]      FIG. 10  is a block diagram schematically showing a communication device according to a forth embodiment of the present invention. 
           [0028]      FIG. 11  is a block diagram schematically showing a communication device according to a fifth embodiment of the present invention. 
           [0029]      FIG. 12  is a circuit diagram showing an example of a switch control section of the communication device of  FIG. 11 . 
           [0030]      FIG. 13  is a view showing action of a switch and an auxiliary switch of the communication device of  FIG. 11  under a condition where a communication section of the communication device of  FIG. 11  is not in the signal transmitting state. 
           [0031]      FIG. 14  is a view showing the action of the switch of  FIG. 11 . 
           [0032]      FIG. 15  is a view showing the action of the auxiliary switch of  FIG. 11 . 
           [0033]      FIG. 16  is a timing chart showing the action of the switch and the auxiliary switch of  FIG. 11 . 
           [0034]      FIG. 17  is a block diagram schematically showing a communication device according to a sixth embodiment of the present invention. 
           [0035]      FIG. 18  is a view showing action of a switch of the communication device of  FIG. 17 . 
           [0036]      FIG. 19  is a block diagram schematically showing a communication device according to a seventh embodiment of the present invention. 
           [0037]      FIG. 20  is a circuit diagram showing an example of a high voltage output circuit of the communication device of  FIG. 19 . 
           [0038]      FIG. 21  is a block diagram showing further detail of an impedance matching section of the communication device of  FIG. 19 , wherein a part of a switch and a part of a communication section of the communication device are schematically illustrated. 
           [0039]      FIG. 22  is a block diagram schematically showing a communication device according to an eighth embodiment of the present invention. 
           [0040]      FIG. 23  is a block diagram schematically showing a communication device according to a ninth embodiment of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0041]    While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. 
       First Embodiment 
       [0042]    As shown in  FIG. 1 , a communication device  1  according to a first embodiment of the present invention comprises a communication antenna  10 , a communication section  20 , a switch  30 , a switch control section  40 , a booster circuit (high voltage output part)  42 , a power source  50  and a central processing unit (CPU)  60 . 
         [0043]    The communication antenna  10  is connected to the communication section  20  via two signal lines  110 . The communication section  20  is capable of communicating an external device (not shown) via the communication antenna  10 . In detail, the communication section  20  according to the present embodiment is capable of transmitting a signal, namely, a transmission signal, to the external device via the communication antenna  10  and is capable of receiving a signal, namely, a reception signal, from the external device. 
         [0044]    The communication antenna  10  is, for example, a loop antenna which can be magnetically coupled with an external antenna (not shown) of the external device. The loop antenna may be provided with a magnetic body such as a soft magnetic sheet. The provision of the magnetic body to the loop antenna can improve the magnetic coupling between the communication antenna  10  and the external antenna. Moreover, the communication section  20  can be prevented from being affected by a magnetic field due to the external device. 
         [0045]    The switch  30  is connected between the communication antenna  10  and the communication section  20 . In other words, the switch  30  is proved on the signal lines  110 . In detail, each of the signal lines  110  is formed of one of signal lines  112  which are connected to opposite ends of the communication antenna  10 , respectively, and one of signal lines  114  which are connected to the communication section  20 . The switch  30  is connected to the communication antenna  10  via the signal lines  112  and is connected to the communication section  20  via the signal lines  114 . 
         [0046]    The switch  30  may be connected the communication antenna  10  via an impedance matching circuit (not shown). The impedance matching circuit can reduce electric potential difference between the signal lines  112  and the signal lines  114 . 
         [0047]    As shown in  FIG. 2 , the switch  30  is formed of semiconductor switches. In detail, the switch  30  according to the present embodiment is formed of two n-type MOSFETs. For each MOSFET, the drain is connected to the signal line  112 , and the source is connected to the signal line  114 . For each MOSFET, the gate is connected to the booster circuit  42 . 
         [0048]    As described above, the source and the drain of each MOSFET of the switch  30  is connected to the signal line  110 . Accordingly, when a signal, namely, a connection command signal, having a voltage sufficiently larger than another voltage of the signal line  110  is input to the gate, the drain and the source are electrically connected with each other. In other words, the switch  30  is turned into an ON-state. On the other hand, when the aforementioned connection command signal is not input to the gate, the drain and the source are electrically disconnected from each other. In other words, the switch  30  is turned into an OFF-state. 
         [0049]    As can be seen from the above explanation, when receiving the connection command signal, the switch  30  is in the ON-state to electrically connect the communication section  20  with the communication antenna  10 . Accordingly, transmission of the signal (transmission signal) by the communication section  20  and reception of the signal (reception signal) via the communication antenna  10  can be enabled. On the other hand, when not receiving the connection command signal, the switch  30  is in the OFF-state to electrically disconnect the communication section  20  from the communication antenna  10 . Accordingly, the communication section  20  is prevented from an overvoltage. 
         [0050]    As shown in  FIG. 1 , the switch control section  40  according to the present embodiment is connected to the communication antenna  10  in parallel to the switch  30 . Moreover, the switch control section  40  is connected to the switch  30  via the booster circuit  42 . As can be seen from this structure, the switch control section  40  is to output the aforementioned connection command signal toward the switch  30 . 
         [0051]    In detail, the switch control section  40  according to the present embodiment includes a rectifier circuit (not shown). Via the rectifier circuit, the switch control section  40  is capable of detecting a DC voltage (hereafter, referred to as “rectified voltage” or “detected voltage”) that is a voltage generated in the communication antenna  10  because of signal transmission/reception (including electric power reception) with use of the communication antenna  10 . In other words, the switch control section  40  is capable of detecting the voltage of the reception signal (including the electric power reception signal) and the voltage of the transmission signal in the communication antenna  10  as the detected voltage. 
         [0052]    The switch control section  40  outputs the connection command signal toward the switch  30  under a specific condition described later. Moreover, the switch control section  40  stops the connection command signal when detecting in advance that the overvoltage, or a predetermined voltage larger than the endurable voltage of the communication section  20 , is to be applied to the communication section  20 . When the switch control section  40  stops the connection command signal, the communication section  20  is electrically disconnected from the communication antenna  10  to be prevented from the overvoltage. 
         [0053]    In particular, the switch control section  40  according to the present embodiment detects the overvoltage in advance depending on the detected voltage. In detail, when the detected voltage is not smaller than a predetermined value and smaller than the overvoltage, the switch control section  40  detects in advance that a voltage equal to or larger than the overvoltage is to be applied to the communication section  20 . This predetermined value is larger than a voltage that is be generated in the communication antenna  10  because of the signal transmission by the communication section  20  via the communication antenna  10  and is smaller than the overvoltage. For example, the predetermined value is slightly smaller than the overvoltage. 
         [0054]    The booster circuit  42  is connected between the switch control section  40  and the switch  30 . As explained below, the booster circuit  42  converts a voltage of the connection command signal, which is received from the switch control section  40  and is to be output to the switch  30 , into another voltage that keeps the communication section  20  in a signal transmitting state from being electrically disconnected from the communication antenna  10 . 
         [0055]    Referring to  FIG. 2 , a voltage is generated in the signal lines  110  because of the transmission signal from the communication section  20  and because of the reception signal from the communication antenna  10 . In general, when the communication section  20  transmits the signal (i.e. when the communication section  20  is in the signal transmitting state), a large voltage tends to be generated in the signal lines  110 . If electric potential difference between the voltage of the connection command signal output to the gate and the voltage of the signal lines  110  is small, the switch  30  might not be properly in the ON-state. In other words, in order to properly turn the switch  30  into the ON-state, the voltage of the connection command signal applied to the gate needs to be sufficiently larger than the voltage of the signal lines  110 . 
