Semiconductor device

It is an object to provide a semiconductor device comprising a short circuit protecting system capable of enhancing the detection precision of a collector current, thereby carrying out a reliable short circuit protection. An IGBT (1) having a collector (C) connected to a terminal (T1) and an emitter (E) connected to a terminal (T2) is provided, and has a sense emitter (SE) connected to a terminal (T2) through a variable resistor (VR1) to be a current and voltage converting section. A sense potential is output from an end on the sense emitter (SE) side of the variable resistor (VR1) and is given to a terminal (T11) of a current ratio detecting section (15). A gate of the IGBT (1) is connected to a terminal (T3) and an output of the current ratio detecting section (15) is connected to a terminal (T4).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device and more particularly to a semiconductor device having a short circuit protecting function for preventing a short circuit of a semiconductor element such as an insulated gate bipolar transistor (IGBT).

2. Description of the Background Art

In an IGBT in which a large current flows as a main current, a sense emitter to be used for measuring a current is provided separately from an emitter in which the main current flows, and the main current can be indirectly monitored by measuring a current (sense current) of the sense emitter with a conversion into a voltage through a current and voltage converting section.

As an example of the current and voltage conversion, FIG. 9 shows a structure using a resistor. In FIG. 9 , an IGBT 1 having a collector C connected to a terminal T 1 and an emitter E connected to a terminal T 2 is provided, and has a sense emitter SE connected to the terminal T 2 through a resistor R 101 to be a current and voltage converting section CV 1 . A terminal T 5 for outputting a sense emitter voltage is connected to an end on the sense emitter SE side of the resistor R 101 . The IGBT 1 has a gate connected to a terminal T 3 .

As an example of the current and voltage conversion, FIG. 10 shows a structure using a current mirror circuit. In FIG. 10 , the same structures as those in FIG. 9 have the same reference numerals and repetitive description will be omitted.

In FIG. 10 , a sense emitter SE of an IGBT 1 is connected to a collector of a transistor Q 2 constituting a current and voltage converting section CV 2 . The transistor Q 2 is paired with a transistor Q 1 to constitute the current mirror circuit, and both gates are connected to the sense emitter SE in common and both emitters are connected to a terminal T 2 in common.

A collector of the transistor Q 1 is connected to a positive electrode of a constant voltage source PS through a resistor R 102 , a negative electrode of the constant voltage source PS is connected to the terminal T 2 , and a terminal T 5 for outputting a sense emitter voltage is connected to an end on the transistor Q 1 side of the resistor R 102 .

The current and voltage converting section CV 2 constituted by the current mirror circuit can lessen a voltage drop with a current and voltage conversion to carry out a measurement.

FIG. 11 is a diagram showing a structure of a short circuit protecting circuit for measuring a sense emitter voltage to protect a short circuit by using a resistor R 101 as a current and voltage converting section.

In FIG. 11 , a sense emitter voltage output from an end on the sense emitter SE side of the resistor R 101 is input to an input terminal on the positive side of a comparator C 101 and a positive electrode of a constant voltage source PS is connected to an input terminal on the negative side of the comparator C 101 .

An output terminal of the comparator C 101 is connected to a cathode of a diode D 101 and an anode of the diode C 101 is connected to a gate of an IGBT 1 .

In a short circuit protecting circuit having such a structure, a predetermined voltage supplied from the constant voltage source PS is compared with the sense emitter voltage. If the sense emitter voltage is higher than the predetermined voltage, it is decided that a sense emitter current abnormally flows. Consequently, a current is caused to flow from the gate of the IGBT 1 toward an emitter through the diode D 101 , thereby dropping a gate voltage and reducing a collector current of the IGBT 1 .

There is a problem in that the sense emitter current is assumed to have a uniquely simple correlation with the collector current.

More specifically, an electric potential of the sense emitter SE is increased with the detection of the sense current in an actual circuit. For example, therefore, in some cases in which an electric potential of a collector C is low, the electric potentials of the collector C and the sense emitter SE become insufficient so that the sense emitter current is measured to be small even if a large collector current flows. For this reason, the detection precision of the collector current is reduced and a sufficient protection cannot be obtained in some cases. In addition, a rise in a collector voltage rapidly increases the sense emitter current with a start of a protecting operation so that the protecting operation is promoted. Furthermore, a positive feedback in which the gate voltage is decreased to increase the collector voltage is generated and the sense emitter current is rapidly increased with a delay of the operation of a protection circuit, thereby causing a malfunction in some cases.

Moreover, the collector current is rapidly reduced by the inaccurate detection of the collector current. In the IGBT to be used for controlling a large current, for example, there is a problem in that a high surge voltage is generated.

In order to solve these problems, a structure for detecting a collector voltage has also been proposed. However, the collector of the IGBT is provided on the high potential side. Therefore, a detection circuit corresponding to a high voltage is required so that a manufacturing cost is increased and a large space for providing the detection circuit is required, resulting in an increase in a size of the device. Moreover, a countermeasure to be taken against a noise made from various circuits on the high potential side becomes complicated.

Japanese Patent Application Laid-Open No. 11-299218 (1999) has disclosed a structure in which a sense emitter and an emitter in an IGBT are imaginarily short-circuited by using an operational amplifier, thereby preventing a fluctuation in a sense emitter voltage. In order to implement such a structure, it is necessary to prepare a power supply having a lower potential than an emitter potential. In addition, in the case in which the structure is to be implemented as a monolithic IC, there is a problem in that a countermeasure to be taken against a noise becomes complicated because an emitter potential to be an original grounding potential is an intermediate potential of a power supply of the monolithic IC.

SUMMARY OF THE INVENTION

A first aspect of the present invention is directed to a semiconductor device comprising a transistor having a sense emitter, and a protecting system for carrying out a protecting operation for detecting a sense current flowing to the sense emitter to control a collector current, the protecting system including a current and voltage converting section having a variable resistor for converting the sense current into different voltages by change of a resistance value thereof, thereby generating a plurality of sense voltages, and a current ratio detecting section for receiving the sense voltages output from the current and voltage converting section and information about a resistance value of the variable resistor corresponding to the sense voltages, thereby calculating a ratio of each of the sense voltages to each of the resistance values and detecting an amount of change of the ratio thus calculated.

