# Live Monitoring of the Blocking Behaviour of an IGBT Module in the HV-H 3 TRB Test

Benedikt Kostka, Axel Mertens

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Benedikt Kostka, Axel Mertens. Live Monitoring of the Blocking Behaviour of an IGBT Module in the HV-H 3 TRB Test. The 26th European Conference on Power Electronics and Applications, GDR SEEDS France & EPE Association, Mar 2025, Paris, France. 10.34746/epe2025-0193 . hal-05101145

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# Live Monitoring of the Blocking Behaviour of an IGBT Module in the HV-H³TRB Test

Benedikt Kostka, Axel Mertens

Institute for Drive Systems and Power Electronics

Welfengarten 1A

30167, Hanover, Germany

Tel.: +49 / 511 – 762 18833

E-Mail: benedikt.kostka@ial.uni-hannover.de

URL: https://www.ial.uni-hannover.de/

***Index Terms***—Condition Monitoring, Breakdown Voltage, Leakage Current, IGBT, Reliability, Degradation, Humidity.

***Abstract***—One of the main electrical characteristics of IGBT modules influenced by humidity-related degradation is the blocking behaviour. This paper presents an approach for online monitoring of the blocking behaviour during a accelerated ageing test based on the high voltage, high humidity, high temperature reverse bias (HV-H³TRB) test by injecting a constant current into the device under test.

## I. INTRODUCTION

Power electronic devices (e.g., IGBT modules) form the centrepiece of a large number of applications. They are increasingly being used in environments with harsh climatic conditions. Nevertheless, the modules are expected to have a service life of several decades. Premature failure of the components can lead to the destruction of the entire system or to high maintenance costs [1].

With regard to IGBT modules, the failures can be classified as overstress and wear-out failures. Overstress failures are usually triggered by a single event such as overload, high-altitude radiation, overtemperature, etc., whereas wear-out failures are caused by aging-related degradation of the component. These include thermomechanical fatigue, insulation degradation and electrochemical corrosion [2]. This work deals with wear-out degradation of IGBT modules caused by electrochemical corrosion.

This work is part of the project ReCoWind II and was funded by the Federal Ministry for Economic Affairs and Climate Action on the basis of a decision by the German Bundestag. Funding number: 03E14066D. The authors are responsible for the content of this publication.

In recent decades, a large number of test procedures have been developed to accelerate the aging of power semiconductors. The aim of these test procedures is to predict the service lifetime of the components under field conditions using a relatively short test period of a few thousand hours under laboratory conditions. With regard to ageing caused by humidity, the so-called *Temperature Humidity Bias* (THB) test is defined as a standard. In this test, the device under test (DUT) is exposed to a climate of 85 °C and 85 % relative humidity, subjected to a voltage of up to 80 volts, and held for up to 1000 hours under these conditions. However, it has been shown that for power semiconductors of higher voltage classes of several hundred volts or even several kilovolts, this low voltage is not sufficient to trigger electrochemical degradation. The *High Voltage High Humidity High Temperature Reverse Bias* (HV-H³TRB) test is therefore presented in [3]. Here, the DUT is held under the same climatic conditions as in the THB test, but a voltage in the range of 60 - 90 % of the nominal reverse voltage is then applied. In addition, monitoring the leakage current alone did not appear to be sufficient to draw conclusions about the condition of the DUT. For this reason, in the HV-H³TRB, intermediate measurements of the blocking characteristic are conducted. This revealed that, in addition to an increase in the leakage current, a reduction in the maximum reverse blocking voltage can also be a result of electrochemical degradation. In recent years, the HV-H³TRB test has thus become established as a quasi-standard for characterising the climatic robustness of power semiconductors. This paper presents a first approach that allows online monitoring of the blocking behaviour of power semiconductors during the HV-H³TRB test.
The structure of this paper is as follows. First, the hardware used in this work is described by its principle of operation and underlying schematic. Next, the test set-up is introduced. In the fourth section the measurement results obtained are presented and discussed. Finally, a conclusion is given in the last section.

## II. MEASUREMENT HARDWARE

This section describes the condition monitoring hardware for online monitoring of the blocking behaviour of an IGBT module. Firstly, the functional principle is explained. The hardware used in this work is then illustrated and explained in more detail.

