# Patent

## Title
Semiconductor switch control device

## Abstract

A semiconductor switch control device includes a current detecting unit that detects a current that flows in a semiconductor switch, a voltage detecting unit that detects a voltage, and a temperature detecting unit that detects a temperature. A transient thermal resistance value providing unit provides a transient thermal resistance value Zth in accordance with an elapsed time from reception of an excess voltage signal to a computing unit. A temperature detecting unit detects an initial temperature TJ0 when an excess voltage is produced. The computing unit calculates a temperature of the semiconductor switch by a following expression when a detected current value is represented by Ids, and a detected voltage value is represented by Vds: (expression): temperature of the semiconductor switch TJ=Ids×Vds×Zth+TJ0.

## Background
BACKGROUND
1. Field of the Invention
The present invention relates to a semiconductor switch control device, a semiconductor switch device, a load drive system, and a semiconductor switch control program. More specifically, the present invention relates to a semiconductor switch control device to protect a semiconductor switch from breakdown due to overload.
2. Description of Related Art
In recent years, computerization has been advancing in order to realize safety, comfort, and energy saving of automobiles. As an example, a mechanical relay has been replaced with a semiconductor switch, which contributes to downsizing of a device mounted on an automobile. Further downsizing has been required in the device, and in accordance therewith, miniaturization of a semiconductor chip design rule of a semiconductor switch has been advancing. More specifically, the size of a semiconductor chip has been gradually miniaturized from 1.2 μm, 1.0 μm, 0.5 μm, 0.35 μm, to 0.25 μm, and further miniaturization will be desired from now on.
Although the miniaturization of the design rule increases a density of a current that can be flowed in the semiconductor chip per unit area, a new problem of heating has been raised due to the increased current density. Especially, decrease of a breakdown tolerance due to heating in a transient state, mainly due to the heating in a load short-circuit, is a major problem. Electronic manufacturers and semiconductor manufacturers conduct research of a temperature detecting means, a current limiting means, a current detecting means and the like in order to protect semiconductor switches.
For example, a power supply control device is disclosed in a patent document 1 (Japanese Unexamined Patent Application Publication No. 2000-299631).
In FIG. 9, an ON resistance of a semiconductor switch 124 is employed as a load current detecting means. Then, a temperature of the semiconductor switch 124 is detected by temperature detecting means (D1 to D4, and a differential amplifier 105), a load current is detected by current detecting means (an ON resistance of the semiconductor switch 124 and a differential amplifier 106), and on the basis of the temperature detected result, the level of the reference potential of a comparator for judging an excess current is shifted by a driving control unit 107. When this temperature corrected reference potential is exceeded, an FET 102 is controlled to OFF. And in this way, an excess current braking function in the control of power supply from a power source 101 to a load 103 is provided.
According to this configuration, (1) heat loss of the power supply control device can be suppressed without using additional current sensing resistor (so-called shunt resistor), (2) the problem of the temperature dependency of the ON resistance of the semiconductor switch can be overcome by compensating the temperature upon detection of the load current and then the high-accuracy detection of the load current is realized, and (3) individual control for each load is enabled.
Further, a patent document 2 (Japanese Unexamined Utility Model Application Publication No. H1-37135) discloses a protection device of a power transistor.
In the patent document 2, each of a current that flows in a power transistor and a voltage that is applied is detected. Then, a product of the detected current and the voltage is calculated. Predicting an increase of the temperature when the value of the product of the current and the voltage is kept for a certain period of time, the power transistor is turned off after a certain delay time in accordance with the value of the product of the current and the voltage. By varying the time for the off control in accordance with the heating value, it is possible not to operate a protective function for the instantaneous excess current that does not cause the thermal destruction of the switch and to execute off control in accordance with time when increase of the temperature of the power transistor would cause destruction.
The off control by the current detection as disclosed in the patent document 1 appears effective. But we have now discovered that it is rarely used in practice due to its insufficiency of the response of a lamp-driven rush current as below. More specifically, when turning on the lamp as a load, the rush current that is six to ten times larger than a nominal rating current of the lamp. This rush current considerably varies depending on the temperature of the lamp itself, the voltage that is applied, and the lamp manufacturer and so on.
Further, a lamp that is not recommended by any manufacturer may be employed as a load by a user of an automobile. When the setting of the reference current Iref was improper, the rush current would exceed Iref in a lamp-driven lighting step, which makes it impossible to light the lamp due to the off control despite its normal state.
Furthermore, if the value of Iref is set to high in order to avoid such a situation, the semiconductor switch whose rated current is high is required to be used, which may increase the cost.
The patent document 2 solves the problem of the patent document 1 by considering the time elapse with the heating value, and turning off the semiconductor switch only when it is expected that the temperature of the semiconductor switch causes the thermal destruction. However we have now discovered that according to the patent document 2, the control is performed based on the heating value which is derived from the power consumption and its duration time. Any problem may be raised, for example, the semiconductor switch is destructed earlier than expected when the ambient temperature is high. On the other hand, when the ambient temperature is low, the semiconductor switch is frequently turned off even when there is no need to perform off control, which makes it impossible to realize stable operation.
SUMMARY
A first exemplary aspect of an embodiment of the present invention is a semiconductor switch control device that performs switching control of a semiconductor switch that switches power supply from a power source to a load, including a current detecting unit that detects a current that flows in the semiconductor switch, a voltage detecting unit that detects a voltage applied to the semiconductor switch, a temperature detecting unit that detects a temperature of the semiconductor switch, a computing unit that calculates the temperature of the semiconductor switch based on a detected current value that is detected by the current detecting unit, a detected voltage value that is detected by the voltage detecting unit, and a detected temperature value that is detected by the temperature detecting unit, and a driver that turns off the semiconductor switch when the temperature of the semiconductor witch calculated in the computing unit exceeds a set threshold value.
According to such a configuration, instead of sorely using a detection temperature and a power consumption, the temperature at that time is considered in addition to a current value and a voltage value, to thereby make it possible to obtain a temperature TJ of a semiconductor switch depending on its situations at that time with high accuracy. As such, as the cutoff control can be executed so that the temperature of the semiconductor switch would not exceed the set value, the semiconductor switch can surely be protected from the thermal destruction. On the other hand, unnecessary cutoff is not performed when the temperature of the semiconductor switch is below the set temperature, whereby the semiconductor switch where the stable operation is possible can be operated with high accuracy and without failure.



