# Silicon IGBT modules with voltage slopes higher than SiC modules

Thomas Zoels, Jorge Mari, Fabio Carastro

► To cite this version:

Thomas Zoels, Jorge Mari, Fabio Carastro. Silicon IGBT modules with voltage slopes higher than SiC modules. The 26th European Conference on Power Electronics and Applications, GDR SEEDS France & EPE Association, Mar 2025, Paris, France. 10.34746/epe2025-0053. hal-05077171

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Submitted on 21 May 2025

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# Silicon IGBT modules with voltage slopes higher than SiC modules

Thomas Zoels, Jorge Mari, Fabio Carastro

SEMIKRON DANFOSS GmbH

Carl-Zeiss-Ring 5, 85737 Ismaning, Germany

E-Mail: thomas.zoels@semikron-danfoss.com, jorge.mari@semikron-danfoss.com, fabio.carastro@semikron-danfoss.com

URL: https://www semikron-danfoss com/

## Keywords

IGBT, Diode, reverse recovery, dV/dt, device characterization, double pulse test, EMC/EMI, bipolar device

## Abstract

This paper presents the issue of generating excessive dV/dt in standard medium voltage IGBT modules with PiN freewheel diodes. Two different application scenarios are discussed where the issue arises. A proposed modified double pulse test allows to investigate the phenomenon under controlled laboratory conditions. Power modules from two different manufacturers were characterized and the test results are presented. Finally, a comparison with an intermediate voltage SiC module is shown and relevant EMI considerations conclude the paper.

## Introduction

The issue of diode snappiness of medium voltage freewheel diodes, co-packed in IGBT modules, have been discussed before [1]. With 1D device simulation it was shown that the root cause of diode snappiness is a condition where the carrier plasma is completely swept out before the diode blocks the full link voltage [2]. Previous investigations on 3.3kV devices showed diode snappiness present either at low switching current or at low diode conduction time [2].

This paper presents a related but different phenomenon observed in high current 4.5kV power devices as shown in Fig. 1, which are available from multiple manufacturers. In these devices, excessive dV/dt is generated at very low current, below 1% of the rated current, and short diode conduction time of just a few microseconds. The application study shows that these switching conditions can't be easily avoided in the application. Astonishingly, the high dV/dt is exceeding the ones measured with SiC modules and hence even for Si IGBT converters becomes

a possible source for EMI issues in the electronics of the power system. High amplitude common mode currents and other interferences caused by the high dV/dt can disturb the gate driver and controller electronics. Therefore, the converter system must be designed accordingly to withstand these harsh EMI conditions. The proposed adapted double pulse test allows to find the worst-case condition with highest dV/dt generated and allows to compare devices from different manufacturers.

<figure><img src="image_2.png" /><figcaption>Fig. 1: Exemplary 4.5kV blocking voltage IGBT module with co-packed freewheel diodes in 190x140mm format [3]</figcaption></figure>

## Soft recovery diodes

The reverse-recovery behavior of medium voltage fast switching diodes has been continuously improved to allow easy application of IGBT power modules with co-packed freewheel diodes [4]. One way to improve the reverse recovery behavior is to structure the cathode-side of the diode with P and N doped regions shunted with the cathode-side contact metallization. The P regions are injecting additional holes when the electric field is approaching the cathode, which prevents an abrupt stop of the reverse current. Examples of this concept are the RFC diode from Mitsubishi Electric [5] and FCE diode from Hitachi Energy, former ABB Semiconductors [6].

The manufacturers of modern IGBT modules claim “ultra soft switching” with “low noise recovery” for their co-packed freewheel diodes in
IGBT modules. Issues with electromagnetic interference caused by high dV/dt and oscillations should accordingly be eliminated.

This may be true for most of the operating conditions. The following investigations however show that under certain conditions these diodes still show very snappy behavior.

## Application scenarios

The effect of short diode conduction was observed in two different applications which are presented next.

### Chopper application

A brake chopper is a typical application in traction or non-generative motor drive systems to dissipate braking energy from the motors. The circuit consists of a half-bridge with at least one IGBT and a freewheel diode in series.

Fig. 2 shows a simplified PLECS® model of a motor drive system with brake chopper. It contains the main chopper circuit with a fix voltage source as DC-link, a chopper IGBT at the high-side, a freewheel-diode at the low-side, and a braking resistor with cable inductance and resistance. The DC-link would be connected to further inverter(s) in the power converter system which allows the power flow from the electrical machine to the brake chopper. A logic signal from the step block is actuating the switching of the chopper IGBT. The phase output between chopper IGBT and freewheel diode is connected to the braking resistor whose return is connected back to the DC-link.

Additionally, there are common-mode (CM) capacitances from a typical motor drive system implemented. The electrical motors have typically quite large CM capacitances up to tens of nano-farads which are switched between DC+ and DC- at each inverter switching transition. Through the inverters, these motor CM capacitances are connected to the DC-link. The model contains a switched capacitance C_switched, which emulates a switched motor phase, and a fixed CM capacitance C fixed, which is representing the capacitance of the non-switched phases and other components. The braking resistor also comprises a CM capacitance C_CM_Rbrake which is however smaller than the motor CM capacitance. A capacitance of approximately 1 nano-farad is expected for the braking resistors due to typically large distance to ground for insulation through air for air-cooled resistors.