         [0056]    As described above, the booster circuit  42  sufficiently boosts the voltage of the connection command signal and applies it to the switch  30 . In other words, the switch  30  is controlled by the boosted connection command signal. Accordingly, the switch  30  can be prevented from being turned into the OFF-state in error. The communication by the communication section  20  can be stably maintained while the communication section  20  is protected from the overvoltage. 
         [0057]    Referring to  FIG. 1 , the power source  50  is a battery which supplies operating power to the switch control section  40 . The illustrated power source  50  is directly connected only to the switch control section  40 . However, the power source  50  may be also connected to the CPU  60  and the communication section  20 . The power source  50  according to the present embodiment supplies the operating power to the booster circuit  42  via the switch control section  40 . According to the present embodiment, the operating power supplied from the power source  50  is mainly consumed by the booster circuit  42 . The booster circuit  42  boosts the voltage of the connection command signal by using the supplied operating power. 
         [0058]    For example, when the supply voltage of the power source  50  is 3.3V and the voltage generated in the signal lines  110  is not larger than 3.3V, the voltage of the connection command signal output by the switch control section  40  may be boosted into 5V by the booster circuit  42  to be output to the switch  30 . 
         [0059]    The power source  50  does not need to be a battery. For example, a part of the electric power generated in the communication antenna  10  may be rectified or converted to be used as the power source  50 . However, if the operating power supplied via the communication antenna  10  is not sufficient, the voltage of the connection command signal might be lowered. If the voltage of the connection command signal is lowered, the switch  30  is turned into the OFF-state so that the communication section  20  is protected from the overvoltage but cannot communicate with the external device (not shown). In contrast, if the power source  50  is a battery, the communicating state can be maintained even under a case where the electric power is not received from the external device. Accordingly, in a view point of stably maintaining the communicating state, the power source  50  is preferred to be a battery. 
         [0060]    The battery used as the power source  50  may be any one of a primary battery and a secondary battery. However, when the communication device  1  has a non-contact charging function (not shown) using an electric power reception antenna, a rectifier circuit, a smoothing circuit, a charging control circuit, etc., the power source  50  is desirable to be a secondary battery which is charged by the non-contact charging function. In this structure, the power source  50  more reliably supplies the operating power to the switch control section  40  and the booster circuit  42 . Accordingly, the communicating state can be more securely maintained. 
         [0061]    As previously described, the power source  50  supplies the operating power also to the switch control section  40 . If the supply of the operating power from the power source  50  is stopped for some reason, the switch control section  40  does not output the connection command signal. As a result, the switch  30  is turned into the OFF-state so that the communication section  20  is protected from the overvoltage. According to the present embodiment, the communication section  20  can be protected even if the power source  50  is broken down. 
         [0062]    The CPU  60  according to the present embodiment is connected to the communication section  20  and the switch control section  40 . The CPU  60  sends a signal, namely, an indication signal, to the switch control section  40  when the communication section  20  transmits the signal, wherein the indication signal indicates that the communication section  20  is in the signal transmitting state. Accordingly, the switch control section  40  is capable of detecting whether the communication section  20  is in the signal transmitting state or not depending on whether the indication signal is sent or not. As described later, the switch control section  40  according to the present embodiment works differently depending on whether the indication signal is sent or not. As can be seen from the above explanation, if the communication section  20  does not transmit the signal but only performs load modulation communication or only receives the signal, the function related to the indication signal is unnecessary. 
         [0063]    Hereafter, further detailed explanation is made about functions of the switch  30  and the switch control section  40  according to the present embodiment as referring to  FIGS. 1 and 3 . 
         [0064]    In the present embodiment, a first threshold is a lower limit (or a value about the lower limit) of a signal voltage necessary to communicate via the communication antenna  10 , and a second threshold is an upper limit (or a value about the upper limit) of a signal voltage which does not apply the overvoltage to the communication section  20 . More specifically, the first threshold a lower limit of the detected voltage which is detected by the switch control section  40  when the communication section  20  receives the signal. The second threshold is the predetermined value which is larger than an upper limit of a voltage that is to be generated because of the transmission of the signal by the communication section  20  via the communication antenna  10 , and which is smaller than the overvoltage. The second threshold is larger than the first threshold. 
         [0065]    As previously described, the switch control section  40  obtains the voltage generated in the communication antenna  10  via the rectifier circuit (not shown) as the rectified voltage (detected voltage). In addition, the switch control section  40  obtains the indication signal from the CPU  60 , wherein the indication signal indicates that the communication section  20  is in the signal transmitting state. The switch control section  40  controls the switch  30  by using the detected voltage and the indication signal. 
         [0066]    Specifically, the switch control section  40  controls the switch  30  as described below under a condition where the communication section  20  is not in the signal transmitting state, or under a case where the indication signal is not received from the CPU  60 . 
         [0067]    The switch control section  40  does not output the connection command signal to the booster circuit  42  under a condition where the detected voltage is not larger than the first threshold, for example, under a case where the communication antenna  10  does not receive the signal. As a result, the switch  30  is in the OFF-state. In the meantime, the consumption of the operating power in the booster circuit  42  is suppressed. The power source  50  may be formed so as not to supply the operating power to the switch control section  40  under the condition where the detected voltage is not larger than the first threshold. For example, the power source  50  may receive the detected voltage to determine whether the operating power needs to be supplied or not. 
         [0068]    The switch control section  40  outputs the connection command signal to the switch  30  via the booster circuit  42  under a condition where the detected voltage is larger than the first threshold and is not larger than the second threshold, for example, under a case where the communication antenna  10  receives the signal. As a result, the switch  30  is turned into the ON-state to enable the communication section  20  to communicate. 
         [0069]    The switch control section  40  does not output the connection command signal to the booster circuit  42  under a condition where the detected voltage is larger than the second threshold, for example, under a case where the communication antenna  10  receives the electric power. As a result, the switch  30  is turned into OFF-state to protect the communication section  20 . 
         [0070]    The switch control section  40  controls the switch  30  as described below under a condition where the communication section  20  is in the signal transmitting state, or under a case where the indication signal is received from the CPU  60 . 
         [0071]    The switch control section  40  outputs the connection command signal to the switch  30  via the booster circuit  42  under a condition where the detected voltage is not larger than the second threshold. As a result, the switch  30  is turned into the ON-state to enable the communication section  20  to communicate. When the communication section  20  is transferred into the signal transmitting state and about to transmit the signal, the communication section  20  is electrically connected with the communication antenna  10  in advance. Moreover, under the condition where the communication section  20  is in the signal transmitting state, the communication section  20  is kept to be electrically connected with the communication antenna  10  even if the detected voltage is temporarily not larger than the first threshold. The signal transmitting state is therefore stably maintained. 
         [0072]    The switch control section  40  does not output the connection command signal to the booster circuit  42  under the condition where the detected voltage is larger than the second threshold. As a result, the switch  30  is turned into the OFF-state to protect the communication section  20 . 
         [0073]    As can be seen from the above explanation, according to the present embodiment, under the condition where the detected voltage is not larger than the first threshold, the switch control section  40  controls the switch  30  depending on whether the communication section  20  is in the signal transmitting state or not. In detail, the switch control section  40  stops the connection command signal under the condition where the communication section  20  is not in the signal transmitting state and the detected voltage is not larger than the first threshold. The switch control section  40  outputs the connection command signal under the condition where the communication section  20  is in the signal transmitting state and the detected voltage is not larger than the first threshold. 