A second aspect of the present invention is directed to the semiconductor device according to the first aspect of the present invention, wherein a resistance value of the variable resistor is varied in response to a voltage signal which is changed on a time basis, the information about the resistance value of the variable resistor is the voltage signal which is changed on a time basis, and the resistance value of the variable resistor and the voltage signal which is changed on a time basis have a proportional relationship.

A third aspect of the present invention is directed to a semiconductor device comprising a transistor having a sense emitter, and a protecting system for carrying out a protecting operation for detecting a sense current flowing to the sense emitter to control a collector current, the protecting system including a current and voltage converting section for converting the sense current into a voltage and outputting the voltage, a forcible voltage changing section for applying external voltages having different values to the sense emitter and forcibly changing a sense emitter potential by the current and voltage converting section, thereby outputting the voltages thus changed as a plurality of sense voltages from the current and voltage converting section, and a current ratio detecting section for receiving the sense voltages output from the current and voltage converting section and information about the external voltages corresponding to the sense voltages, thereby calculating a ratio of each of the sense voltages to each of the external voltages and detecting an amount of change of the ratio thus calculated.

A fourth aspect of the present invention is directed to the semiconductor device according to the third aspect of the present invention, wherein the current and voltage converting section is a current mirror circuit having an input section connected to the sense emitter, the external voltage is generated by a variable resistor connected between an output end of a voltage source of the current mirror circuit and the sense emitter, a resistance value of the variable resistor is changed in response to a voltage signal which is changed on a time basis, the information about the external voltage is the voltage signal which is changed on a time basis, and the resistance value of the variable resistor and the voltage signal which is changed on a time basis have a proportional relationship.

A fifth aspect of the present invention is directed to a semiconductor device comprising a transistor having a sense emitter, and a protecting system for carrying out a protecting operation for detecting a sense current flowing to the sense emitter to control a collector current, the protecting system including a current and voltage converting section having a resistance variable section for converting the sense current into different voltages by change of a resistance value thereof, thereby generating a plurality of sense voltages, a voltage comparator serving to input the sense voltages output from the current and voltage converting section and having two high and low comparison potentials set by a hysteresis, and connecting means for electrically connecting a gate of the transistor to a predetermined electric potential based on an output of the voltage comparator.

A sixth aspect of the present invention is directed to the semiconductor device according to the firth aspect of the present invention, wherein the resistance variable section includes first and second resistors connected in series between the sense emitter and an emitter of the transistor, and a switch element electrically connected in parallel with one of the first and second resistors, an ON/OFF operation of the switch element being controlled based on the output of the voltage comparator.

A seventh aspect of the present invention is directed to the semiconductor device according to the fifth aspect of the present invention, wherein the connecting means is a diode having a cathode connected to the output of the voltage comparator and an anode connected to the gate of the transistor.

An eighth aspect of the present invention is directed to a semiconductor device comprising a transistor having a sense emitter, and a protecting system for carrying out a protecting operation for detecting a sense current lowing to the sense emitter to control a collector current, the protecting system including a current and voltage converting section having a resistance variable section for converting the sense current into different voltages by change of a resistance value thereof, thereby generating a plurality of sense voltages, first and second voltage comparators for inputting the sense voltages output from the current and voltage converting section, an extension circuit connected to an output side of the first voltage comparator and serving to extend a time required for change such that a time in which an output of the first voltage comparator is changed from a second potential to a first potential is longer than a time in which the output of the first voltage comparator is changed from the first potential to the second potential, a variable d.c. power supply for changing a comparison voltage of the second voltage comparator based on the output of the first voltage comparator, and connecting means for electrically connecting a gate of the transistor to a predetermined electric potential based on an output of the second voltage comparator.

A ninth aspect of the present invention is directed to the semiconductor device according to the eighth aspect of the present invention, wherein the resistance variable section includes first and second resistors connected in series between the sense emitter and an emitter of the transistor, and a switch element electrically connected in parallel with one of the first and second resistors, an ON/OFF operation of the switch element being controlled based on the output of the first voltage comparator.

A tenth aspect of the present invention is directed to the semiconductor device according to the eighth aspect of the present invention, wherein the connecting means is a diode having a cathode connected to the output of the second voltage comparator and an anode connected to the gate of the transistor.

An eleventh aspect of the present invention is directed to a semiconductor device comprising a transistor having at least two sense emitters, and a protecting system for carrying out a protecting operation for detecting a sense current flowing to the at least two sense emitters to control a collector current, the protecting system including a current and voltage converting section for converting the sense current flowing to the at least two sense emitters into voltages, thereby generating at least two sense voltages which are different from each other, and a current ratio detecting section for receiving the at least two sense voltages output from the current and voltage converting section, thereby calculating a ratio of the at least two sense voltages.

A twelfth aspect of the present invention is directed to the semiconductor device according to the eleventh aspect of the present invention, wherein the current and voltage converting section includes at least two resistors having different resistance values which are connected between the at least two sense emitters and an emitter of the transistor, respectively.

According to the first aspect of the present invention, the current ratio detecting section receives a plurality of sense voltages output from the current and voltage converting section and information about the resistance value of the variable resistor corresponding to the sense voltages, thereby calculating the ratio of each of the sense voltages to each of the resistance values and detecting the amount of change thereof. Thus, a pattern of a short circuit state of the transistor can be known based on the amount of change. As a result, a short circuit protecting operation is carried out corresponding to the pattern of the short circuit state. Thus, it is possible to implement the short circuit protecting operation in consideration of a difference in the short circuit state depending on a magnitude of a collector voltage. Moreover, it is not necessary to detect the collector voltage. Therefore, a detection circuit corresponding to a high voltage and a large space for providing the detection circuit are not required, and a manufacturing cost can be reduced and a size of the device can be prevented from being increased.

According to the second aspect of the present invention, it is possible to obtain a practical structure for changing the resistance value of the variable resistor. Moreover, the information about the resistance value of the variable resistor is given through the voltage signal which is changed on a time basis and has a proportional relationship with the resistance value of the variable resistor. Therefore, it is possible to easily carry out an operational processing or the like in the current ratio detecting section.

According to the third aspect of the present invention, a plurality of sense voltages output from the current and voltage converting section and information about external voltages corresponding to the sense voltages are received to calculate a ratio of each of the sense voltages to each of the external voltages and to detect the amount of change thereof. Consequently, it is possible to know a pattern of a short circuit state of the transistor based on the amount of change. As a result, a short circuit protecting operation is carried out corresponding to the pattern of the short circuit state. Thus, it is possible to implement the short circuit protecting operation in consideration of a difference in the short circuit state depending on a magnitude of a collector voltage. Moreover, it is not necessary to detect the collector voltage. Therefore, a detection circuit corresponding to a high voltage and a large space for providing the detection circuit are not required, and a manufacturing cost can be reduced and a size of the device can be prevented from being increased.

