JP6569693B2 - Electronic circuit and overheat detection method - Google Patents

Description

  The present invention relates to an electronic circuit in which a heat generating component is mounted on a substrate and an overheat detection method.

  In an electronic circuit in which a heat generating component having a large heat generation amount such as a field effect transistor (FET) is mounted, it is necessary to detect a temperature rise of the heat generating component for overheating protection.

  There has been proposed a technique for detecting overheating by thermally adhering an overheat detecting element to a heat generating component (see, for example, Patent Document 1). In Patent Document 1, a silicon diode or a transistor is used as the overheat detecting element, and the heat generating component and the overheat detecting element are thermally adhered to each other so that the heat generating component and the overheat detecting element are formed on the same chip or soldered. It is described that an element fixed connection means such as the above is employed.

  In addition, a technique has been proposed in which a temperature detection element is disposed in the vicinity of a lead leg of a motor drive IC that is a heat generating component (see, for example, Patent Document 2). Patent Document 2 describes the use of a positive temperature coefficient thermistor as a temperature detection element.

JP-A-4-308416 JP 2000-188850 A

  However, in the prior art, it is necessary to provide a temperature detection element (diode, transistor, thermistor, etc.) for detecting a temperature rise. Therefore, there are problems that the cost of the temperature detecting element is increased and the number of parts is increased.

  Furthermore, when providing a temperature detection element, it is necessary to thermally couple the heat generating component and the temperature detection element, and if the thermal coupling is insufficient, the detected value is delayed with respect to the temperature change of the heat generating component, The temperature cannot be detected correctly. In order to secure thermal coupling, it is necessary to dispose the temperature detection element in the vicinity of the heat-generating component or mechanically contact it. In this case, there are disadvantages such as layout restrictions, an increase in manufacturing processes, and an increase in parts such as adhesives. In addition, variations in mature coupling from substrate to substrate can be a problem.

  An object of the present invention is to provide an electronic circuit and an overheat detection method capable of solving the above-described problems of the prior art and detecting a temperature rise of a heat generating component without providing a special temperature detection element.

The electronic circuit of the present invention is an electronic circuit in which a heat generating component is mounted on a substrate, and a wiring pattern that is a current path of a current flowing through the heat generating component includes a tin wiring portion made of tin or solder and the tin wiring. And a shunt path parallel to the section, a resistance value detecting section for detecting a resistance value of the tin wiring section as a current value flowing through the shunt path, and the temperature rise due to abnormal overheating of the heat generating component When the resistance value of the tin wiring part increases, the current flowing through the tin wiring part decreases, the current flowing through the shunt path increases, and the current value detected by the resistance value detection part exceeds a preset threshold value. And an overheat determination unit that determines abnormal overheating of the heat-generating component.
Furthermore, in the electronic circuit of the present invention, the tin wiring portion may be connected between the divided wiring patterns.
Furthermore, in the electronic circuit of the present invention, the tin wiring portion may be connected in parallel with the wiring pattern.
Furthermore, in the electronic circuit of the present invention, the tin wiring portion may be solder for connecting the electronic component serving as the current path and the wiring pattern.
The overheat detection method of the present invention is an overheat detection method for detecting abnormal overheating of a heat generating component mounted on a substrate, and a wiring pattern which is a current path of a current flowing through the heat generating component is tin or solder. And a shunt path parallel to the tin wiring section is formed, and a resistance value detecting unit detects a resistance value of the tin wiring section as a current value flowing through the shunt path, and an overheat determination unit. Is detected by the resistance value detection unit due to an increase in the resistance value of the tin wiring part due to an increase in temperature due to abnormal overheating of the heat-generating component, and a decrease in the current flowing through the tin wiring part. When the measured current value exceeds a preset threshold value, abnormal overheating of the heat generating component is determined .

  According to the present invention, the volume resistivity of tin increases around the melting temperature, that is, the resistance value of the tin wiring portion increases. Therefore, by determining the abnormal overheating of the heat generating component based on the resistance value of the tin wiring portion, a special temperature is obtained. There is an effect that it is possible to detect the temperature rise of the heat generating component without providing the detection element.

