Patent Publication Number: US-2016241242-A1

Title: Drive unit

Description:
INCORPORATION BY REFERENCE 
     The disclosure of Japanese Patent Application No. 2015-026858 filed on Feb. 13, 2015 including the specification, drawings and abstract is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a drive unit. 
     2. Description of Related Art 
     A drive unit is known that includes a reverse conducting transistor that has a transistor and a first diode that is connected in inverse-parallel to the transistor, the transistor and the first diode being provided on a common semiconductor substrate, and a second diode that has a cathode that is connected to a collector of the transistor (refer to Japanese Patent Application Publication No. 2014-216932 (JP 2014-216932 A), for example). This drive unit has a configuration in which a voltage V CE  between the collector and emitter of the transistor is detected via an anode of the second diode. 
     However, a forward voltage of the first diode and a forward voltage of the second diode respectively have the characteristic of changing with temperature (temperature characteristic). Thus, when the temperature of the first diode and the temperature of the second diode vary independently of each other, the forward voltage of the first diode and the forward voltage of the second diode also change independently of each other and the detection value of the voltage V CE  therefore varies widely. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the invention is to provide a drive unit in which the detection value of the voltage between a collector and an emitter does not vary widely. 
     A drive unit according to an aspect of the invention includes: a reverse conducting transistor including a transistor and a first diode being connected in inverse-parallel to the transistor, the transistor and the first diode being provided on a first semiconductor substrate; a second diode including a cathode being connected to a collector of the transistor, the second diode being provided on the first semiconductor substrate; and a detection portion configured to detect a voltage between the collector and an emitter of the transistor via an anode of the second diode. 
     According to the above aspect, because the first diode and the second diode are provided on a first semiconductor substrate, the difference between the temperature of the first diode and the temperature of the second diode decreases and these temperatures vary in an approximately similar fashion. Thus, even when each of the forward voltages of the first and second diodes changes with variation in temperature, the variation in the detection value of the voltage between the collector and emitter of the transistor decreases as compared to a case where the temperatures of the first and second diodes vary independently of each other. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein: 
         FIG. 1  is a diagram that illustrates one example of the configuration of a drive unit; 
         FIG. 2  is a diagram that illustrates another example of the configuration of a drive unit; 
         FIG. 3  is a diagram that illustrates another example of the configuration of a drive unit; 
         FIG. 4  is a diagram that illustrates one example of an arrangement position of a second diode; and 
         FIG. 5  is a diagram that illustrates one example of the configuration of a power converter that is equipped with a plurality of drive units. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Embodiments of the invention are hereinafter described with reference to the drawings. 
       FIG. 1  is a diagram that illustrates one example of the configuration of a drive unit  1  according to a first embodiment. The drive unit  1  is a semiconductor device that drives an inductive load (such as an inductor, motor or the like) that is connected to a first current path  15  or a second current path  16  by on-off driving of a reverse conducting transistor  14 , for example. 
     The first current path  15  is an electric wiring that is conductively connected to a source voltage VH of a high source potential part, such as a positive electrode of a power source, for example. The first current path  15  may be indirectly connected to the source voltage VH of the high source potential part via another switching element or load. The second current path  16  is an electric wiring that is conductively connected to a low source potential part, such as a negative electrode of a power source (for example, ground), for example. The second current path  16  may be indirectly connected to the low source potential part via another switching element or load. 
     One example of a device in which one or more drive units  1  are used is a power converter that converts electric power between input and output by on-off driving of the reverse conducting transistor  14 , for example. Specific examples of the power converter include a converter that increase or decrease the voltage of DC power, and an inverter that performs power conversion between DC power and AC power. 
     The drive unit  1  includes a semiconductor substrate  10 , and a drive circuit board  20  that is separate from the semiconductor substrate  10 . The semiconductor substrate  10  is a chip that has the reverse conducting transistor  14 , and a protection diode  12 , for example. The drive circuit board  20  is an integrated circuit (IC) that has a detection part  21 , a determination part  31 , and a drive part  27 , for example. 