### A. Principle of Operation

The basic idea of the concept is described in [4] and shown in Fig. 1.

<figure><img src="image_2.png" /><figcaption>Fig. 1: Measurement concept for the online monitoring of the blocking behaviour of an IGBT module [4]</figcaption></figure>

Basically, the concept consists of two current sources ($I_{q1}$ and $I_{q2}$) connected to the DC+, DC- and a phase terminal of an inverter. To conduct the in-situ measurement of the blocking behaviour, all switches have to be switched off and a DC-link voltage $V_{dc}$ above the nominal voltage $V_{dc,N}$ must be applied. Due to the current sources $I_{q1}$ and $I_{q2}$, a measuring current $I_M$ is injected. Thus, the voltage $V_M$ changes according to the blocking characteristic of the highside and lowside switches. In simple words, if the maximum positive current $I_M = I_{M,max}$ is reached, this means that the measured voltage $V_M$ corresponds to the breakdown voltage of the lowside switches ($V_{bd,LS} = V_M(I_{M,max})$). Otherwise, if the maximum negative current $I_M = -I_{M,max}$ is reached, then the difference between $V_M$ and the DC-link voltage $V_{dc}$ is the breakdown voltage of the highside switch ($V_{bd,HS} = V_{dc} - V_M(-I_{M,max})$).

### B. Hardware

For the measurements shown in this paper, the hardware presented in [5] is used. Fig. 2 (right) shows a

picture of the measurement board and in Fig. 2 (right) the simplified schematic of the hardware is sketched.

The current source $I_{q1}$ is implemented as a constant current source (CCS), while the current source $I_{q2}$ is adjustable and can be controlled by a low-pass filtered PWM Signal. To measure the current $I_M$, a shunt resistor $R_{I_M}$ in parallel with an INA186 current-sense amplifier (CSA) from TEXAS INSTRUMENTS is used. The output voltage of the CSA then is captured by a 16-bit analogue-to-digital converter (ADC). For the voltage measurement, a simple voltage divider is used. The voltage is also captured by a 16-bit ADC.

## III. TEST SETUP

The general test set-up is based on the classical high voltage, high humidity, high temperature, reverse bias (HV-H$^3$TRB) test. The DUT is held under defined climatic conditions and a bias voltage is applied. In Fig. 3, the schematic of the test set-up is depicted.

In contrast to the classical approach, a DC-link voltage higher than the breakdown voltage of the DUT is applied. In addition, a CCS (see Section II) is implemented, which limits the current through the DUT. In this work, the duty cycle of the PWM-signal which controls $I_{q2}$ is set to zero (see Fig. 2 (right)). Then the current $I_M$ is set by the current source $I_{q1}$ only. The current of $I_{q1}$ was chosen to be slightly above the maximum leakage current of the DUT stated on the datasheet (1 mA). Thus, the voltage across the DUT is limited to the breakdown voltage and the DUT is assumed to be in breakdown condition at least at the beginning of the test.

As the DUT, an IGBT module with a nominal blocking voltage of 600 V and a current rating of 50 A in a six-pack configuration is used. Note besides that in subsequent tests, only the lowside IGBT of the first phase's half-bridge (SW2) is biased (see Fig. 3). The ceramic substrate consists of aluminium oxide (Al$_2$O$_3$). In contrast to the commercially available version of the IGBT module it is not potted with silicone gel, since it was taken out of the production line before the potting process. Thus, it can be assumed that the climatic conditions in the climate chamber are also experienced by the DUT.