According to the fourth aspect of the present invention, the current mirror circuit is used for the current and voltage converting section. Consequently, it is possible to lessen a drop in a voltage with a current and voltage conversion, thereby measuring an emitter voltage. Moreover, a voltage source of the current mirror circuit is used for generating an external voltage and a value thereof is changed through the variable resistor. Therefore, it is possible to simply obtain a structure required for the application of the external voltage and to prevent a manufacturing cost from being increased.

According to the fifth aspect of the present invention, the voltage comparator compares each of the sense voltages output from the current and voltage converting section with two high and low comparison potentials set by a hysteresis. Consequently, in the case in which the sense voltage for each resistance value exceeds one of the comparison potentials but does not exceed the other comparison potential or exceeds both of the two comparison potentials, a sense voltage characteristic can be analyzed in more detail. Therefore, a short circuit protecting operation can be carried out more properly. Moreover, the gate of the transistor is electrically connected to the predetermined electric potential through the connecting means based on the output of the voltage comparator. In the case in which the short circuit protecting operation is required, consequently, the gate of the transistor is connected to the predetermined electric potential to decrease a gate current. Thus, a collector current can be reduced to carry out the short circuit protection.

According to the sixth aspect of the present invention, it is possible to obtain a practical and simple structure of the resistance variable section.

According to the seventh aspect of the present invention, it is possible to obtain a practical and simple structure of the connecting means.

According to the eighth aspect of the present invention, there are provided the first and second voltage comparators to which the sense voltages output from the current and voltage converting section are input. Based on the output of the first voltage comparator, the comparison voltage of the second voltage comparator is changed through the variable d.c. power supply. Therefore, the sense voltage can be compared with at least two comparison voltages. Thus, it is possible to carry out the short circuit protecting operation more properly. Moreover, the extension circuit is provided. Therefore, it is possible to extend the period for detection of the second comparator to determine the execution and stop of the protecting operation. Thus, it is possible to prevent a malfunction from being caused by a short period for detection.

According to the ninth aspect of the present invention, it is possible to obtain a practical and simple structure of the resistance variable section.

According to the tenth aspect of the present invention, it is possible to obtain a practical and simple structure of the connecting means.

According to the eleventh aspect of the present invention, the current ratio detecting section calculates the ratio of at least two sense voltages output from the current and voltage converting section. Therefore, the sense voltages are directly compared with each other. Consequently, a time required for knowing the pattern of the short circuit state can be reduced and the protecting operation can be carried out in a real time.

According to the twelfth aspect of the present invention, it is possible to obtain a practical and simple structure of the current and voltage converting section.

In order to solve the above-mentioned problems, it is an object of the present invention to provide a semiconductor device comprising a short circuit protecting system capable of enhancing the detection precision of a collector current, thereby carrying out a reliable short circuit protection.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A. First Embodiment

<A-1. Feature of First Embodiment>

First of all, two plain patterns will be described as an example of a short circuit state of an IGBT.

A first pattern implies a short circuit in a state in which the IGBT is completely operated in an active region and a collector-emitter voltage (hereinafter referred to as a collector voltage) is high.

A second pattern implies a short circuit in a state in which the IGBT is operated in a saturation region and the collector voltage is low.

A sense emitter is provided to cause a sense emitter current (hereinafter referred to as a sense current) to flow, the sense current having a predetermined ratio to an emitter current and being much smaller than the emitter current. The ratio of the sense current to the emitter current in such a state that the IGBT is short-circuited is varied between the first and second patterns. More specifically, the sense current ratio is great in the short circuit of the first pattern and is small in the short circuit of the second pattern.

This implies that the sense current is smaller in the second pattern than that in the first pattern if the emitter current is identical. It is desirable that the short circuit protecting operation should be changed depending on the short circuit state.

Moreover, when a sense emitter potential (hereinafter referred to as a sense potential) related to an emitter potential is changed, the sense current is varied. The amount of change is varied between the first and second patterns.

More specifically, in the first pattern having a high collector voltage, the sense current is less changed with a variation in the sense potential. If a value of a resistor (hereinafter referred to as a sense resistor) connected to the sense emitter is reduced, the sense potential is dropped almost proportionally thereto.

On the other hand, in the second pattern having a low collector voltage, the sense current is changed with a variation in the sense potential. When the sense potential is changed, the sense current is varied. Therefore, even if the value of the sense resistor is reduced, the sense potential is not dropped proportionally thereto.

In this respect, a first embodiment is characterized in that the sense resistance value is changed to measure each sense potential, each sense current is calculated from the sense resistance value for each sense potential, the amount of change in the sense current is detected to know a pattern of the short circuit state based on the amount of change, thereby carrying out a short circuit protecting operation corresponding to the pattern of the short circuit state.

<A-2. Structure of Device>

FIG. 1 shows a structure of a semiconductor device 100 according to the first embodiment of the present invention.

In FIG. 1 , an IGBT 1 having a collector C connected to a terminal T 1 and an emitter E connected to a terminal T 2 is provided, and has a sense emitter SE connected to the terminal T 2 through a variable resistor VR 1 to be a current and voltage converting section. A sense potential is output from an end on the sense emitter SE side of the variable resistor VR 1 and is given to a terminal T 11 of a current ratio detecting section 15 . A gate of the IGBT 1 is connected to a terminal T 3 and an output of the current ratio detecting section 15 is connected to a terminal T 4 .

Moreover, an output of a resistance value changing section 14 for changing a resistance value of the variable resistor VR 1 on a time basis is connected to a control terminal of the variable resistor VR 1 and a terminal T 12 of the current ratio detecting section 15 . The variable resistor VR 1 is an element for changing the resistance value according to a predetermined voltage. For example, a field effect transistor (FET) or the like is used for the variable resistor VR 1 , and a gate of the FET acts as a control terminal and a resistance between a drain and a source can be varied by changing a gate voltage.

Next, an example of a structure of the current ratio detecting section 15 will be described with reference to FIG. 2 . The current ratio detecting section 15 has a division circuit 151 on the input side and an amplitude detection circuit 152 on the output side.

