Thermal resistance measuring method and thermal resistance measuring device

A temperature of a semiconductor element is measured based on a temperature coefficient of a voltage between the first electrode and the second electrode when no heat is generated when causing a constant current of an extent such that the semiconductor element does not generate heat to be input wherein current is caused to flow from a third electrode to a second electrode in accordance with voltage applied between a first electrode and the second electrode. Also, a constant current such that the semiconductor element generates heat is input into the third electrode, with voltage applied between the first electrode and second electrode of the semiconductor element kept constant, and power is measured based on the current such that the semiconductor element generates heat and on voltage when heat is generated between the third electrode and second electrode when the semiconductor element generates heat.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from and the benefit of Japanese Application No. 2013-134768, filed Jun. 27, 2013, the entire disclosure of which is incorporated herein by reference.

BACKGROUND

Field of the Invention

The present invention relates to a thermal resistance measuring method and thermal resistance measuring device.

Discussion of the Background

On a semiconductor device incorporated a semiconductor element, thermal resistance is measured in order to measure the heat dissipation capacity of the semiconductor device.

A ΔVgs method, whereby measurement is carried out by controlling gate voltage so that the power of the semiconductor element becomes constant (for example, refer to JP-A-2012-145354), a ΔVds method, whereby measurement is carried out by the gate voltage of the semiconductor element being kept constant (for example, refer to JP-A-11-211786), or the like, are utilized for measuring the thermal resistance of a semiconductor device when the semiconductor element is, for example, a metal-oxide semiconductor field effect transistor (MOSFET).

SUMMARY OF THE INVENTION

However, when measuring thermal resistance value using the ΔVgs method, the heat generation distribution within a MOSFET with particularly high channel resistance becomes localized when the MOSFET is caused to generate heat, it becomes necessary to estimate the size of the MOSFET on the small side, and a problem occurs in that it is not possible to appropriately carry out thermal resistance measuring.

Also, when measuring thermal resistance using the ΔVds method, the accuracy of a temperature coefficient is poor in the case of a MOSFET with particularly low on-state voltage, and a problem occurs in that it is not possible to appropriately carry out thermal resistance measuring.

Embodiments of the invention provide a thermal resistance measuring method and thermal resistance measuring device such that the accuracy of thermal resistance measuring is increased.

In order to resolve the heretofore described problems, an aspect of the invention provides a thermal resistance measuring method including a step of measuring the exterior temperature of a semiconductor device housing a semiconductor element wherein current is caused to flow from a third electrode to a second electrode in accordance with voltage applied between a first electrode and the second electrode, a step of inputting a constant current of an extent such that the semiconductor element does not generate heat, measuring a second voltage between the first electrode and second electrode of the semiconductor element, which is controlled so that a first voltage between the third electrode and second electrode of the semiconductor element is constant, and calculating the element temperature of the semiconductor element based on the second voltage and on a temperature coefficient relating to the second voltage, a step of causing a constant current such that the semiconductor element generates heat to be input between the third electrode and second electrode, with a third voltage applied between the first electrode and second electrode of the semiconductor element kept constant, a step of calculating a power based on the current such that the semiconductor element generates heat, and on a fourth voltage between the third electrode and second electrode when the semiconductor element generates heat, a step of inputting a constant current of an extent such that the semiconductor element does not generate heat, measuring a fifth voltage between the first electrode and second electrode of the semiconductor element, which is controlled so that the first voltage between the third electrode and second electrode of the semiconductor element is constant, and calculating the element temperature of the semiconductor element based on the fifth voltage and on a temperature coefficient relating to the fifth voltage, a step of measuring the exterior temperature of the semiconductor element after heat is generated, and a step of calculating the thermal resistance value of the semiconductor element based on the amount of change in the exterior temperature and the amount of change in the element temperature of the semiconductor element before and after heat is generated, and on the power.

Also, an aspect of the invention provides a thermal resistance measuring device with which the heretofore described thermal resistance measuring method is executed.

This kind of thermal resistance measuring method and thermal resistance measuring device are such that it is possible to measure thermal resistance with high accuracy.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Hereafter, embodiments will be described with reference to the drawings.

First Embodiment

A thermal resistance measuring method of a first embodiment is such that, firstly, the temperature is measured on the exterior of a semiconductor device housing a semiconductor element wherein current is caused to flow from a third electrode to a second electrode in accordance with voltage applied between a first electrode and the second electrode. Furthermore, a constant current of an extent such that the semiconductor element does not generate heat is input, a second voltage between the first electrode and second electrode is controlled so that a first voltage between the third electrode and second electrode of the semiconductor element is constant, and the element temperature of the semiconductor element is measured based on the temperature coefficient of the second voltage between the first electrode and second electrode.