BRIEF DESCRIPTION OF THE DRAWINGS
The above and other exemplary aspects, advantages and features will be more apparent from the following description of certain exemplary embodiments taken in conjunction with the accompanying drawings, in which:
FIG. 1 is a diagram showing the configuration of a load drive system according to a first exemplary embodiment of the present invention;
FIG. 2 is a diagram showing a configuration example of a current detecting unit and a current limiting unit according to the first exemplary embodiment;
FIG. 3 is a diagram showing a configuration example of a voltage detecting unit according to the first exemplary embodiment;
FIG. 4 is a diagram showing a configuration example of a temperature detecting unit according to the first exemplary embodiment;
FIG. 5 is a diagram showing a configuration example of a transient thermal resistance value providing unit according to the first exemplary embodiment;
FIG. 6 is a diagram showing an example of transient thermal resistance characteristics provided by the transient thermal resistance value providing unit according to the first exemplary embodiment;
FIG. 7 is a diagram showing a configuration example of a computing unit according to the first exemplary embodiment;
FIG. 8 is a diagram showing the configuration according to a second exemplary embodiment; and
FIG. 9 is a diagram showing a related art.



DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS
First Exemplary Embodiment
The first exemplary embodiment of the present invention will now be described.
FIG. 1 is a diagram showing the configuration of a load drive system 200.
The load drive system 200 has the configuration in which a current from a battery 201 is supplied to a load (lamp, for example) 202 through a semiconductor switch device 300.
The semiconductor switch device 300 includes a semiconductor switch M0 between the battery 201 and the load 202.
Although an IGBT (Insulated Gate Bipolar Transistor), a power MOSFET, a bipolar transistor, and a MOS transistor and so on may be employed as the semiconductor switch M0, it is not limited to them.
The semiconductor switch device 300 further includes a current detecting unit 210, a current limiting unit 220, a voltage detecting unit 230, a temperature detecting unit 240, a transient thermal resistance value providing unit 250, a computing unit 260, and a driver 270.
These current detecting unit 210, the current limiting unit 220, the voltage detecting unit 230, the temperature detecting unit 240, the transient thermal resistance value providing unit 250, the computing unit 260, and the driver 270 compose a semiconductor switch control device 310.
The current detecting unit 210 detects a current Ids that flows in the semiconductor switch M0 to output a current signal SIds based on the current Ids to the computing unit 260. Further, the current limiting unit 220 limits the current Ids that flows in the semiconductor switch M0 to a current value Ilimit that is set in advance in order to prevent the current Ids from being exceeded.
Now, FIG. 2 shows a configuration example of the current detecting unit 210 and the current limiting unit 220.
In FIG. 2, M0 is a semiconductor switch. Further, a parallel transistor M01 is a transistor having the same configuration as the semiconductor switch M0. A sense resistor RS is connected to a source side of the parallel transistor M01, and the parallel transistor M01 and the semiconductor switch M0 have a common drain and a gate.
As such, a voltage that is proportional to the current Ids that flows in the semiconductor switch M0 is produced in both ends of the sense resistor RS. The voltage is amplified by a differential amplifier A1, and is output to the computing unit 260 as a current value signal SIds.
Further, a switch M1 is in ON state when the voltage that is produced in both ends of the sense resistor RS becomes higher, and a gate potential of the semiconductor switch M0 is limited.
As the gate potential of the semiconductor switch M0 is limited, the current Ids that flows in the semiconductor switch M0 is limited. The limit current value Ilimit is set to a value that does not exceed a fusing current of a bonding wire. The magnitude of the limit current value Ilimit is set and adjusted by adjusting the impedance of the driver 270, the switch M1, and the sense resistor RS.
Now, the current detecting unit 210 is composed of the parallel transistor M01, the sense resistor RS, and the differential amplifier A1, and the current limiting unit 220 is composed of the switch M1.