## IV. MEASUREMENT RESULTS

This section presents the measurement results obtained. First, the measured voltage $V_M$ and the measured current $I_M$ are presented and discussed. Then, the blocking characteristics of all IGBT switches in the module are shown in the initial and the final state after HV-H$^3$TRB testing.
<figure><img src="image_3.png" /><figcaption>Fig. 2: Hardware design used in this work: PCB design (left); Simplified schematic (right) [5]</figcaption></figure>

<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 3: Schematic of the test set-up</figcaption></figure>

## A. HV-H³TRB Results

In total, the HV-H³TRB test was conducted for approximately 1173 h. During the testing, a DC-link voltage above the breakdown voltage of the DUT is applied. This allows the current of the CCS to flow through the DUT. Thus, at least at the beginning of the test, the DUT will be in the breakdown condition. The difference between the measured voltage $V_M$ and the DC-link voltage $V_{dc}$ is present across the CCS. During the test, different climatic conditions and DC-link voltages were applied to the test set-up (see Fig. 3). Thus, the testing time can be divided into several sections (time sections I - IV). Table I summarises the testing parameters of each time section applied to the test set-up shown in Fig. 3. and in Fig. 4 the measurement results obtained during the HV-H³TRB test are depicted.

<table><caption>TABLE I: Applied testing parameters for each time section shown in Fig. 4</caption><thead><tr><th>Time Section</th><th>V<sub>dc</sub> in V</th><th>RH in %</th><th>T in °C</th></tr></thead><tbody><tr><td>I: 0 h - 129.43 h<sup>1</sup></td><td>1000</td><td>85</td><td>85</td></tr><tr><td>II: 129.43 h - 493.50 h</td><td>1200</td><td>90</td><td>90</td></tr><tr><td>III: 493.50 h - 510.37 h<sup>2</sup></td><td>1200</td><td>50</td><td>25</td></tr><tr><td>IV: 510.37 h - 1177.3 h</td><td>1360</td><td>85</td><td>85</td></tr></tbody></table>

Here, the blue line represents the injected measuring current through the DUT ($I_M$). As expected, the current $I_M$ is constant during the whole testing time and is not affected by changes in the climatic conditions or the applied DC-link voltage. In contrast, the measured voltage $V_M$ across the DUT shows some significant changes during the testing time. $I_M$ was set to be approximately 1.14 mA in order to be above the maximum leakage current stated on the datasheet (1 mA). Thus, it can be assumed that the DUT is operated at an impermissible operating point during the whole testing time. In the following, the measured results for each time section will

<footnote n="1">¹During this time section, the climate step of the climate chamber from room conditions to 85%/85°C is also recorded</footnote>

<footnote n="2">²During this time section, the climate step of the climate chamber from 90%/90°C to 50%/25°C is also recorded</footnote>
<figure><img src="image_6.png" /><figcaption>Fig. 4: Measurement results during the HV-H<sup>3</sup>TRB test</figcaption></figure>

be discussed in more detail.

1) *Time Section I:* The first time section lasts approximately 130 h. During this testing period, a DC-link voltage $V_{dc}$ = 1000 V is applied. Based on the classical HV-H<sup>3</sup>TRB test standard, the climatic conditions of the climate chamber are set to $T$ = 85°C and 85% relative humidity (*RH*). In addition, during this testing period, the initial climatic conditions are normal room conditions and $I_M$ and $V_M$ are also recorded during the climate step from room conditions to 85%/85°C. Since the module is not sealed with silicone gel, it can be assumed that the ambient conditions of the climate chamber are also present at the DUT. Fig. 5 shows the measured blocking voltage $V_M$ against the time *t*. Due to the lack of new information, the current $I_M$ is not depicted in Fig. 5.

As can be seen, at the beginning the measured voltage $V_M$ is approximately 740 V and starts to rise for 2.7 h until it settles at approximately 840 V. These measured values of $V_M$ are in good accordance with the measured values for the breakdown voltage at room temperature and at a temperature of $T$ = 85°C, respectively (see TABLE II). From this time onward, the connection to the voltage source was lost; the DC-link voltage was reapplied a few hours later. In this few hours, the DUT was still exposed to the climate of $T$ = 85°C, $RH$ = 85% but no voltage was applied then. After reapplying the DC-link voltage ($t$ = 2.7 h), the measured voltage $V_M$ drops immediately to roughly 530 V. After this, $V_M$ seems to stabilise between 535 V and 525 V until the end of time section I ($t$ = 129.43 h).