Next, with a third voltage applied between first electrode and second electrode of the semiconductor element is kept constant, a constant current such that the semiconductor element generates heat is input into the third electrode, and a power between the third electrode and second electrode when the semiconductor element generates heat is measured. Then, the thermal resistance value is calculated based on the amount of change in each of the exterior temperature and element temperature before and after the semiconductor element generates heat, and on the power.

A description will be given of this kind of thermal resistance measuring method, usingFIG. 1andFIGS. 2A, 2B, and 2C.

FIG. 1is a flowchart showing the thermal resistance measuring method according to the first embodiment.

FIGS. 2A, 2B, and 2Care diagrams showing voltage and current that change in accordance with time according to the first embodiment, whereinFIG. 2Ashows a ΔVgs method,FIG. 2Ba ΔVds method, andFIG. 2Ca case utilizing the thermal resistance measuring method according to the first embodiment. Also, inFIGS. 2A to 2C, Vgs represents the temporal change of voltage between gate and source electrodes, Id the temporal change of a drain current, and Vds the temporal change of voltage between drain and source electrodes.

A semiconductor device that is the subject of thermal resistance value measuring houses a semiconductor element wherein current is caused to flow from a third electrode to a second electrode in accordance with voltage applied to a first electrode. When the semiconductor element is, for example, a MOSFET, it is possible for the first electrode to correspond to a gate electrode, the second electrode to a source electrode, and the third electrode to a drain electrode, while when the semiconductor element is an insulated gate bipolar transistor (IGBT), it is possible for the first electrode to correspond to a gate electrode, the second electrode to an emitter electrode, and the third electrode to a collector electrode.

Hereafter, the description will be given with a case wherein a MOSFET is utilized as the semiconductor element, but this is merely an example as embodiments of the invention may be applied to any applicable semiconductor device.

Firstly, usingFIG. 2A, a description will be given of a thermal resistance measuring method when utilizing only the ΔVgs method in a semiconductor device housing this kind of semiconductor element.

With the ΔVgs method, the Vds between the drain and source electrodes of the semiconductor element is taken to be a constant (VH).

An exterior temperature (Tc0) of the semiconductor device in a condition in which the semiconductor element is not generating heat is measured. Measuring of the exterior temperature of the semiconductor device can be carried out using a thermocouple, an infrared camera, or the like.

A very small constant current (Im) of an extent such that the semiconductor element does not generate heat is input into the semiconductor element, the Vgs between the gate and source electrodes at the time is measured, and an element temperature (Tjo) of the semiconductor element is calculated based on the measured Vgs (=Vm0) and a temperature coefficient (K). The temperature coefficient (K) representing the rate of change of the Vgs with respect to the temperature change of the semiconductor element is measured and calculated in advance.

Next, by a constant Id (=IH) being input into the semiconductor element, the semiconductor element is caused to operate and generate heat. The Vgs is controlled so that the Vds is constant (Va), and power PH (=IH×Va) is calculated.

After the semiconductor element is caused to generate heat, an exterior temperature (Tc1) of the semiconductor device and, based on the temperature coefficient (K) (the Vgs at this time is Vm1), an element temperature (Tj1) of the semiconductor element are measured, with the Id input into the semiconductor element as the very small constant current (Im) again.

Herein, the thermal resistance value (Rth) can generally be calculated using the following Expression 1:
Rth(j−c)={ΔTj(=Tj1−Tj0)−ΔTc(=Tc1−Tc0)}/PH(Expression 1)

By inputting the exterior temperatures (Tc0 and Tc1), element temperatures (Tj0 and Tj1), and power (PH) measured above into Expression 1, it is possible to calculate the thermal resistance value (Rth) according to the ΔVgs method.

However, when using the ΔVgs method to measure the thermal resistance value of a semiconductor device including a semiconductor element, the temperature of a channel region through which a channel current flows rises more than that of other portions in a semiconductor element with particularly high channel resistance. Because of this, the temperature distribution within the semiconductor element becomes localized, and the temperature characteristics of the current when heat is generated become positive, because of which it is not possible to appropriately carry out thermal resistance measuring. Furthermore, when the temperature distribution within the semiconductor element becomes localized, the temperature of generated heat between semiconductor elements is unequal in a semiconductor device in which multiple semiconductor elements are disposed in parallel, and it is not possible to appropriately carry out thermal resistance measuring in this case either.