The voltage detecting unit 230 detects a potential difference Vds between a drain and a source of the semiconductor switch M0 and outputs the potential difference Vds to the computing unit 260. When the load 202 is in a load short-circuit state due to any failure, in other words, when the voltage between the drain and the source of the semiconductor switch M0 becomes high (Vds>3V, for example), the voltage detecting unit 230 outputs an excess voltage signal.
FIG. 3 shows an example of the voltage detecting unit 230.
The voltage Vds between the drain and the source of the semiconductor switch M0 is amplified by a differential amplifier A2 and output as a detected voltage value SVds. Further, a voltage limit value Vroc is set in advance.
The voltage limit value Vroc may be a value that corresponds to the detected voltage value SVds when the voltage Vds between the drain and the source of the semiconductor switch M0 is 3 V as an example. However, it may be set as appropriate in accordance with a withstand voltage of the semiconductor switch M0.
When the detected voltage value SVds exceeds the voltage limit value Vroc, an excess voltage signal SVexc is output from a differential amplifier A3. The excess voltage signal SVexc is an operation trigger signal of the temperature detecting unit 240 and the transient thermal resistance value providing unit 250.
The temperature detecting unit 240 holds an initial temperature TJ0 of the semiconductor switch M0 and outputs the value to the computing unit 260 by the excess voltage signal SVexc as a trigger.
An example of the temperature detecting unit 240 is shown in FIG. 4.
The temperature characteristics of the voltage decrease of a diode D1 arranged near the semiconductor switch M0 is output to a buffer B1 by a differential amplifier A4. The buffer B1 includes a differential amplifier A5 and a capacitor C for fixing the potential that is arranged in a non-inverting input terminal of the differential amplifier A5, and a switch SW is further arranged in a former stage of the capacitor C.
The switch SW is opened by the excess voltage signal SVexc from the voltage detecting unit 230, and the input voltage is held by the capacitor C.
The buffer B1 outputs the voltage of the non-inverting input terminal as a temperature signal STJ0.
The transient thermal resistance value providing unit 250 calculates a transient thermal resistance value Zth that is time-dependent from when the load short-circuit occurs, and outputs the transient thermal resistance value Zth to the computing unit 260.
FIG. 5 shows an example of the transient thermal resistance value providing unit 250.
A MOS transistor Tr0 that is connected to a capacitor C2 in parallel is cut off by the excess voltage signal SVexc from the voltage detecting unit 230, and charge is stored in the capacitor by a constant current. The voltage decrease of the capacitor C2 is amplified by a differential amplifier A6, and is output as a transient thermal resistance signal.
As the transient thermal resistance is non-linear with respect to time, the constant current that is flowed in the capacitor C2 is changed depend on the voltage decrease, so as to adjust the characteristics. In summary, serial units U1, U2, and U3 of a pMOS transistor and an nMOS transistor are arranged in parallel between the capacitor C2 and a line LVbat to which the drain of the semiconductor switch M0 is connected.
Further, four resistors are connected in series between the line LVbat and the ground power supply GND. Then, resistors are called R1, R2, R3, R4 from the ground power supply side, and a part between R1 and R2 is called nod1, a part between R2 and R3 is called nod2, and a part between R3 and R4 is called nod3 . 
The gate of the nMOS transistor of the unit U1 is connected to the nod1, the gate of the nMOS transistor of the unit U2 is connected to the nod2, and the gate of the nMOS transistor of the unit U3 is connected to the nod3.
In this configuration, the potential from the nod1 to the nod3 varies along with the time, and the nMOS transistors are sequentially turned on in accordance with the potential of each nod.
Thus, the transient thermal resistance signal SZ in accordance with the non-linear transient thermal resistance characteristics as shown in FIG. 6 is output from the transient thermal resistance value providing unit.
The computing unit 260 calculates a junction temperature TJ of the semiconductor switch M0, and when the junction temperature TJ of the semiconductor switch M0 exceeds a preset temperature (175° C., for example), the cutoff control is executed in order to protect the semiconductor switch M0 from thermal destruction.
Now, the junction temperature TJ of the semiconductor switch M0 is obtained by the following Expression 1.