         [0074]    Under the condition where the detected voltage is larger than the first threshold, the switch control section  40  controls the switch  30  without depending on whether the communication section  20  is in the signal transmitting state or not. In detail, the switch control section  40  outputs the connection command signal under the condition where the detected voltage is larger than the first threshold and is not larger than the second threshold. The switch control section  40  stops the connection command signal under the condition where the detected voltage is larger than the second threshold. 
         [0075]    According to the present embodiment, when the communication device  1  receives the electric power in a non-contact manner, the switch  30  breaks the signal lines  110  to prevent the communication section  20  from the overvoltage. In addition, even if the communication device  1  does not have the non-contact electric power transmission function, the communication section  20  is prevented from the overvoltage under a case where the communication device  1  is placed in the vicinity of a device transmitting the electric power. Moreover, when the signal lines  110  are broken, impedance between the opposite ends of the communication antenna  10  becomes higher. Accordingly, when the communication device  1  receives the electric power in a non-contact manner, loss of the transmitted electric power is prevented. 
         [0076]    Moreover, according to the present embodiment, by making the voltage of the connection command signal sufficiently higher than the voltage of the signal lines  110 , the communication section  20  can be electrically stably connected with the communication antenna  10  and can be electrically reliably disconnected from the communication antenna  10 . 
         [0077]    Moreover, according to the present embodiment, the signal lines  110  are broken when the connection command signal is not output. Accordingly, when the signal lines  110  are broken, electric power loss due to the switch control section  40  and the booster circuit  42  is reduced. 
         [0078]    The communication device  1  according to the present embodiment can be variously modified in addition to the already described modifications. 
         [0079]    For example, when the communication section  20  does not transmit the signal but only performs the load modulation communication or only receives the signal, the switch control section  40  may stop the connection command signal also under the condition where the detected voltage is not larger than the first threshold without depending on whether the communication section  20  is in the signal transmitting state or not. 
         [0080]    Moreover, the switch control section  40  may be formed to receive a DC voltage while the switch control section  40  is provided with no rectifier circuit (not shown). For example, when an impedance matching circuit (not shown) is provided between the communication antenna  10  and the switch  30 , the switch control section  40  may be connected to the signal lines  112  between the impedance matching circuit and the switch  30 . By this structure, the switch control section  40  can directly detect the voltage applied to the communication section  20 . 
         [0081]    Moreover, the switch control section  40  may obtain the detected voltage without using the rectifier circuit (not shown). For example, the switch control section  40  may obtain the detected voltage by performing envelope detection of the signal on the signal lines  110 . 
       Second Embodiment 
       [0082]    As can be seen from  FIGS. 1 and 4 , a communication device  1 A according to a second embodiment of the present invention is a modification of the communication device  1  according to the first embodiment. Specifically, the communication device  1 A comprises an additional switch  32 . Moreover, the communication device  1 A comprises, instead of the switch control section  40 , a switch control section  40 A slightly different from the switch control section  40 . In detail, the switch control section  40 A is connected not only to the booster circuit  42  but also to the additional switch  32 . The communication device  1 A has structure and function similar to those of the communication device  1  except for the aforementioned difference. Hereafter, explanation is mainly made about this difference. 
         [0083]    As shown in  FIG. 4 , the additional switch  32  is connected between the switch  30  and the communication section  20 . The additional switch  32  is also connected to the switch control section  40 A without the booster circuit  42 . The additional switch  32  is controlled by the connection command signal received from the switch control section  40 A similar to the switch  30 . 
         [0084]    As shown in  FIG. 5 , the switch  30  according to the present embodiment is formed of two n-type MOSFETs like the first embodiment (see  FIG. 2 ). 
         [0085]    The additional switch  32  is formed of semiconductor switches similar to the switch  30 . However, the additional switch  32  is formed differently from the switch  30  by using two n-type MOSFETs. For each MOSFET, the drain is connected to the signal line  114 , and the source is grounded. For each MOSFET, the gate is connected not to the booster circuit  42  but to the switch control section  40 A. 
         [0086]    Since the source of the additional switch  32  is connected to the ground, the additional switch  32  is turned into an ON-state by the connection command signal on the basis of the ground potential. Accordingly, the connection command signal of the switch control section  40 A is directly output to the gate without passing through the booster circuit  42 . When the connection command signal is output to the gate, the additional switch  32  is in the ON-state. In the meantime, the signal lines  114  are connected to the ground so that the communication section  20  is electrically disconnected from the switch  30 . On the other hand, when no connection command signal is applied to the gate, the additional switch  32  is in an OFF-state. In the meantime, the signal lines  114  are not grounded so that the communication section  20  is electrically connected with the switch  30 . 
         [0087]    As can be seen from the above explanation, the connection command signal applied to the additional switch  32  by the switch control section  40 A works as a disconnection command signal. 
         [0088]    Even when the switch  30  is in the OFF-state, the signal lines  114  cannot be electrically completely isolated from the signal lines  112 . In other words, complete electrical disconnection between the communication antenna  10  and the communication section  20  cannot be achieved. However, the additional switch  32  according to the present embodiment electrically disconnects the communication section  20  from the switch  30  when receiving the connection command signal (disconnection command signal). The additional switch  32  can be turned into the ON-state at the same time as the switch  30  is turned into the OFF-state. The communication section  20  can be therefore more securely protected. In addition, the additional switch  32  according to the present embodiment has a protection function using zener diodes (ZD). Accordingly, the communication section  20  can be almost completely protected. 
         [0089]    The additional switch  32  electrically connects the communication section  20  with the switch  30  when not receiving the connection command signal (disconnection command signal). The additional switch  32  can be turned into the OFF-state at the same time as the switch  30  is turned into the ON-state. The communication by the communication section  20  can be therefore stably maintained. 
         [0090]    Hereafter, explanation is made about functions of the additional switch  32  and the switch control section  40 A according to the present embodiment as referring to  FIGS. 4 ,  6  and  7 . The function of the switch  30  is same as the function thereof in the first embodiment (see  FIG. 3 ) and is therefore not explained. 
         [0091]    The switch control section  40 A controls the additional switch  32  as described below not depending on whether the communication section  20  is in the signal transmitting state or not. 
         [0092]    Specifically, the switch control section  40 A outputs the disconnection command signal (connection command signal) to the additional switch  32  under the condition where the detected voltage is larger than the second threshold. As a result, the switch  30  is turned into the ON-state. The communication section  20  is electrically disconnected from the switch  30 . The switch control section  40 A stops the disconnection command signal (connection command signal) directed to the additional switch  32  under the condition where the detected voltage is not larger than the second threshold. As a result, the additional switch  32  is turned into the OFF-state. The communication section  20  is electrically connected with the switch  30 . 
         [0093]    As can be seen from  FIGS. 6 and 7 , when the switch  30  and the additional switch  32  are provided, the overvoltage to the communication section  20  is more securely blocked and the communication section  20  is more securely protected in particular under the condition where the detected voltage is larger than the second threshold. Moreover, under the condition where the detected voltage is not larger than the first threshold, the connection command signal does not need to be output to the additional switch  32  because the additional switch  32  can be available even in the OFF-state. Accordingly, consumption of the electric power can be suppressed. 
       Third Embodiment 
       [0094]    As can be seen from  FIGS. 1 and 8 , a communication device  1 B according to a third embodiment of the present invention is a modification of the communication device  1  according to the first embodiment. Specifically, the communication device  1 B comprises, instead of the switch control section  40 , a switch control section  40 B slightly different from the switch control section  40 . In detail, the switch control section  40 B is not connected to the CPU  60  (not illustrated in  FIG. 8 ) but is connected to the signal lines  114 . The communication device  1 B has structure and function similar to those of the communication device  1  except for the aforementioned difference. Hereafter, explanation is mainly made about this difference. 