Next, usingFIG. 2B, a description will be given of a thermal resistance measuring method when utilizing only the ΔVds method, in place of the ΔVgs method, in a semiconductor device housing this kind of semiconductor element.

With the ΔVds method, the Vgs between the gate and source electrodes of the semiconductor element is taken to be a constant (Bgs).

With the ΔVds method, the exterior temperature (Tc0) of the semiconductor device is measured in a condition in which the semiconductor element is not generating heat, in the same way as with the ΔVgs method. Also, with the ΔVds method, the Vds between the source and drain electrodes is measured with respect to the very small constant current (Im) of an extent such that the semiconductor element does not generate heat, and the element temperature (Tjo) of the semiconductor element is measured based on the measured Vds (=Vm0) and the temperature coefficient (K). In this case, too, the temperature coefficient (K) is measured and calculated in advance.

Next, by the constant Id (=IH) being input into the semiconductor element, the semiconductor element is caused to operate and generate heat. The Vds (=VH), which changes in accordance with the characteristics of the semiconductor element at this time, is measured, and the power PH (=IH×VH) when the semiconductor element generates heat is calculated.

After the semiconductor element is caused to generate heat, the exterior temperature (Tc1) of the semiconductor device is measured, and the element temperature (Tj1) of the semiconductor element is calculated based on the measured Vds (=Vm1) and temperature coefficient (K), with the Id input into the semiconductor element as Im again.

With the ΔVds method, it is possible to calculate the thermal resistance value (Rth) by inputting the exterior temperatures, element temperatures, and power measured in this way into Expression 1.

However, when using the ΔVds method to measure the thermal resistance value (Rth) of a semiconductor element, the accuracy of the calculated temperature coefficient (K) is extremely poor in the case of a semiconductor element with particularly low on-state voltage. Because of this, it is not possible to appropriately carry out thermal resistance measuring based on the temperature coefficient (K).

Therefore, the thermal resistance measuring method according to the first embodiment is carried out using the procedure shown inFIG. 1and the voltage and current shown inFIG. 2C.

Firstly, the exterior temperature (Tc0) of the semiconductor device housing the semiconductor element is measured before the semiconductor element generates heat (step S1).

A very small constant current (=Im) of an extent such that the semiconductor element does not generate heat is input, and the second voltage between the gate electrode (first electrode) and source electrode (second electrode) of the semiconductor element, controlled so that the first voltage between the drain electrode (third electrode) and source electrode (second electrode) of the semiconductor element is constant, is measured. The element temperature (Tj0) of the semiconductor element is calculated based on the second voltage and a temperature coefficient relating to the second voltage (step S2).

That is, in step S2, the element temperature (Tj0) of the semiconductor element is calculated using the ΔVgs method. It is taken that the temperature coefficient (K) representing the rate of change of the second voltage with respect to the temperature change at this time is calculated in advance.

The constant current (IH) such that the semiconductor element generates heat is input between the drain electrode (third electrode) and source electrode (second electrode), with the third voltage applied between the gate electrode (first electrode) and source electrode (second electrode) of the semiconductor element kept constant (step S3).

That is, in step S3, switching from the ΔVgs method, the semiconductor element is caused to generate heat by the semiconductor element being energized, using the ΔVds method.

In the semiconductor element that generates heat in this way, the power (PH=IH*VH) is measured based on the constant current (IH) such that the semiconductor element generates heat and on a fourth voltage (VH) between the drain electrode (third electrode) and source electrode (second electrode) when the semiconductor element generates heat (step S4).

After the power is measured, the current inputted to the drain electrode (third electrode) is returned from the current such that the semiconductor element generates heat to the very small constant current (Im) of an extent such that the semiconductor element does not generate heat, the element temperature (Tj1) is calculated using the ΔVgs method again, and the exterior temperature (Tc1) after the semiconductor element generates heat is measured (steps S5and S6).

Lastly, it is possible to calculate the thermal resistance value (Rth) of the semiconductor device based on the amount of change in the exterior temperature (ΔTc=Tc1−Tc0) and amount of change in the element temperature (ΔTj=Tj1−Tj0) before and after the semiconductor element generates heat, and on the power (PH) (step S7).

