Abstract:
In aspects of the invention, a photocoupler output signal receiving circuit includes a first constant current circuit, connected between an input terminal and the high potential side of a direct current power source, that discharges current, a second constant current circuit, connected between the input terminal and the low potential side of the direct current power source, that takes in current, and switching elements that operate the first and second constant current circuits in a complementary way, wherein the switching elements are operated so that current is taken in by the second constant current circuit after a photocoupler is turned on, and are operated so that current is discharged by the first constant current circuit after the photocoupler is turned off, and a discharge current value in a current discharge period is reduced after a certain period elapses from the start of discharging.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of International Application No. PCT/JP2012/53367, filed on Feb. 14. 2012, which is based on and claims priority to Japanese Patent Application No. JP 2011-223598, filed on Oct. 11, 2011. The disclosure of the Japanese priority application and the PCT application in their entirety, including the drawings, claims, and the specification thereof, are incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    Embodiments of the invention relate to photocoupler output signal receiving circuits used for isolating and transmitting signals in an intelligent power module (hereafter abbreviated to IPM). 
         [0004]    2. Related Art 
         [0005]    PWM (Pulse Width Modulation) inverters are widely used as power conversion devices that drive a three phase alternating current motor. Also, PWM converters are widely used as power conversion devices that obtain direct current voltage from a three phase alternating current power source. These power conversion devices are configured of a power circuit that carries out a direct current to alternating current or alternating current to direct current power conversion, a pre-driver that controls and drives the power circuit, a protective circuit, and a control circuit that centrally controls the circuits. 
         [0006]    The power circuit is a circuit that supplies a drive current to the three phase alternating current motor and generates a main direct current power source, and is configured of a switching element such as an IGBT (insulated gate bipolar transistor), and a free wheel diode for causing energy when the switching element is turned off to flow as current. Also, the pre-driver is a circuit for driving the switching element. Furthermore, the protective circuit is a circuit that has various kinds of protective function, such as overcurrent protection, short circuit protection, overheat protection, and current and voltage drop protection. 
         [0007]    The control circuit is configured of a controller and a central processing unit (CPU/ROM) including a memory such as a ROM (read only memory), carries out processes such as generating a PWM signal and issuing an alarm when detecting a problem, and centrally controls the PWM inverter or PWM converter. 
         [0008]    Recently, a semiconductor device called an IPM in which, of the configurations, the power circuit, drive circuit, and protective circuit are incorporated as one package, has become commercially available, and is being widely used. 
         [0009]    Herein,  FIG. 7  is a block diagram showing a circuit configuration of an inverter device into which an IPM is incorporated. 
         [0010]    The inverter device is connected to a two phase or three phase alternating current power source, and includes a converter  1  that converts alternating current into direct current, an electrolytic capacitor  2  for smoothing, an IPM  3 , a buffer  4 , a controller  5 , a central processing unit (CPU/ROM)  6  including a memory, a switching transistor  7 , a power source circuit formed of transformers  8  and  9  and a switching regulator  10 , and a current transformer CT connected between the output side of the IPM  3  and a three phase alternating current motor M. 
         [0011]    The controller  5  and central processing unit (CPU/ROM)  6  will be collectively called a control circuit  24 . 
         [0012]    The IPM  3  is integrally configured of a three phase inverter  11 , configured of a semiconductor switching element such as an IGBT and a free wheel diode, whose alternating current output side is connected to the motor M, a pre-driver  12  that controls and drives the inverter  11 , a protective circuit  13 , an overcurrent detection sensor  14  and overheat detection sensor  15  connected to the protective circuit  13 , a brake power element  16  and resistor  17  used when controlling the motor M so as to decelerate, and a pre-driver  18  that controls and drives the brake power element  16 . Herein, a depiction of a signal line that sends a drive signal from the pre-driver  12  to the inverter  11  is omitted. 
         [0013]    A control signal to the IPM  3  is sent from the control circuit  24  via the buffer  4  and photocouplers  21  and  23  to the pre-drivers  12  and  18  respectively, while an alarm signal when an overcurrent condition or overheat condition is detected by the sensor  14  or  15  is sent from the protective circuit  13  via the photocoupler  22  to the buffer  4 . 
