JP2014103767A - Switching device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress an influence of a voltage drop of a resistor for current detection upon temperature dependency of a resistor for temperature detection.SOLUTION: The switching device comprises: an inverter INV; power lines L1, L2; a resistor R4 for current detection inserted into the power line L2; a resistor Rth for temperature detection; and a constant current source 3A. One end of the resistor Rth for temperature detection is connected with the power line L2 between the resistor R4 for current detection and the inverter INV. One end of the constant current source 3A is connected with the other end of the resistor Rth for temperature detection, and a DC potential Vcc is supplied to the other end of the constant current source 3A. A potential V2 is obtained at the other end of the resistor Rth for temperature detection.

Description

  The present invention relates to a technique for measuring the temperature of a semiconductor switching circuit.

  Inverters and choppers are known as semiconductor switching circuits that perform switching to convert a DC voltage into another voltage. The inverter converts a DC voltage into an AC voltage, and the chopper boosts / decreases the DC voltage and converts it to another DC voltage. The chopper may be employed as a power factor correction circuit.

  Whether it is an inverter or a chopper, there is a need to detect a current flowing through the semiconductor switching circuit (hereinafter referred to as “switching current”) in order to control switching of the semiconductor switching circuit or to detect an overcurrent.

  As a configuration for detecting the switching current, a resistor (hereinafter referred to as “current detection resistor”) is employed in order to easily realize this. The current detection resistor is provided on one of a pair of power supply lines for supplying a DC voltage input to the semiconductor switching circuit. In order to simplify the configuration for detecting the switching current from the current drop in the current detection resistor, the current detection resistor is often provided on the low-potential-side power line among the power lines.

  On the other hand, there is also a demand for detecting the temperature of the semiconductor switching circuit in order to prevent the semiconductor switching circuit from being destroyed by overheating. As a configuration for detecting the temperature, a resistor (hereinafter referred to as a “temperature detection resistor”) is employed in order to easily realize this. The temperature detection resistor may be a normal resistor (because its resistance value has temperature dependence), but a thermistor (or posistor) with higher sensitivity is often employed.

  The temperature detection resistor is required to be provided near the semiconductor switching circuit in order to detect the temperature of the semiconductor switching circuit. Since a temperature change in the temperature detection resistor can be detected as a temperature change in the resistance value of the temperature detection resistor, a current is supplied to the temperature detection resistor to detect a voltage drop in the temperature detection resistor. In order to easily configure a circuit for supplying such a current and measuring a voltage drop, the temperature detection resistor is often provided between a low-potential power line and a predetermined potential point.

  As described above, Patent Document 1 discloses a technique in which a current detection resistor is provided on a low-potential side power line, and a temperature detection resistor having one end connected to the low-potential side power line.

Japanese Patent No. 4639950

  As shown in Patent Document 1 described above, it is desirable that the temperature detection resistor is provided close to the semiconductor switching circuit in order to detect the temperature of the semiconductor switching circuit. Therefore, it is desirable that one end of the temperature detection resistor is connected to the low potential side power supply line between the current detection resistor and the semiconductor switching circuit.

  However, in such a connection mode, the switching current tends to be an error factor in temperature detection. This is because the change of the switching current changes the voltage drop of the current detection resistor, and the change changes the potential of one end of the temperature detection resistor.

  For example, in FIG. 4 of Patent Document 1, the voltage drop in the current detection resistor is the voltage Vsh, the voltage drop in the series connection of the current detection resistor and the temperature detection resistor is the voltage V1, and the current detection resistor and the temperature detection. A voltage applied to the series connection of the resistor for use and the voltage dividing resistor is indicated by a voltage VDD. Thus, the detected voltage V1 is obtained by the following equation (1) by introducing the resistance value R51 of the temperature detection resistor and the resistance value R53 of the voltage dividing resistor.

  The first term on the right side depends not only on the variation of the resistance value R51 of the temperature detection resistor but also on the variation of the voltage drop Vsh of the current detection resistor. On the other hand, since the voltage VDD is normally maintained at a constant voltage and the temperature dependency of the resistance value R51 of the voltage dividing resistor is small, it can be seen that the second term depends only on the fluctuation of the resistance value R51 of the temperature detection resistor. it can. That is, from the viewpoint of the S / N ratio for the voltage V1, the first and second terms correspond to noise and signal, respectively.