     The reverse conducting transistor  14  is one example of a reverse conducting transistor that has a transistor  13  and a flyback diode  11  that are provided together on the common semiconductor substrate  10 . The transistor  13  has a gate G, a collector C, and an emitter E. The flyback diode  11  has an electrode that uses the emitter E of the transistor  13  as an anode, and an electrode that uses the collector C of the transistor  13  as a cathode. In other words, the reverse conducting transistor  14  is a switching element that has a structure in which a common electrode that serves as the emitter E of the transistor  13  and as the anode of the flyback diode  11  and a common electrode that serves as the collector C of the transistor  13  and the cathode of the flyback diode  11  are formed. The flyback diode  11  is one example of a first diode that is connected in inverse-parallel to the transistor  13 . 
     The reverse conducting transistor  14  is a reverse conducting insulated gate bipolar transistor (RC-IGBT) that uses the transistor  13  as an insulated gate bipolar transistor (IGBT), for example. An RC-IGBT is sometimes referred to as built-in diode IGBT. 
     The protection diode  12  is one example of a second diode that is provided on the common semiconductor substrate  10  on which the reverse conducting transistor  14  is provided. The protection diode  12  has a cathode that is connected to the collector C of the transistor  13 , and an anode that is connected to the detection part  21  of the drive circuit board  20 . The protection diode  12  can protect the drive circuit board  20  (in particular, the detection part  21 ) from a voltage Vce with an increased voltage value. The voltage Vce is the voltage between the collector C and the emitter E of the transistor  13 . 
     The detection part  21  is one example of a detection part that detects whether the flyback diode  11  is electrified by detecting the voltage Vce via the anode of the protection diode  12 . The detection part  21  has a voltage source  25 , a resistance  24 , and a monitor circuit  26 , for example. 
     The anode of the protection diode  12  is in pull-up connection with a voltage VB of the voltage source  25  via the resistance  24 . The resistance  24  may be a constant current source that outputs a constant current. The voltage source  25  shares a ground with the drive circuit board  20 . The ground of the drive circuit board  20  is conductively connected to the emitter E of the transistor  13 . The connecting point between the anode of the protection diode  12  and the resistance  24  is connected to the monitor circuit  26 , and an input voltage Vin is input into the monitor circuit  26  via the connecting point. In other words, the input voltage Vin corresponds to one example of a detection value of the voltage Vce. The detection part  21  detects whether the flyback diode  11  is electrified based on the voltage value of the input voltage Vin that is input into the monitor circuit  26 . 
     For example, when the flyback diode  11  is electrified, a forward current flows through the flyback diode  11  and the voltage Vce is therefore equal to −VF 11  (the emitter E of the transistor  13  is defined to have a reference potential of zero and VF 11  is defined to be a forward voltage of the flyback diode  11 ). Because the voltage Vce (=−VF 11 ) at this time is lower than the voltage VB, the protection diode  12  is electrified in a forward direction. Thus, when the flyback diode  11  is electrified, the input voltage Vin is equal to “−VF 11 +VF 12 ,” which is higher than the voltage Vce by the amount of the forward voltage VF 12  of the protection diode  12 . 
     When the flyback diode  11  is not electrified, the voltage Vce is equal to an on-voltage Von of the transistor  13  if the transistor  13  is electrified. The on-voltage Von is the voltage that is developed between the collector C and the emitter E when the transistor  13  is electrified. Because the voltage Vce (=Von) at this time is also lower than the voltage VB, the protection diode  12  is electrified in a forward direction. Thus, when the flyback diode  11  is not electrified and the transistor  13  is electrified, the input voltage Vin is equal to “Von+VF 12 ,” which is higher than the voltage Vce by the amount of the forward voltage VF 12  of the protection diode  12 . 
     When neither the flyback diode  11  nor the transistor  13  are electrified, the voltage Vce is approximately equal to the source voltage VH of the high source potential part that is directly or indirectly connected to the first current path  15 . Because the voltage Vce (=VH) at this time is higher than the voltage VB, the protection diode  12  is not electrified. Thus, when neither the flyback diode  11  nor the transistor  13  is electrified, the input voltage Vin is equal to the “voltage VB.” It should be noted that the voltage VB is set to a voltage value that is higher than “Von+VF 12 ” and lower than the source voltage VH. 
     As described above, the voltage value of the input voltage Vin that is input into the monitor circuit  26  of the detection part  21  changes depending on whether the flyback diode  11  is electrified. Thus, the detection part  21  can detect whether the flyback diode  11  is electrified by detecting the difference in the voltage value of the input voltage Vin that is input into the monitor circuit  26 . 