This kind of thermal resistance measuring method is such that the element temperature of a semiconductor element wherein current is caused to flow from a third electrode to a second electrode in accordance with the voltage applied between a first electrode and second electrode is measured based on the temperature coefficient of voltage between the first electrode and second electrode when no heat is generated when inputting a constant current of an extent such that the semiconductor element does not generate heat. This kind of ΔVgs method is such that there is an increase in the accuracy of the temperature coefficient representing the amount of change in the voltage with respect to the temperature change when no heat is generated, and there is an increase in the accuracy of ΔTj obtained from the amount of change in the voltage when no heat is generated before and after the semiconductor element generates heat.

Also, this kind of thermal resistance measuring method is such that a constant current such that the semiconductor element generates heat is input between the third electrode and second electrode, with the voltage applied between the first electrode and second electrode of the semiconductor element kept constant, and the power is measured based on the current at which the semiconductor element generates heat and on the voltage between the third electrode and second electrode when heat is generated when the semiconductor element generates heat. When causing the semiconductor element to generate heat using this kind of ΔVds method, an increase in the temperature of a channel region, which is a cause of the temperature distribution becoming localized in the semiconductor element, is suppressed, the temperature changes over the whole of the inside of the semiconductor element, and it is thus no longer necessary to estimate the semiconductor element size on the small side. Also, it is possible to suppress variation in heat generation distribution in a semiconductor device in which multiple semiconductor elements are disposed in parallel.

Consequently, it is possible to measure the thermal resistance of a semiconductor device with high accuracy using the heretofore described thermal resistance measuring method. Also, as it is no longer necessary to add an unnecessary design margin, it is possible to carry out optimum design.

Second Embodiment

In a second embodiment, a more specific description will be given of the first embodiment.

Firstly, usingFIG. 3, a description will be given of an example of a hardware configuration of a thermal resistance measuring device according to the second embodiment.

FIG. 3is a diagram showing an example of a hardware configuration of a thermal resistance measuring device according to the second embodiment.

A thermal resistance measuring device200measures the thermal resistance value of a measurement target module100, which is the subject of measuring.

The measurement target module100includes a switching element such as a MOSFET or IGBT, and is a module (semiconductor device) that is the subject of thermal resistance measuring. A description of a specific example of the measurement target module100will be given, usingFIGS. 4A and 4B.

The thermal resistance measuring device200includes, for example, a control unit210, a display unit220, an input unit230, and a measuring unit300, as shown inFIG. 3.

The control unit210further includes a central processing unit (CPU)210a, a random access memory (RAM)210b, a hard disk drive (HDD)210c, a graphic processing unit210d, and an input-output interface210e. These units are connected to each other by a bus210f.

The CPU210acentrally controls the whole of the computer by executing various kinds of programs stored in a storage medium, such as the HDD210c.

The RAM210btemporarily stores at least one portion of the programs executed by the CPU210a, and various kinds of data necessary for processing by the programs.

The HDD210cstores the programs executed by the CPU210a, and various kinds of data and the like necessary for processing by the programs.

The display unit220, to be described hereafter, is connected to the graphic processing unit210d. The graphic processing unit210dcauses an image to be displayed on a display screen of the display unit220in accordance with a command from the CPU210a.

The input unit230and measuring unit300, to be described hereafter, are connected to the input-output interface210e. The input-output interface210etransmits an input signal from the input unit230to the CPU210avia the bus210f. Also, the input-output interface210enotifies the measuring unit300via the bus210fof a measuring control signal from the CPU210a, thereby causing measuring of the measurement target module100to be executed. Also, the input-output interface210etransmits a signal representing a measuring result from the measuring unit300to the CPU210avia the bus210f.

Also, the display unit220is a display device such as a display or monitor, and can display a result of measuring the thermal resistance value of the measurement target module100, or the like, based on image information from the CPU210a.

The input unit230is an input device such as a keyboard or mouse, receives input information, such as a setting of measuring conditions or a request for a process to be executed, in accordance with an operation input from a user, and notifies the CPU210athereof.

The measuring unit300includes units for measuring the element temperature of a semiconductor element, the exterior temperature of the measurement target module100, and the like, which are used for measuring the thermal resistance value of the measurement target module100. Details of the measuring unit300will be described usingFIG. 5, to be described hereafter.

Next, usingFIGS. 4A and 4B, a description will be given of details of the measurement target module100.

FIGS. 4A and 4Bare diagrams showing the measurement target module according to the second embodiment.

