1. Introduction
Transistors are among the electronic components from which electronic devices are built [
1]. They consist of a die placed on a base plate made of a well-conducting metal, which, in many cases, is copper. The die is encapsulated inside epoxy resin, and connections to die are possible using leads, commonly called “legs”, which are often made of the same material as the base plate. Only one of the leads is directly attached to the base plate. The remaining two leads are placed in the epoxy resin and connected to the die using thin bond wires. The parts of the base plate and leads that are not placed under the epoxy resin layer are covered with a thin layer of tin [
2,
3]. The dimensions of the base plate, leads, and the epoxy resin layer depend on the type of case used; the most commonly used cases are TO 220 and TO 247. The dimensions of the electronic cases are standardized [
4]. In the remainder of this article, the term “transistor” should be understood together with the case in which it is placed.
The most popular materials from which transistor dies are made include silicon (Si), silicon carbide (SiC), and gallium nitride (GaN). Transistors made of these materials differ in their properties [
5]. Electronic components made on the basis of silicon reach the limit operating parameters resulting from the theoretical limitations of the material used. For this reason, electronic components based on SiC and GaN materials, called wide band gap (WBG) semiconductors, are becoming more and more popular. They feature better electrical, mechanical, and thermal properties than the electronic components made on the basis of Si. Replacing Si with WBG semiconductors increases the breakdown voltages, operating temperatures, and switching frequency and reduces switching losses [
6]. Elements made on the basis of SiC deserve special attention. Compared to Si elements, those made on the basis of SiC have higher breakdown voltage values and higher thermal conductivity [
7]. Another feature of this type of semiconductor is low ON-resistance [
8].
Metal oxide semiconductor field effect transistors (MOSFETs) made on the basis of SiC are used in the construction of high conversion ratio converters (HCRCs) [
9], traction converters [
10], wind turbine converters [
11], motor drives for electric vehicles [
12], and DC–DC step-up converters [
13]. The operational reliability of these devices is related to the operational reliability of the SiC MOSFETs placed inside them [
14]. Due to the construction of the die area and the width of the gate oxide, they are susceptible to transient-overloading or short-circuit events [
15]. Other examples of SiC MOSFET damage are related to long-term exposure to high temperature, which can cause interlayer dielectric erosion, electrode delamination, gate-oxide breakdown [
16], and bond-wire lift-off and solder cracks [
17,
18].
The temperature value of SiC MOSFETs depends, among other things, on their switching frequency [
19]. In turn, the switching frequency depends on the operating characteristics of the device in which the transistor is placed and on its energy efficiency [
20]. A good example is the converter. In high-power converters, lower switching frequencies are often used to minimize switching losses and increase energy efficiency [
21]. In turn, in low-power converters, higher switching frequencies are usually used, which may lead to smaller converter sizes and better regulation [
22]. The higher the switching frequency is, the shorter is the switching time of SiC-based MOSFETs. During switching, rapid changes in voltage and current occur, which leads to power losses and heat generation. Therefore, the switching frequency has a direct impact on heat generation in the transistor [
23].
The switching frequency of a SiC MOSFET affects the temperature of its die. In turn, the operation of the die at excessively high temperature may damage the transistor. For this reason, it is necessary to monitor the die temperature of the transistor,
Tj. In the literature, is possible to find three groups of methods that make this possible: electrical, contact, and non-contact methods [
24].
Electrical methods use a selected parameter whose value depends on
Tj. This parameter is called the temperature-sensitive parameter (TSP) [
25]. An example of a TSP that is used to determine the die temperature of a transistor is the drop voltage across the body diode. Knowing the relationship between TSP and
Tj, it is possible to determine the
Tj value based on the measured TSP value. The relationship between TSP and
Tj is individual for each transistor. Additionally, its determination requires removing the transistor from the device in which it was installed and placing it in the measurement system. For this reason, this method is not suitable for real-time monitoring of
Tj values [
26].