     However, the forward voltage VF 11  of the flyback diode  11  and the forward voltage VF 12  of the protection diode  12  both have the characteristic of changing with temperature (temperature characteristic). Thus, when the temperature of the flyback diode  11  and the temperature of the protection diode  12  vary independently of each other, the forward voltage VF 11  and the forward voltage VF 12  change independently of each other and the voltage value of the input voltage Vin therefore varies widely. As a result, the accuracy of the detection of whether the flyback diode  11  is electrified by the monitor circuit  26  of the detection part  21  is lowered. 
     For example, because the process cost of the drive circuit board  20  is lower than the process cost of the semiconductor substrate  10  on which the reverse conducting transistor  14  is provided, a case is assumed where the protection diode  12  that can protect the detection part  21  is provided, together with the detection part  21 , on the drive circuit board  20 . However, because the reverse conducting transistor  14  as a heat source is provided on the semiconductor substrate  10 , the temperatures of the flyback diode  11  and the protection diode  12  vary independently of each other when the flyback diode  11  and the protection diode  12  are provided on different substrates. As a result, the voltage value of the input voltage Vin varies widely, and, consequently, the accuracy of the detection of whether the flyback diode  11  is electrified is lowered. 
     In contrast to this, in this embodiment, the temperature of the flyback diode  11  and the temperature of the protection diode  12  do not vary independently of each other but vary in an approximately similar fashion because the protection diode  12  is provided on the common semiconductor substrate  10  on which the flyback diode  11  is provided. Thus, even when the forward voltage VF 11  and the forward voltage VF 12  independently change with variation in temperature, the variation of the voltage value of the input voltage Vin decreases as compared to a case where the temperatures of the flyback diode  11  and the protection diode  12  vary independently of each other. As a result, the accuracy of the detection of whether the flyback diode  11  is electrified by the monitor circuit  26  of the detection part  21  can be improved. 
     In addition, because the cathode of the protection diode  12  is connected to the collector of the transistor  13  to which the flyback diode  11  is connected in inverse-parallel, the forward direction of the flyback diode  11  and the forward direction of the protection diode  12  are opposite to each other. In other words, the cathode of the flyback diode  11  and the cathode of the protection diode  12  are connected to each other. Therefore, because the variation of the forward voltage VF 11  with temperature and the variation of the forward voltage VF 12  with temperature are cancelled out almost completely, the variation in the voltage value of the input voltage Vin decreases. As a result, the accuracy of the detection of whether the flyback diode  11  is electrified by the monitor circuit  26  of the detection part  21  can be improved. 
     The flyback diode  11  and the protection diode  12  may be different kinds of diodes but are preferably diodes of the same kind. When both the diodes are of the same kind, the temperature characteristics of the forward voltages of both the diodes can be the same. In this case, because the variation of the forward voltage VF 11  with temperature and the variation of the forward voltage VF 12  with temperature can be equalized, the variation in the voltage value of the input voltage Vin further decreases. As a result, the accuracy of the detection of whether the flyback diode  11  is electrified by the monitor circuit  26  of the detection part  21  can be further improved. 
     The detection part  21  outputs a detection signal Vd that indicates the result of the detection of whether the flyback diode  11  is electrified from the monitor circuit  26  based on the voltage value of the input voltage Vin. For example, the monitor circuit  26  has a comparator  22 , and a threshold voltage generation part  23  in order to output a detection signal Vd that indicates the result of the detection of whether the flyback diode  11  is electrified. 
     The comparator  22  has a non-inverting input that is connected to the connecting point between the anode of the protection diode  12  and the resistance  24 , and an inverting input that is connected to the threshold voltage generation part  23 . The threshold voltage generation part  23  generates a threshold voltage Vth using the ground of the drive circuit board  20  as a ground reference, and provides the threshold voltage Vth to the inverting input of the comparator  22 . The comparator  22  compares the magnitude relationship between the input voltage Vin and the threshold voltage Vth to detect whether the flyback diode  11  is electrified. 
     The threshold voltage Vth is set to a voltage value that is higher than “−VF 11 +VF 12 ” and lower than “Von+VF 12 .” Thus, the comparator  22  outputs a low-level detection signal Vd that indicates that the flyback diode  11  is electrified when it detects that the input voltage Vin is lower than the threshold voltage Vth. On the other hand, the comparator  22  outputs a high-level detection signal Vd that indicates that the flyback diode  11  is not electrified when it detects that the input voltage Vin is higher than the threshold voltage Vth. 