FIG. 4Ais a sectional schematic view of the measurement target module100, whileFIG. 4Bshows a circuit configuration of a MOSFET110included in the measurement target module100.

The measurement target module100is such that the MOSFET110, including a gate terminal111, source terminal112, and drain terminal113connected to a gate electrode, source electrode, and drain electrode respectively (FIG. 4B), is disposed on a circuit substrate120across a solder layer130.

The MOSFET110may include silicon carbide (or silicon), and the configuration thereof is such that a diode is incorporated therein.

The circuit substrate120may include an insulating substrate120aand copper patterns120band120cformed on the front and back surfaces of the insulating substrate120a.

Furthermore, the measurement target module100is such that the circuit substrate120on which is disposed this kind of MOSFET110is disposed, via a solder layer140, on a substrate150, which is the exterior of the measurement target module100and includes, for example, copper.

This kind of measurement target module100is such that the element temperature of the MOSFET110at a point P on the front surface of the MOSFET110and the exterior temperature of the measurement target module100at a point Q on the back surface of the substrate150are measured.

Next, usingFIG. 5, a description will be given of an example of functions included in the thermal resistance measuring device according to the second embodiment.

FIG. 5is a diagram showing an example of a function block representing functions included in the thermal resistance measuring device according to the second embodiment.

The control unit210included in the thermal resistance measuring device200includes at least an information storage unit211, a measuring control unit212, a measurement value acquisition unit213, and a calculation unit214.

The information storage unit211holds a measurement value of the measurement target module100measured by the measuring unit300and a calculation result calculated by the calculation unit214, to be described hereafter. Also, the information storage unit211holds information on the temperature coefficient (K), which represents the rate of change of the voltage between the gate and source electrodes with respect to the temperature change of the MOSFET110of the measurement target module100.

The measuring control unit212controls a measuring of the measurement target module100by the measuring unit300. The measuring control unit212causes the measuring unit300to execute a measuring of the element temperature of the measurement target module100using the ΔVgs method, and causes a current such that the MOSFET110generates heat to be input into the measurement target module100, and the voltage and current at the time to be measured, using the ΔVds method. The measuring control unit212causes the measuring unit300to measure the exterior temperature of the measurement target module100using an exterior temperature measuring unit310.

The measurement value acquisition unit213receives from the measuring unit300measurement values of the element temperature, exterior temperature, current, voltage, and the like, of the measurement target module100measured by the measuring unit300, and causes the information storage unit211to hold the received measurement values.

The calculation unit214, based on the measurement values held by the information storage unit211, calculates the element temperature by dividing the voltage by the temperature coefficient, the power by multiplying the voltage with the current, and the thermal resistance value by dividing the difference between the amount of change in the element temperature and the amount of change in the exterior temperature by the power.

The processing functions of at least the measuring control unit212, measurement value acquisition unit213, and calculation unit214of the control unit210are realized by, for example, predetermined programs being executed by the CPU210aincluded in the control unit210, or by being configured of circuits, devices, or the like, that execute the processes.

Also, the measuring unit300includes the exterior temperature measuring unit310, a measuring switching circuit320, a ΔVgs measuring circuit330(first measuring unit), and a ΔVds measuring circuit340(second measuring unit).

The exterior temperature measuring unit310is a temperature measuring device, such as a thermocouple or an infrared camera, that measures the temperature of the Q point (refer toFIG. 4A) of the measurement target module100.

The measuring switching circuit320is configured of a circuit in which an arbitrary element or the like is used and, based on a control signal from the measuring control unit212, causes measuring to be executed by switching connection to the measurement target module100to the ΔVgs measuring circuit330or ΔVds measuring circuit340.

The ΔVgs measuring circuit330is configured of a circuit in which an arbitrary element or the like is used, and measures the voltage between the gate and source electrodes of the MOSFET110of the measurement target module100using the ΔVgs method.

The ΔVds measuring circuit340is configured of a circuit in which an arbitrary element or the like is used, causes a current such that the MOSFET110of the measurement target module100generates heat to be input into the MOSFET110, and measures the voltage and current of the MOSFET110at the time, using the ΔVds method.

Next, referring toFIGS. 6A and 6B, a description will be given of circuit configurations of the ΔVgs measuring circuit330and ΔVds measuring circuit340.

FIGS. 6A and 6Bare diagrams showing examples of configurations of measuring circuits according to the second embodiment that carry out the ΔVgs method and ΔVds method, respectively. These are merely examples of circuits that may be used, as other configurations are possible.