The sense potential obtained from the variable resistor VR 1 is input from a terminal T 11 of the current ratio detecting section 15 to an input terminal on the negative side of an amplifier C 1 constituting the division circuit 151 through a resistor R 1 . An input terminal on the positive side of the amplifier C 1 is connected to a grounding potential.

Moreover, the input terminal on the negative side of the amplifier C 1 is connected to an output terminal through a variable resistor VR 2 , and an output of the resistance value changing section 14 is given to a control terminal of the variable resistor VR 2 through a terminal T 12 . The resistance value changing section 14 is based on an electric potential of the terminal T 2 .

The amplitude detection circuit 152 has a capacitor CP 1 in which an output of the amplifier C 1 is given to one of electrodes thereof, a diode D 1 having an anode connected to the other electrode of the capacitor CP 1 , and an amplifier C 2 in which a cathode of the diode D 1 is connected to an input terminal on the positive side.

Furthermore, the amplitude detection circuit 152 has a constant voltage source PS 1 having a positive electrode connected to an input terminal on the negative side of the amplifier C 2 and a negative electrode connected to a grounding potential, a diode D 2 having a cathode connected to the other electrode of the capacitor CP 1 and an anode connected to the grounding potential, and a capacitor CP 2 and a resistor R 2 which are connected in parallel with each other between a wiring of the diode D 1 and the input terminal on the positive side of the amplifier C 2 and the grounding potential.

In the semiconductor device 100 shown in FIG. 1 , a sense resistance (the variable resistor VR 1 ) is changed on a time basis by using the resistance value changing section 14 to calculate a sense potential (a value obtained by dividing the sense potential by a sense resistance value, that is, a sense current) per sense resistance through the current ratio detecting section 15 . By detecting the amount of change, a pattern of a short circuit state can be known.

It is sufficient that the resistance value changing section 14 changes an output thereof on a time basis, and an AD conversion output device to be controlled by a microcomputer or any waveform generator may be used.

For simplicity of the description, if the resistance value changing section 14 is an oscillator for periodically changing two high and low voltages, the sense resistance value is also changed to be high and low when the output of the resistance value changing section 14 is brought into a high potential state and a low potential state. The sense potential is also changed to be high and low with the variation. For example, in the case in which the change of the sense resistance value is proportional to the output of the resistance value changing section 14 , the sense potential per sense resistance is proportional to a value obtained by dividing the sense potential by the output of the resistance value changing section 14 . Therefore, the sense potential per output of the resistance value changing section 14 is calculated by the division circuit 151 included in the current ratio detecting section 15 . Accordingly, although the division circuit 151 does not accurately calculate the sense current, a value proportional to the sense current is calculated. Therefore, the value thus obtained will be referred to as a sense current for convenience.

In the division circuit 151 , a resistance value of the variable resistor VR 2 provided in a negative feedback path is changed interlockingly with the sense resistance (the variable resistor VR 1 ) through the output of the resistance value changing section 14 , and a ratio of the sense potential per sense resistance, that is, the sense current is converted into a predetermined electric potential to be output.

An output of the division circuit 151 (an output corresponding to the sense current) which is changed corresponding to a variation in the variable resistor VR 1 (that is, a variation in the output of the resistance value changing section 14 ) is given to the amplitude detection circuit 152 . An electric potential to be proportional to a difference (amplitude) between the output potentials of the division circuit 151 in the cases in which the sense resistance is high and low is sent as an output of the amplifier C 2 to the terminal T 4 .

It is possible to know whether the short circuit state has a first pattern or a second pattern depending on an output value at the terminal T 4 , that is, a difference in the amount of change of the sense current.

More specifically, in the case of the first pattern having a high collector voltage, if the value of the sense resistance is reduced, the sense potential is dropped almost proportionally thereto. Therefore, the amount of change of the sense current is increased and an electric potential of the terminal T 4 is raised. In the case of the second pattern having a low collector voltage, even if the value of the sense resistance is reduced, the sense potential is not dropped proportionally thereto. Therefore, the amount of change of the sense current is small and the output potential of the terminal T 4 is dropped.

By utilizing the difference in the output potential, it is possible to change the short circuit protecting operation for the first and second patterns.

As described above, the semiconductor device 100 according to the first embodiment of the present invention comprises the protecting system for changing the sense resistance value to vary the sense potential and for detecting the amount of change to know the pattern of the short circuit state based on the amount of change, thereby carrying out the short circuit protecting operation corresponding to the pattern of the short circuit state. Therefore, it is possible to carry out the short circuit protecting operation in consideration of a difference in the short circuit state through a magnitude of the collector voltage. Moreover, it is not necessary to detect the collector voltage. Therefore, a detection circuit corresponding to a high voltage and a large space for providing the detection circuit are not required, and a manufacturing cost can be reduced and a size of the device can be prevented from being increased.

B. Second Embodiment

<B-1. Feature of Second Embodiment>

While the variable resistor VR 1 is used for the sense resistor to change the sense potential in the semiconductor device 100 according to the first embodiment described with reference to FIG. 1 , means for changing the sense potential is not restricted to the variable resistor. A second embodiment is characterized in that a circuit having a small voltage drop such as a current mirror circuit is used for a current and voltage converting section and means for forcibly changing a sense potential is used separately therefrom.

<B-2. Structure of Device>

FIG. 3 shows a structure of a semiconductor device 200 according to the second embodiment of the present invention. In FIG. 3 , the same structures as those in the semiconductor device 100 shown in FIG. 1 have the same reference numerals and repetitive description will be omitted.

In FIG. 3 , a sense emitter SE of an IGBT 1 is connected to one of ends of a resistor R 11 . The resistor R 11 is connected to a collector of a transistor Q 12 constituting a current mirror circuit to be a current and voltage converting section CV 10 .

The transistor Q 12 is paired with a transistor Q 11 to constitute the current mirror circuit, and both gates are connected to the other end of the resistor R 11 in common and both emitters are connected to a terminal T 2 in common.

Moreover, a collector of the transistor Q 11 is connected to a positive electrode of a constant voltage source PS 11 through a resistor R 12 , and a negative electrode of the constant voltage source PS 11 is connected to the terminal T 2 .

A sense emitter voltage is output from an end on the transistor Q 11 side of the resistor R 12 and is given to a terminal T 11 of a current ratio detecting section 15 .