Contact methods involve applying a temperature sensor to the transistor package (also called a ‘case’ in the literature) or directly to the die transistor. There is thermal resistance of an unknown value between the temperature sensor case and the transistor case (or die). Additionally, touching the transistor (or die) case with the temperature sensor causes a local disturbance of the temperature distribution. Part of the transistor case is made of metal; therefore, incorrect application of the temperature sensor (especially when placed in a metal case) may cause electric shock [
27].
Non-contact methods are based on the absorption of infrared radiation emitted from the surface of the transistor case (indirect non-contact method) or through the transistor die (direct non-contact method). One of these methods is infrared thermography, which is considered safe, as it poses no risk of electric shock (e.g., as a result of touching a metal temperature sensor based on a plate or a radiator to which a transistor is attached). The direct method requires opening the transistor case. It is difficult to close the opened case. For this reason, it is not suitable for real-time application. The use of the indirect non-contact method consists of two steps: measuring the temperature of the transistor case (
Tc) and determining the difference between
Tc and
Tj. Tc can be measured using a pyrometer and a thermographic camera. The use of a thermographic camera makes it possible to determine the temperature distribution on the surface of the transistor case. The differences between
Tc and
Tj can be determined using the finite element method. Knowing the value of the thermographic measurement of the temperature of the transistor case and the relationship between
Tc and
Tj, it is possible to determine the value of
Tj in real time [
28,
29].
After analyzing the available sources, no studies were found on the indirect thermographic temperature measurement of a SiC MOSFET, the temperature of which increases due to the increase in the switching frequency. For this reason, it was decided to undertake research that would result in the development of a method enabling the indirect thermographic measurement of the SiC MOSFET die temperature and monitoring that temperature at variable switching frequencies.
Section 2 describes the tested SiC MOSFET, the methodology, and the measurement system;
Section 3 describes the obtained results of the work;
Section 4 contains a discussion; and
Section 5 presents conclusions.
2. Tested Transistor, Methodology, and Measurement System
The indirect thermographic measurement of the transistor temperature die consists of two parts. The first part consists of performing a thermographic measurement of the transistor case temperature,
Tc. The second part consists of determining the transistor die value
Tj using simulation work. The method of performing a thermographic measurement of
Tc is described in
Section 2.1. The value of
TPt1000, which is used to verify the
Tc value, is also determined and described in
Section 2.1. The method used to determine the
Tj value based on simulation work is described in
Section 2.2. The method for determining the
Tjd value, which is used to verify the
Tj value (determined based on simulation work), is also described in
Section 2.2. The algorithm for the procedure is presented in
Figure 1.
2.1. Tested Transistor and Measurement System
The model C2M0280120D (Cree Inc., Durham, NC, USA) transistor was selected for testing. This transistor is described by the following parameters:
VDSmax = 1200 V (for
VGS = 0 V,
ID = 100 µA),
VGSmax = −10/+25 V,
ID = 10 A (for
VGS = 20 V,
Tc = 25 °C),
ID = 6 A (for
VGS = 20 V,
Tc = 100 °C), and
IDpulse = 20A. The external dimensions of the transistor and schematics are shown in
Figure 2. Three randomly selected C2M0280120D transistors from the same series were selected to carry out the work.
Pt1000 sensors in an SMD 6203 case (Reichelt electronics GmbH & Co. KG, Sande, Germany) were glued to the case of each transistor [
30]. For this purpose, WLK 5 glue with a known thermal conductivity value of
k = 0.836 W/mK (Fischer Elektronik GmbH & Co. KG, Lüdenscheid, Germany) was used [
31]. Additionally, next to the Pt1000 sensor, a measurement marker was painted on the transistor case. Velvet Coating 811-21 (Nextel, Hamburg, Germany) paint was used for this purpose with a known emissivity coefficient value
ε ranging from 0.970 to 0.975 for temperatures within the range from –36 °C to 82 °C. The uncertainty with which the emissivity coefficient value was determined was 0.004 [
32].