  Therefore, it is desirable to reduce the ratio of the first term on the right side to the second term on the right side. For example, it is desirable to set R53 << R51. However, the change in the second term on the right side with respect to the change in the resistance value R51 is reduced. This is undesirable because it reduces the sensitivity to temperature detection of voltage V1.

  Such a decrease in sensitivity is not particularly desirable in the case where digital processing is performed by A / D converting the voltage V1 in that the influence of quantization error linearity in the digital processing increases and temperature detection accuracy deteriorates.

  From the above viewpoint, an object of the present invention is to reduce the influence of the voltage drop of the current detection resistor on the temperature dependence of the temperature detection resistor.

  A first aspect of the switching device according to the present invention includes a semiconductor switching circuit (INV, CH) that performs switching to convert a DC voltage into another voltage, and a high potential side that supplies the DC voltage to the semiconductor switching circuit. The power supply line (L1), the low potential side power supply line (L2), the current detection resistor (R4) inserted into the low potential side power supply line, and the current detection resistor and the semiconductor switching circuit between A temperature detection resistor (Rth) having one end connected to the low potential side power supply line and the other end, a substrate (2) on which at least the semiconductor switching circuit and the temperature detection resistor are mounted, and the temperature detection A constant current source (3) for supplying a constant current (I) to the other end of the resistor (Rth).

  A second aspect of the switching device according to the present invention is the first aspect, in which the constant current sources (3A, 3B) have a potential higher than a first potential (GND) provided by the low potential side power supply line. Has one end connected to a second potential supply point for supplying a high DC second potential (Vcc), the other end for outputting the constant current (I), and current setting resistors (R1, R2). . The constant current is obtained based on a current obtained from the second potential supply point. The value of the constant current is set by the reciprocal of the resistance value of the current setting resistor.

  A third aspect of the switching device according to the present invention is the second aspect, in which the temperature coefficient of the resistance value of the temperature detection resistor (Rth) and the current setting resistors (R1, R2) The temperature coefficient of the resistance value has a different sign from each other, and the current setting resistor is also placed on the substrate (2).

  A fourth aspect of the switching device according to the present invention is the third aspect thereof, wherein a resistance value of the temperature detection resistor (Rth) has a positive temperature coefficient, and the current setting resistor (R1, R1) has a positive temperature coefficient. The resistance value of R2) has a negative temperature coefficient.

  A fifth aspect of the switching device according to the present invention is any one of the second to fourth aspects, wherein the current setting resistor (R1) has one end connected to the second potential supply point. And the other end. The constant current source (3A) further includes an operational amplifier (32) and a current amplification element (31).

  The operational amplifier has one end connected to the second potential supply point and the other end, a constant voltage element (33) supporting a constant voltage (Vd), the other end of the constant voltage element, and the current A pair of input ends connected to the other end of the setting resistor and an output end are included.

  The current amplification element includes a control electrode connected to the output terminal of the operational amplifier, a current input electrode connected to the other end of the current setting resistor, and the other end of the temperature detection resistor (Rth). Includes connected current output electrodes.

  A sixth aspect of the switching device according to the present invention is any one of the second to fourth aspects, wherein the constant current source (3B) includes a three-terminal regulator including an input end, an output end, and a control end ( 34).

  The input terminal of the three-terminal regulator is connected to the second potential supply point. The output terminal of the three-terminal regulator is connected to one end of the current setting resistor (R2). The control terminal of the three-terminal regulator is connected to the other end of the current setting resistor and the other end of the temperature detection resistor (Rth).

  According to the first aspect of the switching device of the present invention, the fluctuation of the resistance value of the temperature detection resistor is estimated from the potential of the other end of the temperature detection resistor, and the temperature fluctuation of the semiconductor switching circuit is estimated. . Moreover, this estimation is not easily affected by the voltage drop at the current detection resistor.

  According to the second aspect of the switching device of the present invention, the value of the constant current can be controlled by the resistance value of the current setting resistor.

  According to the third aspect of the switching device of the present invention, since the current setting resistor is mounted on the same substrate as the semiconductor switch circuit, the dependence of the potential at the other end of the temperature detection resistor on the temperature change is increased. Therefore, the sensitivity of temperature detection increases.

