# Analysis of Aged Power Modules considering the Loss Calculation of a Heavy-Duty Fuel Cell Truck

Yavuz Gürlek, Martin Ackerl, Roland Dold, Patrick Heimler, James Abuogo,

Sören Gesell, Thomas Basler

► To cite this version:

Yavuz Gürlek, Martin Ackerl, Roland Dold, Patrick Heimler, James Abuogo, et al. Analysis of Aged Power Modules considering the Loss Calculation of a Heavy-Duty Fuel Cell Truck. The 26th European Conference on Power Electronics and Applications, GDR SEEDS France & EPE Association, Mar 2025, Paris, France. 10.34746/epe2025-0140. hal-05101611

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Submitted on 6 Jun 2025

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# Analysis of Aged Power Modules considering the Loss Calculation of a
Heavy-Duty Fuel Cell Truck

Yavuz Gürlek, Martin Ackerl, Roland Dold

DAIMLER TRUCK AG, ADVANCED ENGINEERING MERCEDES BENZ TRUCKS

Fasanenweg 10

70771 Leinfelden-Echterdingen, Germany

E-Mail: [Yavuz.Guerlek@daimlertruck.com](mailto:Yavuz.Guerlek@daimlertruck.com)

URL: [https://www.daimlertruck.com/](https://www.daimlertruck.com/)

Patrick Heimler, James Abuogo, Sören Gesell, Thomas Basler

CHEMNITZ UNIVERSITY OF TECHNOLOGY, PROFESSORSHIP OF POWER
ELECTRONICS

Reichenhainer Str. 70

09126 Chemnitz, Germany

## Acknowledgements

For this work, I would like to extend my gratitude to the Advanced Engineering department of Mercedes-Benz Trucks for providing the real measured data of a Heavy-Duty Fuel Cell Truck, which made this analysis possible.

## Keywords

«Fuel Cell Electric Vehicle (FCEV)», «Switching and conduction losses», «Mission profile», «Variable resistance».

## Abstract

The aging of power modules in Heavy-Duty Fuel Cell Trucks (HDFCT) is crucial because, as these modules age, their on-state resistance increases. This increase results mainly in higher switching and conduction losses in case of a Si IGBT, ultimately reducing the efficiency of the truck. This work presents the results of simulated switching and conduction losses based on a real mission profile using actual measured (aged) devices. These devices exhibit varying on-state resistances.

## 1. Introduction

The High Voltage (HV) power network of a Heavy-Duty Fuel Cell Truck (HDFCT), analyzed in this work, comprises three different energy sources and several consumers, as shown in [1]-[4]. The energy sources in the HDFCT include an HV battery system and two fuel cell systems,

<table><thead><tr><th colspan="2">Power module description for simulation-setup</th></tr></thead><tbody><tr><td>Case 1:</td><td>New module according to data sheet</td></tr><tr><td>Case 2:</td><td>Aged module (power cycling, surge-current stress)</td></tr><tr><td>Case 3:</td><td>Worst case: heavily artificial aged module (artificially aged)</td></tr></tbody></table>

<figure><img src="image_2.png" /><figcaption>Figure 1: Power module status and description of methods</figcaption></figure>

along with HV auxiliaries such as electric turbochargers, humidity recirculation blowers, and two Fuel Cell Converters (FCC), which are DC/DC boost converters. The third energy source is the electrical drive system in recuperation
mode, consisting of two Permanent Magnet Synchronous Machines (PMSMs) and a dual inverter [1]. The novelty of the paper is to describe the impact of a drivetrain power module aging (e.g. due to power cycling) on the conduction and switching losses in an HDFCT. To be precise, a real-world mission profile is applied. The ageing can lead to higher conduction losses, due to the increasing on-state resistance.

## 2. Methods

This chapter describes the methods of this work and the structure of the analysis according to Figure 1. First, the simulation model [3] is used to simulate the device losses of a new module according to the datasheet of a chip manufacturer (case 1) with a real mission profile of a HDFCT. Then the determined on-state resistance of the aged module (case 2) is used to simulate the losses according to the same mission profile. Subsequently, the aged module is artificially modified by further cutting bond wires or gate wires (case 3). The same mission profile is simulated again by using the aged + artificially modified on-resistance values. Finally, the evaluation and summary of this work is presented.