         [0014]    Also, an output signal of the current transformer CT is input into the controller  5 . As the current transformer CT carries out various kinds of control by detecting the output current of the IPM  3  and feeding back to the controller  5 , it includes three through holes, and is installed inside the inverter device in a condition wherein wires or bars, which are the three phases of output current line of the inverter  11 , are inserted into the through holes. 
         [0015]    The inverter device with the heretofore described configuration is such that a direct current voltage converted by the converter  1  is converted into a three phase alternating current voltage by the inverter  11 , and supplied to the motor M. The inverter  11  has a bridge circuit formed of a switching element and a free wheel diode, wherein the switching element converts a current eventually caused to flow through the motor M into an alternating current by carrying out a chopping control on the direct current voltage, and controls the motor M at variable speeds by changing the frequency of the alternating current. 
         [0016]    Also, the controller  5  controls so that no distortion occurs in an inverter  11  output current waveform detected by the current transformer CT, and controls so that the output current does not reach or exceed a predetermined value. 
         [0017]    The pre-drivers  12  and  18 , which receive a control signal from the control circuit  24  after it is isolated by the photocouplers  21  and  23 , include a current discharge function and a current intake function with respect to the photocouplers. 
         [0018]      FIG. 8  is a block diagram of an input circuit of, for example, the pre-driver  12 , with the input circuit of the pre-driver  18  also being configured in the same way as in  FIG. 8 . In  FIG. 8 ,  25  indicates a direct current power source (a power source voltage V cc  is, for example, 15V),  121  inside the pre-driver  12  is a constant current circuit (discharge side),  122  is a constant current circuit (intake side),  123  is a power source terminal to which the power source voltage V cc  is applied,  124  is an input terminal to which an output signal V in  of the photocoupler  21  is applied,  125  is a ground terminal, R pu  is a pull-up resistor, ZD is a Zener diode, and SW 1  and SW 2  are switches (semiconductor switching elements) that operate in a mutually complementary way.  21   a  inside the photocoupler  21  is a light emitting diode acting as a light emitting unit, and  21   b  is a phototransistor acting as a light receiving unit. 
         [0019]    The output signal V in  will hereafter be called an “input signal V in ” when seen from the pre-driver  12  side. 
         [0020]    Herein, the current discharge function and current intake function of the pre-driver  12  have an object of improving noise resistance. 
         [0021]    That is, as shown in  FIG. 9 , while the voltage level of the input signal V in  in  FIG. 8  exceeds a threshold value V inL  (before a time t 1 ), a current I inH  flows as a discharge current from the input terminal  124  to the exterior via the constant current circuit  121  and switch SW, of  FIG. 8 , and the photocoupler  21  will not reach the threshold value V inL  unless it draws in more current. 
         [0022]    Meanwhile, when the voltage level of the input signal V in  drops below the threshold value V inL  at the time t 1 , the pre-driver  12  stops the discharge of the current I inH . Then, until the voltage level of the input signal V in  exceeds a threshold value V inH  at a time t 2 , a current I inL  flows as an intake current from the input terminal  124  to the interior (times t 1  to t 2 ) via the constant current circuit  122  and switch SW 2  of  FIG. 8 , and the photocoupler  21  will not reach the threshold value V inH  unless it draws in more current. 
         [0023]    That is, the pre-driver  12  carries out a discharge or intake of current in accordance with the voltage level of the input signal V in  in order to improve the noise resistance of the photocoupler  21 , with a maximum of the discharge current I inH  of  FIG. 9  flowing through the collector of the phototransistor  21   b.    
         [0024]    An isolating type signal transmission circuit, wherein a drive signal to an IPM switching element is isolated and transmitted by a photocoupler, is described in Japanese Patent Publication No. JP-A-2007-150003 (also referred to herein as “Patent Document 1”). The signal transmission circuit is such that, in order to prevent a drop in current conversion efficiency due to temporal depreciation of the photocoupler when routinely increasing the intake current, or the like, a transient current with an amplitude larger than normal is caused to flow through the light emitting diode when a switching element connected in series to the light emitting diode of the photocoupler is off, and in the same way, when a transistor inside a constant current circuit connected in series to the light emitting diode is maintained in a saturated condition, and the switching element is turned on. 
         [0025]    Also, there is disclosed in Japanese Patent Publication No. JP-A-2008-172513 (also referred to herein as “Patent Document 2”) an invention that, with an object of increasing photocoupler lifespan, includes a current compensation circuit that detects a temporal depreciation signal having a correlation with characteristic depreciation of a photocoupler light emitting diode, and increases the on-state current of the light emitting diode in accordance with the condition thereof. 