It is a circuit block diagram which shows the structure of embodiment of the electronic circuit which concerns on this invention. It is XX sectional drawing shown in FIG. It is a figure which shows the temperature-volume resistivity characteristic of the tin contained in the tin wiring part shown in FIG. It is a figure which shows the voltage value detected by the voltage detection part shown in FIG. It is a figure which shows the other example of a connection of the tin wiring part shown in FIG. It is explanatory drawing explaining the example of a connection of the tin wiring part shown in FIG. It is a figure which shows the example of a parallel connection pattern connected in parallel with the tin wiring part shown in FIG. It is a circuit block diagram which shows the structure of other embodiment of the electronic circuit which concerns on this invention. It is a figure explaining the electric current value which flows through the shunt path shown in FIG. It is an equivalent circuit of the electronic circuit shown in FIG. It is a circuit block diagram which shows the structure of other embodiment of the electronic circuit which concerns on this invention. It is a figure which shows the voltage value detected by the voltage detection part shown in FIG. It is a circuit block diagram which shows the structure of other embodiment of the electronic circuit which concerns on this invention. It is a figure which shows the voltage value detected by the voltage detection part shown in FIG.

  Referring to FIG. 1, the electronic circuit 1 of the present embodiment uses a power semiconductor Q1 such as a MOSFET as a switching element, and supplies / currents from the power source Vp to the load L by turning on / off the power semiconductor Q1. The semiconductor relay circuit 2, the drive circuit 3, the voltage detection part 4, the overheat determination part 5, and the protection circuit 6 which are interrupted | blocked are provided. The power semiconductor Q1 is a heat generating component that generates heat during operation. The voltage detection unit 4, the overheat determination unit 5, and the protection circuit 6 function as an overheat protection unit that stops the operation of the power semiconductor Q1 when the temperature of the power semiconductor Q1 rises abnormally.

  The semiconductor relay circuit 2 is disposed on the substrate 10 and the components (the power source Vp, the power semiconductor Q1, and the load L) are connected by a wiring pattern 20 formed of copper foil. This wiring pattern 20 is a current path of a current flowing through the power semiconductor Q1.

  A part of the wiring pattern 20 that connects the positive terminal of the power supply Vp and the power semiconductor Q1 is divided. The released power supply side end and the component side end are connected by a tin wiring portion SP1. The tin wiring portion SP1 is made of tin or a wiring material containing tin, for example, solder containing tin as a main component.

  Referring to FIG. 2, the surface of the wiring pattern 20 is coated with a resist 21. A predetermined area of the resist 21 on the surface is peeled off at the power supply side end and the component side end, and the portions where the resist 21 is peeled off are formed as pad portions 20a and 20b, respectively. Then, the pad portion 20a and the pad portion 20b are connected by solder to form a tin wiring portion SP1.

  The tin wiring part SP1 can be formed with a desired cross-sectional area (width × thickness) by printing using a metal mask and cream solder.

  The drive circuit 3 turns on / off the power semiconductor Q1 by a drive signal generated based on an external on / off signal.

The voltage detection unit 4 detects the voltage value V _SP1 between both ends of the tin wiring portion SP1, and outputs the detection result to the overheat determination unit 5. The voltage value V _SP1 is detected by the voltage detection unit 4 when the power semiconductor Q1 is in an on state, that is, when a constant current value I _Q1 flows through the power semiconductor Q1. Therefore, the detected voltage value V _SP1 is proportional to the resistance value R _SP1 of the tin wiring portion SP1, and the voltage detection unit 4 functions as a resistance value detection unit for detection.

The overheat determination unit 5 functions as an overheat determination unit that determines whether the power semiconductor Q1 is abnormally overheated based on the voltage value V _SP1 detected by the voltage detection unit 4. The overheat determination unit 5 compares the voltage value V _SP1 detected by the voltage detection unit 4 with a preset threshold voltage value V _th, and if the voltage value V _SP1 is equal to or higher than the voltage value V _SP1 , an abnormality is detected. When the voltage value V _SP1 is less than the voltage value V _SP1, it is determined that there is no abnormal overheating, and the determination result is output to the protection circuit 6.

The comparison between the voltage value V _SP1 detected by the voltage detection unit 4 and the threshold voltage value V _th is synonymous with the comparison between the resistance value R _SP1 of the tin wiring portion _SP1 and the threshold resistance value. Therefore, the overheat determination unit 5 can be restated as an overheat determination unit that determines whether or not the power semiconductor Q1 is abnormally overheated based on the resistance value R _SP1 of the tin wiring portion SP1.

