Abstract:
A signal transmitting apparatus that may suppress generation of a noise voltage attributable to a common mode voltage is provided. A transistor P1 is connected between a first terminal of a sending coil and a power supply voltage. A transistor N1 is connected between the first terminal and a ground voltage. A transistor P2 is connected between a second terminal of the sending coil wand the power supply voltage. A transistor N2 is connected between the second terminal and the ground voltage. In a period-PE1, a coil current flowing in a positive direction is generated by turning on the transistors P1 and N2 and turning off the transistors P2 and N1, and then the transistor N1 is turned on in response to turning off the transistor P1. In a period PE2, a coil current flowing in a negative direction is generated by turning off the transistors P1 and N2 and turning on the transistors P2 and N1, and then the transistor N2 is turned on in response to turning off the transistor P2.

Description:
TECHNICAL FIELD 
       [0001]    The present application relates to a signal transmitting apparatus which transmits a signal inputted to a sending circuit to a receiving circuit that is insulated electrically from the sending circuit. 
       BACKGROUND ART 
       [0002]    A technique is known in which a non-optical isolator that sends and receives signals via a separation barrier such as a transformer is driven by an H-bridge. An example of a prior-art signal transmitting apparatus  100  is shown in  FIG. 14 . In the signal transmitting apparatus  100 , a signal inputted to an input terminal IN is transmitted to a receiving circuit  130 . A sending-side ground voltage GNDL and a receiving-side ground voltage GNDH are separated from each other. An H-bridge is formed by transistors P 1 , P 2 , N 1 , and N 2 . A transformer TR comprises a sending coil L 1  and a receiving coil L 2 . The transistors P 1 , P 2 , N 1 , and N 2  of the H-bridge are on-off controlled in response to a signal inputted to the input terminal IN. Accordingly, a current flows through the sending coil L 1  of the transformer TR and, in turn, a current flows through the receiving coil L 2 . The receiving circuit  130  detects the signal inputted to the input terminal from the current flowing through the receiving coil L 2 . Moreover, in the drawing, series resistance components of the sending coil L 1  are denoted by Rs 1  and Rs 2 , and series resistance components of the receiving coil L 2  are denoted by Rs 3  and Rs 4 . In addition, parasitic capacitances between the sending coil L 1  and the receiving coil L 2  are denoted by Cc 1  and Cc 2 . Furthermore, respective capacitance values of the parasitic capacitances Cc 1  and Cc 2  are expressed as a capacitance C and respective resistance values of the series resistance components Rs 3  and Rs 4  are expressed as a resistance R. Techniques related to the present application are disclosed in U.S. Pat. No. 6,720,816, Japanese Translation of PCT Application No. 2003-523147, and Japanese Patent Application Publication No. 2007-123650. 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0003]    A common mode voltage VCM may be applied between a power supply terminal of the sending-side ground voltage GNDL and a power supply terminal of the receiving-side ground voltage GNDH. Let us assume that a voltage changing rate of the common mode voltage VCM at this point is (dv/dt). Accordingly, a displacement current i (=capacitance C×dv/dt) flows through the parasitic capacitances Cc 1  and Cc 2 . In addition, for example, When the transistor N 1  is turned on in a case in which a voltage of the sending-side ground voltage GNDL is lower than a voltage of the receiving-side ground voltage GNDH, the displacement current i flows along a path i 1 , The path it is a path from one terminal of the receiving coil L 2  to the ground voltage GNDL via the parasitic capacitance Cc 1 , the series resistance component Rs 1 , and the transistor N 1 . Furthermore, the path i 1  is an asymmetric current path with respect to the transformer TR. On the other hand, when the transistor N 2  is turned on in the case in which the voltage of the sending-side ground voltage GNDL is lower than the voltage of the receiving-side ground voltage GNDH, the displacement current i flows along a path i 2 . The path i 2  is a path from another terminal of the receiving coil L 2  to the ground voltage GNDL via the parasitic capacitance Cc 2 , the series resistance component Rs 2 , and the transistor N 2 . The path i 2  is also an asymmetric current path with respect to the transformer TR. Due to the displacement current i flowing through the asymmetric current paths, a noise voltage (=displacement current i×resistance R) that is unrelated to a signal component is generated on a side of the receiving coil L 2 . Consequently, since the noise voltage is superimposed on a signal voltage, a signal may be erroneously detected at the receiving circuit  130 . 
       Solution to the Technical Problem 
       [0004]    A signal transmitting apparatus disclosed in the present application may have a sending coil and receiving coil. The sending coil and the receiving coil may be insulated electrically. A signal may be transmitted from the sending coil to the receiving coil, The signal transmitting apparatus may comprise an upper first switching element provided between a first terminal of the sending coil and a power supply terminal of a high reference voltage, and a lower first switching element provided between the first terminal and a power supply terminal of a low reference voltage, The signal transmitting apparatus may comprise an upper second switching element provided between a second terminal of the sending coil and the power supply terminal of the high reference voltage, and a lower second switching element provided between the second terminal and the power supply terminal of the low reference voltage. The signal transmitting apparatus may further comprises a switching element controlling module that controls the upper first switching element, the lower first switching element, the upper second switching element and the lower second switching element. The switching element controlling module may perform a first control and a second control. The first control may generate a coil current flowing through the sending coil in a first direction by turning on the upper first switching element and the lower second switching element and turning off the upper second switching element and the lower first switching element, and then turn on the lower first switching element in response to turning off the upper first switching element. The second control may generate a coil current flowing through the sending coil in a second direction by turning off the upper first switching element and the lower second switching element and turning on the upper second switching element and the lower first switching element, and then turn on the lower second switching element in response to turning off the upper second switching element. 
         [0005]    With the signal transmitting apparatus, the sending coil and the receiving coil are insulated electrically and a low reference voltage of the sending coil and a low reference voltage of the receiving coil are separated from each other. A transformer is formed by the sending coil and the receiving coil. In addition, a parasitic capacitance exists between the sending coil and the receiving coil. When a common mode voltage is applied between a power supply terminal of the low reference voltage of the sending coil and a power supply terminal of the low reference voltage of the receiving coil, a displacement current flows through the parasitic capacitance. For example, when the common mode voltage is applied in a state in which the low reference voltage of the sending coil is lower than the low reference voltage of the receiving coil, the displacement current flows from the receiving coil to the low reference voltage of the sending circuit via the parasitic capacitance and the sending coil. Since series resistance components exist at both terminals of the receiving coil, a noise voltage is generated by the series resistance components when the displacement current flows through the receiving coil. 
         [0006]    With the signal transmitting apparatus according to the present application, the lower first switching element is turned on in response to turning off the upper first switching element during the first control. In addition, the lower second switching element is turned on in response to turning off the upper second switching element during the second control. As a result, there is a period in which both the lower first switching element and the lower second switching element are turned on. During this period, both voltages of the first terminal and the second terminal of the sending coil become low reference voltages and enter a low impedance state. In addition, since both the first terminal and the second terminal are in a low impedance state, the displacement current due to the common mode voltage flows through both a first current path (a path from one terminal of the receiving coil to the low reference voltage of the sending coil via a parasitic capacitance, the first terminal of the sending coil, and the lower first switching element) and a second current path (a path from another terminal of the receiving coil to the low reference voltage of the sending coil via a parasitic capacitance, the second terminal of the sending coil, and the lower second switching element). Accordingly, since displacement currents with directions that are opposite to each other flow through both terminals of the receiving coil, influences of the displacement currents cancel each other out. As a result, since a noise voltage can be suppressed from being generated on a side of the receiving coil, a situation in which a signal is erroneously detected by the receiving circuit can be prevented. 
         [0007]    In the signal transmitting apparatus disclosed in the present application, the switching element controlling module may further comprise a first pulse outputting module that outputs a first pulse signal by detecting a rising edge of an inputted signal, a second pulse outputting module that outputs a second pulse signal by detecting a falling edge of the inputted signal, a first inverting module that generates an inverted first pulse signal that is a signal having inverted the first pulse signal, and a second inverting module that generates an inverted second pulse signal that is a signal having inverted the second pulse signal. The first pulse signal may be supplied to the upper first switching element. The inverted first pulse signal may be supplied to the lower first switching element. The second pulse signal may be supplied to the upper second switching element. The inverted second pulse signal may be supplied to the lower second switching element. 
         [0008]    The first pulse signal is supplied to the upper first switching element and the inverted first pulse signal is supplied to the lower first switching element. The first pulse signal and the inverted first pulse signal are complementary signals. Therefore, during the first control, control can be performed so that the lower first switching element is turned on in response to turning off the upper first switching element. In a similar manner, the second pulse signal is supplied to the upper second switching element and the inverted second pulse signal is supplied to the lower second switching clement. The second pulse signal and the inverted second pulse signal are complementary signals. Therefore, during the second control, control can be performed so that the lower second switching element is turned on in response to turning off the upper second switching element. As a result, switching elements can be controlled so that a period exists in which both the lower first switching element and the lower second switching element are turned on. 