The transistor prepared in this way was placed in a station where the measuring device was a Flir E50 Thermographic Camera (Flir, Wilsonville, OR, USA) [
33]. The selected Flir E50 thermographic camera was equipped with a matrix from an uncooled IR detector (7.5–13 µm) with a resolution of 240 × 180 pixels and an instantaneous field of view (IFOV) value of 1.82 mrad. The noise equivalent differential temperature (NEDT) value of this camera was 50 mK. An additional Close-up 2× lens (T197214, Flir, Wilsonville, OR, USA) was attached to the camera lens [
34]. As a consequence, it was possible to obtain an IFOV value of 67 µm for the above-mentioned detector array (240 × 180 pixels) (thermographic camera with the additional lens). Before starting the work, the correctness of the indications of the camera used was verified using the IRS Calilux thermographic camera calibration standard (AT—Automation Technology GmbH, Bad Oldesloe, Germany) [
35].
The thermographic camera prepared in this way was placed together with the tested C2M0280120D transistor in a chamber made of plexiglass. The external dimensions of the chamber were 45 cm × 35 cm × 35 cm. The internal dimensions of the chamber were 40 cm × 30 cm × 30 cm. The difference resulted from two reasons: the thickness of the plexiglass used (3 mm) and the thickness of the material (black foam made of polyurethane) lining the internal walls of the chamber. The foam used is characterized by porous structure, and every single pore of the foam resembles the black body cavity model. As a consequence, the material used was characterized by having a high emissivity factor
ε = 0.95 [
36].
The distance
d between the tested transistor and the additional lens was adjusted using a stepper motor. In turn, the stepper motor was controlled using a Siemens S7-1200 PLC controller (Siemens AG, Munich, Germany) [
37]. A block diagram of the constructed stand is shown in
Figure 3.
The observed transistor was connected to a circuit that allowed its switching frequency to be changed. The circuit diagram is shown in
Figure 4. In this circuit, the transistor
T1 was turned on by the generator
G1 for 20 s. As a result, the load current
IDS flowed through the tested transistor. During this time, the voltage drop between the drain and the source
VDS was measured using an Agilent 34401A voltmeter. In the next step, the same generator
G1 turned on transistor
T2, allowing the flow of the measuring current
Idi for 200 ms. This operation allowed estimating the die temperature based on the automatic measurement of the drop voltage
Vfd on the diode and the known characteristic between the drop voltage value and the die temperature. During the entire testing process, the tested transistor (DUT) was pulse-controlled using the
G2 generator with a PWM waveform with a duty cycle of 50% and a frequency in the range from 1 kHz to 50 kHz.
The case of the tested transistor, in which the switching frequency was changed, was observed using a thermographic camera. During the tests, first, for a given value of the current
IDS flowing through the die and a given switching frequency
fT, the temperature of the case (
Tc) was measured using a thermographic camera. We then waited until its value increased and stabilized at a specified level. When it was found that the
Tc value had stabilized, its thermographic measurement was performed. At the same time,
Tc was measured using a Pt1000 sensor, which was glued to the case near the thermographic measurement point (
Figure 2). After the measurement was performed, the switching frequency
fT of the transistor was changed for the same
ID current value. The
fT setting was changed for selected values, ranging from 1 kHz to 800 kHz.
2.2. Finite Element Analysis and Measurement of Die Temperature
The relationship between
Tc and
Tj was determined using finite element analysis (FEA), which is a numerical method used to solve problems in engineering and mathematical physics [
38]. The software applied in the work performed was Solidworks 2020 SP05 (Dassault Systèmes, Vélizy-Villacoublay, France), which uses FEA, and the simulation was completed with the use of this software.
The simulation could be carried out after the transistor model had been constructed. Making the model required knowledge of its structure and internal dimensions. In order to determine these, the case of the tested transistor was opened and its interior was measured. For this purpose, a microscope equipped with a Cam 3.3 MP camera (Motic, Xiamen, China) was used. The microscope with the camera was calibrated using a special calibration glass. Based on the measurements taken, a three-dimensional model of the tested transistor was created. The model was created in Solidworks 2020 SP05 software. The created model and internal dimensions of the tested transistor C2M0280120D are shown in
Figure 5.