         [0095]    As can be seen from  FIG. 8 , the switch control section  40 B can directly detect the voltage of the transmission signal of the communication section  20  from the signal lines  114 . In detail, the switch control section  40 B according to the present embodiment smoothes the voltage of the signal lines  114  to obtain a smoothed voltage. As explained below, the switch control section  40 B determines whether the communication section  20  is in the signal transmitting state or not by using this smoothed voltage. 
         [0096]    As shown in  FIG. 9 , the switch control section  40 B controls the switch  30  similar to the first embodiment (see  FIG. 3 ) and the second embodiment (see  FIGS. 6 and 7 ) under the condition where the detected voltage is larger than the first threshold. However, the switch control section  40 B controls the switch  30  by using the aforementioned smoothed voltage under the condition where the detected voltage is not larger than the first threshold. In detail, the switch control section  40 B turns the switch  30  into the OFF-state under a condition where the smoothed voltage is not larger than a predetermined third threshold. The switch control section  40 B turns the switch  30  into the ON-state under a condition where the smoothed voltage is larger than the predetermined third threshold. 
         [0097]    Under the condition where the communication section  20  in not in the signal transmitting state and the detected voltage is not larger than the first threshold, the switch  30  is in the OFF-state. Accordingly, when the communication section  20  starts to transmit the signal, the switch  30  needs to be turned into the ON-state. Since the switch control section  40 B according to the present embodiment works as described above, the switch  30  is turned into the ON-state under a case where the smoothed voltage becomes larger than the third threshold because of the transition of the communication section  20  into the signal transmitting state. As a result, the communication section  20  can transmit the signal. 
         [0098]    As can be seen from the above explanation, the switch control section  40 B according to the present embodiment is capable of detecting whether the communication section  20  is in the signal transmitting state or not by using not the indication signal of the CPU  60  (see  FIG. 1 ) but the smoothed voltage. 
       Fourth Embodiment 
       [0099]    As can be seen from  FIGS. 1 and 10 , a communication device  1 C according to a fourth embodiment of the present invention is a modification of the communication device  1  according to the first embodiment. Specifically, the communication device  1 C comprises an auxiliary antenna  12  in addition to the communication antenna  10 . Moreover, the communication device  1 C comprises, instead of the switch control section  40 , a switch control section  40 C slightly different from the switch control section  40 . In detail, the switch control section  40 C is not connected to the communication antenna  10  but is connected to the auxiliary antenna  12 . The communication device  1 C has structure and function similar to those of the communication device  1  except for the aforementioned difference. Hereafter, explanation is mainly made about this difference. 
         [0100]    The auxiliary antenna  12  may be any antenna, provided that the antenna is other than the communication antenna  10  and is magnetically coupled with the communication antenna  10  during the signal transmission/reception. For example, when the communication device  10  comprises an electric-power-receiving loop antenna which receives the electric power in a non-contact manner, this electric-power-receiving loop antenna may be used as the auxiliary antenna  12 . 
         [0101]    The switch control section  40 C does not directly detect the voltage of the communication antenna  10  as the detected voltage but detects, as the detected voltage, a voltage that is generated in the auxiliary antenna  12  because of the signal transmission/reception with use of the communication antenna  10 . In other words, in the present embodiment, the detected voltage is the voltage that is generated in the auxiliary antenna  12  because of the signal transmission/reception with use of the communication antenna  10 . The thus-formed switch control section  40 C can control the switch  30  similar to the switch control section  40  (see  FIGS. 1 and 3 ). 
         [0102]    Moreover, when the communication antenna  10  and the auxiliary antenna  12  are arranged so as to weaken the magnetic coupling therebetween, the detected voltage can be properly detected from the auxiliary antenna  12  with almost no affection to the voltage in the communication antenna  10 . 
       Fifth Embodiment 
       [0103]    As can be seen from  FIGS. 8 and 11 , a communication device  1 D according to a fifth embodiment of the present invention is a modification of the communication device  1 B according to the third embodiment. Specifically, the communication device  1 D comprises an auxiliary switch  34  in addition to the switch  30 . Moreover, the communication device  1 D comprises, instead of the switch control section  40 B, a switch control section  40 D slightly different from the switch control section  40 B. In detail, the switch control section  40 D is connected not only to the booster circuit  42  but also to the auxiliary switch  34 . The communication device  1 D has structure and function similar to those of the communication device  1 B except for the aforementioned difference. Hereafter, explanation is mainly made about this difference. 
         [0104]    As shown in  FIG. 11 , the auxiliary switch  34  is connected between the communication antenna  10  and the communication section  20  in parallel to the switch  30 . In other words, the auxiliary switch  34  is provided on the signal lines  110  similar to the switch  30 . The auxiliary switch  34  is connected to the switch control section  40 D without the booster circuit  42 . The auxiliary switch  34  can be formed of semiconductor switches similar to the switch  30  (see  FIG. 2 ). For example, the auxiliary switch  34  may be formed of two n-type MOSFETs similar to the switch  30 . 
         [0105]    As can be seen from  FIG. 11 , the switch control section  40 D outputs the connection command signal to the switch  30  and the auxiliary switch  34 . The connection command signal directed to the auxiliary switch  34  is output, for example, to the gate of the MOSFET. 
         [0106]    Specifically, as shown in  FIG. 12 , the switch control section  40 D according to the present embodiment is formed of a circuit using semiconductors. Hereafter, in reference with  FIG. 12 , explanation is made about a function of the switch control section  40 D under a case where a predetermined voltage is generated in the signal lines  112 , for example, under a case where the communication device  1 D receives the signal. 
         [0107]    The switch control section  40 D full-wave rectifies the voltage of the signal lines  112  by using a diode bridge. The switch control section  40 D smoothes the full-wave rectified voltage by using a smoothing circuit formed of a capacitor C 1  so that the full-wave rectified voltage is converted into a rectified voltage Vidc (detected voltage). The rectified voltage Vidc is input to an inverted input of a comparator CA. In addition, a voltage V 2  of the second threshold is input to a non-inverted input of the comparator CA. The output of the comparator CA is output to each of the auxiliary switch  34  and an AND circuit as the connection command signal. 
         [0108]    The switch control section  40 D includes a reception signal detection section  400 . In other words, the communication device  1 D comprises the reception signal detection section  400 . The rectified voltage Vidc (detected voltage) is also input to the reception signal detection section  400 . In detail, the rectified voltage Vidc is input to the gate of an n-type MOSFET (Q 1 ). The source of the MOSFET (Q 1 ) is grounded. Accordingly, electric potential difference between the gate and the source becomes larger because of the input rectified voltage Vidc so that the drain and the source are electrically connected with each other. As a result, a drain voltage of the MOSFET (Q 1 ) is lowered. Since a gate voltage of a p-type MOSFET (Q 2 ) connected to the drain of the MOSFET (Q 1 ) is also lowered, the drain and the source of the MOSFET (Q 2 ) are electrically connected with each. 
         [0109]    Since the reception signal detection section  400  works as described above, a power supply voltage Vcc is input to a non-inverted input of a comparator CB via the MOSFET (Q 2 ) and a diode under a case where the predetermined voltage is generated in the signal lines  112 . The voltage of the signal lines  114  is also smoothed by a diode and a capacitor and boosted as necessary (not shown) to be input to the non-inverted input of the comparator CB as a smoothed voltage at the communication section  20  side. In addition, a voltage V 1  of a predetermined value is input to an inverted input of the comparator CB. The output of the comparator CB is input to the AND circuit. The output of the AND circuit is output to the booster circuit  42 . 