FIG. 6Ashows the ΔVgs measuring circuit330, which measures using the ΔVgs method, whileFIG. 6Bshows the ΔVds measuring circuit340, which measures using the ΔVds method.

The ΔVgs measuring circuit330includes a gate terminal331, source terminal332, and drain terminal333, connected to the gate terminal111, source terminal112, and drain terminal113(refer toFIG. 4B), respectively, of the MOSFET110, as shown inFIG. 6A.

The ΔVgs measuring circuit330includes, between the source terminal332and drain terminal333, a constant current supply334, which supplies current to the MOSFET110between the source electrode and drain electrode of the MOSFET110, and a power supply335for applying a constant voltage.

Also, the ΔVgs measuring circuit330includes, between the source terminal332and drain terminal333, a comparator336that applies voltage to the gate terminal331in response to signals from the source electrode and drain electrode of the MOSFET110so that the voltage to the gate electrode is of a predetermined value.

Furthermore, the ΔVgs measuring circuit330includes, between the gate terminal331and source terminal332, a voltmeter337that measures the voltage between the gate electrode and source electrode of the MOSFET110.

Referring toFIG. 6B, the ΔVds measuring circuit340includes a gate terminal341, source terminal342, and drain terminal343, connected to the gate terminal111, source terminal112, and drain terminal113, respectively, of the MOSFET110.

The ΔVds measuring circuit340includes, between the drain terminal343and source terminal342, a constant current supply344, which supplies current for causing the MOSFET110to generate heat between the drain electrode and source electrode of the MOSFET110, and a voltmeter345that measures the voltage between the drain electrode and source electrode of the MOSFET110.

Furthermore, the ΔVds measuring circuit340includes, between the gate terminal341and source terminal342, a power supply346for applying a constant voltage between the gate electrode and source electrode of the MOSFET110.

Next, referring toFIG. 7, a description will be given of a method of measuring the thermal resistance of the measurement target module100executed by the thermal resistance measuring device200including this kind of configuration.

FIG. 7is a flowchart showing a thermal resistance measuring process executed by the thermal resistance measuring device according to the second embodiment.

The temperature coefficient (K) is calculated in advance before the process of the flowchart shown inFIG. 7is executed. This kind of temperature coefficient (K) is calculated, for example, as described below.

The ΔVgs measuring circuit330applies voltage so that the voltage (Vds) between the source and drain electrodes of the MOSFET110becomes 20V, and causes a constant current Im (for example, 100 mA) of an extent such that the MOSFET110does not generate heat to be input (refer toFIG. 6A).

In this kind of situation, the ΔVgs measuring circuit330causes the voltage (Vgs) between the gate and source electrodes to change, and measures the voltage (Vgs) with respect to the corresponding temperature. The description of the temperature measuring device is omitted fromFIG. 6A. The measurement value acquisition unit213acquires information on the temperature corresponding to each voltage (Vgs) from the ΔVgs measuring circuit330, and causes each item of information to be held in the information storage unit211. The calculation unit214can calculate the temperature coefficient (K) by dividing the amount of change in the voltage (Vgs) by the amount of change in the temperature, based on this kind of information on the voltage (Vgs) and temperature corresponding to the voltage (Vgs) held by the information storage unit211. This kind of temperature coefficient (K) is held by the information storage unit211.

The thermal resistance measuring device200wherein this kind of advance preparation is completed, with the measurement target module100set in a predetermined position, receives a measuring start operation input with respect to the input unit230from the user. Then, the thermal resistance measuring device200is such that the control unit210executes an initial setting, such as clearing as appropriate information held by the information storage unit211, and starts the process described below.

In step S10, the measuring control unit212of the control unit210notifies the measuring switching circuit320of a signal requesting measuring using the ΔVgs method.

The measuring switching circuit320, on being notified of this kind of signal, connects the ΔVgs measuring circuit330(FIG. 6A) to the measurement target module100, and sets measuring using the ΔVgs method.

In step S20, the calculation unit214of the control unit210calculates the temperature (Tj0) at the point P of the MOSFET110, based on the voltage (Vgs=Vm0) between the gate and source electrodes of the MOSFET110measured by the ΔVgs measuring circuit330and on the temperature coefficient (K).

The calculation unit214of the control unit210causes the calculated temperature (Tj0) to be held in the information storage unit211.

Also, details of the process of step S20will be described hereafter.

In step S30, the measuring control unit212of the control unit210notifies the exterior temperature measuring unit310of a request to measure the temperature (Tc0) at the point Q of the measurement target module100.