2) *Time Section II:* Before time section II, the DUT was stored at $T$ = 90°C, $RH$ = 90% for several hours before the DC-link voltage of $V_{dc}$ = 1200 V was applied

<figure><img src="image_7.png" /><figcaption>Fig. 5: Measured voltage $V_M$ during the first testing period with $T$ = 85°C, $RH$ = 85% and $V_{dc}$ = 1000 V, while also the climate step from room conditions to $T$ = 85°C and $RH$ = 85% is recorded</figcaption></figure>

at $t$ = 129.43 h. Fig. 6 shows the voltage $V_M$ during the time section II.

<figure><img src="image_8.png" /><figcaption>Fig. 6: Measured voltage $V_M$ during time section II with $T$ = 90°C, $RH$ = 90% and $V_{dc}$ = 1200 V</figcaption></figure>

At the beginning of time section II, $V_M$ is about 10 to 20 volts lower compared to $V_M$ in time section I (see Fig. 5). During the first 70 h of time section II, $V_M$ remains almost constant at 515 V. This difference might result from the higher temperature present at the DUT during this testing period. Since a higher temperature of an IGBT in general results in a higher leakage current and a higher breakdown voltage [6], [7], it can be as-
sumed that an increased leakage current is the underlying
degradation mechanism. Since the current through the
DUT $I_M$ remains constant, a rising temperature would
result in an increasing voltage $V_M$ if the DUT would be
in a breakdown condition. However, after $t = 200$ h, $V_M$
rises until it reaches 530 V within the next 300 h. This
suggests that the blocking capability recovers slightly
during this testing period.

3) *Time Section III:* During the third time section, a climate step from 90°C/90% to 25°C/50% is applied to the DUT via the climate chamber, while the DC-link voltage is kept constant at 1200 V. $V_M$ during time section III is depicted in Fig. 7.

<figure><img src="image_9.png" /><figcaption>Time Section III</figcaption></figure>

Fig. 7: Measured voltage $V_M$ during time section III during a climate step from $T = 90°C$, $RH = 90\%$ to $T = 25°C$, $RH = 50\%$ with $V_{dc} = 1200$ V

In the first half hour, the voltage $V_M$ rises from approximately 530 V to almost 580 V. After this, $V_M$ remains between 570 V and 580 V until the end of time section III ($t = 510.37$ h). Most likely, this is due to the change of temperature from 90°C to 25°C. Since the temperature at the DUT falls and the measured voltage $V_M$ rises at a constant current ($I_M \approx 1.14$ mA), this suggests that the blocking capability improves with a falling temperature. This is in good agreement with the observations made during time section II and is also an indicator that the underlying degradation symptom of the DUT is an increased leakage current and not a reduced breakdown voltage.

4) *Time Section IV:* The climate present at the DUT during the fourth time section shown in Fig. 4 is $T = 85°C$ and $RH = 85\%$ while a DC-link voltage of $V_{dc} = 1360$ V is applied. In addition, the DUT was stored at

$85°C/85\%$ for several hours before the DC-link voltage was applied. In Fig. 8, the measured voltage $V_M$ during this time section is depicted.

<figure><img src="image_10.png" /><figcaption>Time Section IV</figcaption></figure>

Fig. 8: Measured voltage $V_M$ during the fourth testing period with $T = 85°C$, $RH = 85\%$ and $V_{dc} = 1360$ V

Just at the beginning of time section IV, after applying the DC-link voltage ($V_{dc} = 1360$ V), the measured voltage $V_M$ decreases from 510 V to approximately 125 V within the first 10 min. After this, $V_M$ remains almost constant at approximately 125 V for 40 min. Then the DUT seems to recover at $t = 511.25$ h and $V_M$ rises again until it reaches the initial value of $V_M \approx 510$ V again at $t = 513.5$ h. Subsequently, $V_M$ seems to stabilise and remains between 510 V and 550 V until the end of the test is reached ($t = 1177.3$ h).

## B. Blocking Characteristics

First, an initial characterisation of the blocking characteristic is conducted. The schematic of the measurement setup for the blocking curves is shown in Fig. 9.

<figure><img src="image_11.png" /></figure>

Fig. 9: Schematic of the test set-up for characterisation of the blocking behaviour
The resistor $R_{lim}$ is used, in order to limit the maximum leakage current through the measured IGBT in breakdown conditions. During the measurement the input voltage $V_{in}$ is increased stepwise. The leakage current $I_{leak}$ and the blocking voltage $V_{block}$ is captured by a digital multimeter (BEHAMPROBE 38XR-A). The voltage drop accros the current measurement is neglectible compared to the measured values of the blocking voltage.