         [0110]    In the switch control section  40 D shown in  FIG. 12  as an example, the first threshold of the rectified voltage Vidc (detected voltage) is equal to the gate voltage necessary to electrically connect the drain and the source of the MOSFET (Q 1 ) with each other. When the drain and the source of the MOSFET (Q 1 ) are electrically connected with each other, a power supply voltage Vcc, which is larger than the voltage V 1  of the predetermined value, is input to the comparator CB. Accordingly, even if the rectified voltage Vidc is so weak as the comparator CB cannot directly detect it, the comparator CB can detect the rectified voltage Vidc by using the power supply voltage Vcc. For example, even when the communication antenna  10  receives a weak signal, the switch  30  can be controlled so that the signal lines  112  are electrically connected with the signal lines  114 , respectively. 
         [0111]    The switch control section  40 D may be formed so that a level of the voltage (predetermined voltage), or the power supply voltage Vcc in  FIG. 12 , input to the comparator CB changes depending on another level of the rectified voltage Vidc (detected voltage). According to this structure, the rectified voltage Vidc (detected voltage) is converted into the predetermined voltage by the reception signal detection section  400  to be input to the comparator CB. In this structure, the comparator CB can indirectly compare the rectified voltage Vidc with the first threshold when the voltage V 1  is set to a value corresponding to the first threshold. In other words, the switch control section  40 D compares the rectified voltage Vidc and the first threshold with each other by using the rectified voltage Vidc which is amplified by the reception signal detection section  400 . Accordingly, even if the rectified voltage Vidc is as weak as the comparator CB cannot directly detect, the comparator CB can detect the rectified voltage Vidc by using the predetermined voltage. A small first threshold can be therefore set for the rectified voltage Vidc. For example, even when the communication antenna  10  receives a weak signal, the switch  30  can be controlled to electrically connect the signal lines  112  to the signal lines  114 , respectively. 
         [0112]    As described above, the rectified voltage Vidc (detected voltage) detected by the switch control section  40 D is input to the gate of the MOSFET (Q 1 ). When the switch control section  40 D is thus formed of the circuit using the semiconductors, the lower detectable limit of the rectified voltage Vidc is often restricted to a barrier voltage about 0.6V in a p-n junction of a semiconductor. Accordingly, the first threshold needs to be set larger than the barrier voltage. However, for example, when the communication section  20  is formed of an IC chip which is in compliant with the ISO/IEC 18092 standard, the communication section  20  often uses some reception signal having a voltage smaller than 0.6 V in order to determine whether the signal transmission is allowed or not. The voltage of the reception signal is therefore sometimes smaller than the first threshold. In order to allow the communication section  20  to receive such weak reception signal, the communication section  20  and the communication antenna  10  need to be electrically connected with each other even when the switch  30  is in the OFF-state. 
         [0113]    According to the present embodiment, the connection command signal is output to the auxiliary switch  34 , provided that the rectified voltage Vidc (detected voltage) is not larger than the second threshold. Accordingly, the auxiliary switch  34  continues to electrically connect the communication section  20  with the communication antenna  10  even if the rectified voltage Vidc is smaller than the barrier voltage. Since the auxiliary switch  34  is thus provided, the communication section  20  can receive the weak reception signal for determining whether the transmission of the signal is allowed or not even under a case where the switch  30  is in the OFF-state. 
         [0114]    Moreover, no circuit such as the booster circuit  42  which consumes large electric power is not provided between the auxiliary switch  34  and the switch control section  40 D. Accordingly, the auxiliary switch  34  consumes only slight electric power for continuing to electrically connect the communication section  20  with the communication antenna  10 . Moreover, when the power source  50  does not supply the electric power to the switch control section  40 D, the auxiliary switch  34  does not work. In other words, the auxiliary switch  34  is in the OFF-state. Accordingly, even if the electric power from the power source  50  is stopped, the communication section  20  is protected from the overvoltage. 
         [0115]    Hereafter, explanation is made about functions of the switch  30 , the auxiliary switch  34  and the switch control section  40 D according to the present embodiment as referring to  FIGS. 11 and 13  to  16 . 
         [0116]    Referring to  FIGS. 11 and 13 , when the communication section  20  is in a signal receiving state, the auxiliary switch  34  is in an ON-state even under a condition where the rectified voltage Vidc (detected voltage) is not larger than the first threshold. Accordingly, the communication section  20  can determine whether a weak reception signal exists or not, wherein the weak reception signal is used for determination of whether the signal transmission is allowed or not. 
         [0117]    Referring to  FIGS. 11 and 14 , the switch  30  according to the present embodiment works similar to the switch  30  according to the third embodiment (see  FIG. 9 ). However, when the switch control section  40 D is formed as shown in  FIG. 12 , the first threshold is equal to the third threshold. 
         [0118]    As can be seen from  FIG. 12 , the auxiliary switch  34  works with no direct relation with the smoothed voltage at the communication section  20  side according to the present embodiment. In other words, the auxiliary switch  34  basically works only depending on the rectified voltage Vidc (detected voltage). However, as can be seen from  FIG. 11 , the rectified voltage Vidc and the smoothed voltage are related to each other. Accordingly, the function of the auxiliary switch  34  has indirect relation with the smoothed voltage. More specifically, referring to  FIGS. 11 and 15 , the auxiliary switch  34  works as described below. 
         [0119]    The switch control section  40 D outputs the connection command signal to the auxiliary switch  34  under a condition where the rectified voltage Vidc (detected voltage) due to the communication antenna  10  is not larger than the second threshold. The auxiliary switch  34  is basically in the ON-state when receiving the connection command signal. In detail, the auxiliary switch  34  is in the ON-state under a condition where the rectified voltage Vidc is not larger than the first threshold. In addition, the auxiliary switch  34  is basically in the ON-state under a condition where the rectified voltage Vidc is larger than the first threshold and is not larger than the second threshold. 
         [0120]    However, under the condition where the rectified voltage Vidc (detected voltage) is larger than the first threshold and is not larger than the second threshold, electric potential between the voltage of the connection command signal output to the auxiliary switch  34  and the voltage of the signal lines  110  sometimes becomes small. At that time, the auxiliary switch  34  cannot keep the ON-state and is turned into an OFF-state. For example, in some cases, the voltage of the signal lines  110  increases because of the signal transmission by the communication section  20  so that the auxiliary switch  34  is turned into the OFF-state. As a result, the auxiliary switch  34  electrically disconnects the communication section  20  from the communication antenna  10 . When the auxiliary switch  34  according to the present embodiment receives the connection command signal, the auxiliary switch  34  electrically connects the communication section  20  with the communication antenna  10  at least under the condition where the rectified voltage Vidc is not larger than the first threshold. 
         [0121]    As described above, under the condition where the rectified voltage Vidc (detected voltage) of the switch control section  40 D is larger than the first threshold and is not larger than the second threshold, the switch  30  electrically connects the communication section  20  with the communication antenna  10 . Accordingly, the communication section  20  continues to be electrically connected with the communication antenna  10  regardless of whether the auxiliary switch  34  is in the ON-state or in the OFF-state. In other words, according to the present embodiment, the auxiliary switch  34  may be in any one of the ON-state and the OFF-states under the condition where the rectified voltage Vidc is larger than the first threshold and is not larger than the second threshold. 