Moreover, a variable resistor VR 11 (forcible voltage changing section) is provided between an end on the sense emitter SE side of the resistor R 11 and an end connected to the constant voltage source PS 11 in the resistor R 12 , and an output of a resistance value changing section 14 for changing a resistance value of the variable resistor VR 11 on a time basis is connected to a control terminal of the variable resistor VR 11 and a terminal T 12 of the current ratio detecting section 15 . The resistance value changing section 14 is based on an electric potential of the terminal T 2 .

In the semiconductor device 200 shown in FIG. 3 , the resistance value of the variable resistor VR 11 connected to the sense emitter SE is changed on a time basis by using the resistance value changing section 14 . Thus, an electric potential of the sense emitter SE is forcibly changed.

Consequently, the amount of change of a sense current with a variation in the sense potential can be detected more directly through the current ratio detecting section 15 .

More specifically, if the resistance value changing section 14 is an oscillator for periodically changing two high and low voltages, the sense potential is proportionally changed to be high and low when the output of the resistance value changing section 14 is brought into a high potential state and a low potential state.

However, the foregoing can be applied to the case of the first pattern in which the sense current is not changed with a variation in the sense potential but cannot be applied to the case of the second pattern in which the sense current is changed with the variation in the sense potential. Even if the value of the variable resistor VR 11 is changed to forcibly apply a voltage to the sense emitter SE, the sense potential is not varied proportionally thereto.

In the case in which the change of the variable resistor VR 11 is proportional to the output of the resistance value changing section 14 , a sense potential per value of the variable resistor VR 11 is proportional to a value obtained by dividing the sense potential by the output of the resistance value changing section 14 .

Accordingly, a division circuit 151 included in the current ratio detecting section 15 calculates a sense potential for the variable resistor VR 11 (that is, a value obtained by dividing the sense potential by the output of the resistance value changing section 14 ).

An output of the division circuit 151 (a value obtained by dividing the sense potential by the output of the resistance value detecting section 14 ) for each value of the variable resistor VR 11 (that is, the output of the resistance value changing section 14 ) is given to an amplitude detection circuit 152 , and an electric potential which is proportional to the amount of change (amplitude) of an output potential is sent as an output of an amplifier C 2 to a terminal T 4 .

It is possible to know whether a short circuit state has the first pattern or the second pattern depending on an output value at the terminal T 4 , that is, a difference in the amount of change of the sense potential for the variation in the variable resistor VR 11 .

More specifically, in the case of the first pattern having a high collector voltage, if the value of the variable resistor VR 11 is changed, the sense potential is varied almost proportionally thereto. Therefore, the amount of change (amplitude) is increased and an electric potential of the terminal T 4 is raised. In the case of the second pattern having a low collector voltage, even if the value of the variable resistor VR 11 is changed, the sense potential is not varied proportionally thereto and the amount of change (amplitude) is small. Therefore, the electric potential of the terminal T 4 is dropped.

By utilizing the difference in the potential, it is possible to change the short circuit protecting operation for the first and second patterns.

As described above, the semiconductor device 200 according to the second embodiment of the present invention comprises the protecting system for forcibly changing the sense potential and detecting the amount of change to know the pattern of the short circuit state based on the amount of change, thereby carrying out the short circuit protecting operation corresponding to the pattern of the short circuit state. Therefore, it is possible to carry out the short circuit protecting operation in consideration of a difference in the short circuit state depending on a magnitude of the collector voltage. Moreover, it is not necessary to detect the collector voltage. Therefore, a detection circuit corresponding to a high voltage and a large space for providing the detection circuit are not required, and a manufacturing cost can be reduced and a size of the device can be prevented from being increased.

<C-1. Feature of Third Embodiment>

In the semiconductor device 100 according to the first embodiment described with reference to FIG. 1 , the variable resistor VR 1 to be a sense resistor is changed by the output of the resistance value changing section 14 on a time basis, thereby detecting the sense potential which is changed on a time basis. A third embodiment is characterized in that it is sufficient that the sense potential is changed to be at least two electric potentials in order to know a difference in a short circuit state depending on a magnitude of a collector voltage and a protecting operation can be implemented with a more simplified structure by detecting the two sense potentials.

<C-2. Structure of Device>

FIG. 4 shows a structure of a semiconductor device 300 according to the third embodiment of the present invention. In FIG. 4 , the same structures as those in the semiconductor device 100 shown in FIG. 1 have the same reference numerals and repetitive description will be omitted.

In FIG. 4 , resistors R 21 and R 22 connected in series are provided between a sense emitter SE of an IGBT 1 and a terminal T 2 , and the sense emitter SE is connected to one of ends of the resistor R 21 . A terminal T 2 is connected to a common potential.

An end on the sense emitter SE side of the resistor R 21 is connected to an input terminal on the positive side of a comparator C 21 having a hysteresis, an input terminal on the negative side of the comparator C 21 is connected to a positive electrode of a constant voltage source PS 21 , and a negative electrode of the constant voltage source PS 21 is connected to the common potential. Moreover, an output of the comparator C 21 is connected to a cathode of a diode D 21 through a logic inverter IV and a resistor R 23 , and an anode of the diode D 21 is connected to a gate of the IGBT 1 .

An N-channel MOS transistor Q 21 is connected in parallel with the resistor R 22 and a gate of the N-channel MOS transistor Q 21 is connected to the output of the comparator C 21 . The resistors R 21 and R 22 and the N-channel MOS transistor Q 21 constitute a current and voltage converting section CV 20 .

Next, an operation of the semiconductor device 300 will be described with reference to FIGS. 5 and 6 . In the semiconductor device 300 , the comparator C 21 having a hysteresis includes two comparison potentials based on a hysteresis characteristic.

More specifically, the comparison potentials have a first detection potential to be a comparison potential which is determined by a voltage of the constant voltage source PS 21 and a voltage having a higher hysteresis of the comparator C 21 and a second detection potential to be a comparison potential which is determined by the voltage of the constant voltage source PS 21 and a voltage having a lower hysteresis of the comparator C 21 .

In the case in which a short circuit state is set to a first pattern having a high collector voltage, a sense potential is dropped almost proportionally if a value of a sense resistance is reduced. In the case of a second pattern having a low collector voltage, even if the value of the sense resistance is reduced, the sense potential is not dropped proportionally thereto. These matters have been described above. In the case in which the value of the sense resistance is equal and a collector current is also identical, the short circuit state set to the first pattern has a larger sense current and a higher sense potential than those of the short circuit state set to the second pattern.