### 2.1 Measurements with HDFCT

A series of driving sessions involving accelerations and braking maneuvers were conducted to identify the specific maneuvers leading to the highest current ripples. Several tests with the HDFCT were carried out on the test road track. A short window of an acceleration phase is regarded in the loss calculation of this work. In this test, the switching frequency of the drivetrain inverter is set to $f_s = 6.4$ kHz. The performed measurements of the HDFCT are recorded with 200 kHz resolution transducers at full throttle acceleration and deceleration with maximum recuperation. The first test is made with the initial setup of the HDFCT as a reference with a space vector pulse width modulation (SVPWM). The measurements are predominantly used to identify the best method to reduce the current ripples according to [3]-[4]. Besides, it has to be taken care that the measures are not introducing too much additional losses (e.g. higher switching losses by increased switching frequency). Therefore, the measurement results are also used for the study on aged modules in this paper.

### 2.2 Measurements of the on-state resistances of a power module

For this study, a commercially available 3-phase 2-level (B6) power module with a rated current of 500 A and a blocking class of 1700 V was selected. The logic switch per position consists of 3 parallel IGBT chips and diodes, which use ten bond wires with double stitch configuration for

<figure><img src="image_3.png" /><figcaption>Figure 3: a) Layout of the full power module with the six switch positions and b) Image of a switch in the module</figcaption></figure>

every IGBT/diode pair. Figure 3: a) illustrates the schematic circuit of this module and Figure 3 b) shows the image of one single switch in the module. The forward voltage of the individual switches of the module was measured with the measuring system DM 659 from Schuster-Elektronik and the pulse length was set to 200 µs. The primary objective of this approach is to

<figure><img src="image_4.png" /><figcaption>Figure 2: Output characteristic of an aged module for each switch at a) 25 °C and b) 125 °C with a gate voltage of 15 V; Note: The determination of the equivalent resistance is only shown schematically for one switch (case 2 module)</figcaption></figure>
measure the total on-resistance of the entire package, thereby ensuring that the boundary conditions used in the simulation closely replicate real-world conditions of aged modules. The measurement results of the output characteristics of an aged module are shown in Figure 2 for two different temperatures at the same gate voltage. The measurement setup was standardized across all systems, in accordance with Figure 1. The aging effects are due to two different durability tests, like power cycling and surge currents. As can be seen, the high-side system of the 3rd halfbridge is strongly aged compared to the other systems, showing approximately 30% higher equivalent resistance values at the interesting current range. This is already a quite high ageing. It is higher than in a classical power-cycling test with 5% increase in forward voltage as a stop criteria.

<table><caption>**Table I: Results of the R<sub>eq</sub> - resistance measurements in mΩ of an aged module (case 2) at different gate voltages and temperatures at a current range 150 – 300 A**</caption><thead><tr><th>Logic switch position</th><th>25 °C<br>V<sub>GE</sub><br>=<br>15 V<br>[mΩ]</th><th>125 °C<br>V<sub>GE</sub><br>=<br>15 V<br>[mΩ]</th><th>25 °C<br>V<sub>GE</sub><br>=<br>18 V<br>[mΩ]</th><th>125 °C<br>V<sub>GE</sub><br>=<br>18 V<br>[mΩ]</th></tr></thead><tbody><tr><td>1_HS</td><td>3.12</td><td>4.36</td><td>2.89</td><td>4.01</td></tr><tr><td>1_LS</td><td>2.86</td><td>4.12</td><td>2.66</td><td>3.79</td></tr><tr><td>2_HS</td><td>3.11</td><td>4.36</td><td>2.89</td><td>4.01</td></tr><tr><td>2_LS</td><td>2.88</td><td>4.07</td><td>2.61</td><td>3.72</td></tr><tr><td>3_HS</td><td>4.07</td><td>5.81</td><td>3.72</td><td>5.20</td></tr><tr><td>3_LS</td><td>2.84</td><td>4.07</td><td>2.67</td><td>3.74</td></tr></tbody></table>

<figure><img src="image_5.png" /><figcaption>Figure 4: a) Schematic structures and cuts of the load bond or gate wires of the chips b) Example of cut gate wires (blue cross); Note: The red cross indicates that the bond wires have been cut from a full emitter segment of an IGBT chip</figcaption></figure>