TJ=Ids×Vds×Zth+TJ0  (Expression 1)

A configuration example of the computing unit 260 is shown in FIG. 7.
A first stage St1 represents a logarithm converter, which is supplied with the current signal SIds, the voltage signal SVds, and the transient thermal resistance signal SZ. A second stage St2 is an adder, which adds values obtained by logarithm-converting the aforementioned signals.
The voltage is inversed in a third stage St3, and an inverse logarithm converter in a fourth stage St4 realizes the following multiplication. (current signal SIds)×(voltage signal SVds)×(transient thermal resistance signal SZ)
In a fifth stage St5, the temperature signal STJ0 is added, and the voltage is inversed in a sixth stage St6. In a seventh stage St7, the value is compared with a predetermined temperature threshold value (VrJt). If the value exceeds the predetermined value, a junction temperature detection signal Sdet is output to the driver 270.
The driver 270 cuts off the semiconductor switch M0 upon receiving the junction temperature detection signal Sdet.
According to the first exemplary embodiment having such a configuration, the following effects can be achieved.
(1) In the first exemplary embodiment, the junction temperature TJ of the semiconductor switch M0 is obtained from the (Expression 1) by computing in the computing unit 260 not only from the voltage value Vds and the current value Ids flowing in the semiconductor switch M0 but also with the transient thermal resistance value Zth that depends on the time from the occurrence of the load short-circuit and the temperature TJ0 of the semiconductor switch M0 when the excess voltage is started to flow (when the load short-circuit occurs).
From the aforementioned description, the transient thermal resistance characteristics and the temperature when the excess voltage occurs are considered in addition to the electric energy by the current and the voltage, whereby the junction temperature TJ of the semiconductor switch M0 can be obtained in high accuracy depending on its situations.
Accordingly, the cutoff control can be carried out so that the junction temperature TJ of the semiconductor switch M0 does not exceed the set value, whereby the semiconductor switch M0 can surely be protected from the thermal destruction. On the other hand, unnecessary cutoff is not performed when the junction temperature TJ of the semiconductor switch M0 is below the set temperature, whereby the semiconductor switch M0 where the stable operation is possible can be operated with high accuracy and without failure.
There may be also a method in which a temperature sensor is provided in a vicinity of the semiconductor switch to turn off the semiconductor switch when the temperature detected by the temperature sensor exceeds the heat resistant temperature of the semiconductor switch. However, even when the temperature sensor is arranged and formed in the vicinity of the semiconductor switch, the temperature sensor cannot follow the instantaneous heating speed of the semiconductor switch due to the power consumption in the load short-circuit. Thus, such a configuration does not function in a real load short-circuit, and the semiconductor switch is often damaged by the thermal destruction. Even when it functions, serious damage may be caused to the semiconductor switch, which results in rapid deterioration of the semiconductor switch.
On the other hand, according to the exemplary embodiment, instead of simply performing the temperature detection, the temperature of the semiconductor switch M0 is accurately estimated from the transient thermal resistance Zth, the voltage decrease Vds, the current value Ids, and the junction temperature TJ0 in the occurrence of the load short-circuit of the semiconductor switch, whereby it is possible to respond to the instantaneous heating speed of the semiconductor switch M0 and to prevent the thermal destruction and the degradation of the semiconductor switch M0.
(2) Further, as the OFF control of the semiconductor switch based simply on the result of the current sensing is not performed in the first exemplary embodiment, the driving of the load under normal environment may be definitely performed.
(3) Further, when the excess current flows in the semiconductor switch M0, the current that flows in the semiconductor switch M0 is limited by the current limiting unit 220. Thus, it is possible to suppress the excess current and to protect the semiconductor switch M0. For example, the bonding wire of the semiconductor switch M0 may be protected.
Second Exemplary Embodiment
Now, the second exemplary embodiment of the present invention will be described.
FIG. 8 shows the configuration of the second exemplary embodiment.
Although the basic configuration of the second exemplary embodiment is similar to that of the first exemplary embodiment, the second exemplary embodiment has a characteristic feature in that the transient thermal resistance value providing unit and the computing unit are formed in programmable ways with software.