         [0009]    In the signal transmitting apparatus disclosed in the present application, the switching element controlling module may further comprise a delay circuit that delays rising edges of the first pulse signal, the second pulse signal, the inverted first pulse signal and the inverted second pulse signal by a predetermined time. The first pulse signal and the second pulse signal outputted from the delay circuit may be supplied to the upper first switching element and the upper second switching element. The inverted first pulse signal and the inverted second pulse signal outputted from the delay circuit may be supplied to the lower first switching element and the lower second switching element. 
         [0010]    When a rising edge of the inverted fat pulse signal is delayed by the delay circuit, a dead time period in which both the first pulse signal and the inverted first pulse signal are at low levels is created bet a falling edge of the first pulse signal and a rising edge of the inverted first pulse signal. Since both the upper first switching element and the lower first switching element are turned off during the dead time period, a through current can be prevented from flowing from the upper first switching element to the lower first switching element. In a similar manner, the delay circuit can create the dead time period between a falling edge of the second pulse signal and a rising edge of the inverted second pulse signal. Therefore, the through current can be prevented from flowing from the upper second switching element to the lower second switching element. 
         [0011]    In the signal transmitting apparatus disclosed in the present application, the signal transmitting apparatus may further comprise a lower first current path that is connected in parallel to the lower first switching element, and a lower second current path that is connected in parallel to the lower second switching element. Impedance of the lower first current path may be higher than impedance of the lower first switching element that is in an on-state. Impedance of the lower second current path may be higher than impedance of the lower second switching element that is in an on-state. 
         [0012]    The lower first current path constantly connects the first terminal of the sending coil to the power supply terminal of the low reference voltage. In addition, the lower second current. path constantly connects the second terminal of the sending coil to the power supply terminal of the low reference voltage. Therefore, even during a period in which the lower first switching element and the lower second switching element are turned off, both the first terminal and the second terminal of the sending coil can be constantly maintained in a low impedance state, Consequently, the noise voltage due to the common mode voltage can be prevented from being generated on the side of the receiving coil over an entire operation period of the signal transmitting apparatus. 
         [0013]    In addition, impedance of the lower first current path is set higher than impedance of the lower first switching element that is in an on-state, and impedance of the lower second current path is set higher than impedance of the lower second switching element that is in an on-state. Consequently, a value of the through current flowing from the upper first switching element through the lower first current path and a value of the through current flowing from the upper second switching element through the lower second current path can be sufficiently reduced. 
         [0014]    In the signal transmitting apparatus disclosed in the present application, each of the lower first switching element, the lower second switching element, the lower first current path and the lower second current path may comprise an NMOS transistor. A size of the NMOS transistor in the lower first current path may be smaller than a size of the NMOS transistor in the lower first switching element. A size of the NMOS transistor in the lower second current path may be smaller than a size of the NMOS transistor in the lower second switching element. The switching element controlling module may control the NMOS transistors in the lower first current path and the lower second current path in an on-state at all times. 
         [0015]    The lower first current path and the lower second current path may be formed by transistors. Therefore, since a high-impedance current path need not be separately created, circuit design and manufacturing processes may he simplified. In addition, the impedance of the NMOS transistor in the lower first current path that is in the on-state is set higher than the impedance of the NMOS transistor in the lower first switching element that is hi the on-state. Furthermore, the impedance of the NMOS transistor in the lower second current path that is in the on-state is set higher than the impedance of the NMOS transistor in the lower second switching element that is in the on-state. Consequently, the value of the through current flowing from the upper first switching element through the lower first current path and the value of the through current flowing from the upper second switching element through the lower second current path may be sufficiently reduced. 
         [0016]    In the signal transmitting apparatus disclosed in the present application, each of the lower first switching element, the lower second switching clement, the lower first current path and the lower second current path may comprise an NMOS transistor. The lower first current path may further comprise a first resistive element connected to a drain terminal of the NMOS transistor of the lower first current path. The lower second current path may further comprise a second resistive clement connected to a drain terminal of the NMOS transistor of the lower second current path. The switching element controlling module may control the NMOS transistors in the lower first current path and the lower second current path in an on-state at all times. 
         [0017]    By connecting the resistive elements to the drain terminals of the NMOS transistors in the lower first current path and the lower second current path, the impedance of the NMOS transistors in the on-state can be increased. Consequently, a value of the through current flowing from the upper first switching element through the lower first current path and a value of the through current flowing from the upper second switching element through the lower second current path can be sufficiently reduced. 
         [0018]    In the signal transmitting apparatus disclosed in the present application, the signal transmitting apparatus may further comprise a parallel first switching element that is connected in parallel to the upper first switching element, and a parallel second switching element that is connected in parallel to the upper second switching element. In the first control, the switching element controlling module may turn on the upper first switching element and the parallel first switching element at the same time, and then turn off the upper first switching element and the parallel first switching element at different timings. In the second control, the switching element controlling module may turn on the upper second switching element and the parallel second switching element at the same time, and then turn off the upper second switching element and the parallel second switching element at different timings. 
         [0019]    A receiving coil voltage is proportional to a time rate of change (di/dt) of a current flowing through the sending coil. In addition, in the first control, the upper first switching element and the parallel first switching element are controlled so as to be turned on at the same time when entering the on-state and turned off at different timings when entering the off-state. in a similar manner, in the second control, the upper second switching element and the parallel second switching element are controlled so as to be turned on at the same time when entering the on-state and turned off at different timings when entering the off-state. As a result, the time rate of change of a current flowing through the sending coil may be controlled so that a rate of decrease in an off-state is reduced with respect to a rate of increase in an on-state. Accordingly, an amplitude value of a pulse voltage that is generated in the receiving coil in response to the on-state of the switching element may be increased in comparison to an amplitude value of a pulse voltage that is generated in the receiving coil in response to the off-state of the switching element. Therefore, the on-state of the switching element may be more easily detected at the receiving coil. 
         [0020]    Furthermore, When controlling the time rate of change of the current flowing through the sending coil by gradually turning off the switching element, a midpoint voltage of on/off voltages of the switching element must he used. Since the midpoint voltage is a state in which the switching element is unstable, noise may be generated and a signal may be erroneously transmitted. On the other hand, since the control of gradually turning off the switching clement need not be performed with the signal transmitting apparatus according to the present application, the midpoint voltage of on/off voltages of the switching element need not be used. Accordingly, since control of the time rate of change of the current flowing through the sending coil may be realized in the state in which the switching element is stable, generation of noise may be suppressed. 
         [0021]    In the signal transmitting apparatus disclosed in the present application, each of the upper first switching element, the parallel first switching element, the upper second switching element and the parallel second switching element may comprise a PMOS transistor. A size of the PMOS transistor in the parallel first switching element may be smaller than a size of the PMOS transistor in the upper first switching element A size of the PMOS transistor in the parallel second switching element may be smaller than a size of the PMOS transistor in the upper second switching element. In the first control, the switching element controlling module may turn off the upper first switching element after the parallel first switching element is turned off. In the second control, the switching element controlling module may turn off the upper second switching element after the parallel second switching element is turned off. 
         [0022]    The size of the transistor in the upper first switching element is set larger than the size of the transistor in the parallel first switching element. In addition, the parallel first switching element is turned of first and then the upper first switching element is turned off. In a similar manner, the size of the transistor in the upper second switching element is set larger than the size of the transistor in the parallel second switching element. Furthermore, the parallel second switching element is turned off first and then the upper second switching element is turned off. Consequently, the time rate of change of the current flowing through the sending coil may be controlled so that a rate of decrease in the off-state is reduced with respect to a rate of increase in the on-state. 
         [0023]    In the signal transmitting apparatus disclosed in the present application, a number of coil windings of the receiving coil may be larger than a number of coil windings of the sending coil. 
         [0024]    A deterioration of a coupling coefficient due to downsizing of the transformer causes a deterioration of the signal component and makes signal transmission more difficult, in consideration thereof, with the signal transmitting apparatus according to the present application, by increasing a number of coil windings of the receiving coil, inductance of the receiving coil may be increased and the signal component may be increased. Therefore, downsizing of the transformer may be realized and a cost of the signal transmitting apparatus may be reduced. Moreover, increasing the number of coil windings of the receiving roil increases the series resistance component which, in turn, increases the noise voltage. However, in the signal transmitting apparatus according to the present application, since generation of a noise voltage itself may be suppressed, the number of coil windings of the receiving coil can be increased. 