     For example, when “VF 11 =VF 12 =Von=1[V],” the threshold voltage Vth is set to a voltage value that is higher than 0[V] and lower than 2[V] because “−VF 11 +VF 12 =0[V]” and “Von+VF 12 =2[V]”. In this case, the detection part  21  can detect electrification of the flyback diode  11  even when a minute current which is slightly higher than 0 ampere flows through the flyback diode  11 . 
     The determination part  31  determines whether to permit the transistor  13  to be turned on based on the result of the detection of whether the flyback diode  11  is electrified by the detection part  21 . When the detection part  21  detects that the flyback diode  11  is electrified (for example, when a low-level detection signal Vd is input into the determination part  31 ), the determination part  31  prohibits the transistor  13  from being turned on. On the other hand, when the detection part  21  detects that the flyback diode  11  is not electrified (for example, when a high-level detection signal Vd is input into the determination part  31 ), the determination part  31  permits the transistor  13  to be turned on. 
     The determination part  31  has an AND circuit (AND gate) into which a command signal Vg and the detection signal Vd are input, for example. The command signal Vg is a pulse width modulation (PWM) signal that is provided from a controller outside of the drive circuit board  20 , for example. A high-level command signal Vg represents an on-command for the transistor  13 , and a low-level command signal Vg represents an off-command for the transistor  13 . The controller that outputs the command signal Vg is a microcomputer that includes a central processing unit (CPU), for example. It should be noted that the controller that outputs the command signal Vg may be provided on the drive circuit board  20 . 
     When the transistor  13  is prohibited from being turned on by the determination part  31 , the drive part  27  maintains a gate voltage Vge of the transistor  13  at a voltage value at which the transistor  13  is fixed in an off state even if a command signal Vg that commands turn-on of the transistor  13  is input. On the other hand, the drive part  27  turns on or off the transistor  13  according to the command signal Vg when the transistor  13  is permitted to be turned on by the determination part  31 . In other words, the drive part  27  changes the gate voltage Vge to a voltage value at which the transistor  13  is turned on when the command signal Vg is an on-command for the transistor  13  and changes the gate voltage Vge to a voltage value at which the transistor  13  is turned off when the command signal Vg is an off-command for the transistor  13 . 
     In the reverse conducting transistor  14 , when the transistor  13  is turned on while a current is flowing through the flyback diode  11 , the forward voltage VF 11  increases and the forward loss of the flyback diode  11  increases. This phenomenon is sometimes referred to as “gate interference.” However, when the transistor  13  is prohibited from being turned on by the determination part  31 , the transistor  13  is maintained in an off state even if a command signal Vg that commands turn-on of the transistor  13  is input. Thus, an increase in forward loss of the flyback diode  11  can be prevented. This can lead to a reduction of power consumption of the drive unit  1  and, consequently, contribute to improvement of the fuel efficiency of the vehicle that is equipped with the drive unit  1 , for example. 
       FIG. 2  is a diagram that illustrates one example of the configuration of a drive unit  2  according to a second embodiment. As for the same configurations and effects as those of the above-mentioned drive unit  1 , the description of the drive unit  1  is incorporated. The drive unit  2  has a monitor circuit  26  that is different in configuration from that of the drive unit  1 . The monitor circuit  26  of the drive unit  2  has an ADC  32  and a processing circuit  28  in order to output a detection signal Vd that indicates the result of the detection of whether the flyback diode  11  is electrified. 
     The ADC  32  is an AD (Analog-to-Digital) converter that has an input that is connected to the connecting point between the anode of the protection diode  12  and the resistance  24 . The ADC  32  converts an analog value of the input voltage Vin into a digital value and outputs the digital value to the processing circuit  28 . The processing circuit  28  compares the magnitude relationship between the digital value of the input voltage Vin and a digital value of the threshold voltage Vth, and outputs a detection signal Vd that indicates the result of the detection of whether the flyback diode  11  is electrified. 
       FIG. 3  is a diagram that illustrates one example of the configuration of a drive unit  3  according to a third embodiment. As for the same configurations and effects as those of the above-mentioned drive unit  1 , the description of the drive unit  1  is incorporated. The drive unit  3  has a monitor circuit  26  that is different in configuration from that of the drive unit  1 . The monitor circuit  26  of the drive unit  3  has a buffer circuit  29  in order to output a detection signal Vd that indicates the result of the detection of whether the flyback diode  11  is electrified. 