In FIG. 8, a semiconductor switch device 300 includes a computing control unit 280, and the computing control unit 280 includes an A/D converter (analog/digital converter) 281, a memory 282, a timer 283, and an MCU (micro controller unit) 284.
The A/D converter 281 A/D-converts the detection signals from the current detecting unit 210, the temperature detecting unit 240, and the voltage detecting unit 230 and outputs the signals to the MCU 284.
The characteristics of the transient thermal resistance value of the semiconductor switch M0 is stored in the memory 282 in advance. The memory thus forms a transient thermal resistance value providing unit. Further, the heat resistant temperature threshold value of the semiconductor switch M0 is set in the memory 282.
The timer 283 is activated as the MCU 284 has received the excess voltage signal SVexc, and constantly outputs the elapsed time from the start-up to the MCU 284.
The MCU (micro controller unit) includes a CPU and a main memory, for example, and executes the control program of the semiconductor switch. More particularly, the junction temperature TJ of the semiconductor switch M0 is calculated by the control program of the semiconductor switch, and the control signal is output to the driver 270 in accordance with the junction temperature TJ.
The computing unit is composed of the MCU.
In such a configuration, each output of the current detecting unit 210, the voltage detecting unit 230, and the temperature detecting unit 240 is transmitted to the micro controller unit (MCU) 284 through the A/D converter 281. When the load 202 becomes in a load short-circuit state due to any failure, in other words, when the voltage Vds between the drain and the source of the semiconductor switch M0 becomes high (Vds>3V, for example), the following processing is executed.
In summary, the temperature detecting unit 240 detects the temperature TJ0 of the semiconductor switch M0 by the excess voltage signal SVexc by the voltage detecting unit 230 as a trigger, and supplies the temperature TJ0 to the micro controller unit 284 through the A/D converter 281. Then, the micro controller unit 284 holds the initial junction temperature TJ0 in the micro controller unit. The micro controller unit 284 activates the timer 283 at the same time as the hold of the initial junction temperature TJ0. The timer 283 constantly outputs the elapsed time from the start-up to the micro controller unit 284. The micro controller unit 284 obtains the transient thermal resistance value Zth that corresponds to the elapsed time received from the timer 283 by referring to the transient thermal resistance characteristics of the memory 282. Then, the micro controller unit 284 calculates the junction temperature TJ of the semiconductor switch by the following calculation.

TJ=Ids×Vds×Zth+TJ0 

When the junction temperature TJ of the semiconductor switch M0 which is the calculation result exceeds the temperature that is set in advance, the micro controller unit 284 indicates the driver 270 to cut off the semiconductor switch M0.
Note that limiting the current volume flowing in the semiconductor switch M0 to the predetermined current value Ilimit to prevent the overcurrent by the current limiting unit 220 at the same time as the above control operation is similar to the operation of the first exemplary embodiment.
According to the second exemplary embodiment, as the computing control unit 280 is formed by software, various parameters can be readily set or changed to optimal values in accordance with the characteristics of the load, the semiconductor switch, and requirement by a user. As a result, optimal control in accordance with the application can be performed and products with increased availability that meet the requirement of a user can be offered.
The present invention is not limited to the above-described exemplary embodiments, but various modifications may be made without departing from the spirit of the present invention that is already described.
Although the configuration including the current limiting unit is shown as the semiconductor switch controlling unit (semiconductor switch control device), the current limiting unit may be omitted as long as the main purpose of protecting the semiconductor switch from the thermal destruction is attained without the current limiting unit.
Although the temperature detecting unit detects the temperature by the diode that is arranged near the semiconductor switch in the above exemplary embodiments, a plurality of diodes may be arranged instead of one, or a bipolar transistor may be employed instead of the diode.
The first and second exemplary embodiments can be combined as desirable by one of ordinary skill in the art.
While the invention has been described in terms of several exemplary embodiments, those skilled in the art will recognize that the invention can be practiced with various modifications within the spirit and scope of the appended claims and the invention is not limited to the examples described above.
Further, the scope of the claims is not limited by the exemplary embodiments described above.
Furthermore, it is noted that, Applicant's intent is to encompass equivalents of all claim elements, even if amended later during prosecution.