       Effect of the Invention 
       [0025]    According to the present application, in a signal transmitting apparatus which transmits a signal inputted to a sending circuit to a receiving circuit that is insulated electrically from the sending circuit, generation of the noise voltage attributable to the common mode voltage can be suppressed. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0026]      FIG. 1  is a circuit diagram of a motor drive system according to a first embodiment. 
           [0027]      FIG. 2  is a circuit diagram of a signal transmitting circuit according to a first embodiment. 
           [0028]      FIG. 3  is a circuit diagram of a rising delay circuit according to a first embodiment. 
           [0029]      FIG. 4  is an operational waveform diagram of a rising delay circuit according to a first embodiment. 
           [0030]      FIG. 5  is a circuit diagram of a receiving circuit according to a first embodiment. 
           [0031]      FIG. 6  is an operational waveform diagram of a signal transmitting circuit according to a first embodiment using an operating method according to the present application. 
           [0032]      FIG. 7  is an operational waveform diagram of a signal transmitting circuit according to a first embodiment using a general method. 
           [0033]      FIG. 8  is a circuit diagram of a sending circuit according to a second embodiment. 
           [0034]      FIG. 9  is a circuit diagram of a sending circuit according to a third embodiment. 
           [0035]      FIG. 10  is a circuit diagram of a falling delay circuit according to a third embodiment. 
           [0036]      FIG. 11  is an operational waveform diagram of a falling delay circuit according to a third embodiment. 
           [0037]      FIG. 12  is an operational waveform diagram of a sending circuit according to a third embodiment. 
           [0038]      FIG. 13  is an example of modification of an H-bridge circuit. 
           [0039]      FIG. 14  is a circuit diagram of a prior-art signal transmitting apparatus. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0040]    Primary characteristics of embodiments will be listed below. 
         [0041]    (First mode) A switching element controlling module may perform control so as to generate a coil current flowing through a sending coil in a first direction by turning on an upper first switching element and a lower second switching element and turning off an upper second switching element and a lower first switching element, and to subsequently turn on the upper second switching element in response to turning of the lower second switching element. In addition, the switching element controlling module may also perform control so as to generate a aril current flowing through the sending coil in a second direction by turning off the upper first switching element and the lower second switching element and turning on the upper second switching element and the lower first switching element, and to subsequently turn on the upper first switching element in response to turning off the lower first switching element. The object of setting voltages of both terminals of the sending coil to low impedance can also be achieved by this configuration. 
       First Embodiment 
       [0042]    A first embodiment of the present application will he described with reference to the drawings.  FIG. 1  shows a motor drive system  50 . The motor drive system  50  comprises a low voltage system circuit  60  and a high voltage system circuit  70 . The low voltage system circuit  60  and the high voltage system circuit  70  are insulated from each other. The low voltage system circuit  60  comprises a low-voltage battery  61  and a microcomputer  62 . The microcomputer  62  outputs a control signal CS. The control signal CS is a signal for controlling switching operations of a switching circuit  72 . 
         [0043]    The high voltage system circuit  70  comprises a control circuit  71 , the switching circuit  72 , a motor  73 , and a high-voltage battery  74 . The control circuit  71  comprises a signal transmitting circuit  1  and a driving circuit  75 . The signal transmitting circuit  1  is a circuit comprising an isolated signal device. The signal transmitting circuit  1  transmits the control signal CS outputted from the microcomputer  62  to the driving circuit  75  while maintaining an insulated state. In addition, the control circuit  71  is formed as an integrated IC by using a small-sized device such as an on-chip transformer as the isolated signal device. Therefore, downsizing and cost reduction are achieved. In response to the control signal CS, the driving circuit  75  drives the switching circuit  72 . Accordingly, rotation of the motor  73  is controlled. 
         [0044]      FIG. 2  shows a detailed circuit diagram of the signal transmitting circuit  1 . The signal transmitting circuit  1  comprises an input terminal IN, a sending circuit  10 , a transformer TR, a receiving circuit  30 , and an output terminal OUT. The sending circuit  10  and the receiving circuit  30  are insulated by the transformer TR. In addition, a ground voltage GNDL of the sending circuit  10  and a ground voltage GNDH of the transformer TR are separated from each other. An input voltage VIN is inputted to the input terminal IN and an output voltage VOUT is outputted from the output terminal OUT. 
         [0045]    The sending circuit  10  comprises a switching element controlling circuit  20 , inverters  15  and  16 , buffers  17  and  18 , and an H-bridge circuit  19 . The switching element controlling circuit  20  will now be described. The switching element controlling circuit  20  comprises pulse generators  11  and  12 , an inverter  13 , and a delay controlling circuit  14 . The input voltage VIN is inputted to the pulse generator  11  and a pulse signal PS 1  is outputted from the pulse generator  11 . The input voltage VIN inverted by the inverter  13  is inputted to the pulse generator  12  and a pulse signal PS 2  is outputted from the pulse generator  12 . 
         [0046]    The delay controlling circuit  14  comprises rising delay circuits  21  to  24  and inverters  25  and  26 . The pulse signal PS 2  is inputted to the rising delay circuit  21  and a gate controlling signal SP 2  is outputted from the rising delay circuit  21 . The pulse signal PS 1  is inputted to the rising delay circuit  22  and a gate controlling signal SP 1  is outputted from the rising delay circuit  22 . The pulse signal PS 1  is inputted to the inverter  25  and an inverted pulse signal PS 1 B is outputted from the inverter  25 . The inverted pulse signal PS 1 B is inputted to the rising delay circuit  23  and a gate controlling signal SN 1  is outputted from the rising delay circuit  23 . The pulse signal PS 2  is inputted to the inverter  26  and an inverted pulse signal PS 2 B is outputted from the inverter  26 , The inverted pulse signal PS 2 B is inputted to the rising delay circuit  24  and a gate controlling signal SN 2  is outputted from the rising delay circuit  24 . 
         [0047]      FIG. 3  shows a block diagram of the rising delay circuit  21 . The rising delay circuit  21  comprises an inverter  41 , a capacitor  42 , and a Schmitt trigger inverter  43 . The inverter  41  comprises a transistor P 41  that is a PMOS transistor and a transistor N 41  that is an NMOS transistor. A gate terminal of the transistor P 41  and a gate terminal of the transistor N 41  are commonly connected and are arranged as input terminals. A supply voltage VDDL is inputted to a source terminal of the transistor P 41 . A ground voltage GNDL is inputted to a source terminal of the transistor N 41 . A drain terminal of the transistor P 41  and a drain terminal of the transistor N 41  are commonly connected and are arranged as output terminals. In addition, a size of the transistor P 41  is set larger than a size of the transistor N 41 . Therefore, impedance of the transistor P 41  in an on-state is set lower than impedance of the transistor N 41  in an on-state. 
         [0048]    The pulse signal PS 2  is inputted to an input terminal of the inverter  41 . An output terminal of the inverter  41  and an input terminal of the Schmitt trigger inverter  43  are connected to a first terminal of the capacitor  42 . The supply voltage VDDL is inputted to a second terminal of the capacitor  42 . The gate controlling signal SP 2  is outputted from the Schmitt trigger inverter  43 . 
         [0049]      FIG. 4  shows an operational waveform diagram of the rising delay circuit  21 . At time t 41 , as the pulse signal PS 2  transitions from a low level to a high level, an output of the inverter  41  transitions from a high level to a low level. Consequently, the capacitor  42  is discharged through a current path constituted by the capacitor  42 , the transistor N 41 , and the ground voltage GNDL. Subsequently, when an output voltage of the capacitor  42  drops below a threshold voltage of the Schmitt trigger inverter  43 , at time t 42 , the gate controlling signal SP 2  transitions from a low level to a high level. 
         [0050]    In a similar manner, at time t 43 , as the pulse signal PS 2  transitions from a high level to a low level, an output of the inverter  41  transitions from a low level to a high level. Consequently, the capacitor  42  is charged through a current path constituted by the supply voltage VDDL, the transistor P 41 , and the capacitor  42 , Subsequently, when an output voltage of the capacitor  42  rises above the threshold voltage of the Schmitt trigger inverter  43 , at time t 43 , the gate controlling signal SP 2  transitions from a high level to a low level. 
         [0051]    Moreover, impedance of the transistor P 41  in the on-state is set lower than impedance of the transistor N 41  in an on-state. Therefore, a discharge period of the capacitor  42  can be set longer than a charge period of the capacitor  42 . Accordingly, the rising delay circuit  21  can perform an operation in which only a rising edge of the pulse signal PS 2  is outputted delayed by a delay time DT 1  while a falling edge of the pulse signal PS 2  is outputted without delay. Moreover, a length of the delay time DT 1  can be set to any value by adjusting a resistance value of the transistor N 41  and a capacitance value of the capacitor  42 . Since configurations of the rising delay circuits  22  to  24  are similar to that of the rising delay circuit  21 , a detailed description will be omitted herein. 