Therefore, two comparison potentials are set to the sense potential. Whether the sense potential exceeds either or both of the two comparison potentials is detected by the comparator C 21 .

FIGS. 5 and 6 show the characteristics of the sense potential detected through the first and second detection potentials set by the comparator C 21 .

In FIGS. 5 and 6 , an axis of abscissa indicates a sense resistance value and an axis of ordinate indicates a sense potential, the sense potential being shown with a variation in the sense resistance value. FIG. 5 shows a characteristic X 1 with a high collector voltage V CE and a characteristic X 2 with a low collector voltage V CE in the case in which such a collector current as not to require a protecting operation flows. FIG. 6 shows a characteristic Y 1 with the high collector voltage V CE and a characteristic Y 2 with the low collector voltage V CE in the case in which such a collector current as to require the protecting operation flows.

In FIGS. 5 and 6 , moreover, an axis of ordinate indicates first and second detection potentials P 1 and P 2 set by the comparator C 21 , and an axis of abscissa indicates a first resistance value Z 1 defined by the resistors R 21 and R 22 and a second resistance value Z 2 defined by the resistors R 21 and R 22 and an ON-state resistance of an N-channel MOS transistor Q 31 .

In the semiconductor device 300 , the sense resistance value is only switched into two values, that is, the first and second resistance values Z 1 and Z 2 . Therefore, a maximum data point is two for each characteristic. In FIGS. 5 and 6 , data are set to be continuous for convenience.

Description will be given with reference to FIG. 5 . First of all, in the case in which the sense potential is sufficiently low as an initial state, an output potential of the comparator C 21 is L and the sense resistor has the first resistance value Z 1 with the N-channel MOS transistor set in an OFF state. When a short circuit of the IGBT 1 is generated in this state, the sense potential is set to S 1 for the characteristic X 1 and is set to S 2 for the characteristic X 2 . In this case, an output of the logic inverter IV is H . Therefore, a protecting operation for extracting a gate current of the IGBT 1 through the diode D 21 is not executed.

Since the sense potential S 1 exceeds the first detection potential P 1 , the comparator C 21 is operated so that the output is inverted, the output potential is set to H and the output of the logic inverter IV is set to L . Therefore, the protecting operation for extracting the gate current of the IGBT 1 through the diode D 21 is executed. Moreover, the N-channel MOS transistor Q 21 is turned ON. Consequently, the sense resistor is set to have the second resistance value Z 2 and the sense potential is set to S 11 at that time.

The sense potential S 2 of the characteristic X 2 does not exceed the second detection potential P 2 . Therefore, the output potential of the comparator C 21 maintains L and the sense resistor is not switched to have the second resistance value Z 2 .

The sense potential S 1 and the sense potential S 11 make a great difference and are changed almost proportionally to a variation in the sense resistance. Therefore, it is apparent that the characteristic X 1 is set in the short circuit state having the first pattern in which the collector voltage V CE is high.

In the short circuit state having the first pattern, if the collector current is not so high as to require the protecting operation, the sense potential is greatly dropped when the sense resistance value is reduced. As shown in FIG. 5 , the sense potential S 11 with the second resistance value Z 2 does not reach the second detection potential P 2 . Therefore, the output of the comparator C 21 is inverted to L so that the protecting operation for extracting the gate current of the IGBT 1 through the diode D 21 is stopped.

Next, description will be given with reference to FIG. 6 . In the case in which the sense resistor has the first resistance value Z 1 , the sense potential is set to S 3 for the characteristic Y 1 and is set to S 4 for the characteristic Y 2 .

Since the sense potential S 3 for the characteristic Y 1 exceeds the first detection potential P 1 , the comparator C 21 is operated so that the output is inverted, the output potential is set to H and the output of the logic inverter IV is set to L . Therefore, the protecting operation for extracting the gate current of the IGBT 1 through the diode D 21 is executed. Moreover, the N-channel MOS transistor Q 21 is turned ON. Consequently, the sense resistor is set to have a second resistance value Z 2 and the sense potential is set to S 31 at that time.

The sense potential S 3 and the sense potential S 31 make a great difference and are changed almost proportionally to a variation in the sense resistance. Therefore, it is apparent that the characteristic Y 1 is set in the short circuit state having the first pattern in which the collector voltage V CE is high.

In the short circuit state having the first pattern, if the collector current is so high as to require the protecting operation, the sense potential is also increased wholly. As shown in FIG. 6 , the sense potential S 31 with the second resistance value Z 2 exceeds the second detection potential P 2 . Therefore, the output of the comparator C 21 maintains H so that the protecting operation for extracting the gate current of the IGBT 1 through the diode D 21 is maintained. The protecting operation is maintained until the collector current of the IGBT 1 is decreased and the sense potential is reduced to be lower than the second detection potential P 2 .

The diode D 21 can be referred to as connecting means for electrically connecting a gate of the IGBT 1 to a predetermined electric potential based on the output of the comparator C 21 . In place of the diode D 21 , for example, a switch element may be used to carry out a switching operation through the output of the comparator C 21 .

Moreover, since the sense potential S 4 for the characteristic Y 2 also exceeds the first detection potential P 1 , the comparator C 21 is operated so that the output is inverted, the output potential is set to H and the output of the logic inverter IV is set to L . Therefore, the protecting operation for extracting the gate current of the IGBT 1 through the diode D 21 is executed. Furthermore, the N-channel MOS transistor Q 21 is turned ON. Consequently, the sense resistor is set to have a second resistance value Z 2 and the sense potential is set to S 41 at that time.

The sense potential S 4 and the sense potential S 41 make a small difference and are not changed proportionally to a variation in the sense resistance. Therefore, it is apparent that the characteristic Y 2 is set in the short circuit state having the second pattern in which the collector voltage V CE is low.

In the short circuit state having the second pattern, if the collector current is so high as to require the protecting operation, the sense potential is also increased wholly. As shown in FIG. 6 , the sense potential S 41 with the second resistance value Z 2 exceeds the second detection potential P 2 . Therefore, the output of the comparator C 21 maintains H so that the protecting operation for extracting the gate current of the IGBT 1 through the diode D 21 is maintained. The protecting operation is maintained until the collector current of the IGBT 1 is decreased and the sense potential is reduced to be lower than the second detection potential P 2 .