## Claims

1. A semiconductor switch control device that performs switching control of a semiconductor switch that switches power supply from a power source to a load, comprising:
a current detecting unit that detects a current that flows in the semiconductor switch;
a voltage detecting unit that detects a voltage applied to the semiconductor switch;
a temperature detecting unit that detects a temperature of the semiconductor switch;
a computing unit that calculates the temperature of the semiconductor switch based on a detected current value that is detected by the current detecting unit, a detected voltage value that is detected by the voltage detecting unit, and a detected temperature value that is detected by the temperature detecting unit;
a driver that turns off the semiconductor switch when the temperature of the semiconductor switch calculated in the computing unit exceeds a set threshold value;
a transient thermal resistance value providing unit that obtains transient thermal resistance characteristics of the semiconductor switch along with time elapse and outputs the transient thermal resistance characteristics,
wherein
the voltage detecting unit outputs an excess voltage signal when the detected voltage exceeds a set threshold value;
the transient thermal resistance value providing unit provides a transient thermal resistance value Zth in accordance with an elapsed time from reception of the excess voltage signal to the computing unit;
when the temperature detecting unit receives the excess voltage signal from the voltage detecting unit, the temperature detecting unit detects a temperature of the semiconductor switch at this time, and outputs the detected temperature to the computing unit as an initial temperature TJ0, and

the computing unit calculates a temperature TJ of the semiconductor switch by a following expression, where the detected current value is represented by Ids, and the detected voltage value is represented by Vds:
(expression): temperature of the semiconductor switch TJ=Ids×Vds×Zth×TJ0.

2. The semiconductor switch control device according to claim 1, comprising:
a current limiting unit including a switch that is connected to a gate of the semiconductor switch, the switch being in an ON state in accordance with a value of a current that flows in the current detecting unit; and
the current limiting unit makes the switch be in an ON state to limit a gate potential of the semiconductor switch when the current flowing in the current detecting unit exceeds a predetermined limit current value, whereby the current limiting unit limits a current value flowing in the semiconductor switch.

3. A semiconductor switch device that comprises the semiconductor switch control device according to claim 1, and a semiconductor switch controlled by the semiconductor switch device.

4. A load drive system that comprises the semiconductor switch device according to claim 3, and a load that is supplied with power through the semiconductor switch device.

5. A semiconductor switch control device that comprises a current detecting unit that performs switching control of a semiconductor switch that switches power supply from a power source to a load to detect a current that flows in the semiconductor switch; a voltage detecting unit that detects a voltage applied to the semiconductor switch; a temperature detecting unit that detects a temperature of the semiconductor switch; and a driver that turns off the semiconductor switch when the temperature of the semiconductor switch exceeds a set threshold value; and a computer,
wherein a semiconductor switch control program product stored in a computer readable medium causes the computer to operate as:
a transient thermal resistance value providing unit that calculates a transient thermal resistance value Zth of the semiconductor switch in accordance with an elapsed time from when it is detected by the voltage detecting unit that an excess voltage is applied to the semiconductor switch;
a computing unit that computes the temperature of the semiconductor switch based on a detected current value, a detected voltage value, a detected temperature value, and the transient thermal resistance value;
when the temperature detecting unit receives the excess voltage signal from the voltage detecting unit, the temperature detecting unit detects a temperature of the semiconductor switch at this time, and outputs the detected temperature to the computing unit as an initial temperature TJ0, and
the computing unit calculates a temperature TJ of the semiconductor switch by a following expression, where the detected current value is represented by Ids, and the detected voltage value is represented by Vds:

(expression): temperature of the semiconductor switch TJ=Ids×Vds×Zth×TJ0.