         [0052]    The H-bridge circuit  19  ( FIG. 2 ) comprises transistors P 1 , P 2 , N 1 , and N 2 . The transistors P 1  and P 2  are PMOS transistors and the transistors N 1  and N 2  are NMOS transistors. A drain terminal of the transistor Pi is connected to a first input terminal T 11  of the transformer TR. The supply voltage VDDL is inputted to a source terminal of the transistor P 1 . The gate controlling signal SP 1  inverted by the inverter  16  is inputted to a gate terminal of the transistor P 1 . The transistor P 1  enters the on-state when the gate controlling signal SP 1  is at a high level and enters an off-state when the gate controlling signal SP 1  is at a low level. A drain terminal of the transistor N 1  is connected to the first input terminal T 11  of the transformer TR. The ground voltage GNDL is inputted to a source terminal of the transistor N 1 . The gate controlling signal SN 1  is inputted to a gate terminal of the transistor N 1  via the buffer  17 . The transistor N 1  enters the on-state when the gate controlling signal SN 1  is at the high level and enters an off-state when the gate controlling signal SN 1  is at the low level. 
         [0053]    A drain terminal of the transistor P 2  is connected to a second input terminal T 12  of the transformer TR. The supply voltage VDDL is inputted to a source terminal of the transistor P 2 . The gate controlling signal SP 2  inverted by the inverter  15  is inputted to a gate terminal of the transistor P 2 . The transistor P 2  enters the on-state when the gate controlling signal SP 2  is at the high level and enters the off-state when the gate controlling signal SP 2  is the a low level. A drain terminal of the transistor N 2  is connected to the second input terminal T 12  of the transformer TR. The ground voltage GNDL is inputted to a source terminal of the transistor N 2 . The gate controlling signal SN 2  is inputted to a gate terminal of the transistor N 2  via the huller  18 . The transistor N 2  enters the on-state when the gate controlling signal SN 2  is at the high level and enters the off-state when the gate controlling signal SN 2  is at the low level. In addition, a drain terminal of the transistor P 1  and a drain terminal of the transistor N 1  are connected by a node ND 1 . In a similar manner, a drain terminal of the transistor P 2  and a drain terminal of the transistor N 2  are connected by a node ND 2 . 
         [0054]    The transformer TR will be described. The transformer TR shown in  FIG. 2  is an equivalent circuit. The transformer TR comprises a sending coil L 1  and a receiving coil L 2 . The sending coil L 1  and the receiving coil L 2  are insulated electrically. In addition, a number of coil windings of the receiving coil L 2  is set larger than a number of coil windings of the sending coil L 1 . The node ND 1  is connected to the first input terminal T 11  of the transformer TR and the node ND 2  is connected to the second input terminal T 12  of the transformer TR. Furthermore, the receiving circuit  30  is connected to a first output terminal T 21  of the transformer TR. In addition, a ground voltage GNDH is supplied to a second output terminal T 22 . Moreover, a current flowing through the sending coil L 1  is defined as a coil current I 1  and a current flowing through the receiving coil L 2  is defined as a coil current I 2 . 
         [0055]    A series resistance component Rs 1  exists in a connection path of a first terminal E 11  of the sending coil L 1  and the first input terminal T 11 . A series resistance component Rs 2  exists in a connection path of a second terminal E 12  and the second input terminal T 12 . In a similar manner, a series resistance component Rs 3  exists in a connection path of a first terminal E 21  the receiving coil L 2  and the first output terminal T 21 . A series resistance component Rs 4  exists in a connection path of a second terminal E 22  and the second output terminal T 22 . In addition, a parasitic capacitance Cc 1  exists between the first terminal E 11  of the sending coil L 1  and the first terminal E 21  of the receiving coil L 2 . A parasitic capacitance Cc 2  exists between the second terminal E 12  of the sending coil L 1  and the second terminal E 22  of the receiving coil L 2 . Furthermore, a distributed capacitance Cs 1  exists between wirings of the sending coil L 1  and a distributed capacitance Cs 2  exists between wirings of the receiving coil L 2 . In this case, each of capacitance values of the parasitic capacitances Cc 1  and Cc 2  is defined as a capacitance C. In addition, each of resistance values of the series resistance components Rs 3  and Rs 4  is defined as a resistance R. 
         [0056]      FIG. 5  shows the receiving circuit  30 . The receiving circuit  30  comprises a low pass filter  31 , a high pass filter  32 , comparators cmp 1  and cmp 2 , a signal processing circuit  33 , and an RS flip flop  34 , The first output terminal T 21  of the transformer TR is connected to an input terminal of the low pass filter  31 . An output terminal of the low pass filter  31  is connected to an input terminal of the high pass filter  32 . A receiving coil voltage Vd is outputted from an output terminal of the high pass filter  32 . The receiving coil voltage Vd is inputted to a non-inverting input terminal of the comparator cmp 1 , a threshold Vthp is inputted to an inverting input terminal of the comparator cmp 1 , and an output signal Vc 1  is outputted from an output terminal of the comparator cmp 1 . In addition, the receiving coil voltage Vd is inputted to a non-inverting input terminal of the comparator cmp 2 , a threshold Vthn is inputted to an inverting input terminal of the comparator cmp 2 , and an output signal Vc 2  is outputted from an output terminal of the comparator cmp 2 . 
         [0057]    The output signals Vc 1  and Vc 2  are inputted to the signal processing circuit  33 , and a pulse signal Vs and a pulse signal Vr are outputted from the signal processing circuit  33 . The signal processing circuit  33  is a circuit that detects a rising edge and a falling edge of the input voltage VIN. Specifically, a case in which the output signals Vc 1  and Vc 2  are inputted in succession to the signal processing circuit  33  with the output signal Val inputted first and the output signal Vc 2  inputted second is determined to be a case in which the coil current I 1  is generated in a positive direction (in  FIG. 2 , a direction coinciding with an arrow of the coil current I 1 ). Therefore, a determination is made that the rising edge of the input voltage VIN has been inputted to the sending circuit  10  and the pulse signal Vs is outputted from the signal processing circuit  33 . In addition, a case in which the output signals Vc 1  and Vc 2  arc inputted in succession to the signal processing circuit  33  with the output signal Vc 2  inputted first and the output signal Vc 1  inputted second is determined to be a case in which the coil current I 1  is generated in a negative direction (in  FIG. 2 , a direction opposite to the arrow of the coil current I 1 ). Therefore, a determination is made that the falling edge of the input voltage VIN has been inputted to the sending circuit  10  and the pulse signal Yr is outputted from the signal processing circuit  33 . 
         [0058]    The pulse signal Vs is inputted to a set terminal of the RS flip flop  34  and the pulse signal Vr is inputted to a reset terminal of the RS flip flop  34 , The RS flip flop  34  outputs a high-level output voltage VOUT when the pulse signal Vs is inputted and outputs a low-level output voltage VOUT when the pulse signal Vr is inputted. 
         [0059]    Operations of the signal transmitting circuit  1  will be described using an operational waveform diagram shown in  FIG. 6 . A period PET is a period during which the input voltage VIN is at a high level and a period PE 2  is a period during which the input voltage VIN is at a low level. 
         [0060]    Operations during the period PEI will now be described. At time t 1 , in response to the rising edge of the input voltage VIN, the pulse signal PS 1  transitions to a high level (arrow Y 1 ). In response to a rising edge of the pulse signal PS 1 , the gate controlling signal SN 1  transitions to a low level (arrow Y 2 ). Therefore, the transistor N 1  is turned off at time t 1 . In addition, the gate controlling signal SP 1  transitions to the high level (arrow Y 3 ) after a delay of a delay time DT 1  from the rising edge of the pulse signal PS 1 . Therefore, the transistor P 1  is turned on at time t 2 . Furthermore, at time t 2 , the transistor P 2  is turned off and the transistor N 2  is turned on. Therefore, a current path of the supply voltage VDDL, the transistor P 1 , the node ND 1 , the sending coil L 1 , the node ND 2 , the transistor N 2 , and the ground voltage GNDL is formed and the coil current I 1  starts to increase in the positive direction. In other words, the coil current I 1  in the positive direction flows in accordance with the riding edge of the input voltage VIN. 
         [0061]    At the receiving coil L 2 , a secondary voltage that is proportional to a rate of increase (di/dt) of the coil current I 1  is generated by electromagnetic induction. The secondary voltage is inputted to the low pass filter  31  and the high pass filter  32  and noise is removed. The secondary voltage after noise reduction is outputted from the high pass filter  32  as the receiving coil voltage Vd. In addition, during a period in which the receiving coil voltage Vd rises above the threshold Vthp, the output signal Vc 1  of the comparator cmp 1  has a high level (arrow Y 4 ). 