After creating the model, all of its elements were assigned the material from which it was made, along with the thermal conductivity values k. Next, the simulation was started in the Solidworks 2020 SP05 environment. In the initial stage, we checked whether the temperature distribution (measured at the surface) changes after removing individual parts of the model (e.g., leads). The temperature distribution was also checked, depending on the given mesh size. After simplifying the model and selecting the mesh size, it was possible to determine the Tj value based on the simulation work.
The die temperature (Tj) of the tested transistors obtained as a result of simulation work was verified for the same conditions using the electrical method. In order to perform a reliable temperature measurement of the die using the electrical method, it was necessary to select the appropriate temperature-sensitive parameter (TSP). The voltage drop Vfd across the body diode was chosen as the TSP. In order to use the TSP to determine the Tj value, the relationship Tj = f(Vfd) had to be determined. For this reason, a measuring system was designed, the main element of which was a climatic chamber. The chamber used allowed for changing the temperature Ta inside it. The Ta value was changed in the range from 20 °C to 180 °C. Additionally, a Pt1000 sensor was placed inside the chamber, which was used to measure the temperature there. The sensor was connected in a four-wire circuit for measuring resistance using the technical method. A current of 100 µA flowed through the sensor.
In order to determine the relationship
Tj =
f(
Vfd), three tested transistors were placed inside the described chamber. They were connected in such a way that the current
Idi (
Idi = 100 mA) forcing the voltage drop
Vfd on the body diode flowed through all diodes of the tested transistors (the body diodes of the three transistors were connected in series). The measurement setup is shown in
Figure 6. The
Vfd values of all tested transistors were measured using an Agilent 34401A multimeter (Agilent, Santa Clara, CA, USA) [
39]. The measurement was performed for a given temperature
Ta at the moment when the
Vfd voltage value stabilized. The constant values of the
Vfd voltage in time indicated that the temperature
Ta set in the chamber was equal to the die temperature
Tj of the transistors located in this chamber. In turn, the voltage drop
VPt1000 on the Pt1000 sensor was measured using a UT51 multimeter (UNI-T, Dongguan City, China) [
40].
2.3. Power Dissipated in Die and Ambient Conditions
The correct simulation work using Solidworks 2020 SP05 requires determining the power
P that has been released in the die and defining the boundary condition. The power released in the die can be determined using Equation (1):
where:
P—power (in W) dissipated in the die,
VDS—drop voltage (in V) between drain and source,
IDS—current (in A) flowing between drain and source.
The
VDS and
IDS values were measured with measurement errors, which can be determined using the UT51 multimeter documentation (UNI-T, Dongguan City, China). Therefore, the
p value will also be within the range defined by the measurement error limit ∆
P, which can be determined from Equation (2):
where: ∆
VDS—limiting error of the
VDS value (in V), ∆
IDS—limiting error of the
IDS value (in A). The ∆
IDS and ∆
VDS values can be determined using the formulas in the UT51 user manual [
40].
The increase in die temperature is related to the distribution of effective power,
PRMS, in the die. For this reason, the Equation (3) should be used:
where:
t0—beginning of the period,
Tk—duration of the period.
The PRMS value is also within the range that is determined by the limiting error ∆PRMS. The limits of the range determined by ∆PRMS can be determined using Equation (2).
The temperature gradient in the radiative heat flux path between the transistor’s die and the transistor’s case can be determined using Equation (4):
where:
J—radiative heat flux (W∙m
−2),
—Nabla operator.
Equation (4) can be written as Equation (5):
where:
x—distance between the points where the temperature values of the die and diode case were measured (m),
J—radiative heat flux (W∙m
−2).