  The protection circuit 6 is a circuit that protects the power semiconductor Q1. When the voltage detection unit 4 determines that the overheating is abnormal, the protection circuit 6 safely turns off the power semiconductor Q1.

As shown in FIG. 3, the tin contained in the tin wiring portion SP1 has a melting point of 231.93 ° C., and the temperature-volume resistivity characteristic is that the volume resistance increases when the temperature rises from 100 ° C. to 300 ° C. The rate is more than three times (see Science Chronology 2002 version). As indicated by the dotted line in FIG. 3, it is considered that the volume resistivity suddenly increases near the melting point at which melting starts. Therefore, the resistance value _Rspl of the tin wiring part SP1 _containing tin also rapidly increases in the vicinity of the melting point of tin.

The electronic circuit 1 according to the present embodiment _detects the change in the resistance value R _spl of the tin wiring portion SP1 as a change in the voltage drop (voltage value V _SP1 ) of the tin wiring portion SP1, thereby abnormally overheating the power semiconductor Q1. Is detected.

Here, the current value _{I Q1} flowing through the power semiconductor Q1 is 1000A, the resistance value _{R spl} at room temperature tin wire portion SP1 and 0.1Emuomega. In this case, the voltage value V _SP1 detected by the voltage detector 4 is 0.1V.

  Next, consider a case where abnormal heat generation occurs in the power semiconductor Q1, heat of the power semiconductor Q1 is transmitted to the tin wiring part SP1 through the wiring pattern 20, and the temperature of the tin wiring part SP1 rises to the vicinity of the melting point of tin.

When the temperature of the tin wiring part SP1 rises near the melting point of tin, the resistance value _Rspl also rises. For example, when the resistance value R _spl increases from 0.1 mΩ to 0.2 mΩ, the voltage value V _SP1 detected by the voltage detection unit 4 is 0.2V.

Therefore, the overheat determination unit 5, as shown in FIG. 4, to set the threshold voltage value _{V th} to be compared with the voltage value _{V SP1} to a value of 0.1～0.2V (e.g., 0.18 V) Thus, the temperature of the tin wiring portion SP1 can be detected near the melting point of tin, that is, abnormal overheating of the power semiconductor Q1 can be detected.

  In addition, you may make it connect the tin wiring part SP1 in parallel with the surface of the wiring pattern 20 which is not parted, as shown in FIG. For example, as shown in FIG. 5A, two pad portions are formed on the surface of the wiring pattern 20 by removing the resist 21 at a predetermined interval, and the two pad portions are connected by soldering. Tin wiring part SP1 is formed. Further, as shown in FIG. 5B, a pad part from which the resist 21 having a predetermined length is peeled off is formed on the surface of the wiring pattern 20, and solder is put on the formed pad part to form a tin wiring part SP1.

As shown in FIG. 6A, the thickness of the copper foil as the wiring pattern 20 is 70 μm, the thickness of the tin wiring part SP1 is 2 mm, and the wiring connected in parallel with the tin wiring part SP1 and the tin wiring part SP1 The pattern 20 (hereinafter referred to as a parallel connection pattern PP) has an outer shape of 10 mm × 10 mm. In this case, the resistance value _{R SP1} tin wire section SP1, and the resistance value _{R PP} parallel connection pattern PP, at 100 ° C. and 300 ° C., is as shown in Figure 6 (b). The tin wiring portion SP1 and the parallel connection pattern PP can be regarded as resistors connected in parallel as shown in FIG. Accordingly, the combined resistance value of the tin wiring portion SP1 and the parallel connection pattern PP is 0.063 mΩ at 100 ° C. and 0.17 mΩ at 300 ° C. When the temperature changes from 100 ° C. to 300 ° C., the combined resistance is 4 .7-fold change.

Here, if the current value I _Q1 flowing through the power semiconductor Q1 is 1000 A, the voltage value V _SP1 detected by the voltage detector 4 is 0.063 V at 100 ° C. and 0.17 V at 300 ° C. Accordingly, the overheat judgment section 5, the value of 0.063~0.17V the threshold voltage value _{V th} to be compared with the voltage value _{V SP1} (e.g., 0.15V) By setting the, tin wire portion SP1 The temperature rises near the melting point of tin, that is, abnormal overheating of the power semiconductor Q1 can be detected.