         [0122]    The switch control section  40 D stops the connection command signal directed to the auxiliary switch  34  under a condition where the rectified voltage Vidc (detected voltage) is larger than the second threshold. The auxiliary switch  34  electrically disconnects the communication section  20  from the communication antenna  10  when not receiving the connection command signal. As a result, both the switch  30  and the auxiliary switch  34  are turned into the OFF-state, and the communication section  20  is therefore protected. 
         [0123]    As shown in  FIG. 16 , for example, when the rectified voltage Vidc (detected voltage) is uniformly increased over time, the state of each of the switch  30  and the auxiliary switch  34  is transferred as described below. 
         [0124]    Before the rectified voltage Vidc (detected voltage) exceeds the first threshold, the switch  30  is in the OFF-state but the auxiliary switch  34  is kept to be in the ON-state. Accordingly, the communication section  20  is electrically connected with the communication antenna  10 . 
         [0125]    After the rectified voltage Vidc (detected voltage) exceeds the first threshold, the electric potential difference between the gate and the source of the auxiliary switch  34  (MOSFET) gradually decreases. Accordingly, the auxiliary switch  34  cannot keep the ON-state and is turned into the OFF-state. However, the switch  30  keeps the ON-state because of the booster circuit  42 . Accordingly, the communication section  20  continues to be electrically connected with the communication antenna  10  with no affection of the action of the auxiliary switch  34 . 
         [0126]    When the rectified voltage Vidc (detected voltage) exceeds the second threshold, both the switch  30  and the auxiliary switch  34  are in the OFF-state. Accordingly, the communication section  20  is electrically disconnected from the communication antenna  10 , and the communication section  20  is therefore protected. 
         [0127]    The communication device  1 D according to the present embodiment can be variously modified in addition to the already described modifications. For example, the reception signal detection section  400  of the switch control section  40 D may be replaced by any amplifying circuit which can amplify a weak voltage, an operational amplifier, a comparator or the like. 
         [0128]    Moreover, as can be seen from the above explanation, each of the aforementioned switch control sections according to the first to fourth embodiments can be formed similar to the switch control section  40 D according to the present embodiment. For example, the switch control section  40 B (see  FIG. 8 ) according to the third embodiment can be formed by omitting lines directed to the auxiliary switch  34  from the switch control section  40 D. 
       Sixth Embodiment 
       [0129]    As can be seen from  FIGS. 11 and 17 , a communication device  1 E according to a sixth embodiment of the present invention is a modification of the communication device  1 D according to the fifth embodiment. Specifically, the communication device  1 E does not comprise the auxiliary switch  34 . Moreover, the communication device  1 E comprises, instead of the switch control section  40 D, a switch control section  40 E slightly different from the switch control section  40 D. The communication device  1 E has structure and function similar to those of the communication device  1 D except for the aforementioned difference. Hereafter, explanation is mainly made about this difference. 
         [0130]    As shown in  FIG. 17 , the switch control section  40 E is connected to the switch  30  without the booster circuit  42  via a first diode (diode)  402  in addition to connection via the booster circuit  42 . The booster circuit  42  is connected to the switch  30  via a second diode (diode)  422  other than the first diode  402 . The switch control section  40 E outputs the connection command signal to the diode  422  via the booster circuit  42  while outputting the connection command signal to the diode  402 . In other words, the connection command signal is output to the switch  30  via an OR circuit formed of the diode  402  and the diode  422 . 
         [0131]    The switch control section  40 E is formed similar to the switch control section  40 D (see  FIG. 12 ) according to the fifth embodiment. However, the output, or the connection command signal, of the comparator CA is output not to the auxiliary switch  34  but to the diode  402 . 
         [0132]    Referring to  FIG. 14 , the switch  30  according to the present embodiment is turned into the ON-state by the connection command signal via the diode  422  under a condition same as that of the switch  30  according to the fifth embodiment. Moreover, referring to  FIG. 15 , the switch  30  according to the present embodiment is turned into the ON-state by the connection command signal via the diode  402  under a condition same as that of the auxiliary switch  34  according to the fifth embodiment. Accordingly, the switch  30  works as shown in  FIG. 18 . Specifically, regardless of the level of the smoothed voltage at the communication section  20  side, the switch  30  is turned into the ON-state under the condition where the rectified voltage (detected voltage) at the communication antenna  10  side is not larger than the second threshold while being turned into the OFF-state under the condition where the detected voltage is larger than the second threshold. 
         [0133]    In detail, the switch control section  40 E outputs the connection command signal to the switch  30  via the first diode  402  and/or the second diode  422  under the condition where the detected voltage is not larger than the second threshold. Moreover, the switch control section  40 E stops the connection command signal directed to the first diode  402  and the connection command signal directed to the second diode  422  under the condition where the detected voltage is larger than the second threshold. The switch  30  electrically connects the communication section  20  with the communication antenna  10  when receiving the connection command signal from one of the first diode  402  and the second diode  422 . On the other hand, the switch  30  electrically disconnects the communication section  20  from the communication antenna  10  when not receiving the connection command signal from any one of the first diode  402  and the second diode  422 . 
         [0134]    Accordingly, under the condition where the detected voltage is not larger than the first threshold and the smoothed voltage at the communication section  20  side is not larger than the third threshold, or under the condition where the communication section  20  is not in the signal transmitting state, the switch  30  is turned into the ON-state by the connection command signal which does not pass through the booster circuit  42 . At that time, as previously described, the switch control section  40 E does not output the connection command signal to the booster circuit  42 . Accordingly, electric power consumption in the booster circuit  42  is suppressed. 
         [0135]    Under the condition where the detected voltage is larger than the first threshold or the smoothed voltage at the communication section  20  side is larger than the third threshold, or under the condition where the communication section  20  is in the signal transmitting state, the switch  30  is turned into the ON-state by the connection command signal which passes through the booster circuit  42 . Accordingly, even when the voltage of the signal lines  110  increases, the electrical connection between the communication antenna  10  and the communication section  20  is stably maintained. 
         [0136]    As can be seen from the above explanation, according to the sixth embodiment, the communication section  20  can be electrically connected with the communication antenna  10  and can be electrically disconnected from the communication antenna  10  similar to the fifth embodiment while the auxiliary switch  34  (see  FIG. 11 ) is not provided. 
       Seventh Embodiment 
       [0137]    As can be seen from  FIGS. 11 and 19 , a communication device  1 F according to a seventh embodiment of the present invention is a modification of the communication device  1 D according to the fifth embodiment. Specifically, the communication device  1 F comprises a high voltage output circuit (high voltage output part)  44  instead of the booster circuit  42 . Moreover, the communication device  1 F comprises a high voltage power source  52  and an impedance matching section  70  which are not comprised in the communication device  1 D. The communication device  1 F has structure and function similar to those of the communication device  1 D except for the aforementioned difference. Hereafter, explanation is mainly made about this difference. 
         [0138]    Referring to  FIG. 19 , the high voltage output circuit  44  according to the present embodiment works as the high voltage output part similar to the booster circuit  42  according to the first to sixth embodiments. In detail, the high voltage output circuit  44  is directly connected to the high voltage power source  52 . The high voltage power source  52  supplies operating power to the high voltage output circuit  44 . The high voltage output circuit  44  applies a voltage supplied from the high voltage power source  52  to the switch  30  depending on the connection command signal of the switch control section  40 D. 