In order to solve Equation (5), we need to separate the differentials that are on the right-hand side of the equation. Consequently, it is possible to integrate the equation on both sides. The constant of integration can be found using Equation (6):
where:
xk—end point of the analyzed heat flow path (m),
T1—temperature at the starting point of the analyzed heat flow path (K),
T2—temperature at the end point of the analyzed heat flow path (K).
Consequently, it is possible to determine Equation (7):
where:
Pc—total power (in W) applied to the wall,
S—area (m
2) of the wall penetrated by
J (W∙m
−2).
Determining the correct temperature distribution in the transistor’s case (using Solidworks 2020 SP05 Software) requires determining the radiation coefficient
hr. The
hr coefficient defines the amount of thermal energy transferred to the environment by radiation per unit time, per unit area, and per unit temperature difference between the body radiating energy and the environment. The value of
hr can be determined using Equation (8):
where: σ—Stefan–Boltzmann constant equal to 5.67 × 10
−8 (W∙m
−2∙K
−4),
TS—surface temperature (K),
Ta—air temperature (K).
It is also necessary to determine the value of the convection coefficient
hcf, which defines the amount of thermal energy transferred to the environment by convection per unit time, per unit area, and per unit temperature difference between the body emitting the energy and the environment. To determine the
hcf value for a flat surface, Equation (9) can be used:
where:
hcf—convection coefficient of flat surfaces,
Nu—Nusselt number (-),
L—characteristic length in meters (for a vertical wall, this value represents height).
The Nusselt number can be determined using Equation (10).
where:
Gr—Grashof number (-),
Pr—Prandtl number (-),
a and
b—dimensionless coefficients. The values of coefficients
a and
b are provided in
Table 1.
The Grashof number can be obtained from Equation (11):
where:
g—gravitational acceleration (9.8 m∙s
−2),
α—coefficient of expansion (0.0034 K
−1),
—air density (1.21 kg∙m
−3) at 273.15 K,
η—dynamic air viscosity (1.75 × 10
−5 kg∙m
−1∙s
−1) at 273.15 K.
Prandtl’s number is determined from Equation (12):
where:
Pr—Prandtl’s number,
c—specific heat of air (1005 J·kg
−1·K
−1) at 293.15 K.
When the value of the average linear velocity of the fluid flow is greater than 0 m/s, the Reynolds number must also be taken into account, which can be obtained using Equation (13):
where:
V—average linear velocity of the fluid flow (m/s).
In order to enable a better understanding of the boundary condition, the analyzed heat flow path and its emission by the observed surface (by convection
hcf and radiation
hr) are shown in
Figure 7.
2.4. Uncertainties
The method by which the uncertainty of the thermographic temperature measurement
Tc can be determined is described in the document
Evaluation of the Uncertainty of Measurement in calibration (EA-4/02 M: 2022) [
41]. This is a method for determining the uncertainty of type B. In order to use this method, all input quantities
Xi that affect the result of the
Tc measurement and the range of their variability must be determined. This can be done based on experience and the literature. In this work, the thermographic camera processing equation from publication [
42] was used (Equation (14)):
where:
Tcam—temperature indicated by the thermographic camera without taking into account the influence of other factors,
—Stefan-Boltzmann constant equal to 5.67 × 10
−8 (W∙m
−2∙K
−4),
τa—atmosphere transmittance coefficient,
τ1—transmittance of the thermographic camera lens,
Ta—air temperature,
—reflected temperature,
Tl—thermographic camera lens temperature.
The next step is to determine the sensitivity coefficient
cs for all input quantities from Equation (14). This is a derivative described in Equation (15):
where:
fi—all input quantities from Equation (14).
In order to determine the uncertainty of the
Tc value, estimates of
xi of the input quantities
Xi (for all above input quantities) must be determined. This is possible using Equation (16) (rectangular probability distribution):
where:
—upper limit of the input quantity range,
—lower limit of the input quantity range.