         [0026]    As is also described in Patent Documents 1 and 2, the photocoupler characteristics depreciate with the passing of time, and when the internal light emitting diode is used for a long time, the light emitting efficiency decreases, and a current conversion efficiency CTR (=I c /I f ) drops. Herein, I f  is the input current flowing through the light emitting diode, and I c  is the output current (collector current) of the phototransistor on the output side. 
         [0027]    When actually using the photocoupler, the estimated lifespan of the photocoupler is calculated from the current conversion efficiency CTR, the load design values, and the like, and I f  is determined. In order to increase the lifespan of the photocoupler, it is necessary to suppress the size of I f . However, when I f  is too small, noise resistance drops, and operating speed drops. 
         [0028]      FIG. 10  shows V ce  (the voltage between the collector and emitter of the phototransistor)—I c  (the phototransistor collector current) characteristics, with the current I f  flowing through the photocoupler light emitting diode as a parameter. 
         [0029]    As previously described, in order to increase the phototransistor collector current I c  to improve the pre-driver noise resistance, it is necessary to increase the current I f  flowing through the light emitting diode. 
         [0030]    Meanwhile,  FIG. 11  shows lifespan depreciation characteristics of the photocoupler current conversion efficiency CTR, wherein the tendency is that the more the current I f  increases, the shorter the time in which the current conversion efficiency CTR drops. That is, as the current conversion efficiency CTR drops in a short time when routinely increasing the phototransistor collector current, in other words, the light emitting diode current I f , in order to improve noise resistance, the current drawing capability of the photocoupler after the photocoupler is turned on drops. Because of this, a long time is needed until the input signal voltage level drops below the threshold value V inL  of  FIG. 9 , as a result of which, a PWM signal for driving the switching element is not accurately transmitted from the pre-driver, causing malfunction. 
         [0031]    Consequently, it cannot be said that the method whereby the light emitting diode current I f  is increased, as in the heretofore known technology according to Patent Document 1 or Patent Document 2, is optimal for preventing a drop in the current conversion efficiency CTR or preventing malfunction. Thus, there are needs in the art for an improved a photocoupler output signal receiving circuit. 
       SUMMARY OF THE INVENTION 
       [0032]    Embodiments of the invention address these and other needs. Embodiments of the invention provide a photocoupler output signal receiving circuit such that, with noise resistance secured without increasing the light emitting diode current, it is possible to increase photocoupler lifespan, prevent circuit malfunction, and the like. 
         [0033]    In some embodiments, an output signal receiving circuit that receives an output signal of a photocoupler wherein a signal output from a light emitting unit is transmitted to a light receiving unit in an isolated state, and a pull-up resistor is connected to the output side of the light receiving unit. 
         [0034]    In some embodiments, a first constant current circuit, connected between an input terminal into which the output signal is input and the high potential side of a direct current power source, for discharging current to the light receiving unit, a second constant current circuit, connected between the input terminal and the low potential side of the direct current power source, for taking in current from the direct current power source, and switching elements that operate the first and second constant current circuits in a complementary way. 
         [0035]    In some embodiments, the switching elements are operated so that current is taken in by the second constant current circuit after the photocoupler is turned on, and are operated so that current is discharged by the first constant current circuit after the photocoupler is turned off, and a discharge current value in a current discharge period is reduced after a certain period elapses from the start of discharging. 
         [0036]    In some embodiments, in the output signal receiving circuit, the discharge current value after the certain period elapses is reduced to zero. 
         [0037]    In some embodiments, in the output signal receiving circuit, the first constant current circuit includes a current mirror circuit, and the discharge current is reduced by changing the bias current of the current mirror circuit. 
         [0038]    According to some embodiments, in a receiving circuit into which a photocoupler output signal is input, and which has a current discharge function and intake function with respect to the photocoupler, it is possible to reduce the value of current to be drawn in by the photocoupler by reducing the value of current discharged in order to pull up the voltage level of the input signal from the photocoupler after the elapse of a certain period from the current discharge point. Consequently, even with a photocoupler wherein temporal depreciation is advanced and a large current cannot be caused to flow, it is possible to reduce the input signal voltage level to the threshold value, and thus possible to increase the photocoupler lifespan and prevent circuit malfunction. 