  Further, when the tin wiring portion SP1 is formed in parallel with the parallel connection pattern PP, the shapes of the tin wiring portion SP1 and the parallel connection pattern PP can be appropriately set. For example, as shown in FIG. 7A, a part of the parallel connection pattern PP may be a thin pattern. Further, as shown in FIG. 7B, a part of the parallel connection pattern PP may be formed in a lattice shape in which thin patterns are formed at intervals. In this case, the amount of solder that forms the tin wiring portion SP1 can be stabilized. Further, as shown in FIG. 7C, the parallel connection pattern PP may be wider than the width of the other wiring pattern 20. In this case, the solder forming the tin wiring portion SP1 can be easily placed on the parallel connection pattern PP.

Further, as in the electronic circuit 1a shown in FIG. 8, a shunt path 22 is formed in the wiring pattern 20 in parallel with the tin wiring portion SP1, and the current value I ₂ flowing through the shunt path 22 is monitored by the current detecting section 7 such as the hall IC. By doing so, it is possible to detect an increase in the resistance value R _SP1 of the tin wiring portion SP1, that is, an abnormal overheating of the power semiconductor Q1. The current value I ₂ flowing through the shunt path 22 may be detected by providing a resistance in the shunt path 22 and voltage drop.

The current detection unit 7 detects a current value I ₂ flowing through the shunt path 22 and outputs a voltage value corresponding to the current value I ₂ to the overheat determination unit 5 as a detection result.

The current value I _Q1 flowing through the power semiconductor Q1 is a value obtained by adding the current value I ₁ flowing through the tin wiring portion SP1 and the current value I ₂ flowing through the shunt path 22. Thus, with increasing temperature of the power semiconductor Q1, the resistance value R _SP1 tin wiring portion SP1 increases, as shown in FIG. 9, reduces the current I ₁ flowing through the tin wire section SP1, the current flowing through the shunt path 22 value _{I 2} is increased. Therefore, the current value I ₂ flowing through the shunt path 22 is monitored, and the voltage value output from the current detection unit 7 according to the current value I ₂ is compared with the threshold voltage value V _th set in advance in the overheat determination unit 5. By performing the overheat determination, the abnormal overheating of the power semiconductor Q1 can be monitored.

FIG. 10 is an equivalent circuit of a parallel circuit composed of the tin wiring portion SP1 and the shunt path 22. In addition, R _I1 shown in FIG. 10 is a resistance value of the wiring pattern 20 that configures the parallel pattern connected in parallel with the shunt path 22 together with the tin wiring portion SP1, and R _I2 is a resistance value of the shunt path 22. . According to this equivalent circuit, the current value I ₁ flowing through the tin wire unit SP1, shunt ratio between the current value I ₂ flowing through the shunt path 22 is determined by the ratio of the respective wiring impedance. Therefore, by increasing the thin to the wiring impedance pattern of the shunt path 22, it is possible to reduce the current value I ₂ flowing through the shunt path 22. By reducing the current value I ₂ flowing through the shunt path 22, it can be advantageously reduced scale of the current sensor used as the current detector 7.

Here, the resistance value R _I1 is O.I. It is assumed that lmΩ, resistance value R _I2 is 10 mΩ, and resistance value R _SP1 is 0.1 mΩ. When the current value I _Q1 flowing through the power semiconductor Q1 is 1000 A, the current value I ₁ flowing through the tin wiring portion SP1 is distributed to 980 A, and the current value I ₂ flowing through the shunt path 22 is distributed to 20 A. That is, by dividing the current in this way, it is sufficient to monitor 20 A instead of monitoring the current of 1000 A. Therefore, a small and inexpensive current sensor can be used as the current detector 7.

  Next, consider a case where abnormal heat generation occurs in the power semiconductor Q1, heat of the power semiconductor Q1 is transmitted to the tin wiring part SP1 through the wiring pattern 20, and the temperature of the tin wiring part SP1 rises to the vicinity of the melting point of tin.

When the temperature of the tin wiring part SP1 rises near the melting point of tin, the resistance value _Rspl also rises. For example, when the resistance value R _spl increases from 0.1 mΩ to 0.2 mΩ, the current value I ₁ flowing through the tin wiring portion SP1 is distributed from 980A to 971A, and the current value I ₂ flowing through the shunt path 22 is distributed from 20A to 29A. Is changed. Therefore, it can be detected that the power semiconductor Q1 is abnormal overheating by monitoring the increase in the current value I ₂ flowing through the shunt path 22.