  According to the fourth aspect of the switching device of the present invention, the temperature of the semiconductor switching circuit rises by operation, but there is an upper limit in the allowable range of temperature. Therefore, the estimated temperature of the semiconductor switching circuit needs to be improved on the high temperature side. The resistance value of the temperature detection resistor has a positive temperature coefficient, so that the resistance value increases as the temperature increases, and the resistance value of the current setting resistor has a negative temperature coefficient, so that the constant current increases as the temperature increases. Increase. Therefore, the potential at the other end of the temperature detection resistor is less susceptible to the voltage drop at the current detection resistor.

  According to the fifth aspect of the switching device of the present invention, the current obtained by dividing the constant voltage supported by the constant voltage element by the resistance value of the current setting resistor due to the virtual short circuit between the pair of input terminals of the operational amplifier is a constant current. As obtained.

  According to the sixth aspect of the switching device of the present invention, the constant current source can be configured with a small number of parts.

It is a circuit diagram which shows the structure of the switching apparatus concerning 1st Embodiment. It is a top view which shows the structure of the switching apparatus concerning 1st Embodiment. It is a circuit diagram which shows the structure of the switching apparatus concerning 2nd Embodiment. It is a top view which shows the structure of the switching apparatus concerning 2nd Embodiment. It is a circuit diagram which shows the structure of the switching apparatus concerning 3rd Embodiment. It is a top view which shows the structure of the switching apparatus concerning 3rd Embodiment. It is a circuit diagram which shows the structure of the switching apparatus concerning 4th Embodiment. It is a top view which shows the structure of the switching apparatus concerning 4th Embodiment.

First embodiment.
FIG. 1 is a circuit diagram showing the configuration of the switching device according to the first embodiment. The inverter INV as a semiconductor switching circuit has switching elements Q1 to Q6. The DC voltage Vdc between the power supply lines L1 and L2 is applied to the series connection of the current detection resistor R4 and the inverter INV. The DC voltage supplied to the inverter INV is converted into three-phase AC voltages Vu, Vv, Vw by switching of the switching elements Q1 to Q6.

  A current detection resistor R4 is inserted into the power line L2 on the low potential side. The end of the current detection resistor R4 opposite to the inverter INV is grounded. The potential of the ground GND is zero.

  One end of the temperature detection resistor Rth is connected to the power supply line L2 between the current detection resistor R4 and the inverter INV. A constant current I is supplied from the constant current source 3A to the other end of the temperature detection resistor Rth.

  The substrate 2 mounts at least the inverter INV and the temperature detection resistor Rth.

  A voltage drop V4 occurs in the current detection resistor R4 due to the current flowing through the inverter INV, and the potential at one end of the temperature detection resistor Rth takes the value V4. Since the constant current I is supplied to the temperature detection resistor Rth, if the symbol Rth is also used as the resistance value of the temperature detection resistor Rth, the voltage drop at the temperature detection resistor Rth is obtained as the product I · Rth. Therefore, the potential V2 at the other end of the temperature detection resistor Rth is obtained by the following equation (2).

  Here, the first term is a voltage drop at the temperature detection resistor Rth, and in addition to the contribution of the constant current I, there is also the contribution of the current flowing through the inverter INV. Therefore, from the viewpoint of the S / N ratio with respect to the potential V2, the first term and the second term correspond to noise and a signal, respectively.

  Even if the constant current I and the resistance value R51 are selected so as to increase the product I · Rth, which is the second term on the right side, they do not increase the first term. This is because the current flowing through the inverter INV is usually much larger than the constant current I, and even if the constant current I is increased, the contribution of the constant current I to the voltage drop V4 is very small.

  Thus, by using the constant current source 3A, the product I · Rth can be made larger than the voltage drop V4, and the S / N ratio for the potential V2 can be set large.

  As described above, the fluctuation of the resistance value Rth of the temperature detection resistor Rth from the potential V2 at the other end of the temperature detection resistor Rth, and the temperature fluctuation of the inverter INV are estimated. Moreover, this estimation is less susceptible to fluctuations in the voltage drop at the current detection resistor R4.

  In particular, it is desirable to increase the constant current I in order to increase the variation of the product I · Rth with respect to the variation of the resistance value Rth. That is, the sensitivity to temperature detection of the potential V2 can be controlled by the constant current I. In other words, according to the present embodiment, it is possible to control to improve both the S / N ratio and the sensitivity by controlling the constant current I.

  Of course, as described above, it is desirable to set the constant current I to a negligible magnitude with respect to the current flowing through the inverter INV, but it is desirable to limit it to a value smaller than that. This is a request from the viewpoint of A / D conversion of the potential V2.