The exterior temperature measuring unit310measures the temperature (Tc0) at the point Q of the measurement target module100, and notifies the measurement value acquisition unit213of the control unit210of the measured temperature (Tc0).

The measurement value acquisition unit213causes the information storage unit211to hold information on the temperature (Tc0) of which the measurement value acquisition unit213has been notified. The process of step30may be carried out at any time, provided that it is before step S40, to be described hereafter.

In step S40, the measuring control unit212of the control unit210notifies the measuring switching circuit320of a signal requesting measuring using the ΔVds method.

The measuring switching circuit320, on being notified of this kind of signal, switches the connection to the measurement target module100from the ΔVgs measuring circuit330to the ΔVds measuring circuit340(FIG. 6B).

In step S50, the ΔVds measuring circuit340applies 20V as the voltage (Vgs) between the gate and source electrodes of the MOSFET110, and causes a constant current (IH=100 A) to be input into the MOSFET110, thus causing the MOSFET110to generate heat.

At this time, the calculation unit214of the control unit210calculates the power (PH) based on the constant current and on the voltage (Vds) between the drain and source electrodes of the MOSFET110measured by the ΔVds measuring circuit340.

Details of the process of step S50will be described hereafter.

In step S60, the measuring control unit212of the control unit210notifies the measuring switching circuit320of a signal requesting measuring using the ΔVgs method.

The measuring switching circuit320, on being notified of this kind of signal, switches the connection to the measurement target module100from the ΔVds measuring circuit340to the ΔVgs measuring circuit330(FIG. 6A).

In step S70, in the same way as in step S20, the calculation unit214of the control unit210calculates the temperature (Tj1) at the point P of the MOSFET110, based on the voltage (Vgs=Vm1) between the gate and source electrodes of the MOSFET110measured by the ΔVgs measuring circuit330after heat is generated, and on the temperature coefficient (K).

The calculation unit214of the control unit210causes the calculated temperature (Tj1) to be held in the information storage unit211.

Also, details of the process of step S70will be described hereafter.

In step S80, in the same way as in step S30, the measuring control unit212of the control unit210notifies the exterior temperature measuring unit310of a request to measure the temperature (Tc1) at the point Q of the measurement target module100, including the MOSFET110, after heat is generated.

The exterior temperature measuring unit310measures the temperature (Tc1) at the point Q of the measurement target module100, and notifies the measurement value acquisition unit213of the control unit210of the measured temperature (Tc1). The measurement value acquisition unit213causes the information storage unit211to hold information on the temperature (Tc1) of which the measurement value acquisition unit213has been notified.

The process of step S80should be executed as soon as possible, for example, within 100 μs, after the process of step S50ends. In step S90, the calculation unit214of the control unit210, referring to the information storage unit211, inputs the element temperatures (Tj0 and Tj1) and exterior temperatures (Tc0 and Tc1) measured and calculated in steps S20, S70, S30, and S80, and the power (PH), into Expression 1, and calculates the thermal resistance value (Rth).

The calculation unit214causes the information storage unit211to hold information on the calculated thermal resistance value (Rth). By executing the processes in accordance with the heretofore described flowchart, it is possible to calculate the thermal resistance value (Rth) of the measurement target module100including the MOSFET110.

Next, referring toFIGS. 8A and 8B, a description will be given of details of the processes executed in steps S20, S50, and S70of the heretofore described flowchart.FIGS. 8A and 8Bare flowcharts showing details of the thermal resistance measuring process executed by the thermal resistance measuring device according to the second embodiment.

FIG. 8Ashows a flowchart of step S20(the process of calculating the element temperature of the module before heat generation) and step S70(the process of calculating the element temperature of the module after heat generation), whileFIG. 8Bshows a flowchart of step S50(the module heat generating process). Also, as the same process is executed in steps S20and S70, steps S20and S70are shown together inFIG. 8A.

Firstly, a description will be given of the process of steps S20and S70. In steps S21and S71, the ΔVgs measuring circuit330measures the voltage (Vgs=Vm0, Vm1) between the gate and source electrodes of the MOSFET110.

The ΔVgs measuring circuit330notifies the measurement value acquisition unit213of the control unit210of the measured voltage (Vgs=Vm0, Vm1). The measurement value acquisition unit213causes the information storage unit211to hold the voltage (Vgs=Vm0, Vm1) of which the measurement value acquisition unit213has been notified.