         [0039]    Embodiments of the invention provide an advantage of reducing circuit current consumption. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0040]      FIG. 1  is a circuit diagram of an output signal receiving circuit according to an embodiment of the invention; 
           [0041]      FIG. 2  is a circuit diagram of a condition wherein the circuit of  FIG. 1  is installed in a pre-driver; 
           [0042]      FIG. 3  is a diagram showing an input signal voltage level and a current waveform in the embodiment and an Example 1 of the invention; 
           [0043]      FIG. 4  is a circuit diagram of an output signal receiving circuit according to the Example 1 of the invention; 
           [0044]      FIG. 5  is a circuit diagram of an output signal receiving circuit according to an Example 2 of the invention; 
           [0045]      FIG. 6  is a diagram showing an input signal voltage level and a current waveform in the Example 2 of the invention; 
           [0046]      FIG. 7  is a block diagram showing an inverter device circuit configuration; 
           [0047]      FIG. 8  is a circuit diagram of an input unit of a pre-driver in  FIG. 7 ; 
           [0048]      FIG. 9  is a diagram showing an input signal voltage level and a current waveform of a pre-driver in  FIG. 8 ; 
           [0049]      FIG. 10  shows V ce -I c  characteristics, with a current I f  flowing through a photocoupler light emitting diode as a parameter; and 
           [0050]      FIG. 11  is a diagram showing lifespan depreciation characteristics of a photocoupler current conversion efficiency. 
       
    
    
     DETAILED DESCRIPTION 
       [0051]    Hereafter, referring to the drawings, a description will be given of an embodiment of the invention. As an output signal receiving circuit according to the embodiment is applied to an input circuit of pre-drivers  12 ,  18  and the like in the previously described  FIG. 7 , circuit components having the same functions as those of previously described circuit components are given the same reference numerals and signs. 
         [0052]    First, heretofore known technology of  FIG. 8  is such that, when a photocoupler  21  is off and an input signal voltage level exceeds a threshold value V inH  at a time t 2  of  FIG. 9 , a switch SW 1  is turned on, a constant current circuit  121  discharges a current I inH  to the exterior, and this condition is continued until the input signal voltage level becomes lower than a threshold value V inL  at a time t 1 . That is, the heretofore known technology is such that, as the large current I inH  flows for a long time in a current discharge period, the current value to be drawn in by the photocoupler  21  the next time the photocoupler  21  is turned on, in other words, the current flowing through a light-emitting diode  21   a,  also increases. 
         [0053]    As opposed to this, the embodiment of the invention is such that, by reducing the discharge current value after a certain period has elapsed from a time t 2 , as shown in  FIG. 3  to be described hereafter, the current value to be drawn in the next time the photocoupler  21  is turned on is reduced. Because of this, even in the event of the photocoupler  21  depreciating with time, a current conversion efficiency CTR decreasing, and the collector current of a phototransistor  21   b  decreasing, the embodiment is such that it is possible to realize a more stable operation than in a case of maintaining the discharge current at a large value, as heretofore known. Also, the embodiment is such that it is possible to prevent a circuit malfunction without increasing the current caused to flow through the light emitting diode  21   a,  and to increase the lifespan of the photocoupler. 
         [0054]    At the same time, according to the embodiment, as the discharge current is reduced overall, it is also possible to reduce power consumption. 
         [0055]      FIG. 1  shows a configuration of an output signal receiving circuit  30  according to the embodiment of the invention. The output signal receiving circuit  30  configures one kind of constant current circuit driven by an input signal V in  input into a pre-driver from the phototransistor  21   b  of the photocoupler  21 . The constant current circuit has a current discharge function and a current intake function with respect to the photocoupler  21 . 
         [0056]    In  FIG. 1 , a power source terminal  123  to which a power source voltage V cc  is applied is connected via the source and drain of a first switching element MP 1  to first and second current sources I 1  and I 2 , and the output sides of the current sources I 1  and I 2  are connected via the drain and source of third and fourth switching elements NM 1  and NM 2  to a ground terminal  125 . Also, the power source terminal  123  is connected via the source and drain of a second switching element MP 2  to a pre-driver input terminal  124  and a third current source I 3 , and the output side of the current source I 3  is connected via the drain and source of a fifth switching element NM 3  to the ground terminal  125 . Furthermore, the gates of the first and second switching elements MP 1  and MP 2  are collectively connected to a contact point of the first and second current sources I 1  and I 2 . 