  For example, the constant current source 3A is connected to a potential supply point to which the DC potential Vcc is supplied (also using the symbol Vcc). The DC potential Vcc is higher than the potential provided by the power supply line L2 (here, the potential 0 of the ground GND). The DC potential Vcc used for driving the constant current source 3A in this way is also used as a potential for driving a processing device that performs signal processing on the potential V2. Therefore, the width of the fluctuation range of the potential V2 is preferably within the range of 0 to Vcc.

  That is, when the resistance value R51 corresponding to the temperature detection range to be detected is represented by the values R51a to R51b, it is desirable that the following expression (3) is satisfied in view of the second term on the right side of the expression (2). .

  The constant current source 3A has one end connected to the potential supply point Vcc and the other end that outputs the constant current I. The other end is connected to the other end of the temperature detection resistor Rth, and the constant current I is obtained based on the current obtained from the potential supply point Vcc.

  The constant current source 3A further includes a current setting resistor R1. The constant current value I is set by the reciprocal of the resistance value of the current setting resistor R1. It is desirable that the constant current source 3A has the current setting resistor R1 from the viewpoint that the value of the constant current I can be controlled by the resistance value.

  More specifically, the constant current source 3 </ b> A further includes a transistor 31, an operational amplifier 32, and a constant voltage element 33.

  The operational amplifier 32 has a positive input terminal, a negative input terminal, and an output terminal. The constant voltage element 33 is a Zener diode, for example, and one end thereof is connected to the potential supply point Vcc. The other end is connected to the positive input end of the operational amplifier 32, and a potential lower than the DC potential Vcc by a constant voltage Vd is applied thereto.

  One end of the current setting resistor R1 is connected to the potential supply point Vcc, and the other end is connected to the negative side input terminal of the operational amplifier 32.

  The transistor 31 is a current amplifying element, and includes an emitter as a current input electrode, a collector as a current output electrode, and a base as a control electrode. The base is connected to the output terminal of the operational amplifier 32, and the emitter is connected to the other end of the current setting resistor R1 (and thus to the negative input terminal of the operational amplifier 32). The collector is connected to the other end of the temperature detection resistor Rth and outputs a constant current I.

  Note that the other end of the constant voltage element 33 is grounded via a resistor R3 in order to cause the current to flow through the constant voltage element 33 to maintain the constant voltage Vd.

  In such a constant current source 3A, a constant current I is obtained by dividing the constant voltage Vd supported by the constant voltage element 33 by the resistance value of the current setting resistor R1 due to a virtual short circuit between the pair of input terminals of the operational amplifier 32. can get.

  The temperature coefficient of the resistance value of the current setting resistor R1 is different from the sign of the temperature coefficient of the resistance value of the temperature detection resistor Rth, and the current setting resistor R1 is also placed on the substrate 2. desirable. This is because the dependence of the potential V2 on the temperature change is increased, and the sensitivity of temperature detection is increased.

  In particular, it is desirable that the temperature coefficient of the resistance value of the current setting resistor R1 is negative and the temperature coefficient of the resistance value of the temperature detection resistor Rth is positive. The reason is as follows.

  In general, the temperature of a semiconductor switching circuit (in this case, the inverter INV) rises due to operation. And there is an upper limit in the allowable range of the temperature. Therefore, the estimated temperature of the semiconductor switching circuit needs to be improved on the high temperature side. When the resistance value of the temperature detection resistor Rth has a positive temperature coefficient, the resistance value increases as the temperature increases, and when the resistance value of the current setting resistor R1 has a negative temperature coefficient, the resistance value increases as the temperature increases. The current I increases. Thereby, the potential V2 becomes less susceptible to the influence of the voltage drop V4 at the current detection resistor R4.

  FIG. 2 is a plan view showing the configuration of the switching device according to this embodiment. In order to increase the temperature measurement accuracy of the inverter INV, it is desirable that the inverter INV and the temperature detection resistor Rth are mounted on the same substrate 2. However, FIG. 2 illustrates the case where the current setting resistor R1 and the current detection resistor R4 are also mounted on the same substrate 2.

  Thus, the inverter INV and the temperature detection resistor Rth are mounted on the same substrate 2 and connected by the conductor pattern 21. Since the conductor pattern 21 is made of, for example, a metal whose main component is copper, the heat conduction is also good, and the resistance value of the temperature detection resistor Rth reflects the temperature of the inverter INV well.