         [0062]    At time t 3 , in response to a falling edge of the pulse signal PS 1 , the gate controlling signal SP 1  transitions to the low level (arrow Y 5 ), Therefore, at time t 3 , since the transistor P 1  is turned off and the current path is blocked, the coil current I 1  starts to decrease. In addition, the gate controlling signal SN 1  transitions to the high level (arrow Y 6 ) after a delay of the delay time DT 1  from the falling edge of the pulse signal PS 1 . Therefore, the transistor N 1  is turned on at time t 4 . 
         [0063]    The receiving coil voltage Vet that is proportional to a rate of decrease (di/dt) of the coil current I 1  is outputted from the high pass filter  32 . In addition, during a period in which the receiving coil voltage Vd falls below the threshold Vthn, the output signal Vc 2  of the comparator cmp 2  has the low level (arrow Y 7 ). 
         [0064]    The signal processing circuit  33  ( FIG. 5 ) of the receiving circuit  30  detects that pulses of the output signals Vc 1  and Vc 2  have been inputted in succession with the pulse of the output signal Vc 1  inputted first and the pulse of the output signal Vc 2  inputted second. Therefore, a determination is made by the signal processing circuit  33  that a rising edge of the input voltage VIN has been inputted to the sending circuit  10  and the output voltage VOUT is set to the high level (arrow Y 8 ). Accordingly, the rising edge of the input voltage VIN at time t 1  is restored as the output voltage VOUT to achieve signal transmission. 
         [0065]    In addition, an effect of the delay controlling circuit  14  will be described. The delay controlling circuit  14  forms a dead time period of the delay time DT 1  between the falling edge of the gate controlling signal SN 1  at time t 1  and the rising edge of the gate controlling signal SP 1  at time t 2 . Furthermore, the delay controlling circuit  14  forms a dead time period of the delay time DT 1  between the falling edge of the gate controlling signal SP 1  at time t 3  and the rising edge of the gate controlling signal SN 1  at time t 4 . Since both transistors P 1  and N 1  are turned off during the dead time periods, a through current can be prevented from flowing from the transistor P 1  to the transistor N 1 . 
         [0066]    Next, operations during the period PE 2  will now be described. At time t 6 , in response to the falling edge of the input voltage VIN, the pulse signal PS 2  transitions to the high level (arrow Y 11 ). In response to the rising edge of the pulse signal PS 2 , the gate controlling signal SN 2  transitions to the low level (arrow Y 12 ). Therefore, the transistor N 2  is turned off at time t 6 . In addition, the gate controlling signal SP 2  transitions to the high level (arrow Y 13 ) after the delay of the delay time DT 1  from the rising edge of the pulse signal PS 2 . Therefore, the transistor P 2  is turned on at time t 7 . Furthermore, at time t 7 , the transistor P 1  is turned off and the transistor N 1  is turned on. Therefore, the current path of the supply voltage VDDL, the transistor P 2 , the node ND 2 , the sending coil L 1 , the node ND 1 , the transistor N 1 , and the ground voltage GNDL is formed and the coil current I 1  starts to increase in the negative direction. In other words, the coil current I 1  in the negative direction flows in accordance with the falling edge of the input voltage VIN. 
         [0067]    The receiving coil voltage Vd that is proportional to the rate of increase (di/dt) of the coil current I 1  is outputted from the high pass filter  32 . In addition, during a period in which the receiving coil voltage Vd fails below the threshold Vthn, the output signal Vc 2  of the comparator cmp 2  has the low level (arrow Y 14 ). 
         [0068]    At time t 8 , in response to a falling edge of the pulse signal PS 2 , the gate controlling signal SP 2  transitions to a low level (arrow Y 15 ). Therefore, at time t 8 , since the transistor P 2  is turned off and the current path is blocked, the coil current I 1  starts to decrease. In addition, the gate controlling signal SN 2  transitions to a high level (arrow Y 16 ) after a delay of the delay time DT 1  frown the falling edge of the pulse signal PS 2 . Therefore, the transistor N 2  is turned on at time t 9 . 
         [0069]    The receiving coil voltage that is proportional to the rate of decrease (di/dt) of the coil current I 1  is outputted from the high pass filter  32 . In addition, during a period in which the receiving coil voltage Vd rises above the threshold Vthp, the output signal Vc 1  of the comparator cmp 1  has a high level (arrow Y 17 ). 
         [0070]    The signal processing circuit  33  ( FIG. 5 ) of the receiving circuit  30  detects that pulses of the output signals Vc 2  and Vc 1  have been inputted in succession with the pulse of the output signal Vc 2  inputted first and the pulse of the output signal Vc 1  inputted second. Therefore, a determination is made by the signal processing circuit  33  that the falling edge of the input voltage VIN has been inputted to the sending circuit  10  and the output voltage VOUT is set to the low level (arrow Y 18 ). Accordingly, the falling edge of the input voltage VIN at time t 6  is restored as the output voltage VOUT to achieve signal transmission. 
         [0071]    A first effect of the signal transmitting circuit  1  according to the first embodiment will be described. As an example, a case will be described in Which a common mode voltage VCM is applied between a power supply terminal of a sending-side ground voltage GNDL and a power supply terminal of a receiving-side ground voltage GNDH. In addition, a case will be described in which a voltage changing rate of the common mode voltage VCM is (dv/dt). Furthermore, as an example, a case will be described in which a voltage of the sending-side ground voltage GNDL, is lower than a voltage of the receiving-side ground voltage GNDH. 
         [0072]    First, for comparison, a general operation method will be described.  FIG. 7  is a waveform diagram showing a case in which the signal transmitting circuit  1  is operated by a general method. In the general method, during a period PE 1 , since the gate controlling signal SN 1  is maintained at a low level and the gate controlling signal SN 2  is maintained at a high level, the transistor N 1  is maintained in an off-state and the transistor N 2  is maintained in an on-state. In addition, during a period PE 2 , since the gate controlling signal SN 1  is maintained at a high level and the gate controlling signal SN 2  is maintained at a low level, the transistor N 1  is maintained in an on-state and the transistor N 2  is maintained in an off-state. In other words, with the general method, the transistors N 1  and N 2  do not enter an on-state (low-impedance state) at the same time. 
         [0073]    Furthermore, due to the common mode voltage VCM, a displacement current i (=C×dv/dt) flows through the parasitic capacitances Cc 1  and Cc 2 . During the period PEI, a path through Which the displacement current i flows is a path Ri 2  Shown in  FIG. 2 . The path Ri 2  is a path from the second terminal E 22  of the receiving coil L 2  to the ground voltage GNDL via the parasitic capacitance Cc 2 , the series resistance component Rs 2 , the second input terminal T 12 , the node ND 2 , and the transistor N 2 . In addition, the path Ri 2  is an asymmetric current path with respect to the transformer TR. Moreover, during the period PE 2 , a path through which the displacement current i flows is a path Ri 1  shown in  FIG. 2 . The path Ri 1  is a path from the first terminal E 21  of the receiving coil L 2  to the ground voltage GNDL via the parasitic capacitance Cc 1 , the series resistance component Rs 1 , the first input terminal T 11 , the node ND 1 , and the transistor N 1 . In addition, the path Ri 1  is an asymmetric current path with respect to the transformer TR. 
         [0074]    Due to the displacement current i flowing through the asymmetric current paths, a noise voltage (=displacement current i×resistance R) is generated on a side of the receiving coil L 2  when the input voltage VIN switches between a high level and a low level. Consequently, since the noise voltage is superimposed on the receiving coil voltage Vd (in  FIG. 7 , area A 11  and area A 12 ), a rising edge and a falling edge of the input voltage VIN may be erroneously detected at the receiving circuit  30 . 
         [0075]    On the other hand, with the operating method according to the present application ( FIG. 6 ), the gate controlling signals SN 1  and SN 2  are both at a high level and the transistors N 1  and N 2  are both in an on-state during a period from time t 4  to time t 6  in the period PE 1  (area A 1 ) and a period from time t 9  to time t 10  in the period PE 2  (area A 2 ). During these periods, both voltages of the first terminal E 11  and the second terminal E 12  of the sending coil L 1  are in a low impedance state. 
         [0076]    Furthermore, since both the first terminal E 11  and the second terminal E 11  are in a low impedance state, the displacement current i due to the common mode voltage VCM flows through both routes Ri 1  and Ri 2 . In other words, with the signal transmitting circuit  1  according to the present application, a current path of the displacement current is never limited to one of the paths Ri 1  and Ri 2  as was the case of the general operating method shown in  FIG. 7 . Therefore, the current path of the displacement current i can be set symmetrical with respect to the transformer TR. Accordingly, since displacement currents with directions that are opposite to each other flow through both terminals of the receiving coil L 2 , influences of the displacement currents cancel each other out. Consequently, generation of a noise voltage on the side of the receiving coil L 2  can be suppressed (in  FIG. 7 , area A 3  and area A 4 ), As a result, since the noise voltage can be suppressed from being superimposed on the receiving coil voltage Vd, a rising edge and a falling edge of the input voltage VIN can be more accurately detected at the receiving circuit  30 . 