The blocking curves obtained for all IGBT switches in the module used at room temperature and at $T = 85°C$ are shown in Fig. 10. In addition, the blocking characteristic of the modules' IGBTs after approximately 1177.3 h of HV-H$^3$TRB testing time are also depicted.

In the initial state, the breakdown voltage at room temperature is between 746 and 785 V, while the leakage current at the nominal blocking voltage (600 V) is between 150 and 380 nA. At $85°C$, a higher breakdown voltage between 810 and 845 V is observed. Compared to the leakage current at room temperature, the leakage current at 600 V is increased by an order of magnitude to approx. 2 µA for a temperature of $T = 85°C$. Table II summarises the measured values for each IGBT switch of the module.

As can be seen in Fig. 10, a clear degradation of the blocking characteristic after the whole testing time can be observed in some switches. Obviously, the IGBT under test (SW2) exhibits a major increase in leakage current. It is conspicuous that the leakage current rises strongly with the applied blocking voltage $V_{block}$. For a voltage of more than 500 V, the increase in leakage current is of the order of 3 to 4 magnitudes. As expected, SW2 shows the strongest signs of degradation, since it was biased during the whole testing time.

Surprisingly, some of the non-biased IGBTs also exhibit clear signs of degradation, even though this is not so pronounced as for IGBT SW2. Interestingly, for the IGBTs SW1, SW3 and SW4, no clear increase in leakage current is observed; instead, a decreasing breakdown voltage is seen. The decrease in breakdown voltage is more pronounced at room temperature and is about 60 V.

In contrast, the IGBTs of the third phase (SW5 and SW6) show only a minor deterioration of the blocking characteristic. This might be due to the fact that these two IGBTs are furthest away from the DUT, which is the lowside IGBT of the first phase leg. The physical distance might result in a lower electrical field and thus in less pronounced corrosion, which affects the blocking characteristic.

V. CONCLUSION

In this work, an accelerated ageing test based on the HV-H$^3$TRB test scheme is conducted. As the DUT, the lowside IGBT of the first phase of an IGBT module in a six-pack configuration is used, where in contrast to the commercially available version the silicone gel is removed. This might result in a more accelerated degradation of the DUT than it should be expected for a module with silicone gel. Anyway, in order to enable live monitoring of the blocking behaviour, a constant current source is used in series with the DUT togehter with the DC-link voltage source, which injects a measuring current ($I_M$) into the DUT. Beside $I_M$, the voltage across the DUT ($V_M$) is recorded and used as an indicator of the remaining blocking capability of the DUT. During the testing time of approximately 1173 h (almost 49 days), the climate present at the DUT and the DC-link voltage applied to the test set-up were both varied. Note that although only one IGBT is electrically biased during the testing time, all module IGBTs were exposed to the chosen climate.

After a few hours of testing, the remaining blocking capability (e.g., $V_M$) drops immediately from approximately 840 V to approximately 535 V just after the DC-link voltage is reapplied to the test set-up, while a climate of $RH = 85\%$ and $T = 85°C$ is present at the DUT. After this, there are only minor changes in $V_M$ as long as the temperature is not changed. Since $V_M$ increases with a decreasing temperature while the current through the DUT is kept constant, this suggests that the underlying degradation is expressed by an increased leakage current caused by electrochemical degradation. The results obtained also indicate that reapplying the bias voltage to the DUT while it is kept in a high-humidity environment is most critical for the DUT.