     The buffer circuit  29  has an input that is connected to the connecting point between the anode of the protection diode  12  and the resistance  24 . A threshold value of the input of the buffer circuit  29  is set to the threshold voltage Vth. The buffer circuit  29  compares the magnitude relationship between the input voltage Vin and the threshold voltage Vth, and outputs a detection signal Vd that indicates the result of the detection of whether the flyback diode  11  is electrified. 
       FIG. 4  is a diagram that illustrates one example of an arrangement position of the protection diode  12 .  FIG. 4  is a plan view that schematically illustrates the semiconductor substrate  10 . The semiconductor substrate  10  has element active regions  17  and  18  in which the reverse conducting transistor  14  is located. In the illustrated case, the protection diode  12  is located at a central part  34  of the rectangular-shaped semiconductor substrate  10  (specifically, in a region between the first element active region  17  and the second element active region  18 ). The difference in temperature between the central part  34  and the element active regions  17  and  18  is relatively small. 
     Thus, because the difference in temperature between the flyback diode  11  and the protection diode  12  is small as the protection diode  12  is located at the central part  34 , the temperatures of both the diodes do not vary independently of each other but vary in an approximately similar fashion. Thus, because the variation in the voltage value of the input voltage Vin decreases, the accuracy of the detection of whether the flyback diode  11  is electrified by the monitor circuit  26  of the detection part  21  can be further improved. 
     It should be noted that the protection diode  12  does not necessarily have to be located at the central part  34  of the semiconductor substrate  10  and may be located in a region other than the central part  34  (for example, in a region between an element active region and an edge of the semiconductor substrate  10 ). 
       FIG. 5  is a diagram that illustrates one example of the configuration of a power converter  101  that is equipped with a plurality of drive units. As for the same configurations and effects as those of the above-mentioned drive unit  1 , the description of the drive unit  1  is incorporated. The power converter  101  includes a pair of drive units  1 L and  1 H, each of which has the same configuration as the drive unit  1 . The power converter  101  includes the drive unit  1 L that is provided on a low side with respect to an intermediate node  19 , and the drive unit  1 H that is provided on a high side with respect to the intermediate node  19 . An inductive load  30  is connected to the intermediate node  19 . 
     A current path  15 L is connected to a high source potential part of a source voltage VH via a reverse conducting transistor  14 H, and a current path  16 L is connected to a ground. A current path  15 H is connected to the high source potential part of the source voltage VH, and a current path  16 H is connected to the ground via a reverse conducting transistor  14 L. 
     The power converter  101  includes an arm circuit  33  in which the reverse conducting transistor  14 L of the drive unit  1 L and the reverse conducting transistor  14 H of the drive unit  1 H are connected in series. When used as an inverter that drives a three-phase motor, the power converter  101  includes three arm circuits  33 , i.e., as many arm circuits  33  as the number of phases of the three-phase motor, that are provided in parallel. 
     The drive unit  1 L includes a semiconductor substrate  10 L, and a drive circuit board  20 L. The semiconductor substrate  10 L is a chip that has the reverse conducting transistor  14 L, and a protection diode  12 L. A voltage Vcel is the voltage between a collector C and an emitter E of a transistor  13 L. On the other hand, the drive unit  1 H includes a semiconductor substrate  10 H, and a drive circuit board  20 H. The semiconductor substrate  10 H is a chip that has the reverse conducting transistor  14 H, and a protection diode  12 H. A voltage Vceh is the voltage between a collector C and an emitter E of a transistor  13 H. 
     While a high level command signal Vgl that commands turn-on of the transistor  13 L is being input into the drive unit  1 L, a low level command signal Vgh that commands turn-off of the transistor  13 H is being input into the drive unit  1 H. On the other hand, while a high level command signal Vgh that commands turn-on of the transistor  13 H is being input into the drive unit  1 H, a low level command signal Vgl that commands turn-off of the transistor  13 L is being input into the drive unit  1 L. 