To calculate the on-state resistance (R<sub>eq</sub> - resistance) required for the simulation, the following formula according to [6] was applied:

$$R_{\text{eq - resistance}} = \frac{\Delta V_{\text{CE}}}{\Delta I_{\text{C}}}$$

where ΔI<sub>C</sub> is the difference of the collector current and ΔV<sub>CE</sub> is the difference of the forward voltage. The determination of the on-state resistance was made in the current range between 150 – 300 A, which includes the operation range of the real truck. All on-state resistances in this work are determined within the same current range. This determination is shown schematically for a single switch in Figure 2 a) and the results of the R<sub>eq</sub> - resistance for all switches are presented in Table I. Due to the positive temperature coefficient behavior of the IGBT, the forward voltage increases with increasing temperature [5]. For the used half-bridge simulation model, only the same values can be specified for the LS (low side) and HS (high side) of a system. In the measurement, the high and low side was determined. For the simulation model, the average value of LS and HS was used. The results are shown in Table II, half-bridge 3 shows the highest values.

<table><caption>**Table II: Mean value of LS and HS for the simulation of the R<sub>eq</sub> - resistance in mΩ of an aged module (case 2) at different gate voltages and temperatures**</caption><thead><tr><th>Half-bridge</th><th>25 °C<br>V<sub>GE</sub><br>=<br>15 V<br>[mΩ]</th><th>125 °C<br>V<sub>GE</sub><br>=<br>15 V<br>[mΩ]</th><th>25 °C<br>V<sub>GE</sub><br>=<br>18 V<br>[mΩ]</th><th>125 °C<br>V<sub>GE</sub><br>=<br>18 V<br>[mΩ]</th></tr></thead><tbody><tr><td>1</td><td>2.99</td><td>4.24</td><td>2.78</td><td>3.89</td></tr><tr><td>2</td><td>2.99</td><td>4.21</td><td>2.76</td><td>3.86</td></tr><tr><td>3</td><td>3.45</td><td>4.94</td><td>3.19</td><td>4.47</td></tr></tbody></table>
<figure><img src="image_6.png" /><figcaption>Figure 5: Measured DC current of drivetrain inverter a) entire acceleration and deceleration current, a)* part for simulation according to [3] b) zoom-in: acceleration mode for loss calculation c) deep zoom-in of highest torque demand of electric machine showing strong current ripples</figcaption></figure>

To simulate an even more critical situation e.g. when a bond wire is lifted completely from the metallization or has been modified by cracks, for example caused by vibration during the driving process of the truck, bond wires of the same module have been cut on top in a random manner (case 3). The values are increased for each switch and the results are shown in Table III. The reason is for example that a segment of the chip is no longer contacted, or the gate bond wire was removed. Figure 4 a) shows the systems and bond wires that have been cut and Figure 4 b) an example of how the gate bond wires have been cut. Figure 4 shows an example to illustrate what happens, if one or two chips fail completely. This

can mean that the $R_{eq}$-resistance for this system is more than doubled. The effect of the cut bond wires on the $R_{eq}$-resistance is shown in Table III. The worst-case scenario was of great significance in this study. Also in this case, the mean value of LS and HS required for the simulation was determined from the measured data (see Table II and III), and Table IV shows the mean values of $R_{eq}$-resistance for each system after cutting some bond wires, according to Figure 6.

<table><caption>Table III: $R_{eq}$-resistance in mΩ of an aged module with additional cut bond wires at a gate voltage of 15 V and room temperature, bond wires are artificially modified randomly (case 3)</caption><thead><tr><th>Logic switch position 25 °C</th><th>Cut of single bond wire [mΩ]</th><th>Cut of one segment of the IGBT [mΩ]</th><th>Cut of one IGBT chip via gate cut [mΩ]</th><th>Cut of two IGBT chips via gate cut [mΩ]</th></tr></thead><tbody><tr><td>1_HS</td><td>3.177</td><td>3.183</td><td>3.467</td><td>-</td></tr><tr><td>1_LS</td><td>-</td><td>3.403</td><td>-</td><td>-</td></tr><tr><td>2_HS</td><td>-</td><td>-</td><td>3.977</td><td>7.35</td></tr><tr><td>2_LS</td><td>2.863</td><td>2.88</td><td>3.367</td><td>-</td></tr></tbody></table>