  The conductor pattern 22 causes a constant current I to flow through the temperature detection resistor Rth.

Second embodiment.
FIG. 3 is a circuit diagram showing a configuration of a switching device according to the second embodiment. The chopper CH, which is a semiconductor switching circuit, has a switching element Q0 and a diode D. The DC voltage Vdc is applied to a series connection of the coil 4 inserted in the power supply line L1, the chopper CH, and the current detection resistor R4. A current detection resistor R4 is inserted into the power supply line L2, and the end of the current detection resistor R4 opposite to the chopper CH is grounded.

  One end of the temperature detection resistor Rth is connected to the power supply line L2 between the current detection resistor R4 and the chopper CH. The connection relationship between the constant current source 3A and the temperature detection resistor Rth is the same as in the first embodiment.

  The chopper CH forms a step-up chopper together with the coil 4 and applies a DC voltage V0 to the capacitor C connected to the outside. That is, it can be understood that the chopper CH cooperates with the coil 4 to convert the DC voltage.

  Also in this case, a voltage drop V4 occurs in the current detection resistor R4 due to the current flowing through the chopper CH. Therefore, by adopting the constant current source 3A, the same effect as that of the first embodiment can be obtained.

  FIG. 4 is a plan view showing the configuration of the switching device according to this embodiment. In order to increase the temperature measurement accuracy of the chopper CH, it is desirable that the chopper CH and the temperature detection resistor Rth are mounted on the same substrate 2. However, FIG. 4 illustrates the case where the current setting resistor R1 and the current detection resistor R4 are also mounted on the same substrate 2.

  Since the coil 4 and the capacitor C are larger in shape than the current setting resistor R1, the current detection resistor R4, and the temperature detection resistor Rth, they may not be mounted on the substrate 2.

  Similarly to the inverter INV shown in the first embodiment, the chopper CH is mounted on the same substrate 2 as the temperature detection resistor Rth and is connected by the conductor pattern 21. Therefore, the resistance value of the temperature detection resistor Rth favorably reflects the temperature of the chopper CH.

Third embodiment.
FIG. 5 is a circuit diagram showing a configuration of a switching device according to the third embodiment. The configuration of the switching device according to the present embodiment is obtained by replacing the constant current source 3A in the configuration of the switching device according to the first embodiment with a constant current source 3B. That is, the constant current I is supplied from the constant current source 3B to the other end of the temperature detection resistor Rth.

  For example, the constant current source 3B has one end connected to the potential supply point Vcc and the other end that outputs the constant current I. The other end is connected to the other end of the temperature detection resistor Rth, and the constant current I is obtained based on the current obtained from the potential supply point Vcc.

  The constant current source 3B further has a current setting resistor R2. The constant current value I is set by the reciprocal of the resistance value of the current setting resistor R2. It is desirable that the constant current source 3B has the current setting resistor R2 from the viewpoint that the value of the constant current I can be controlled by the resistance value.

  More specifically, the constant current source 3B further includes a three-terminal regulator 34. The three-terminal regulator 34 has an input terminal (terminal labeled “I” in the drawing), an output terminal (terminal labeled “O” in the drawing), and a control terminal (labeled “G” in the drawing). Terminal).

  The input end is connected to the potential supply point Vcc, the output end is connected to one end of the current setting resistor R2, and the control end is connected to the other end of the current setting resistor R2 and the other end of the temperature detection resistor Rth. . In such a constant current source 3B, the voltage between the output terminal and the control terminal of the three-terminal regulator 34 is maintained at the constant voltage Vg. The current flowing through the control end is very small and can be ignored. Therefore, a current obtained by dividing the constant voltage Vg by the resistance value of the current setting resistor R2 is obtained as the constant current I.

  Even in such a configuration, it is obvious that the same effect as in the first embodiment can be obtained by detecting the potential V2 at the other end of the temperature detection resistor Rth. Moreover, the constant current source 3B can be configured with fewer parts than the constant current source 3A.

  In addition, the constant current I can be set as in the first embodiment. For example, by setting the constant current I to a value that satisfies Equation (3), the width of the fluctuation range of the potential V2 is set to a range of 0 to Vcc. Can fit.

  Further, as described in the first embodiment, the temperature coefficient of the resistance value of the current setting resistor R2 is different from the sign of the temperature coefficient of the resistance value of the temperature detection resistor Rth, and is for current setting. It is desirable that the resistor R2 is also placed on the substrate 2. In particular, it is desirable that the temperature coefficient of the resistance value of the current setting resistor R2 is negative and the temperature coefficient of the resistance value of the temperature detection resistor Rth is positive.