         [0057]    Herein, the first and second switching elements MP 1  and MP 2  are, for example, p-channel enhancement type MOSFETs (Metal Oxide Semiconductor Field Effect Transistors), and the switching elements MP 1  and MP 2  configure a current mirror circuit. Also, the third, fourth, and fifth switching elements NM 1 , NM 2 , and NM 3  are configured of n-channel enhancement type MOSFETs. 
         [0058]    The input signal V in  to the pre-driver is applied via a delay circuit DLY to the gate of the third switching element NM 1 , and is applied directly to the gate of the fourth switching element NM 2 . Also, the input signal V in  is also applied via a NOT circuit NOT to the gate of the fifth switching element NM 3 . 
         [0059]    In the heretofore described configuration, the switching elements MP 1  and MP 2  and power sources I 1  and I 2  correspond to a first constant current circuit in claim  1 , and in the same way, the current source I 3  corresponds to a second constant current circuit. Also, the first and second constant current circuits also correspond to the constant current circuits  121  (discharge side) and  122  (intake side) respectively in  FIG. 8 . 
         [0060]    Furthermore, the switching elements NM 1  to NM 3  correspond to switching elements that operate the first and second constant current circuits in a complementary way. 
         [0061]    Next,  FIG. 2  is a circuit diagram showing a condition wherein the output signal receiving circuit  30  of  FIG. 1  is installed in a pre-driver  40 . Although the pre-driver  40  includes circuits that generate a drive signal (gate signal) of an inverter  11  in  FIG. 7 , a depiction of these circuits is omitted from  FIG. 2 . 
         [0062]    In  FIG. 2 ,  26  is a PWM signal generator circuit connected to the light emitting diode  21   a,  which acts as a light emitting unit of the photocoupler  21 , R pu  is a pull-up resistor connected between a collector of the phototransistor  21   b,  which acts as a light receiving unit of the photocoupler  21 , and the positive electrode of a direct current power source  25 , and ZD is a Zener diode, in the same way as in  FIG. 8 . 
         [0063]    Also, on the input side of the pre-driver  40 ,  27  and  28  are resistors that divide the power source voltage V cc , and the voltage dividing point thereof is input into an inverting input terminal of a comparator  29 . Furthermore, the output of the collector of the phototransistor  21   b  is input into the non-inverting input terminal of the comparator  29 , and the output thereof is sent to the output signal receiving circuit  30  as the input signal V in . 
         [0064]    Next, an operation of the embodiment will be described, while referring to  FIG. 3 .  FIG. 3  shows the voltage level of the input signal V in  and a current waveform. 
         [0065]    When the photocoupler  21  of  FIG. 2  switches from on to off, the voltage level of the input signal V in  rises, and as the input signal voltage level is pulled up by the pull-up resistor R pu  when the voltage level exceeds a threshold value V inH  at a time t 2  of  FIG. 3 , the operation of the output signal receiving circuit  30  switches from a current intake to a current discharge. In the circuits of  FIG. 1  and  FIG. 2 , the switching element NM 3  to whose gate the input signal V in  is applied via the NOT circuit NOT switches from on to off, and conversely, the switching elements NM 1  and NM 2  switch from off to on. 
         [0066]    As the input signal V in  is input via the delay circuit DLY into the gate of the switching element NM 1 , the switching element NM 1  is turned off at a time t 3  at which a certain period (a delay circuit DLY delay time) has elapsed from the time t 2 . 
         [0067]    Herein, as the switching elements MP 1  and MP 2  configure a current mirror circuit, a current (k×I bias ) of a size k (k is a proportional invariable) proportional to a current I bias  flowing through the switching element MP 1  flows through the switching element MP 2  when current is discharged, and the current is discharged to the input terminal  124  as a discharge current I inH . 
         [0068]    As the embodiment is such that, immediately after the input signal voltage level exceeds the threshold value V inH  at the time t 2  of  FIG. 3  and the operation switches to a current discharge, the operation is such as to discharge a greater current in order to pull the input signal voltage level up more quickly, the switching elements NM 1  and NM 2  are turned on, and a current that is the sum of the current sources I 1  and I 2  is caused to flow as I bias  through the switching element MP 1 . By causing a greater current to flow through the switching element MP 1  in this way, a current flowing through the other switching element MP 2 , that is, the discharge current I inH , also increases. 