<figure><img src="image_7.png" /><figcaption>Figure 6: Aged + artificially modified module - effect of cutting the gate wire on the forward voltage in the system for 2_HS (case 3)</figcaption></figure>
<table><caption>Table IV: Mean value of LS and HS for the simulation of the R<sub>eq</sub> - resistance in mΩ of an aged + artificially modified module (case 3) with cut bond wires at room temperature and a gate voltage of 15 V. Note: Value for system 3 from Table II as no further artificial change took place</caption><thead><tr><th>Half-bridge</th><th>25°C<br>V<sub>GE</sub> = 15 V<br>[mΩ]</th></tr></thead><tbody><tr><td>1</td><td>3.435</td></tr><tr><td>2</td><td>5.358</td></tr><tr><td>3</td><td>3.452</td></tr></tbody></table>

## 2.3 Loss calculation by self-built simulation model

For calculating the device losses of the power modules of the drivetrain inverter, an additional simulation model is also constructed with Matlab Simulink Simscape Electric. An inverter model [3] and [4] is characterized to analyze the pulse pattern of a measured mission profile of a HDFCT. With this, the loss and temperature calculation can be performed as shown in the results section. Finally, the total losses and the calculation of the device temperature of the drivetrain inverter semiconductors are shown.

## 3. Results

To determine the dependency of the different test setups, cases 1-3 are used to simulate the device losses according to the mission profile of the HDFCT (see Figure 5). In Figure 9, Figure 10 and Figure 11 the device losses are shown according to the mission profile of a HDFCT with the values of cases 1-3 for the R<sub>eq</sub> - resistance and the simulated

<figure><img src="image_8.png" /><figcaption>Figure 8: Amount of required power ratings according to test track Stuttgart-Hamburg and Hamburg-Stuttgart (SHHS) [2]</figcaption></figure>

currents of Figure 7. The losses increase due to higher on-state resistance and higher resulting temperatures. In our previous work, we analyzed the excess losses due to current ripples for a given mission profile on a highway in Germany [2]. The averaged current for one module of the drivetrain inverter according to short interval (see Figure 7 d)) from the whole mission profile is I<sub>module,avg</sub> = 182 A. The amount of this required current at a full test track route is n = 1551, which is normalized according to the mission profile with 1 s, see Figure 8. A new module generates for this example the following conduction losses at 25° C chip temperature:

<figure><img src="image_9.png" /><figcaption>Figure 7: Simulated currents according to measured mission profile: a) AC current of one phase - interval Figure 5 b), b) zoom-in of AC current interval Figure 5 c), c) DC I<sub>mean</sub> value of simulated current d) zoom-in of DC I<sub>mean</sub> value</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>Figure 9: Simulated device losses of case 2 module at Tj = 25 °C starting temperature according to Figure 5 b); entire acceleration phase</figcaption></figure>

$$
\begin{align*}
E_{\text{cond,case1}} &= (V_{\text{CE,bi}} + R_{\text{Half-bridge,case1}} \cdot I_{\text{avg}}) \\
& \quad \cdot I_{\text{avg}} \cdot t \cdot n \\
&= (0.6 \text{ V} + 1.78 \text{ m}\Omega \cdot 182 \text{ A}) \cdot \\
& \quad 182 \text{ A} \cdot 1 \text{ s} \cdot 1551 \cdot \frac{1}{60 \cdot 60 \cdot 1000} \\
&= 0.0724 \text{ kWh},
\end{align*}
$$

with the knee voltage (built-in) V<sub>CE,bi</sub> and the
resistance R<sub>Half-bridge,case1</sub> of the data sheet
value. For an aged module according to Table II
(averaged R<sub>eq</sub> - resistance values of all half-bridges at
25° C and V<sub>GE</sub>=15 V), the conduction loss energy
of one module increases by 27.1 %.

$$
\begin{align*}
R_{\text{Half-bridge,case2}} &= \frac{\sum \text{Half-bridge}_{1-3}}{3} \\
E_{\text{cond,case2}} &= (V_{\text{CE,bi}} + R_{\text{Half-bridge,case2}} \cdot I_{\text{avg}}) \\
& \quad \cdot I_{\text{avg}} \cdot t \cdot n
\end{align*}
$$

$$
\begin{align*}
&= (0.6 \text{ V} + 3.15 \text{ m}\Omega \cdot 182 \text{ A}) \cdot \\
& \quad 182 \text{ A} \cdot 1 \text{ s} \cdot 1551 \cdot \frac{1}{60 \cdot 60 \cdot 1000} \\
&= 0.092 \text{ kWh.}
\end{align*}
$$