         [0139]    As shown in  FIG. 20 , the high voltage output circuit  44  according to the present embodiment has an n-type MOSFET (Q 3 ) and a p-type MOSFET (Q 4 ). The source of the MOSFET (Q 3 ) is grounded, and the drain is connected to the gate of the MOSFET (Q 4 ). The gate of the MOSFET (Q 3 ) is connected to the switch control section  40 D. The source of the MOSFET (Q 4 ) receives the voltage applied from the high voltage power source  52  via a diode, and the drain is connected to the switch  30 . 
         [0140]    As can be seen from  FIG. 20 , when the connection command signal of the switch control section  40 D is input to the gate of the MOSFET (Q 3 ), electric potential difference between the source and the gate becomes larger so that the source is electrically connected with the drain to lower a drain voltage. At that time, a gate voltage of the MOSFET (Q 4 ) is also lowered so that the source is electrically connected with the drain. As a result, the voltage applied from the high voltage power source  52  via the diode is output to the switch  30  as the connection command signal. 
         [0141]    According to the present embodiment, the high voltage output part is formed of the high voltage output circuit  44  which is directly connected to the high voltage power source  52 . Accordingly, the function equivalent to that of the booster circuit  42  can be more reliably obtained. 
         [0142]    Referring to  FIG. 19 , the impedance matching section  70  according to the present embodiment is connected between the communication antenna  10  and the switch  30 . In other words, the impedance matching section  70  is proved on the signal lines  112 . 
         [0143]    In detail, as shown in  FIGS. 19 and 21 , the impedance matching section  70  is connected to the communication antenna  10 . In addition, the impedance matching section  70  is connected to the switch control section  40 D (not illustrated in  FIG. 21 ), the switch  30  (schematically illustrated in  FIG. 21 ) and the auxiliary switch  34  (not illustrated in  FIG. 21 ). The impedance matching section  70  is connected to the communication section  20  via the switch  30 . 
         [0144]    As shown in  FIG. 21 , the communication section  20  has two terminals (transmission/reception terminals)  212  and  214  for general communication, or for transmitting and receiving the signal, and two terminals (load modulation communication terminals)  222  and  224  for load modulation communication. The communication section  20  receives the reception signal and transmits the transmission signal from the terminals  212  and  214 . Moreover, the communication section  20  performs load modulation communication by changing impedance at the terminals  222  and  224 . 
         [0145]    The impedance matching section  70  includes a resonance circuit  72 , a first matching circuit (impedance matching circuit)  722  and a second matching circuit (impedance matching circuit)  724 . The resonance circuit  72  is connected to the communication antenna  10 . The resonance circuit  72  has a resonance frequency which is designed to be equal to a frequency of the transmission/reception signal of the communication section  20 . Accordingly, the voltage of the reception signal received by the communication antenna  10  is amplified by the resonance circuit  72 . 
         [0146]    The resonance circuit  72  is connected to the terminals  212  and  214  of the communication section  20  via the first matching circuit  722  and the switch  30 . In addition, the resonance circuit  72  is connected to the terminals  222  and  224  of the communication section  20  via the second matching circuit  724  and the switch  30 . In general, impedance at each of the terminals  212  and  214  is lower than impedance at each of the terminals  222  and  224 . According to the present embodiment, the first matching circuit  722  matches the impedance at the terminals  212  and  214 , and the second matching circuit  724  matches the impedance at the terminals  222  and  224 . According to the present embodiment, voltage amplitude at the terminals  212  and  214  is made smaller than voltage amplitude at the terminals  222  and  224 . 
         [0147]    According to the present embodiment, when the communication antenna  10  receives the signal and the switch  30  electrically connects the communication section  20  with the communication antenna  10 , the voltage amplitude at the terminals  212  and  214  of the communication section  20  is smaller than the voltage amplitude at the communication antenna  10 . Moreover, when the communication antenna  10  receives the signal and the switch  30  electrically disconnects the communication section  20  from the communication antenna  10 , the voltage amplitude at the switch  30  is smaller than the voltage amplitude at the communication antenna  10 . 
         [0148]    According to the present embodiment, the impedance matching section  70  can lower the voltage applied to the switch  30  to some extent. More specifically, the switch  30  can be prevented from receiving a voltage exceeding the power supply voltage of the high voltage output circuit  44  from the communication antenna  10 . Accordingly, even though the switch  30  is formed of the semiconductor switches, the switch  30  is more reliably turned into the OFF-state, and the communication section  20  can be more securely protected. 
         [0149]    According to the present embodiment, when an electric power transmission signal which is received by the communication antenna  10  has a frequency different from another frequency of the transmission/reception signal, the frequency of the electric power transmission signal is different from the resonance frequency of the resonance circuit  72 . Accordingly, the electric power transmission signal is blocked by the resonance circuit  72  to some extent. The first matching circuit  722  is designed to properly work for the transmission/reception signal having a supposed frequency. Accordingly, the first matching circuit  722  might output the overvoltage when receiving the electric power transmission signal of the frequency different from the supposed frequency. However, even if the first matching circuit  722  outputs the overvoltage, the switch  30  is turned into the OFF-state. As a result, the communication section  20  is electrically disconnected from the first matching circuit  722 , and the communication section  20  is protected from the overvoltage. 
         [0150]    The communication section  20  according to the present embodiment performs the load modulation communication by switching each of the terminals  222  and  224  between a high-impedance state and a low-impedance state. The second matching circuit  724  might output the overvoltage similar to the first matching circuit  722  when receiving the electric power transmission signal of the frequency different from that of the transmission/reception signal. However, also in this case, the switch  30  is turned into the OFF-state. As a result, the communication section  20  is electrically disconnected from the second matching circuit  724 , and the communication section  20  is protected from the overvoltage. 
         [0151]    The impedance matching section  70  may have a frequency filter function and/or an impedance conversion function in addition to the aforementioned function, wherein the frequency filter function blocks a target signal, or a signal in a frequency band of the electric power transmission signal, and the impedance conversion function lowers the voltage amplitude of the target signal. The communication section  20  is more securely protected when such protection functions are provided in addition to the protection of the communication section  20  by the switch  30 . 
         [0152]    According to the present embodiment, the switch  30  (in detail, the semiconductor switch such as the MOSFET in the switch  30 ) is connected with every one of the terminals  212  and  214  and the terminals  222  and  224 . However, if a voltage applied to a specific terminal does not become excessive even upon the reception of the electric power transmission signal, the semiconductor switch for this terminal does not need to be provided. 
         [0153]    More specifically, in general, the impedance at each of the terminals  212  and  214  matched by the first matching circuit  722  is lower than the impedance at each of the terminals  222  and  224  matched by the second matching circuit  724 . Accordingly, in some cases, the overvoltage is not applied to the terminals  212  and  214  even if the switch  30  is not provided. However, in many cases, the switch  30  needs to protect the terminals  222  and  224  because the impedance at each of the terminals  222  and  224  repeatedly becomes high and low. In such cases, the switch  30  may be connected only with the terminals  222  and  224 . By not providing the semiconductor switches for the terminals  212  and  214  but providing the semiconductor switches for the terminals  222  and  224 , it is possible to reduce the number of the components of the switch  30  while protecting the communication section  20  from the overvoltage. 
         [0154]    As can be seen from the above explanation, the first to seventh embodiments are applicable even to a communication device which does not have the non-contact electric power transmission function. However, the present invention including the first to seventh embodiments is also applicable to a communication device having the non-contact electric power transmission function. Hereafter, explanation is made in further detail about the communication device having the non-contact electric power transmission function. 