         [0069]    Subsequently, by the input signal voltage level being sufficiently pulled up, and by the switching element NM 1  being turned off by an operation of the delay circuit DLY at the time t 3  within a time shorter than one half of a PWM signal cycle, the current flowing through the switching element MP 1  is reduced, and the discharge current of the switching element MP 2  is reduced as far as I inHD  of  FIG. 3 . 
         [0070]    When the photocoupler  21  switches from off to on, the voltage level of the input signal from the photocoupler  21  drops, and when the voltage level drops below a threshold value V inL  at a time t 1  of  FIG. 3 , the operation of the output signal receiving circuit  30  switches from a current discharge to a current intake. In the circuits of  FIG. 1  and  FIG. 2 , the switching element NM 1  to whose gate the input signal V in  is applied via the delay circuit DLY, and the switching element NM 2  to whose gate the input signal V in  is applied directly, switch from on to off, and the switching element NM 3  to whose gate the input signal V in  is applied via the NOT circuit NOT switches from off to on. 
         [0071]    As the switching elements NM 1  and NM 2  are off at this time, no current flows through the switching elements MP 1  and MP 2 , and meanwhile, as the switching element NM 3  is on, an intake current I inL  flows from the input terminal  124  to the switching element NM 3  side for a period of the times t 1  to t 2  of  FIG. 3 . 
         [0072]    Herein, immediately after the photocoupler  21  switches from off to on, the input signal voltage level must be lowered by the phototransistor  21   b  of the photocoupler  21  drawing in the discharge current I inHD  for pulling up the input signal voltage level and a current I R  flowing through the pull-up resistor R pu . In this case, provided that the photocoupler is a sound one that has not suffered temporal depreciation, it is possible to cause the flow of a current sufficient to lower the input signal voltage level, but the value of current that can be caused to flow decreases with the advance of temporal depreciation, and the time needed to lower the input signal voltage level increases. Then, it eventually becomes impossible to lower the input signal voltage level as far as V inL  within the PWM signal cycle, which reduces the lifespan of the photocoupler. 
         [0073]    However, the embodiment is such that, as the value of current discharged in order to pull up the voltage level is reduced from I inH  to I inHD , as previously described, the value of current to be drawn in by the photocoupler  21  decreases. Consequently, even with a photocoupler in which temporal depreciation is advanced, it is possible to lower the input signal voltage level, as a result of which, it is possible to increase the lifespan of the photocoupler  21 . 
       EXAMPLE 1 
       [0074]    Next,  FIG. 4  is a circuit diagram showing an output signal receiving circuit  31  according to an Example 1 of the invention. 
         [0075]    The example is such that n-channel depression type MOSFETs ND 1  to ND 3  are used as the current sources I 1  to I 3  of  FIG. 1 . The input signal voltage level and current waveform in the example are the same as those of  FIG. 3 . 
         [0076]    In the Example 1, V cc =15(V), the pull-up resistor R pu =90 (kΩ), the discharge current I inH =1 (mA), the discharge current I inHD  after the below-mentioned delay time has elapsed=200 (μA), the intake current I inL =50 (μA), V inL =1.5 (V), V inH =2 (V), and the delay circuit DLY delay time=3 (μsec). 
         [0077]    As is clear from the embodiment of  FIG. 1  to  FIG. 3 , when the photocoupler  21  switches from on to off, the circuit operation switches from a current intake to a current discharge, and 1 (mA) is discharged as I inH . At this time, the switching element NM 3  switches from on to off, and the switching elements NM 1  and NM 2  switch from off to on. Subsequently, the switching element NM 1  is turned off 3 (μsec) later by an operation of the delay circuit DLY. 3 (μsec) is a sufficiently short time when compared to the cycle of the PWM signal used in an IPM. 
         [0078]    Herein, the gate channel lengths of the switching elements MP 1  and MP 2  configuring a current mirror circuit are the same, and the MP 1  to MP 2  gate channel width ratio is 1 to 5. In this case, when current is discharged, a current five times the current I bias  (=200 (μA)) flowing through the switching element MP 1 , that is 1 (mA), flows through the switching element MP 2 , and this current is discharged as the discharge current I inH  to the input terminal  124 . 