  FIG. 6 is a plan view showing the configuration of the switching device according to this embodiment. In order to increase the temperature measurement accuracy of the inverter INV, it is desirable that the inverter INV and the temperature detection resistor Rth are mounted on the same substrate 2. However, FIG. 6 illustrates the case where the current setting resistor R2, the current detection resistor R4, and the three-terminal regulator 34 are also mounted on the same substrate 2.

  Thus, the inverter INV and the temperature detection resistor Rth are mounted on the same substrate 2 and connected by the conductor pattern 21. Since the conductor pattern 21 is made of, for example, a metal whose main component is copper, the heat conduction is also good, and the resistance value of the temperature detection resistor Rth reflects the temperature of the inverter INV well.

Fourth embodiment.
FIG. 7 is a circuit diagram showing a configuration of a switching device according to the fourth embodiment. The configuration of the switching device according to the present embodiment is obtained by replacing the constant current source 3A in the configuration of the switching device according to the second embodiment with the constant current source 3B described in the third embodiment. It is done.

  The constant current source 3B is described in the third embodiment, and other configurations (chopper CH, switching element Q0, diode D, coil 4, capacitor C) are described in the second embodiment. Therefore, detailed description is omitted here.

  FIG. 8 is a plan view showing the configuration of the switching device according to this embodiment. In order to increase the temperature measurement accuracy of the chopper CH, it is desirable that the chopper CH and the temperature detection resistor Rth are mounted on the same substrate 2. However, FIG. 8 illustrates the case where the current setting resistor R2, the current detection resistor R4, and the three-terminal regulator 34 are also mounted on the same substrate 2.

2 Substrate 3A, 3B Constant current source L1, L2 Power line R1 Current setting resistor R4 Current detection resistor Rth Temperature detection resistor

Claims (6)

A semiconductor switching circuit (INV, CH) that performs switching to convert a DC voltage into another voltage;
A high potential side power supply line (L1) and a low potential side power supply line (L2) for supplying the DC voltage to the semiconductor switching circuit;
A current detection resistor (R4) inserted into the low potential side power supply line;
A temperature detection resistor (Rth) having one end connected to the low-potential side power line between the current detection resistor and the semiconductor switching circuit, and the other end;
A substrate (2) on which at least the semiconductor switching circuit and the temperature detection resistor are placed;
A switching device comprising: a constant current source (3) for supplying a constant current (I) to the other end of the temperature detection resistor (Rth). The constant current source (3A, 3B)
One end connected to a second potential supply point for supplying a second DC potential (Vcc) having a higher potential than the first potential (GND) provided by the low potential power line;
The other end that outputs the constant current (I);
Current setting resistors (R1, R2) for setting the constant current value by the reciprocal of its resistance value;
The switching device according to claim 1, wherein the constant current is obtained based on a current obtained from the second potential supply point. The temperature coefficient of the resistance value of the temperature detection resistor (Rth) and the temperature coefficient of the resistance value of the current setting resistor (R1, R2) have different signs.
The switching device according to claim 2, wherein the current setting resistor is also placed on the substrate.   The switching device according to claim 3, wherein a resistance value of the temperature detection resistor (Rth) has a positive temperature coefficient, and a resistance value of the current setting resistors (R1, R2) has a negative temperature coefficient. The current setting resistor (R1) has one end connected to the second potential supply point and the other end,
The constant current source (3A)
A constant voltage element (33) having one end connected to the second potential supply point and the other end and supporting a constant voltage (Vd);
An operational amplifier (32) including a pair of input terminals connected to the other end of the constant voltage element and the other end of the current setting resistor, and an output terminal;
A control electrode connected to the output terminal of the operational amplifier, a current input electrode connected to the other end of the current setting resistor, and a current output electrode connected to the other end of the temperature detection resistor (Rth) 5. The switching device according to claim 2, further comprising a current amplifying element (31) including: The constant current source (3B)
An input terminal connected to the second potential supply point, an output terminal connected to one end of the current setting resistor (R1), the other end of the current setting resistor (R2), and the temperature detection resistor ( Three-terminal regulator (34) including a control end connected to the other end of Rth)
The switching device according to any one of claims 2 to 4, further comprising:

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