As described above, the semiconductor device 300 according to the third embodiment of the present invention comprises the protecting system for comparing the sense potential with the two comparison potentials by using the comparator C 21 having a hysteresis for the detection of the sense potential. Therefore, it is possible to know the difference in the short circuit state depending on a magnitude of the collector current as well as a magnitude of the collector voltage. Thus, the short circuit protecting operation can be carried out more properly.

<D-1. Feature of Fourth Embodiment>

The semiconductor device 300 according to the third embodiment described with reference to FIG. 4 has such a structure that the sense potential is compared with the two comparison potentials by using the comparator having a hysteresis for detecting the sense potential. However, the sense current often includes a noise and a shift for the noise is generated if integration is not carried out through a low-pass filter or the like. Therefore, it is impossible to optionally change a period for detection.

A fourth embodiment is characterized in that means for detecting the sense potential by the two comparators, thereby extending a period for detection of a comparator to determine the execution and stop of a protecting operation is provided to prevent a malfunction from being caused by a short period for detection.

<D-2. Structure of Device>

FIG. 7 shows a structure of a semiconductor device 400 according to the fourth embodiment of the present invention. In FIG. 7 , the same structures as those in the semiconductor device 100 shown in FIG. 1 have the same reference numerals and repetitive description will be omitted.

In FIG. 7 , resistors R 31 and R 32 connected in series are provided between a sense emitter SE of an IGBT 1 and a terminal T 2 , and the sense emitter SE is connected to one of ends of the resistor R 31 . The terminal T 2 is connected to a common potential.

An end on the sense emitter SE side of the resistor R 31 is connected to an input terminal on the positive side of a comparator C 31 (a first comparator), an input terminal on the negative side of the comparator C 31 is connected to a positive electrode of a constant voltage source PS 31 , and a negative electrode of the constant voltage source PS 31 is connected to the common potential. Moreover, an output of the comparator C 31 is connected to an anode of a diode D 31 .

An N-channel MOS transistor Q 31 is connected in parallel with the resistor R 32 and a gate of the N-channel MOS transistor Q 31 is connected to a cathode of the diode D 31 . The resistors R 31 and R 32 and the N-channel MOS transistor Q 31 constitute a current and voltage converting section CV 30 .

Moreover, a capacitor CP 31 and a resistor R 33 are connected in parallel between the cathode of the diode D 31 and the common potential, and constitutes a pulse extension circuit 8 together with the diode D 31 .

Moreover, the cathode of the diode D 31 is connected to a gate of a P-channel MOS transistor Q 32 .

An input terminal on the positive side of a comparator C 32 (a second comparator) is connected to a drain of the P-channel MOS transistor Q 32 , a source of the P-channel MOS transistor Q 32 is connected to a positive electrode of a constant voltage source PS 32 , and a negative electrode of the constant voltage source PS 32 is connected to a common potential. Moreover, resistors R 34 and R 35 connected in series are provided between a source of the P-channel MOS transistor Q 32 and the common potential and the input terminal on the positive side of the comparator C 32 is connected to a node of the resistors R 34 and R 35 .

The P-channel MOS transistor Q 32 , the resistors R 34 and R 35 and the constant voltage source PS 32 constitute a variable d.c. voltage source 9 .

Moreover, an input terminal on the negative side of the comparator C 32 is connected to the input terminal on the positive side of the comparator C 31 , and an output of the comparator C 32 is connected to a cathode of a diode D 32 . An anode of the diode D 32 is connected to a gate of the IGBT 1 .

Next, an operation of the semiconductor device 400 will be described. A comparison potential to be used for the comparator C 31 is set to a first detection potential, and lower and higher comparison potentials which are to be used for the comparator C 32 are set to second and third detection potentials, respectively. The third detection potential is the highest and the second detection potential is the lowest.

First of all, in the case in which a sense potential is lower than the first detection potential of the comparator C 31 as an initial state, an output potential of the comparator C 31 is L , a capacitor CP 31 is sufficiently discharged, the N-channel MOS transistor Q 31 is set in an OFF state and a sense resistance is set to be so high as to be defined by the resistors R 31 and R 32 .

On the other hand, the P-channel MOS transistor Q 32 is set in an ON state and a high potential which is almost equivalent to a voltage of the constant voltage source PS 32 is given as the third detection potential to the input terminal on the positive side of the comparator C 32 so that the output of the comparator C 32 is set to H . Therefore, a gate current of the IGBT 1 is not extracted through the diode D 32 .

Then, when a collector current is increased due to a short circuit or the like so that the sense potential exceeds the first detection potential of the comparator C 31 , the output potential of the comparator C 31 is set to H , the N-channel MOS transistor Q 31 is turned ON, and the sense resistance is set to be so low as to be defined by the resistors R 31 and R 32 and an ON-state resistance of the N-channel MOS transistor Q 31 . Thus, the sense potential is dropped. At this time, the P-channel MOS transistor Q 32 is brought into an OFF state, and an electric potential obtained by dividing and dropping the voltage of the constant voltage source PS 32 by the resistors R 34 and R 35 is given as the second detection potential to the input terminal on the positive side of the comparator C 32 .

When the output potential of the comparator C 31 is set to H so that the capacitor CP 31 is charged, H is given to the gate of the P-channel MOS transistor Q 32 for some period (before an electric charge of the capacitor CP 31 is discharged with a time constant of an RC circuit constituted by the capacitor CP 31 and the resistor R 33 in the pulse extension circuit 8 ) and an OFF state is continuously maintained even if the output potential of the comparator C 31 is then changed to L . Thus, the pulse extension circuit 8 immediately outputs H if a logic to be given to an input thereof is changed from L to H , and does not immediately output L but extends a period for H if the logic to be given to the input is changed from H to L . In order to prevent the start of a protecting operation from being delayed, a logic of extension through the diode D 31 is restricted.

For this period, a short circuit state of the IGBT 1 is detected based on the second detection potential through the comparator C 32 .

In the case in which the short circuit state is set to a second pattern having a low collector voltage, a ratio of a sense current to a collector current is low. Therefore, when the comparator C 31 is operated, the collector current has already reached such a level as to be protected. Accordingly, when the sense resistance is changed to be low so that the sense potential is dropped, the ratio of the sense current to the collector current is increased and a decrease in the sense current is suppressed so that a reduction in the sense potential is stopped.