         [0079]    Immediately after the input signal voltage level exceeds the threshold value V inH , and the operation switches to a current discharge, the switching elements NM 1  and NM 2  are turned on. Because of this, a current (=200 (μA)) wherein a current I 1  (=160 (μA)) flowing through the switching element NM 1  and a current I 2  (=40 (μA)) flowing through the switching element NM 2  are added together flows through the switching element MP 1  as I bias . 3 (μsec) after this, by turning off the switching element NM 1  using an operation of the delay circuit DLY, the current I bias  flowing through the switching element MP 1  becomes only the 40 (μA) of the switching element NM 2 . Consequently, the current flowing through the switching element MP 2  becomes I inHD =40 (μA)× 5 =200 (μA) in accordance with the current mirror circuit ratio, and the discharged current decreases to one-fifth. 
         [0080]    When the photocoupler  21  switches from off to on, the input signal voltage level drops, and when the voltage level drops below the threshold value V inL , the circuit operation switches from a discharge to an intake. In  FIG. 4 , the switching element NM 2  switches from on to off, and the switching element NM 3  switches from off to on. As the switching elements NM 1  and NM 2  are off at this time, no current flows through the switching elements MP 1  and MP 2 , and as the switching element NM 3  is on, an intake current I inL =50 (μA) flows from the input terminal  124  to the switching element NM 3  side. 
         [0081]    Herein, immediately after the photocoupler  21  switches from off to on, the input signal voltage level must be lowered by the photocoupler  21  drawing in the total value of the current I inHD =200 (μA) discharged in order to pull up the input signal voltage level and the current I R =750 (μA) flowing through the pull-up resistor R pu . An initial condition photocoupler that has not suffered temporal depreciation has a PWM signal circuit designed so that a current in the region of 3 (mA) flows, and in an initial condition, it is possible to cause the flow of a current sufficient to lower the input signal voltage level. However, when temporal depreciation advances, and CTR is, for example, 50%, the current that can be drawn in by the photocoupler is in the region of 1.5 (mA). 
         [0082]    An operation of  FIG. 9  described as heretofore known technology is such that the current that must be drawn in by the photocoupler  21  is a current of 1.75 (mA) wherein I inH =1 (mA) and I R =750 (μA) are added together, and it is not possible to draw in sufficient current at 1.5 (mA), which is the limit that can be caused to flow in a photocoupler that has depreciated to a CTR of 50%. Because of this, it is not possible to reduce the input signal voltage level to the threshold value V inL , the PWM signal is not accurately transmitted by the pre-driver, and a malfunction occurs. As opposed to this, according to the circuit of the Example 1, the current that must be drawn in by the photocoupler  21  immediately after the photocoupler  21  switches from off to on is 950 (μA), which is the value of I inHD =200 (μA) and I R =750 (μA) added together. Therefore, response is quite sufficient even when the current that can be caused to flow in a photocoupler that has depreciated to a CTR of 50% is 1.5 (mA), and it is possible to reduce the input signal voltage level, and thus use the photocoupler for an increased lifespan. 
         [0083]    Also, the example also has the advantage of reducing current consumption. 
         [0084]    That is, in  FIG. 4 , I bias =200 (μA) flows as a current mirror circuit bias current for a period for which the output signal receiving circuit  31  discharges current. However, as the discharge current decreases to one-fifth 3 (μsec) after the circuit operation switches to the current discharge side, the bias current I bias  for causing the discharge current to flow also becomes one-fifth, after which it is possible to reduce the bias current consumed in a period until the operation switches to the current intake side. 
       EXAMPLE 2 
       [0085]      FIG. 5  is a circuit diagram showing an output signal receiving circuit  32  according to an Example 2 of the invention. The example is such that the switching elements ND 2  and NM 2  in the Example 1 of  FIG. 4  are eliminated. The Example 2 of  FIG. 5  is such that the reference numerals and signs of the switching elements ND 3  and NM 3  in  FIG. 4  are changed to ND 2  and NM 2  respectively, but the functions of the switching elements ND 3  and NM 3  in  FIG. 4  are the same as the functions of the switching elements ND 2  and NM 2  respectively in  FIG. 5 . 
         [0086]      FIG. 6  is a diagram showing the input signal voltage level and current waveform of the Example 2. 
         [0087]    In the Example 2, V cc =15(V), the pull-up resistor R pu =20 (kΩ), the discharge current I inH =1 (mA), the discharge current I inHD  after the below-mentioned delay time has elapsed=0 (A), the intake current I inL =50 (μA), V inL =1.5 (V), 
         [0088]    V inH =2 (V), and the delay circuit DLY delay time=5 (μsec). 