       Eighth Embodiment 
       [0155]    As can be seen from  FIGS. 19 ,  21  and  22 , a communication device  1 G according to an eighth embodiment of the present invention is a modification of the communication device  1 F according to the seventh embodiment. Specifically, the communication device  1 G comprises the resonance circuit  72  and the first matching circuit  722  of the impedance matching section  70  while not comprising the second matching circuit  724 . In addition, the communication device  1 G comprises a rectifier circuit  80  and a load  90  which are not comprised in the communication device  1 F. Moreover, the communication device  1 G comprises, instead of the switch control section  40 D, a switch control section  40 G slightly different from the switch control section  40 D. The communication device  1 G has structure and function similar to those of the communication device  1 F except for the aforementioned difference. Hereafter, explanation is mainly made about this difference. 
         [0156]    The rectifier circuit  80  is connected between the resonance circuit  72  and the first matching circuit  722 . The load  90  is connected to the rectifier circuit  80 . In other words, the load  90  is connected to the communication antenna  10  via the rectifier circuit  80  and the resonance circuit  72 . The load  90  according to the present embodiment is, for example, a secondary battery. As can be seen from this structure, the signal received in the communication antenna  10  is rectified by the rectifier circuit  80  to be supplied to the load  90  as the electric power. In other words, the communication device  1 G has the non-contact electric power transmission function. 
         [0157]    The switch control section  40 G according to the present embodiment is not directly connected to the signal lines  112  but indirectly connected to the signal lines  112  via the rectifier circuit  80 . The switch control section  40 G detects the voltage rectified by the rectifier circuit  80  as the rectified voltage (detected voltage). Accordingly, the switch control section  40 G does not include an internal rectifier circuit. 
         [0158]    According to the present embodiment, the electric power can be transmitted to the load  90  while no electric power reception antenna (not shown) other than the communication antenna  10  is provided. Moreover, the rectifier circuit inside the switch control section  40 G can be omitted. 
         [0159]    As already explained, the switch control sections according to the first to eighth embodiments detect in advance that the voltage equal to or larger than the overvoltage is to be applied to the communication section  20  when the rectified voltage (detected voltage) is not smaller than the predetermined value and is smaller than the overvoltage. In other words, an advance signal for notifying that the overvoltage is to be applied to the communication section  20  is the detected voltage that is not smaller than the predetermined value and is smaller than the overvoltage. However, such advance signal does not need to be the detected voltage according to the first to eighth embodiments. For example, the advance signal may be an electric power transmission notice signal which is transmitted from an external device (not shown) prior to the electric power transmission. 
         [0160]    Moreover, the advance signal may be obtained from a circuit, etc. other than the communication antenna  10 . For example, a signal of Bluetooth communication or the like may be used as the advance signal. When a time interval between the communication and the electric power transmission is scheduled, a timing control signal generated by an internal timer (not shown) may be used as the advance signal. 
         [0161]    Moreover, the advance signal may be a frequency component of the electric power transmission signal included in the reception signal. Hereafter, explanation is made about a communication device which uses the frequency of the electric power transmission signal as the advance signal under a case where the frequency of the transmission/reception signal of the communication section  20  is different from the frequency of the electric power transmission signal. 
       Ninth Embodiment 
       [0162]    As can be seen from  FIGS. 22 and 23 , a communication device  1 H according to a ninth embodiment of the present invention is a modification of the communication device  1 G according to the eighth embodiment. Specifically, the communication device  1 H comprises a frequency detection section  46  which is not comprised in the communication device  1 G. Moreover, the communication device  1 H comprises, instead of the switch control section  40 G, a switch control section  40 H slightly different from the switch control section  40 G. In detail, the switch control section  40 H is connected not to the rectifier circuit  80  but to the frequency detection section  46 . The communication device  1 H has structure and function similar to those of the communication device  1 G except for the aforementioned difference. Hereafter, explanation is mainly made about this difference. 
         [0163]    The frequency detection section  46  according to the present embodiment is connected to the signal lines  112 . Accordingly, the frequency detection section  46  is connected to the communication antenna  10  via the resonance circuit  72 . The frequency detection section  46  detects the frequency of the signal on the signal lines  112 . If the detected frequency is equal to the frequency of the electric power transmission, the frequency detection section  46  transmits the detected signal to the switch control section  40 H. The frequency detection section  46  only needs to detect amplitude of a signal having a specific frequency component, or the frequency of the electric power transmission signal in the present embodiment. For example, the frequency detection section  46  can be formed of a band pass filter or the like. 
         [0164]    The switch control section  40 H stops the connection command signal directed to the switch  30  and the connection command signal directed to the auxiliary switch  34  when receiving the signal detected by the frequency detection section  46 . As a result, the switch  30  and the auxiliary switch  34  electrically disconnect the communication section  20  from the communication antenna  10  to protect the communication section  20 . As can be seen from the above explanation, according to the present embodiment, when the signal received by the communication antenna  10  has the frequency same as the frequency of the electric power transmission signal for receiving the electric power in a non-contact manner, the switch control section  40 H detects in advance that a voltage equal to or larger than the overvoltage is to be applied to the communication section  20 . In other words, the signal that has the frequency same as that of the electric power transmission signal is used as the advance signal which notifies that the overvoltage is to be applied to the communication section  20 . 
         [0165]    The communication device  1 H according to the present embodiment can be variously modified. For example, although the communication device  1 H according to the present embodiment is capable of receiving the electric power in a non-contact manner similar to the communication device  1 G, the communication device  1 H does not need to be capable of receiving the electric power in a non-contact manner. In other words, the communication device  1 H does not need to comprise the rectifier circuit  80  and the load  90 . 
         [0166]    The communication device explained above can be installed in various electronic apparatus. For example, when an electronic apparatus having the non-contact charging function or the like comprises the communication device according to the present invention, the effects of the present invention are more effectively shown. Moreover, the embodiments explained above can be variously combined. For example, the communication device may comprise both the auxiliary switch and the additional switch. 
         [0167]    The present application is based on a Japanese patent applications of JP2013-105858 and JP2013-179045 filed before the Japan Patent Office on May 20, 2013 and Aug. 30, 2013, respectively, the contents of which are incorporated herein by reference. 
         [0168]    While there has been described what is believed to be the preferred embodiment of the invention, those skilled in the art will recognize that other and further modifications may be made thereto without departing from the spirit of the invention, and it is intended to claim all such embodiments that fall within the true scope of the invention. 
       REFERENCE SIGNS LIST 
       [0000]    
       
         
           
               1 ,  1 A,  1 B,  1 C,  1 D,  1 E,  1 F,  1 G,  1 H communication device 
               10  communication antenna 
               110  signal line 
               112  signal line 
               114  signal line 
               12  auxiliary antenna 
               20  communication section 
               212 ,  214  terminal (transmission/reception terminal) 
               222 ,  224  terminal (load modulation communication terminal) 
               30  switch 
               32  additional switch 
               34  auxiliary switch 
               40 ,  40 A,  40 B,  40 C,  40 D,  40 E,  40 G,  40 H switch control section 
               400  reception signal detection section 
               402  first diode (diode) 
               42  booster circuit (high voltage output part) 
               422  second diode (diode) 
               44  high voltage output circuit (high voltage output part) 
               46  frequency detection section 
               50  power source 
               52  high voltage power source 
               60  CPU 
               70  impedance matching section 
               72  resonance circuit 
               722  first matching circuit (impedance matching circuit) 
               724  second matching circuit (impedance matching circuit) 
               80  rectifier circuit 
               90  load 
             C 1  capacitor 
             CA comparator 
             CB comparator 
             Q 1  MOSFET 
             Q 2  MOSFET 
             Q 3  MOSFET 
             Q 4  MOSFET 
             Vcc power supply voltage 
             Vidc rectified voltage