     When the transistor  13 L is prohibited from being turned on by the determination part  31  of the drive unit  1 L, the drive part  27  of the drive unit  1 L maintains a gate voltage Vgel of the transistor  13 L at a voltage value at which the transistor  13 L is fixed in an off state even if a command signal Vgl that commands turn-on of the transistor  13 L is input. On the other hand, the drive part  27  of the drive unit  1 L turns on or off the transistor  13 L according to the command signal Vgl when the transistor  13 L is permitted to be turned on by the determination part  31  of the drive unit  1 L. 
     When the transistor  13 H is prohibited from being turned on by the determination part  31  of the drive unit  1 H, the drive part  27  of the drive unit  1 H maintains a gate voltage Vgeh of the transistor  13 H at a voltage value at which the transistor  13 H is fixed in an off state even if a command signal Vgh that commands turn-on of the transistor  13 H is input. On the other hand, the drive part  27  of the drive unit  1 H turns on or off the transistor  13 H according to the command signal Vgh when the transistor  13 H is permitted to be turned on by the determination part  31  of the drive unit  1 H. 
     When a flyback diode  11 L is electrified, the voltage Vcel is equal to −VF 11  due to the electrification of the flyback diode  11 L. Because the voltage Vcel is lower than the voltage VB, the protection diode  12 L is electrified. Thus, when the flyback diode  11 L is electrified, the input voltage Vin is equal to “−VF 11 +VF 12 .” 
     On the other hand, when the flyback diode  11 L is not electrified, the voltage Vcel is equal to the on voltage Von of the transistor  13 L if the transistor  13 L is electrified. Because the voltage Vcel is lower than the voltage VB, the protection diode  12 L is electrified. Thus, when the flyback diode  11 L is not electrified and the transistor  13 L is electrified, the input voltage Vin is equal to “Von+VF 12 .” 
     Further, when neither the flyback diode  11 L nor the transistor  13 L are electrified, the voltage Vcel is approximately equal to the source voltage VH due to turn-on of the transistor  13 H or electrification of the flyback diode  1114 . Because the voltage Vcel is higher than the voltage VB, the protection diode  12 L is not electrified. Thus, when neither the flyback diode  11 L nor the transistor  13 L is electrified, the input voltage Vin is equal to the “voltage VB.” 
     Thus, the detection part  21  of the drive unit  1 L can detect whether the flyback diode  11 L is electrified by detecting the difference in the voltage value of the input voltage Vin that is input into the monitor circuit  26  of the drive unit  1 L. In addition, because the protection diode  12 L is provided on the common semiconductor substrate  10 L on which the flyback diode  11 L is provided, the variation in the voltage value of the input voltage Vin decreases. As a result, the accuracy of the detection of whether the flyback diode  11 L is electrified by the monitor circuit  26  of the detection part  21  of the drive unit  1 L can be improved. 
     Because the drive unit  1 H also operates in the same manner as the drive unit  1 L, the accuracy of the detection of whether the flyback diode  11 H is electrified by the monitor circuit  26  of the detection part  21  of the drive unit  1 H can be improved. 
     While the drive unit is described with reference to its embodiments in the foregoing, the invention is not limited to the above embodiments. Various modifications and improvements, such as a combination or replacement with a part or whole of another embodiment, can be made. 
     For example, the RC-IGBT is one example of the reverse conducting transistor, and the reverse conducting transistor may be a different kind of switching element. 
     In addition, the detection part that detects whether a diode that is connected in inverse-parallel to the transistor is electrified does not necessarily have to be provided on a substrate that is different from the semiconductor substrate on which the reverse conducting transistor is provided and may be provided on the semiconductor substrate on which the reverse conducting transistor is provided. 
     Alternatively, the detection part  21  may detect the electrification direction of the reverse conducting transistor  14  by detecting the voltage Vce via the anode of the protection diode  12 . A current that flows through the reverse conducting transistor  14  in a positive direction from the collector to the emitter flows through the transistor  13 , and a current that flows through the reverse conducting transistor  14  in a negative direction from the emitter to the collector flows through the flyback diode  11 . Thus, when the direction of the current that flows through the reverse conducting transistor  14  is positive (in other words, when the transistor  13  is electrified), the input voltage Vin is equal to “Von+VF 12 .” On the other hand, when the direction of the current that flows through the reverse conducting transistor  14  is negative (in other words, when the flyback diode  11  is electrified), the input voltage Vin is equal to “−VF 11 +VF 12 .” 