         [0089]    When the photocoupler  21  switches from on to off, the circuit operation switches from a current intake to a current discharge, and 1 (mA) is discharged as I inH  at a time t 2  of  FIG. 6 . At this time, the switching element NM 2  switches from on to off, and the switching element NM 1  switches from off to on. Subsequently, the switching element NM 1  is turned off of a time t 3  which is 5 (μsec) after the time t 2  by an operation of the delay circuit DLY. 5 (μsec) is a sufficiently short time when compared to the cycle of the PWM signal used in an IPM. 
         [0090]    The characteristics of the switching elements MP 1  and MP 2  configuring a current mirror circuit being the same as in the Example 1, a current of 1 (mA), five times the current I bias  (=200 (μA)) flowing through the switching element MP 1 , flows through the switching element MP 2 , and is discharged as the current I inH  to the input terminal  124 . 
         [0091]    Immediately after the input signal voltage level exceeds the threshold value V inH , and the operation switches to a current discharge, a current (=200 (μA)) flows via the switching element ND 1  through the switching element MP 1  as I bias  by the switching element NM 1  being turned on. When 5 (μsec) have elapsed after this, at a time t 3  of  FIG. 6 , the current I bias  flowing through the switching element MP 1  becomes zero by turning off the switching element NM 1  using an operation of the delay circuit DLY. Because of this, no current flows through the switching element MP 2  either, and the discharge current I inH  also becomes zero (I inHD =0), as shown in  FIG. 6 . 
         [0092]    When the photocoupler  21  switches from off to on, the input signal voltage level drops, and when the voltage level drops below the threshold value V inL , at a time t 1  of  FIG. 6 , the circuit operation switches from a discharge to an intake, and the switching element NM 2  switches from on to off. As the switching element NM 1  is off at this time, no current flows through the switching elements MP 1  and MP 2 , and as the switching element NM 2  is on, an intake current I inL =50 (μA) flows from the input terminal  124  to the switching element NM 2  side. 
         [0093]    Herein, immediately after the photocoupler  21  switches from off to on, the input signal voltage level must be lowered by the photocoupler  21  drawing in the current I R =750 (μA) flowing through the pull-up resistor R pu  in order to pull up the input signal voltage level. 
         [0094]    In the same way as previously described, in a condition in which the design is such that a current in the region of 3 (mA) flows through a photocoupler in an initial condition, the upper limit value of the current that can be drawn in by the photocoupler is 900 (μA) when temporal depreciation advances and CTR is, for example, 30%. 
         [0095]    In this case, as the example is such that the current that must be drawn in by the photocoupler  21  is only the previously mentioned I R =750 (μA), it is possible to draw in a sufficient current, thereby reducing the input signal voltage level, even when the photocoupler has depreciated to a CTR of 30%, and it is thus possible to use the photocoupler for an increased lifespan. 
         [0096]    However, as the Example 2 is such that a malfunction due to the effect of external noise is likely to occur because of reducing the discharge current to zero, it is necessary to use the photocoupler in a circuit wherein external noise is sufficiently suppressed. 
         [0097]    Also, although the example is such that 200 (μA) flows as the bias current I bias  when current is discharged, the discharge current I inH  becomes zero 5 (μsec) after the operation switches to the current discharge side. Consequently, the bias current I bias  for causing the discharge current to flow also becomes zero, after which it is possible to reduce the bias current consumed in a period until the operation switches to the current intake side, in the same way as in the Example 1. 
         [0098]    Embodiments of the invention can be utilized for increasing photocoupler lifespan and preventing malfunction not only in a pre-driver input circuit in an IPM, but in any kind of signal processing circuit that processes a signal isolated and transmitted by a photocoupler. 
         [0099]    Examples of specific embodiments are illustrated in the accompanying drawings. While the invention is described in conjunction with these specific embodiments, it will be understood that it is not intended to limit the invention to the described embodiments. On the contrary, it is intended to cover alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims. In the above description, specific details are set forth in order to provide a thorough understanding of embodiments of the invention. Embodiments of the invention may be practiced without some or all of these specific details. Further, portions of different embodiments and/or drawings can be combined, as would be understood by one of skill in the art.