In steps S22and S72, the calculation unit214of the control unit210, referring to the information storage unit211, calculates the temperature (Tj0, Tj1) at the point P of the MOSFET110by dividing the voltage (Vgs=Vm0, Vm1) by the temperature coefficient (K).

The calculation unit214causes the information storage unit211to hold information on the calculated temperature (Tj0, Tj1). In steps S20and S70, by executing the processes in accordance with the heretofore described flowchart, it is possible to calculate the temperatures (Tj0, Tj1) at the point P of the MOSFET110before and after heat generation.

Next, a description will be given of the process of step S50. In step S51, the ΔVds measuring circuit340applies 20V as the voltage (Vgs) between the gate and source electrodes of the MOSFET110, and causes a constant current (IH=100 A) to be input into the MOSFET110, thus causing the MOSFET110to operate and generate heat.

The ΔVds measuring circuit340notifies the measurement value acquisition unit213of the control unit210of information on the current (IH=100 A). The measurement value acquisition unit213causes the information storage unit211to hold the current (IH=100 A) of which the measurement value acquisition unit213has been notified.

In step S52, the ΔVds measuring circuit340measures the voltage (Vds) between the drain and source electrodes at the time. The ΔVds measuring circuit340notifies the measurement value acquisition unit213of the control unit210of the measured voltage (Vds).

The measurement value acquisition unit213causes the information storage unit211to hold the voltage (Vds) of which the measurement value acquisition unit213has been notified. In step S53, the calculation unit214of the control unit210, referring to the information storage unit211, calculates the power (PH) between the drain and source electrodes in accordance with the product of the current (IH=100 A) and the voltage (Vds).

The calculation unit214uses either the average value of the voltage (Vds) while the current (IH=100 A) is being input into the MOSFET110or the voltage (Vds) at the point at which half of the input time elapses as the voltage (Vds) used when calculating the power (PH).

The calculation unit214causes the information storage unit211to hold the calculated power (PH). In step S50, by executing the process in accordance with the heretofore described flowchart, it is possible to calculate the power (PH) between the source and drain electrodes of the MOSFET110when heat is generated.

The thermal resistance measuring method executed with this kind of thermal resistance measuring device200is such that the element temperature (Tj0, Tj1) of the MOSFET110is calculated based on the temperature coefficient (K) of the voltage (Vgs) between the gate and source electrodes. In particular, as the on-state voltage is extremely low when the MOSFET110is configured of silicon carbide or gallium nitride, the accuracy of the temperature coefficient (K) calculated using the ΔVds method is extremely poor. Therefore, the thermal resistance measuring device200is such that, by utilizing this kind of ΔVgs method, there is an increase in the accuracy of the temperature coefficient (K) representing the rate of change of the voltage with respect to the temperature change (ΔTj=Tj1−Tj0).

Also, the thermal resistance measuring method executed with this kind of thermal resistance measuring device200is such that a current such that the MOSFET110generates heat is input between the drain and source electrodes, with the voltage applied to the gate electrode of the MOSFET110kept constant, and the power (PH) is measured based on the current (IH) such that the MOSFET110generates heat and on the voltage (Vds) between the drain and source electrodes when the MOSFET110generates heat. In particular, as the channel resistance is high when the MOSFET110is composed of silicon carbide or gallium nitride, the heat generation distribution within the MOSFET110becomes localized when generating heat using the ΔVgs method, and furthermore, the temperature of generated heat between the MOSFETs110is unequal in the measurement target module100in which multiple MOSFETs110are disposed in parallel, and it is therefore not possible to appropriately carry out thermal resistance measuring. Therefore, the thermal resistance measuring device200is such that, when causing the MOSFET110to generate heat using this kind of ΔVds method, an increase in the temperature of a channel region, which is a cause of the temperature distribution becoming localized in the MOSFET110, is suppressed, as the temperature changes over the whole of the inside of the MOSFET110, and it is thus no longer necessary to estimate the MOSFET110size on the small side. Furthermore, it is possible to suppress variation in heat generation distribution in the MOSFETs110in the measurement target module100in which multiple MOSFETs110are disposed in parallel.

Consequently, it is possible to measure the thermal resistance of the measurement target module100with high accuracy using the thermal resistance measuring method executed with the thermal resistance measuring device200. Also, it is no longer necessary to add an unnecessary design margin when multiple MOSFETs110are disposed in parallel in the measurement target module100, and it is thus possible to carry out optimum design.