Note that the current value I ₁ flowing through the tin wire unit SP1, shunt ratio between the current value I ₂ flowing through the shunt path 22, the impedance ratio with resistance in the path to shunt path 22 and tin wire section SP1 is connected May be confirmed.

  When there is a current detection resistor Rsen connected in series to the power semiconductor Q1 as in the electronic circuit 1b shown in FIG. 11, solder for connecting the current detection resistor Rsen and the wiring pattern 20 is used as tin wiring portions SP2, SP3. May be used to detect abnormal overheating of the power semiconductor Q1. In this case, the temperature resistance characteristic of the tin wiring portion SP2 close to the power semiconductor Q1 that is a heat generating component is mainly used. Moreover, the current detection resistor Rsen is an example, and solder for connecting the wiring pattern 20 to another electronic component connected in series to the power semiconductor Q1 may be used as the tin wiring portion.

In general, when current detection is performed using the current detection resistor Rsen, the voltage across the current detection resistor Rsen is detected in order to eliminate the influence of the voltage drop in the wiring pattern 20 or the solder portion. However, in the electronic circuit 1b shown in FIG. 11, the potential difference Vsen including the tin wiring portions SP2 and SP3 is detected. Thereby, as shown in FIG. 12, by setting the threshold voltage value _Vth including the voltage drop of the current detection resistor Rsen, abnormal overheating can be detected simultaneously with current detection.

As shown in FIG. 13, the voltage detection unit 4a detects the potential difference Vsen1 between both ends of the current detection resistor Rsen and the potential difference Vsen2 including the tin wiring portions SP2 and SP3, and subtracts the potential difference Vsen1 from the potential difference Vsen2. You may make it output to the overheat determination part 5. FIG. The potential difference Vsen1 is a voltage drop value of the current detection resistor Rsen. Therefore, as shown in FIG. 14, the voltage drop from the current detection resistor Rsen is subtracted from the voltage detection unit 4a to the overheat determination unit 5, and the voltage value V _SP2 across the tin wiring portion _SP2 and the tin wiring portion The total value of the voltage value V _{SP, 3} between both ends of SP3 is output. Therefore, even when changing the current value I _Q1 of the power semiconductor Q1 is, it is more reliably overheat detection.

The current value _{I Q1} is detected by the potential difference VSEN 1. Therefore, even when changing the current value I _Q1 of the power semiconductor Q1 is, the total resistance of the tin wire section SP2 and SP3 from potential Vsen2 can be accurately calculated, based on the total resistance value, the overheat judgment section 5 May perform the overheat determination.

As described above, according to the present embodiment, the electronic circuit 1, 1a, 1b in which the power semiconductor Q1, which is a heat-generating component, is mounted on the substrate 10, and the current path of the current flowing through the power semiconductor Q1. Tin wiring portions SP1, SP2, SP3 made of a wiring material containing tin or tin provided in a part of a certain wiring pattern 20, and a resistance value detecting portion (voltage) for detecting the resistance values of the tin wiring portions SP1, SP2, SP3 A detection unit 4, a current detection unit 7), and an overheat determination unit 5 that determines abnormal overheating of the power semiconductor Q <b> 1 based on the resistance value detected by the resistance value detection unit.
With this configuration, melting starts near the melting point (230 ° C.) of tin, where heat is applied to the tin wiring portions SP1, SP2, and SP3 made of tin or a wiring material containing tin. Then, the volume resistivity of tin, that is, the resistance values of the tin wiring portions SP1, SP2, and SP3 increase near the melting temperature. Therefore, by determining the abnormal overheating of the power semiconductor Q1 based on the resistance values of the tin wiring portions SP1, SP2, SP3, it is possible to detect the temperature rise of the power semiconductor Q1 without providing a special temperature detection element. Further, the temperature can be detected without special consideration of the thermal coupling of the temperature detecting element, and even when the power semiconductor Q1 suddenly generates heat, it can be immediately detected as abnormal overheating.

Further, in the present embodiment, the power semiconductor Q1 includes a protection circuit 6 that turns on and off the current supply to the load L and stops the operation of the power semiconductor Q1 when the overheat determination unit 5 determines that abnormal overheating is detected. Yes.
With this configuration, even when the power semiconductor Q1 suddenly generates heat, it can be protected immediately.