In the comparator C 32 , the sense potential is compared with the second detection potential. If the sense potential is higher, the output of the comparator C 32 is set to L and a protecting operation for extracting the gate current of the IGBT 1 through the diode D 32 is executed. Even if a gate voltage is dropped so that the collector current is reduced, the protecting operation is continuously maintained until the electric charge of the capacitor CP 31 is discharged with the time constant of the RC circuit constituted by the capacitor CP 31 and the resistor R 33 in the pulse extension circuit 8 .

On the other hand, in the case in which the short circuit state is set to a first pattern having a high collector voltage, a ratio of a sense current to a collector current is high. Therefore, when the comparator C 31 is operated, there is a possibility that the collector current has not reached such a level as to be protected. Accordingly, even if the sense resistance is changed to be low so that the sense potential is dropped, the ratio of the sense current to the collector current is less increased and the sense current is considerably decreased. Therefore, the sense potential is also dropped considerably.

In the comparator C 32 , the sense potential is compared with the second detection potential. The second detection potential is previously set based on the short circuit characteristic of the IGBT 1 such that the sense potential becomes lower in this case. Consequently, the output of the comparator C 32 maintains H so that the protecting operation for extracting the gate current of the IGBT 1 through the diode D 32 is executed.

Also in the case in which the short circuit state is set to the first pattern having a high collector voltage, the sense current is also increased sufficiently if the collector current reaches such a magnitude as to be protected. Therefore, the sense potential is also high to reach the second detection potential of the comparator C 32 , and the output of the comparator C 32 is set to L so that the protecting operation for extracting the gate current of the IGBT 1 through the diode D 32 is executed.

While the protecting operation is reset by discharging the electric charge of the capacitor CP 31 as described above, an inductive load is often used for a load to be connected to a power semiconductor element such as an IGBT. In the case in which the inductive load is used, a collector current is reduced if a gate potential of the power semiconductor element is dropped. Thus, a collector voltage is increased at a stretch. Consequently, the comparator C 32 is always operated so that the protecting operation is continuously maintained.

As described above, the semiconductor device 400 according to the fourth embodiment of the present invention comprises the protecting system for detecting the sense potential by the two comparators C 31 and C 32 and for extending, through the pulse extension circuit 8 , the period for detection of the comparator C 32 to determine the execution and stop of the protecting operation. Therefore, it is possible to prevent a malfunction from being caused by a short period for detection.

<E-1. Feature of Fifth Embodiment>

In the semiconductor device 100 according to the first embodiment described with reference to FIG. 1 , the sense potential is changed by using the variable resistor VR 1 as a sense resistor, the sense potentials for various sense resistances are generated with a time division and a variation in the sense potential for the change of the sense resistance is detected to know the short circuit state. Different sense resistors are connected to a plurality of sense emitters to obtain various sense potentials at the same time. By directly comparing the different sense potentials with each other, it is possible to know a difference in the short circuit state more efficiently.

<E-2. Structure of Device>

FIG. 8 shows a structure of a semiconductor device 500 according to a fifth embodiment of the present invention. In FIG. 8 , the same structures as those in the semiconductor device 100 shown in FIG. 1 have the same reference numerals and repetitive description will be omitted.

In FIG. 8 , an IGBT 1 A having a collector C connected to a terminal T 1 and an emitter E connected to a terminal T 2 is provided, and has sense emitters SE 1 and SE 2 connected to the terminal T 2 through resistors R 41 and R 42 having different resistance values which constitute a current and voltage converting section CV 40 . Sense potentials V SE1 and V SE2 are output from ends on the respective sense emitter SE sides of the resistors R 41 and R 42 and are given to terminals T 11 and T 12 of a current ratio detecting section 15 . The sense potential V SE1 is also given to a terminal T 5 .

In the semiconductor device 500 shown in FIG. 8 , the resistors R 41 and R 42 are connected to the sense emitters SE 1 and SE 2 respectively to obtain different sense potentials corresponding to resistance values, and the respective sense potentials are given to the current ratio detecting section 15 to detect their ratio. Thus, a pattern of a short circuit state can be known.

More specifically, the IGBT has such a characteristic that a sense current is also changed with a variation in the sense potential as described above. Therefore, if a sense resistance is varied, the sense potential is also changed. The ratio is varied for first and second patterns of the short circuit state.

For example, a ratio of the sense potentials S 3 and S 31 of the characteristic Y 1 (the short circuit state having the first pattern) is different from that of the sense potentials S 4 and S 41 of the characteristic Y 2 (the short circuit state having the second pattern) in FIG. 6 . If information about the ratio of the sense potentials for each pattern of the short circuit state is prepared, the pattern of the short circuit state can be known easily based on the ratio thus obtained.

In the division circuit 151 ( FIG. 2 ) of the current ratio detecting section 15 , the resistance value of the variable resistor VR 2 provided in the negative feedback path is changed depending on the sense potential V SE2 and a ratio of the sense potential V SE1 to the sense potential V SE2 is converted into a predetermined electric potential to be output.

The output of the division circuit 151 is given to the amplitude detection circuit 152 and an electric potential proportional to an output potential is sent as an output of the amplifier C 2 to a terminal T 4 . In the present embodiment, it is sufficient that the ratio of the sense potential V SE1 to the sense potential V SE2 is apparent. Therefore, the amplitude detection circuit 152 may not be provided and the output of the division circuit 151 may be set to the output of the amplifier C 2 .

By utilizing a difference in the output potential, it is possible to vary a short circuit protecting operation for the first and second patterns.

As described above, the semiconductor device 500 according to the fifth embodiment of the present invention comprises the protecting system for connecting the resistors R 41 and R 42 to the sense emitters SE 1 and SE 2 respectively to obtain different sense potentials corresponding to resistance values and for giving each of the sense potentials to the current ratio detecting section 15 to know the pattern of the short circuit state based on their ratio. Therefore, the different sense potentials corresponding to the resistance values are obtained at the same time and are directly compared with each other. Consequently, it is possible to reduce a time required for knowing the pattern of the short circuit state. Thus, a protecting operation can be carried out in a real time.

While the resistors R 41 and R 42 have the different resistance values in the above description, the resistance values may be equal to each other and a current ratio of the sense emitters SE 1 and SE 2 may be varied.

More specifically, the sense emitter is changed into independent electrodes obtained by electrically isolating a part of an emitter electrode on an area basis, and a current is fetched therefrom. Therefore, if each area of the electrodes to be isolated is changed, a value of the current thus fetched can also be varied.

When the current value is changed, the sense potential is varied with the same resistance value. Therefore, it is possible to obtain the same effects as those in the connection of resistors having different resistance values.