The recorded blocking characteristic of the DUT after the whole testing time shows a huge increase in the leakage current of several orders of magnitude at room temperature as well as at $T = 85°C$. The leakage current at the nominal blocking voltage of the IGBT is above the specified value on the datasheet (1 mA). Surprisingly, the IGBTs of the module which were not electrically biased during the HV-H$^3$TRB test but were exposed to the same climatic conditions as the DUT also show a degradation in blocking capability. In contrast to the DUT, the breakdown voltage is reduced after the testing time, suggesting a different underlying degradation mechanism than for the DUT.
<figure><img src="image_12.png" /><figcaption>Fig. 10: Initial (circles) and final (crosses) blocking characteristics of the IGBTs at room temperature (blue) and at 85 °C (red)</figcaption></figure>

<table><caption>TABLE II: Measured values of the blocking characteristics at the initial state (pre HV-H³TRB test) and at the final state (post HV-H³TRB test)</caption><thead><tr><th rowspan="3">IGBT</th><th colspan="4">initial</th><th colspan="4">final</th></tr><tr><th colspan="2">I<sub>leak</sub> in µA</th><th colspan="2">U<sub>BD</sub> in V</th><th colspan="2">I<sub>leak</sub> in µA</th><th colspan="2">U<sub>BD</sub> in V</th></tr><tr><th>room temperature</th><th>85 °C</th><th>room temperature</th><th>85 °C</th><th>room temperature</th><th>85 °C</th><th>room temperature</th><th>85 °C</th></tr></thead><tbody><tr><td>SW 1</td><td>0.24</td><td>2.15</td><td>785</td><td>829</td><td>0.24</td><td>2.35</td><td>720</td><td>796</td></tr><tr><td>SW 2</td><td>0.2</td><td>1.85</td><td>762</td><td>845</td><td>≥ 1000</td><td>≥ 1000</td><td>-</td><td>-</td></tr><tr><td>SW 3</td><td>0.21</td><td>1.95</td><td>775</td><td>819</td><td>0.85</td><td>2.31</td><td>712</td><td>800</td></tr><tr><td>SW 4</td><td>0.26</td><td>2.13</td><td>775</td><td>822</td><td>0.79</td><td>2.9</td><td>719</td><td>796</td></tr><tr><td>SW 5</td><td>0.15</td><td>2.06</td><td>778</td><td>810</td><td>0.33</td><td>2.33</td><td>760</td><td>786</td></tr><tr><td>SW 6</td><td>0.38</td><td>1.95</td><td>746</td><td>825</td><td>0.61</td><td>2.32</td><td>748</td><td>826</td></tr></tbody></table>

REFERENCES

[1] K. Fischer, M. Steffes, K. Pelka, B. Tegtmeier, and M. Dörenkämper. 2021. "Humidity in Power Converters of Wind Turbines—Field Conditions and Their Relation with Failures" Energies 14, no. 7: 1919. https://doi.org/10.3390/en14071919

[2] A. Abuelnaga, M. Narimani and A. S. Bahman, "A Review on IGBT Module Failure Modes and Lifetime Testing," in IEEE Access, vol. 9, pp. 9643-9663, 2021, doi: 10.1109/ACCESS.2021.3049738.

[3] C. Zorn and N. Kaminski, "Temperature Humidity Bias (THB) Testing on IGBT Modules at High Bias Levels," CIPS 2014; 8th International Conference on Integrated Power Electronics Systems, Nuremberg, Germany, 2014, pp. 1-7.

[4] B. Kostka, D. Herwig, M. Hanf, C. Zorn and A. Mertens, "A Concept for Detection of Humidity-Driven Degradation of IGBT Modules," IEEE Transactions on Power Electronics, vol. 36, no. 12, pp. 13355-13359, Dec. 2021, doi: 10.1109/TPEL.2021.3090149.
[5] B. Kostka and A. Mertens, "Practical Implementa-
tion of a Concept for In-Situ Detection of Humidity-
Related Degradation of IGBT Modules," 2022 24th Euro-
pean Conference on Power Electronics and Applications
(EPE'22 ECCE Europe), Hanover, Germany, 2022, pp.
1-9.

[6] B. Wang, Y. Tang and M. Chen, "Study on Electric Characteristic of IGBT at Different Junction Temperature," 2011 Asia-Pacific Power and Energy Engineering Conference, Wuhan, China, 2011, pp. 1-4, doi: 10.1109/APPEEC.2011.5749117.

[7] B. Wang, Y. Tang and M. Chen, "Experimental Study on Voltage Breakdown Characteristic of IGBT," 2011 Asia-Pacific Power and Energy Engineering Conference, Wuhan, China, 2011, pp. 1-4, doi: 10.1109/APPEEC.2011.5749116.