Furthermore, in the present embodiment, the tin wiring portion SP1 is connected between the divided wiring patterns 20.
With this configuration, the resistance value of the tin wiring portion SP1 can be set to a desired value depending on the length and cross-sectional area of the tin wiring portion SP1, and variations between devices can be reduced.

Furthermore, in the present embodiment, the tin wiring portion SP1 is connected in parallel with the wiring pattern 20.
With this configuration, the tin wiring portion SP <b> 1 can be easily formed simply by placing tin or a wiring material containing tin on the wiring pattern 20.

Further, in the present embodiment, the tin wiring portions SP <b> 2 and SP <b> 3 are solders that connect the electronic component (current detection resistor Rsen) serving as a current path and the wiring pattern 20.
With this configuration, the tin wiring portion SP1 can be easily formed simply by connecting the electronic component (current detection resistor Rsen) and the wiring pattern 20 with solder.

Further, in the present embodiment, the resistance value detection unit is a voltage detection unit 4 that detects the resistance value of the tin wiring unit SP1 as a voltage drop value by the tin wiring unit SP1, and the overheat determination unit 5 is the voltage detection unit 4 When the voltage drop value detected by the above exceeds a preset threshold value, an abnormal overheating of the power semiconductor Q1 is determined.
With this configuration, abnormal overheating of the power semiconductor Q1 can be easily determined by voltage comparison.

Furthermore, according to the present embodiment, the wiring pattern 20 is formed with a shunt path 22 in parallel with the tin wiring portion SP1, and the resistance value detection section flows the resistance value of the tin wiring portion SP1 through the shunt path 22. It is the current detection part 7 detected as a current value, and the overheat determination part determines abnormal overheating of the power semiconductor Q1 when the current value detected by the current detection part 7 exceeds a preset threshold value.
With this configuration, the value of the current flowing through the diversion path 22 can be controlled to a small value, and a small and inexpensive current sensor can be used as the current detection unit 7.

  As mentioned above, although this invention was demonstrated by specific embodiment, the said embodiment is an example and it cannot be overemphasized that it can change and implement in the range which does not deviate from the meaning of this invention.

DESCRIPTION OF SYMBOLS 1, 1a, 1b Electronic circuit 2 Semiconductor relay circuit 3 Drive circuit 4, 4a Voltage detection part 5 Overheating determination part 6 Protection circuit 7 Current detection part 10 Substrate 20 Wiring pattern 21 Resist 22 Shunt path L Load SP1, SP2, SP3 Tin wiring Part Q1 Power semiconductor Vp power supply

Claims (5)

An electronic circuit having a heat generating component mounted on a substrate,
In the wiring pattern that is the current path of the current flowing through the heat generating component, a tin wiring portion made of tin or solder and a shunt path parallel to the tin wiring portion are formed,
A resistance value detecting unit for detecting a resistance value of the tin wiring part as a current value flowing through the shunt path;
Due to an increase in the resistance value of the tin wiring portion due to a temperature rise due to abnormal overheating of the heat generating component, the current flowing through the tin wiring portion decreases, and the current flowing through the shunt path increases and is detected by the resistance value detecting unit. An electronic circuit comprising: an overheat determination unit that determines abnormal overheating of the heat-generating component when a current value exceeds a preset threshold value.   The electronic circuit according to claim 1, wherein the tin wiring portion is connected between the divided wiring patterns.   The electronic circuit according to claim 1, wherein the tin wiring portion is connected in parallel with the wiring pattern.   The electronic circuit according to claim 1, wherein the tin wiring portion is solder that connects the electronic component serving as the current path and the wiring pattern. An overheat detection method for detecting abnormal overheating of a heat generating component mounted on a board,
In the wiring pattern that is the current path of the current flowing through the heat generating component, a tin wiring portion made of tin or solder and a shunt path parallel to the tin wiring portion are formed,
The resistance value detection unit detects the resistance value of the tin wiring part as a current value flowing through the shunt path,
Overheat determination unit, by an increase in the resistance of the tin wire section with increasing temperature due to abnormal overheating of the heat generating component, reduces the current flowing through the tin wire portion, the resistance value detected by increasing the current flowing in the shunt path When the current value detected by the unit exceeds a preset threshold value, an abnormal overheating of the heat-generating component is determined.

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