$R_{\text{Half-bridge,case3}}$ is the averaged on-state
resistance of case 3 module of Table IV:

$$
R_{\text{Half-bridge,case3}} = \frac{\sum \text{Half-bridge}_{1-3}}{3}
$$

Comparing case 2 with case 3, the energy loss
increases by 14.1 %.

$$
\begin{align*}
E_{\text{cond,case3}} &= (V_{\text{CE,bi}} + R_{\text{Half-bridge,case3}} \cdot I_{\text{avg}}) \cdot I_{\text{avg}} \cdot t \cdot n \\
&= (0.6 \text{ V} + 4.08 \text{ m}\Omega \cdot 182 \text{ A}) \cdot 182 \text{ A} \cdot 1 \text{ s} \cdot 1551 \cdot \frac{1}{60 \cdot 60 \cdot 1000} \\
&= 0.105 \text{ kWh.}
\end{align*}
$$

This energy loss is calculated only for one power
demand according to mission profile of Figure 8.

<figure><img src="image_11.png" /><figcaption>Figure 10: Simulated device losses of case 3 module at T<sub>j</sub> = 25 °C starting temperature: according to Figure 5 c)</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>Figure 11: Comparison of simulated losses of HS according to cases 1-3 at T<sub>j</sub> = 25 °C according to Figure 5 c)</figcaption></figure>

According to Figure 8, there are several power demands in higher classes, which means a higher total loss energy. Regarding the expensive battery of the HDFCT, the impact of an aged module should be regarded with attention. The simulated losses of the entire acceleration mode are shown for case 2 module in Figure 9. The switching losses increase up to P<sub>L,SW</sub> = 444.5 W at t<sub>sim</sub> = 244 s for 1_HS. Whereas the conduction reaches a steady state with P<sub>L,cond</sub> = 89.3 W. The simulation of case 3 according to the mission profile of Figure 5 c) is shown in Figure 10 exemplarily for a shorter time interval during acceleration. A comparison of all cases is shown in Figure 11, that are defined with cases 1-3 and simulated for the given mission profile of Figure 5 c). According to Figure 11 it is clear, that case 3 (red) shows the highest conduction losses due to the higher on-state resistance in comparison to case 1 (green) and 2 (black). With the temperature rise the switching losses increase slowly as well (see Figure 11). Considering the entire loss amount of a given mission profile from Figure 8, the energy loss by means of aged and aged + artificially modified module increase up to 115.3 %, namely from 20.9 kWh to 35.2 kWh and 45 kWh, see Figure 12.

<figure><img src="image_13.png" /><figcaption>Figure 12: Total energy loss according to mission profile from Figure 8: according to cases 1-3</figcaption></figure>

## 4. Conclusion

This paper shows a comparison of the module losses according to a mission profile of an HDFCT. The on-state resistances are determined at different boundary conditions for example aged modules, aged + artificially modified modules versus data sheet values at a respective current range to evaluate an assumed aging of the modules. The simulation results of this work show that the losses are significantly increased by the aged and modified modules. Important is to mention that the aged chips (case 2) and the artificially modified chips (case 3) in the module increase losses significantly (see Figure 12): But we expect that in reality at higher temperatures, the chips will damage the entire module due to the increasing on state resistance. This phenomenon needs to be considered in the lifetime of a HDFCT especially regarding the battery cells and the range of this truck. With aged modules, the effective amount of stored energy decreases by at least 35.2 kWh (worst-case 3 – full mission profile). Therefore, we recommend for HDFCT and battery electric long haul trucks to check on the actual health state of the power modules. Besides, the thermal resistance R<sub>th,JC</sub> (Junction to Case) should be determined as well after a specific interval, as an increased R<sub>th,JC</sub> causes higher junction temperatures and thus losses due to an aging effect in the solder layer or backside connection which results in reduced vehicle range if on-state monitoring is not performed. Finally, it has to be mentioned that the modules for this investigation were strongly aged. Typically much
more than is to be expected after performing e.g. an power cycling test.

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