         [0077]    In addition, a second effect of the signal transmitting circuit  1  according to the first embodiment will be described. A deterioration of a coupling coefficient due to downsizing of the transformer TR causes a deterioration of a received signal component and makes signal transmission more difficult. Therefore, with the signal transmitting circuit  1  according to the first embodiment, by setting a number of coil windings of the receiving coil L 2  larger than a number of coil windings of the sending coil L 1 , an inductance of the receiving coil L 2  can be increased and a strength of a received signal can be enhanced. Therefore, downsizing of the transformer TR can be realized and a cost of the signal transmitting circuit  1  can be reduced. Moreover, increasing the number of coil windings of the receiving coil L 2  increases series resistance components Rs 3  and Rs 4  which, in turn, increases a noise voltage attributable to the common mode voltage VCM. However, with the signal transmitting circuit  1  according to the first embodiment, since generation of a noise voltage attributable to the common mode voltage VCM itself can be suppressed, the number of coil windings of the receiving coil L 2  can be increased. 
       Second Embodiment 
       [0078]    A second embodiment of the present application will be described with reference to the drawings.  FIG. 8  shows a detailed circuit diagram of a sending circuit  10   a  according to the second embodiment. Since configurations of the transformer TR and the receiving circuit  30  are similar to those of the first embodiment ( FIG. 2 ), a detailed description will be omitted herein. 
         [0079]    The sending circuit  10   a  comprises a switching element controlling circuit  20   a , inverters  15  and  16 , buffers  17  and  18 , and an H-bridge circuit  19   a . In addition, the switching element controlling circuit  20   a  comprises pulse generators  11  and  12 , an inverter  13 , and a delay controlling circuit  14   a . The delay controlling circuit  14   a  outputs a supply voltage VDDL as a gate controlling signal SN 3 . Moreover, since other parts of the configuration of the delay controlling circuit  14   a  are similar to those of the delay controlling circuit  14  according to the first embodiment ( FIG. 2 ), a detailed description will be omitted herein. 
         [0080]    The H-bridge circuit  19   a  comprises transistors P 1  and P 2  and transistors N 1  to N 4 . The transistors P 1  and P 2  are PMOS transistors and the transistors N 1  to N 4  are NMOS transistors. The H-bridge circuit  19   a  is configured by adding transistors N 3  and N 4  to the H-bridge circuit  19  according to the first embodiment ( FIG. 2 ). 
         [0081]    The transistor N 3  is connected in parallel with the transistor N 1 . A drain terminal of the transistor N 3  is connected to a node ND 1 . A ground voltage GNDL is inputted to a source terminal of the transistor N 3  and the gate controlling signal SN 3  is inputted to a gate terminal of the transistor N 1  The transistor N 3  is constantly set to an on-state. The transistor N 4  is connected in parallel with the transistor N 2 . A drain terminal of the transistor N 4  is connected to a node ND 2 . The ground voltage GNDL is inputted to a source terminal of the transistor N 4  and the gate controlling signal SN 3  is inputted to a gate terminal of the transistor N 4 . The transistor N 4  is constantly set to an on-state. 
         [0082]    A size of the transistor N 3  is set smaller than a size of the transistor N 1 . Therefore, impedance of the transistor N 3  in the on-state is set higher than impedance of the transistor N 1  in the on-state. Accordingly, during a period in which the transistor P 1  is in an on-state, a through current flowing from the transistor P 1  to the transistor N 3  can be reduced. In addition, a size of the transistor N 4  is set smaller than a size of the transistor N 2 . Therefore, impedance of the transistor N 4  in the on-state is set higher than impedance of the transistor N 2  in the on-state. Accordingly, during a period in which the transistor P 2  is in the on-state, a through current flowing from the transistor P 2  to the transistor N 4  can be reduced. 
         [0083]    Moreover, while the smaller the sizes of the transistors N 3  and N 4 , the smaller the through current, a suppression effect of generation of a noise voltage (to be described later) is also reduced. Therefore, the sizes of the transistors N 3  and N 4  must be determined so as to obtain a balance between a permissible value of the through current and a permissible value of the noise voltage. For example, the sizes of the transistors N 3  and N 4  are favorably set to approximately 1/10 to 1/50 of the sizes of the transistors N 1  and N 2 . Moreover, since other parts of the configuration are similar to those of the H-bridge circuit  19  according to the first embodiment ( FIG. 2 ), a detailed description will be omitted herein. 
         [0084]    An effect of the sending circuit  10   a  according to the second embodiment will be described. Driving of the transistors P 1  and N 1  require a dead time period in which both the transistors P 1  and N 1  are turned off. This is required in order to prevent a through current from flowing from the transistor P 1  to the transistor N 1 . However, when the transistor N 3  is in an off state, a voltage of a first terminal E 11  of the sending coil L 1  enters a high-impedance state during the dead time period. Consequently, a displacement current i generated during the dead time period only flows through a path Ri 2 . Since the path Ri 2  is an asymmetric current path with respect to the transformer TR, a noise voltage is generated. In a similar manner, driving of the transistors P 2  and N 2  also require a dead time period in which both the transistors P 2  and N 2  are turned off. In addition, when the transistor N 4  is in an off-state, the displacement current i generated during the dead time period only flows through a path Ri 1  and a noise voltage is generated. From the above, it is shown that the longer the dead time period, the more susceptible to noise voltage. 
         [0085]    However, with the sending circuit  10   a  according to the second embodiment, transistors N 3  and N 4  are constantly in an on-state. Therefore, both the first terminal E 11  and the second terminal E 12  of the sending coil L 1  can constantly be kept in a low impedance state. Accordingly, a displacement current i due to a common mode voltage VCM flows through both a path Ri 1   a  and a path Ri 2   a  ( FIG. 8 ). As a result, since the current path of the displacement current i becomes symmetrical with respect to the transformer TR, the generation of a noise voltage on the side of the receiving coil L 2  can be suppressed. Accordingly, the generation of a se voltage attributable to the common mode voltage VCM can be constantly suppressed at the receiving circuit  30 . Therefore, a length of a dead time period can be freely set. 
         [0086]    Moreover, in the sending circuit  10   a , the path Ri 1   a  is formed by the transistor N 3  and the path Ri 2   a  is formed by the transistor N 4 . Accordingly, the paths Ri 1   a  and Ri 2   a  can be formed using a similar process to a process for forming the transistors N 1  and N 2 . Therefore, since a dedicated process or the like for funning the paths Ri 1   a  and Ri 2   a  need not be separately prepared, circuit design and manufacturing processes can be simplified. 
       Third Embodiment 
       [0087]    A third embodiment of the present application will be described with reference to the drawings.  FIG. 9  Shows a detailed circuit diagram of a sending circuit  10   b  according to the third embodiment. Since configurations of the transformer TR and the receiving circuit  30  are similar to those of the first embodiment ( FIG. 2 ), a detailed description will be omitted herein. 
         [0088]    The sending circuit  10   b  comprises a switching element controlling circuit  20   b , inverters  15 ,  16 ,  57 , and  58 , buffers  17 ,  18 , and  53  to  56 , and an H-bridge circuit  19   b . In addition, the switching element controlling circuit  20   b  comprises pulse generators  11  and  12 , an inverter  13 , and a delay controlling circuit  14   b.    
         [0089]    The delay controlling circuit  14   b  comprises rising delay circuits  21  to  24 ,  27 , and  28 , falling delay circuits  51  and  52 , and inverters  25  and  26 . 
         [0090]    A pulse signal PS 1  is inputted to the rising delay circuit  27  and a gate controlling signal SP 1   b  is outputted from the rising delay circuit  27 . The pulse signal PS 1  is inputted to the falling delay circuit  51  and a delayed pulse signal PS 1 D is outputted from the falling delay circuit  51 . The delayed pulse signal PS 1 D is inputted to the rising delay circuit  22  and a gate controlling signal SP 1  is outputted from the rising delay circuit  22 . The delayed pulse signal PS 1 D is inputted to the inverter  25  and an inverted delayed pulse signal PS 1 DB is outputted from the inverter  25 . The inverted delayed pulse signal PS 1 DB is inputted to the rising delay circuit  23  and a gate controlling signal SN 1  is outputted from the rising delay circuit  23 . 