Then, for each
Xi, the uncertainty standard
u(
xi) should be determined as per Equation (17):
By multiplying the values of
u(
xi) and
cs, we can obtain the uncertainty contribution
u(
y). The standard uncertainty
u(
Tc) of the
Tc value can be obtained as the square root of the sum of squares of the values of
u(
y) as per Equation (18):
In order to determine the expanded uncertainty U(Tc), the value of u(Tc) should be multiplied by the coverage factor k.
To determine ∆
TPt1000, Equations (19)–(23) can be used:
where:
—limit error of
VPt1000,
—relative error of
VPt1000,
—limit error of
IPt1000, —relative error of
IPt1000,
—relative error of
RPt1000,
—limit error of
RPt1000,
RPt1000—resistance of Pt1000 [
42].
Then, by inserting the upper and lower range of Δ
RPt1000 into Equation (23), it is possible to obtain the upper and lower range of Δ
TPt1000 values:
To determine the Δ
VPt1000 and Δ
IPt1000 values, the documentation of the multimeter describe in reference [
40] can be used.
3. Results
Using the measurement system shown in
Figure 6, the relationship
Tjd =
f(
Vfd) was determined. This relationship was approximated by the linear equation
y = e∙x + f. As a result, the individual equations
Tjd = TC + e∙Vfd +
f were obtained for each transistor. The values of the coefficients for each transistor are given in
Table 2.
Then, each of the tested transistors was connected according to the diagram shown in
Figure 4. The black part of the transistor case was observed with a thermographic camera. In order to minimize the factors disturbing the thermographic measurements, the observed transistors and the thermographic camera were placed in a chamber whose connection layout is shown in
Figure 3. Additionally,
Vfd values were measured using a voltmeter. Using these values, the junction temperature
Tjd values were determined based on the previously determined relationship
Tjd =
f(
Vfd) (
Table 2). The
Tjd values determined in this way are given in
Table 2, and sample recorded thermograms are shown in
Figure 8.
In the next stage of the research, simulations were carried out using the FEM method. In the first step, a model of the tested transistor was designed. Materials and thermal conductivity values
k are given in
Table 3.
The selected values of the convection coefficients hcf of the observed surface (black part of the case) were in the range of 15.3 W/m2 K to 24.8 W/m2 K for the tested temperature ranges.
Additionally, the relationship between the mesh size
l specified in the simulation parameters, the duration of a single simulation
ts, and the accuracy of the determined temperature values D
TS was checked. The obtained results are presented in
Table 4.
Based on the data presented in
Table 4, a mesh size was selected at which the simulation duration was sufficiently short and the accuracy of the Δ
TS temperature value obtained as a result of the simulation work was 0.1 °C (
Table 4, No. 5). As a result, the
TS temperature values obtained from the simulation work were close to the temperature
Tc recorded in the thermographic measurement for a given value of the power dissipated in the
PRMS transistor. The selected mesh size was
l = 1.0 mm. The example temperature distributions obtained from the simulation are shown in
Figure 9.
Table 5,
Table 6,
Table 7 and
Table 8 present the values of
Tc and
Tj recorded during measurements and the values of
TS (transistor case) obtained as a result of simulation work, depending on the set value of the switching frequency
fT of the transistor. The measurements were carried out for four current values.
In order to determine the uncertainty of the
Tc value, the range of all variables was determined from Equation (14). The adopted ranges of values and the determined
xi are given in
Table 9.
The Tcam value was also taken into account. The Tcam value limits were selected individually for each case (Tcam ± 2 °C).
Then, using equations from
Section 2.4, the standard uncertainty
u(
xi) and sensitivity coefficient
cs (for all input quantities from Equation (14)) were determined. For each
Xi, the uncertainty contribution
u(
y) was determined. The
Tcam value was added to the budget with
cs equal to 1. After constructing the uncertainty budget, the standard uncertainty
u(
Tc) was determined. An example uncertainty budget for
ft = 1 kHz and
IDS = 0.25 A.
Tcc = 304.3 °C is shown in
Table 10.
The value of
U(
Tc) = 2.36 °C was obtained by multiplying the value of 1.18 by
k = 2. Using the formulas presented in
Section 2.4, the maximum value of ∆
TPt1000 of 1.73 °C was also determined.