     As described above, the voltage value of the input voltage Vin that is input into the monitor circuit  26  of the detection part  21  changes depending on the difference in the direction of the current that flows through the reverse conducting transistor  14 . Thus, the detection part  21  can detect the electrification direction of the reverse conducting transistor  14  by detecting the difference in the voltage value of the input voltage Vin that is input into the monitor circuit  26 . 
     For example, the comparator  22  outputs a low-level detection signal Vd that indicates that the electrification direction of the reverse conducting transistor  14  is negative (in other words, the flyback diode  11  is electrified) when it detects that the input voltage Vin is lower than a first threshold voltage Vth 1 . The first threshold voltage Vth 1  is set to a voltage value that is higher than “−VF 11 +VF 12 ” and lower than “Von+VF 12 .” On the other hand, the comparator  22  outputs a high-level detection signal Vd that indicates that the electrification direction of the reverse conducting transistor  14  is positive (in other words, the transistor  13  is electrified) when it detects that the input voltage Vin is higher than the first threshold voltage Vth 1  and lower than a second threshold voltage Vth 2 . The second threshold voltage Vth 2  is higher than the first threshold voltage Vth 1 . The second threshold voltage Vth 2  is set to a voltage value that is higher than “Von+VF 12 ” and lower than “VB.” 
     The determination part  31  determines whether to permit the transistor  13  to be turned on based on the result of the detection of the electrification direction of the reverse conducting transistor  14  by the detection part  21 . When the detection part  21  detects that the electrification direction of the reverse conducting transistor  14  is negative (in other words, the flyback diode  11  is electrified) (for example, when a low-level detection signal Vd is input into the determination part  31 ), the determination part  31  prohibits the transistor  13  from being turned on. On the other hand, when the detection part  21  detects that the electrification direction of the reverse conducting transistor  14  is positive (in other words, the transistor  13  is electrified) (for example, when a high-level detection signal Vd is input into the determination part  31 ), the determination part  31  permits the transistor  13  to be turned on. 
     Alternatively, the detection part  21  may detect whether the transistor  13  is electrified by detecting the voltage Vce via the anode of the protection diode  12 . When the transistor  13  is electrified, the input voltage Vin is equal to “Von+VF 12 .” On the other hand, when the transistor  13  is not electrified, the input voltage Vin is equal to “−VF 11 +VF 12 ” or “voltage VB.” 
     As described above, the voltage value of the input voltage Vin that is input into the monitor circuit  26  of the detection part  21  changes depending on whether the transistor  13  is electrified. Thus, the detection part  21  can detect whether the transistor  13  is electrified by detecting the difference in the voltage value of the input voltage Vin that is input into the monitor circuit  26 . 
     For example, the comparator  22  outputs a low-level detection signal Vd that indicates that the transistor  13  is not electrified when it detects that the input voltage Vin is lower than a first threshold voltage Vth 1  or higher than a second threshold voltage Vth 2 . The second threshold voltage Vth 2  is higher than the first threshold voltage Vth 1 . The first threshold voltage Vth 1  is set to a voltage value that is higher than “−VF 11 +VF 12 ” and lower than “Von+VF 12 .” The second threshold voltage Vth 2  is set to a voltage value that is higher than “Von+VF 12 ” and lower than “VB.” On the other hand, the comparator  22  outputs a high-level detection signal Vd that indicates that the transistor  13  is electrified when it detects that the input voltage Vin is higher than the first threshold voltage Vth 1  and lower than the second threshold voltage Vth 2 . It should be noted that, in this case, when the transistor  13  is not electrified, the detection signal Vd is not used for the determination of whether to permit the transistor  13  to be turned on by the determination part  31  because the transistor  13  cannot be turned on even if a command signal Vg that commands turn-on of the transistor  13  is input. 
     As described above, because the protection diode  12  is provided on the common semiconductor substrate  10  on which the flyback diode  11  is provided, the variation in the voltage value of the input voltage Vin decreases. In addition, because the flyback diode  11  and the protection diode  12  are diodes of the same kind as described above, the variation in the voltage value of the input voltage Vin decreases. In addition, because the protection diode  12  is located at the central part  34  of the semiconductor substrate  10 , the variation in the voltage value of the input voltage Vin decreases. Thus, according to this embodiment, the accuracy of the detection of the electrification direction of the reverse conducting transistor  14  or the accuracy of the detection of whether the transistor  13  is electrified can be improved.