         [0091]    In addition, a pulse signal PS 2  is inputted to the rising delay circuit  28  and a gate controlling signal SP 2   b  is outputted from the rising delay circuit  28 . The pulse signal PS 2  is inputted to the falling delay circuit  52  and a delayed pulse signal PS 2 D is outputted from the falling delay circuit  52 . The delayed pulse signal PS 2 D is inputted to the rising delay circuit  21  and a gate controlling signal SP 2  is outputted from the rising delay circuit  21 . The delayed pulse signal PS 2 D is inputted to the inverter  26  and an inverted delayed pulse signal PS 2 DB is outputted from the inverter  26 . The inverted delayed pulse signal PS 2 DB is inputted to the rising delay circuit  24  and a gate controlling signal SN 2  is outputted from the rising delay circuit  24 . 
         [0092]      FIG. 10  shows a block diagram of the falling delay circuit  51 . The falling delay circuit  51  comprises an inverter  45 , a capacitor  46 , and a Schmitt trigger inverter  47 . The inverter  45  comprises a transistor P 45  that is a PMOS transistor and a transistor N 45  that is an NMOS transistor. A size of the transistor N 45  is set larger than a size of the transistor P 45 . Therefore, impedance of the transistor N 45  in an on-state is set lower than impedance of the transistor P 45  in an on-state. 
         [0093]    The pulse signal PS 1  is inputted to an input terminal of the inverter  45 . An output terminal of the inverter  45  and an input terminal of the Schmitt trigger inverter  47  are connected to a first terminal of the capacitor  46 . A ground voltage GNDL is inputted to a second terminal of the capacitor  42 . The delayed gate controlling signal PS 1 D is outputted from the Schmitt trigger inverter  47 . 
         [0094]      FIG. 11  shows an operational waveform diagram of the falling delay circuit  51 . At time t 51 , as the pulse signal PS 1  transitions from a low level to a high level, the capacitor  42  is discharged through a current path constituted by the capacitor  46 , the transistor N 45 , and the ground voltage GNDL. In a similar manner, at time t 52 , as the pulse signal PS 1  transitions from a high level to a low level, the capacitor  46  is charged through a current path constituted by the supply voltage VDDL, the transistor P 45 , and the capacitor  46 . 
         [0095]    Furthermore, impedance of the transistor N 45  in the on-state is set lower than impedance of the transistor P 45  in an on-state. Therefore, a charge period of the capacitor  46  can be set longer than a discharge period of the capacitor  46 . Accordingly, the falling delay circuit  51  can perform an operation in which only a falling edge of the pulse signal PS   1   is outputted delayed by a delay time DT 2  while a rising edge of the pulse signal PS   1   is outputted without delay, Moreover, a length of the delay time DT 2  can be set to any value by adjusting a resistance value of the transistor P 45  and a capacitance value of the capacitor  46 . Since a configuration of the falling delay circuit  52  is similar to that of the falling delay circuit  51 , a detailed description will be omitted herein. In addition, since other parts of the configuration of the delay controlling circuit  14   b  are similar to those of the delay controlling circuit  14  according to the first embodiment ( FIG. 2 ), a detailed description will be omitted herein. 
         [0096]    The H-bridge circuit  19   b  ( FIG. 9 ) comprises transistors P 1 , P 2 , P 1   b , P 2   b , N 1 , and N 2 . The transistors P 1   b  and P 2   b  are PMOS transistors. The H-bridge circuit  19   b  is configured by adding transistors P 1   b  and P 2   b  to the H-bridge circuit  19  according to the first embodiment ( FIG. 2 ). 
         [0097]    The transistor P 1   b  is connected in parallel with the transistor P 1 . A drain terminal of the transistor P 1   b  is connected to a node ND 1 . The supply voltage VDDL is inputted to a source terminal of the transistor P 1   b . The gate controlling signal SP 1   b  inverted by the inviter  57  is inputted to a gate terminal of the transistor P 1   b  via the buffer  53 . The transistor P 1   b  enters an on-state when the gate controlling signal SP 1   b  is at a high level and enters an off-state when the gate controlling signal SP 1   b  is at a low level. The transistor P 2   b  is connected in parallel with the transistor P 2 . A drain terminal of the transistor P 2   b  is connected to a node ND 2 . The supply voltage VDDL is inputted to a source terminal of the transistor P 2   b . The gate controlling signal SP 2   b  inverted by the inverter  58  is inputted to a gate terminal of the transistor P 2   b  via the buffer  55 . 
         [0098]    A size of the transistor P 1  b is set smaller than a size of the transistor P 1 . Therefore, impedance of the transistor P 1   b  in the on-state is set higher than impedance of the transistor P 1  in the on-state. In a similar manner, a size of the transistor P 2   b  is set smaller than a size of the transistor P 2 . Therefore, impedance of the transistor P 2   b  in the on-state is set higher than impedance of the transistor P 2  in the on-state. Moreover, since other parts of the configuration of the H-bridge circuit  19   b  are similar to those of the H-bridge circuit  19  according to the first embodiment ( FIG. 2 ), a detailed description will be omitted herein. 
         [0099]    Operations of the sending circuit  10   b  will be described using an operational waveform diagram shown in  FIG. 12 . A period PE 1  is a period during which an input voltage VIN is at a high level and a period PE 2  is a period during which the input voltage VIN is at a low level. Moreover, a dead time period of a delay time D 11  is formed by the rising delay circuits  21  to  24 ,  27 , and  28  of the delay controlling circuit  14   b . A mechanism of a formation of the dead time period is similar to the mechanism described in the first embodiment. Therefore, for the sake of simplicity, a description of the dead time period will he omitted in the description of  FIG. 12 . 
         [0100]    Operations during the period PE 1  will now be described. At time t 21 , in response to a rising edge of the input voltage VIN, the pulse signal PS 1  transitions to a high level (arrow Y 20 ). In response to a rising edge of the pulse signal PS 1 , the gate controlling signals SP 1  and SP 1   b  transition to a high level at the same time (arrow Y 21 ). Therefore, the transistors P 1  and P 1   b  are turned on. In addition, in response to a rising edge of the pulse signal PSI, the gate controlling signal SN 1  transitions to a low level. Therefore, the transistor N 1  is turned off. Accordingly, a first current path via the transistor P 1  and a second current path via the transistor P 1   b  are formed. The first current path is a path from the supply voltage VDDL, to the ground voltage GNDL via the transistor P 1 , the node ND 1 , the transformer TR, the node ND 2 , and the transistor N 2 . In addition, the second current path is a path from the supply voltage VDDL to the ground voltage GNDL via the transistor P 1   b , the node ND 1 , the transformer TR, the node ND 2 , and the transistor N 2 . Furthermore, a coil current I 1  flows through both the first current path and the second current path. At this point, since the coil current I 1  flows through two paths, joint impedance of the current paths is in a low state. Therefore, a rate of increase (di/dt) of the coil current I 1  increases. 
         [0101]    At a receiving coil L 2 , a receiving coil voltage Vd is obtained (arrow Y 22 ) in proportion to the rate of increase of the coil current I 1  flowing through a sending coil L 1 . A waveform of the receiving coil voltage Vd takes a ridge shape protruding upward as shown in the drawing and has an amplitude of AM 1 . Subsequently, when a cmp 1  detects that the receiving coil voltage Vd has exceeded a threshold Vthp, the receiving circuit  30  determines that the coil current I 1  in a positive direction (in  FIG. 2 , a direction coinciding with an arrow of the coil current I 1 ) has been generated. Therefore, a detection is made that a rising edge of the input voltage VIN has been inputted to the sending circuit  10   b  and the output voltage VOUT is set to a high level (arrow Y 23 ). 
         [0102]    At time t 22 , in response to a falling edge of the pulse signal PS 1 , the gate controlling signal SP 1   b  transitions to a low level (arrow Y 24 ). Accordingly, since the transistor P   1   b is turned off and the second current path is blocked, a current path of the coil current I 1  is limited to only the first current path. As a result, since joint impedance of the current paths is in a high state, the coil current I 1  starts to decrease. 
         [0103]    In addition, at time t 23 , the gate controlling signal SP 1  transitions to a low level after a delay of a delay time DT 2  from a falling edge of the pulse signal PS 1   b , As a result, the transistor P 1  is turned off and the first current path is blocked. Accordingly, since both the first current path and the second current path are blocked, the coil current I 1  starts to decrease at an even greater gradient. In addition, at time t 23 , since the gate controlling signal SN 1  transitions to a high level, the transistor N 1  is turned on. Furthermore, at time t 24 , the coil current I 1  becomes 0. 
         [0104]    The receiving coil voltage Vd is obtained in proportion to a rate of decrease (di/dt) of the coil current I 1  from the high pass filter  32  (arrow Y 25 ). A waveform of the receiving coil voltage Vd takes a valley shape protruding downward as shown in the thawing and has an amplitude of AM 2 . 