4. Discussion
During the experimental work, an additional lens (Close-up 2×) was used with the thermographic camera. This enabled the thermographic camera used during the measurements (equipped with a 240 × 180 pixels detector matrix) to obtain such spatial resolution for which the edge of the field of view of a single detector was 67 µm. This value, taking into account the dimensions of the transistor shown in
Figure 2, guaranteed that 25 fields of the view of a single detector of the thermographic camera (fields of the view placed in a rectangle of 5 × 5 pixels) were placed on the transistor case during the measurement. For this reason, the result of the thermographic temperature measurement can be considered reliable.
Before starting the measurements, the performance of the thermographic camera was compared with to the IRS Calilux radiation standard (Automation Technology, Bad Oldesloe, Germany). The results were compared in the range of 30–90 °C with a step of 5 °C. The largest difference between the standard and the camera was 0.72 °C (the camera error was ±2 °C or ±2%, whichever is greater). For this reason, the output from the thermographic camera can be considered reliable.
The results of the thermographic temperature measurements were comparable to those obtained using the Pt1000 sensor and to the results obtained during simulation work using the FEM method. During the work carried out, three transistor specimens were tested. Similar measurement results were obtained for each. Brand new Pt1000 sensors were used.
Analyzing the data from
Table 5,
Table 6,
Table 7 and
Table 8 (and especially comparing the die temperature (
Tj) determined based on the simulation and the voltage drop
Tjd) it can be seen that the largest difference was 4 °C. The conducted studies prove that the use of the transistor body diode during measurements allows for obtaining reliable results. They also prove that the results obtained by simulation work are confirmed in real conditions. Comparing the case temperature determined by simulation work (
TS) with the temperature measured by means of a thermographic camera (
Tc), it can be seen that these values are the same. This proves that the model created is reliable.
Analyzing the data from
Table 5,
Table 6,
Table 7 and
Table 8, it can be seen that the difference between all results for
Tc1–
Tc3 are within the limit defined by the uncertainty
U(
Tc). It can also be seen that the values of
Tc1–Tc3 and
TS and
TPt1000 are within the range defined by ∆
TPt1000 and
U(
Tc). For this reason, it can be assumed that the thermographic temperature measurement is reliable.
5. Conclusions
The aim of this research was to develop a method for performing indirect thermographic measurement of a SiC MOSFET and monitoring the SiC MOSFET temperature at variable switching frequencies.
Analyzing the transistor case temperatures measured with a thermographic camera (Tc) at a frequency ft, it can be seen that despite the constant value of the IDS current, the Tc value increases. The increase in the Tc value depends on the IDS value. For the value of IDS = 0.25 A and ft in the range of 1 kHz–800 kHz, the Tc value increased by 4.5 °C. For the value of IDS = 0.5 A and ft in the range of 1 kHz–800 kHz, the Tc value increased by 5.6 °C. For the value of IDS = 1 A and ft in the range of 1 kHz–800 kHz, the Tc value increased by 3 °C. For the value of IDS = 1.5 A and ft in the range of 1 kHz–800 kHz, the Tc value increased by 1.5 °C. The Tc value depends on the value of IDS and ft. With the increase in IDS, the Tc value is set at increasingly lower values of fT.
The largest recorded difference between the case temperature and the die temperature was 27.3 °C. The use of a thermographic camera allows determining the temperature of the transistor die, which allows selecting the optimal control of the C2M0280120D transistor.
Due to the use of thermographic, there is no risk of electric shock as a result of touching the base plate or radiator, and the measurement result is obtained immediately. Based on a properly performed thermographic measurement of the temperature of the black part of the case (made of epoxy mold compound), it is possible to determine the temperature of the transistor die. As a result, its optimal operating point can be selected even more precisely. It is also possible to capture the operating point at which the transistor begins to operate incorrectly. This will prevent damage and save funds that would have to be spent in the event of a failure.