         [0105]    In addition, a gradient of decrease of the coil current I 1  from time t 22  to time t 24  is set smaller than a gradient of increase of the coil current I 1  from time t 21  to time t 22 . Therefore, with the receiving coil voltage Vd, the amplitude AM 2  during the decrease of the coil current I 1  becomes smaller than the amplitude AM 1  during the increase of the coil current I 1 . 
         [0106]    Moreover, a size of the transistor P 1  is set larger than a size of the transistor P 1   b . In addition, the transistor P 1   b  is turned off first and then the transistor P 1  is turned off. Therefore, control can be performed so that a gradient of decrease of the coil current I 1  from time t 22  to time t 23  becomes smaller than a gradient of decrease of the coil current I 1  from time t 23  to time t 24 . 
         [0107]    Moreover, operations similar to the period PE 1  are performed during the period PE 2 . Therefore, a gradient of decrease of the coil current I 1  from time t 27  to time t 29  is set smaller than a gradient of increase of the coil current I 1  from time t 26  to time t 27 . As a result, with the receiving coil voltage Vd, the amplitude AM 2  during the decrease of the coil current I 1  becomes smaller than the amplitude AM 1  during the increase of the coil current IL Since contents of operations during the period PE 2  are similar to contents of operations during the period PE 1 , a detailed description will be omitted herein. 
         [0108]    An effect of the sending circuit  10   a  according to the third embodiment will be described. The receiving coil voltage Vd is proportional to a time rate of change (di/dt) of the coil current I 1  flowing through the sending roil L 1 . In addition, during the period PE 1 , the transistors P 1  and P 1   b  are controlled so as to be turned on at the same time when entering an on-state (time t 21 ) and turned of at different timings (time t 22  and time t 23 ) when entering an off-state. In a similar manner, during the period PE 2 , the transistors P 2  and P 2   a  are controlled so as to be turned on at the same time when entering an on state (time t 21 ) and turned off at different timings (time t 22  and time t 23 ) when entering an off-state. 
         [0109]    As a result, the time rate of change of the coil current I 1  flowing through the sending coil L 1  can be controlled so that a rate of decrease in an off-state of a transistor is reduced with respect to a rate of increase in an on-state of the transistor. Accordingly, the amplitude AM 1  of the receiving coil voltage Vd generated during an on-state of a transistor can be set greater than the amplitude AM 2  of the receiving coil voltage Vd generated during an off-state of the transistor. Therefore, a waveform that appears in the receiving coil voltage Vd in accordance with a rising (time t 21 ) of the input voltage VIN can he arranged so as to have a ridge shape (from time t 21  to time t 22 ) having a large amplitude. In addition, a waveform that appears in the receiving coil voltage Vd in accordance with a falling (time t 26 ) of the input voltage VIN can be arranged so as to have a valley shape (from time t 26  to time t 27 ) having a large amplitude. Consequently, by having a comparator cmp 1  with a threshold of Vthp detect the receiving coil voltage Vd exceeding the threshold Vthp, a rising of the input voltage VIN can be detected. In addition, by having a comparator cmp 2  with a threshold of Vth detect the receiving coil voltage Vd falling below the threshold Vthn, a falling of the input voltage VIN can be detected. Therefore, an on-state of a transistor can be more easily detected at the receiving coil L 2 . Consequently, an output voltage VOUT can be more reliably restored. 
         [0110]    Furthermore, when controlling the time rate of change of the coil current I 1  flowing through the sending coil L 1  by gradually turning off a transistor, generally, a midpoint voltage of on/off voltages of the transistor must be used. Since the midpoint voltage is a state in which the transistor is unstable, noise may be generated and a signal may be erroneously transmitted. On the other hand, since a control of gradually turning off a transistor need not be performed with the sending circuit  10   b  according to the present application, a midpoint voltage of on/off voltages of the transistor need not be used. Accordingly, since control of the time rate of change of the coil current H flowing through the sending coil L 1  can be realized in a state in which the transistor is stable, generation of noise can be suppressed. 
         [0111]    While specific embodiments of the present application have been described in detail above, such description is for illustrative purposes only and is not intended to limit the scope of claims. Techniques described in the claims include various modifications and changes made to the specific examples illustrated above. 
         [0112]    The H-bridge circuit  19   a  according to the second embodiment comprises the transistor N 3  connected in parallel with the transistor N 1  and the transistor N 4  connected in parallel with the transistor N 2 . In addition, the transistors N 3  and N 4  are controlled to constantly be in the on-state. In this case, as shown in an H-bridge circuit  19   c  in  FIG. 13 , resistive elements R 3  and R 4  may be added to drain terminals of the transistors N 3  and N 4 . Accordingly, impedance of the transistors N 3  and N 4  in the on-state can be increased. Consequently, a value of a through current flowing from the transistor P 1  to the transistor N 3  and a value of a through current flowing from the transistor P 2  to the transistor N 4  can be sufficiently reduced. In addition, the impedance of the transistors N 3  and N 4  in the on-state can be adjusted by the resistive elements R 3  and R 4 . Therefore, the on-state impedance need not be adjusted by adjusting the size of the transistor. As a result, sizes of the transistors N 3  and N 4  can be set arbitrarily. 
         [0113]    In addition, while a case of the H-bridge circuit  19   c  shown in  FIG. 13  comprising transistors N 3  and N 4  which are constantly turned on has been described, modes are not limited thereto. The H-bridge circuit  19   c  may be configured without the transistor N 3  so that a node ND 1  is connected to a ground voltage GNDL solely by the resistive element R 3 . Alternatively, a configuration may be adopted in which the transistor N 4  is not provided and the node ND 1  is connected to the ground voltage GNDL solely by the resistive clement R 4 . Even with these configurations, both the first terminal E 11  and the second terminal E 12  of the sending coil L 1  can constantly be kept in a low impedance state. 
         [0114]    Furthermore, while a case has been described in the first embodiment ( FIG. 6 ) in which control is performed so that both transistors N 1  and N 2  are turned on during a period from tune t 4  to time  16  in the period PE 1  (area A 1 ) and a period from time t 9  to time t 10  in the period PE 2  (area A 2 ), modes are not limited thereto. Control may be performed so that both transistors P 1  and P 2  are turned on during these periods. The object of setting voltages of both terminals of the sending coil L 1  to low impedance can also be achieved by this configuration. However, it is more favorable to turn on the transistors N 1  and N 2  during these periods. This is because source voltages of the transistors N 1  and N 2  are fixed by the ground voltage GNDL. Accordingly, since the transistors N 1  and N 2  can be reliably operated even in a state in which the supply voltage VDDL is unstable such as during activation or falling of the signal transmitting circuit  1 , the H-bridge circuit  19  can be expected to operate in a stable manner. 
         [0115]    Moreover, while a case has been described in the second embodiment in which voltages of both terminals of the sending coil L 1  is maintained at the ground voltage GNDL, modes are not limited thereto. The voltages of both terminals of the sending coil L 1  may be constantly maintained at the supply voltage VDDL. The object of setting voltages of both terminals of the sending coil L 1  to low impedance can also be achieved by this configuration. Consequently, an advantageous effect of the present application of suppressing generation of the noise voltage due to the common mode voltage VCM can be achieved. 
         [0116]    In addition, while a case has been described in the third embodiment in which a single transistor P lb is connected in parallel with the transistor P 1  and a single transistor P 2   b  is connected in parallel with the transistor P 2 , modes are not limited thereto. Two or more transistors may be connected in parallel with the transistor P 1  and two or more transistors may be connected in parallel with the transistor P 2 . Furthermore, the transistors connected in parallel may be turned off at respectively different timings. Accordingly, the time rate of change of the coil current I 1  can be precisely controlled. Moreover, by arranging sizes of the transistors connected in parallel with the transistors P 1  and P 2  so as to differ from each other, the time rate of change of the coil current I 1  can be more precisely controlled. 
         [0117]    In addition, while a case has been described in the third embodiment in which a time rate of change in an off-state is controlled by turning on the transistors P 1  and P 1   b  at the same time when entering an on-state and turning off the transistors P 1  and P 1   b  at different timings when entering an off-state, modes are not limited thereto. The time rate of change in an on-state can also be controlled by turning on the transistors P 1  and P 1   b  at different timings when entering an on-state and turning off the transistors P 1  and P 1   b  at the same time when entering an off-state. 
         [0118]    Furthermore, it is to he understood that the technical elements described in the present specification and the drawings exhibit technical usefulness solely or in various combinations thereof and shall not be limited to the combinations described in the claims at the time of filing. The techniques illustrated in the present specification and the drawings are to achieve a plurality of objectives at the same time, and technical usefulness is exhibited by attaining any one of such objectives.