<figure><img src="image_3.png" /><figcaption>Fig. 2: PLECS® model of chopper application</figcaption></figure>

Also important for the correct response in the simulation model are the output capacitances of the power devices. The point of interest with the short conduction phase of the freewheel diode occurs when the chopper IGBT is initially in off-state and the voltage across the freewheel-diode is pulled to zero volt through the braking resistor R brake (shunting). Out of this condition, a specific timing between a switching event of an inverter and the following turn-on of the chopper IGBT is causing the diode snappiness. The inverter switching, modeled with C_switched, leads to a shift of the CM voltage, which is the voltage between DC-link and ground. This is causing an injection of CM current into the chopper bridge through C_CM_Rbrake. Part of the CM current is converting into a differential mode (DM) current, which is charging the output capacitance of the freewheel diode and brings it in reverse-bias mode. Following, the voltage is pulled down again by the formed resonance circuit which forces the diode in forward-bias mode with very low forward current. A turn-on of the chopper IGBT, after the freewheel diode was for short time in this low current conduction state, is causing the diode snappiness. The freewheel diode capacitance is modeled within the C(v) block in parallel with the freewheel diode. It contains a voltage dependent capacitance model:

$$C_{diode} = \frac{1}{\left(1 + \frac{V_{diode}}{V_0}\right)^n} \quad (1)$$

The parameters n and V₀ are fitted with a least square approach to the datasheet capacitance characteristic.
One specific of the circuit is the resistive grounding through 2x resistors R-ground. This is typical for traction motor drive systems with isolated power source. Additional Y-capacitors connected in parallel to R-ground can change the system behavior but are not considered for this investigation.

The following simulation results show the interaction of the switched motor CM capacitance with the brake chopper freewheel diode conduction. The current through the braking resistor is normally completely quenched during the off-time in the PWM. That means the condition before turn-on of the chopper IGBT is zero current and zero voltage at the freewheel diode. The inverter switching events are normally not time correlated with the chopper switching events, therefore any timing between an inverter phase switching and chopper turn-on is possible. Fig. 3 shows the simulation results. Initially the chopper IGBT is off and the voltage across the freewheel diode is zero volts. At t=3µs C_switched is disconnected from the positive and connected to the negative DC-link voltage rail, which is emulating an inverter phase switching. The upper plot shows the CM voltage of the DC-link referred to ground which is drooping with an oscillation after the switching event. The mid plot shows the freewheel diode cathode-to-anode voltage, which is initially pulled up through C_CM_Rbrake, hence the diode is in reverse-bias mode. Shortly after, the voltage is swinging down, and the diode goes in conduction state which is shown in the diode current in the bottom plot. The conducted current is simulated to approx. 0.1A and is expected to reach approx. 10A depending on the system parameters. The simulation is illustrating a chopper IGBT turn-on at t=11µs. This short time conducting with such low current leads to an excessive dV/dt switching event in the freewheel diode.

Exemplary voltage waveforms from a traction system for this case are shown in Fig. 4. The voltage axis is normalized to the operating DC-link voltage. At t=-6µs a switching event in the inverter causes a droop in the CM voltage which results in a 2µs long voltage pulse in the diode cathode-to-anode voltage. Between t=-4µs and t=0µs the diode voltage is again at zero volts which indicates that the diode is in conduction state. At t=0µs the chopper IGBT is turning-on which causes an excessive dV/dt on the diode due to the short conduction time.

<figure><img src="image_4.png" /><figcaption>Fig. 3: Simulation results of chopper circuit with short diode conduction</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 4: Exemplary chopper high dV/dt case waveform in motor drive application</figcaption></figure>

## Inverter application

A short diode conduction can also appear during certain operating conditions of an inverter in a traction / motor drive application. The turn-off dV/dt of a medium voltage IGBT can get very slow close to zero load current. If this condition coincides with a short off-command from the motor controller, then we can get into the condition of a short diode conduction time.
Fig. 5 shows exemplary voltage waveforms for this case. The voltage across two IGBT modules with co-packed freewheel diodes within the same converter phase are shown, as also illustrated in Fig. 6 [7]. The low-side IGBT was conducting current before t=-20µs and experiences a short off-time between t=-20µs and t=0µs commanded by the motor controller. At t=-20µs the low-side IGBT is turning off with approximately zero phase current which results in a very slow voltage transition. At t=-2µs the voltage across the high-side module reaches zero volt and its diode is conducting a very low current. At t=0µs the low-side IGBT turns-on again which consequently turns-off the high-side diode and results into excessive dV/dt due to the short diode conduction time.

<figure><img src="image_6.png" /><figcaption>Fig. 5: Exemplary high dV/dt case in inverter for motor drive application</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 6: Inverter with high-side and low-side power modules comprising co-packed IGBT and freewheel diode devices</figcaption></figure>

## Modified double pulse test

The double pulse test is well known as a standard test to characterize the switching characteristics of power modules [8]. The circuit consists of two power devices connected to a DC-link capacitor and a load inductor (Fig. 7). The device under test (DUT) is actively switched while the AUX device is operating as a freewheel diode.

Fig. 8 shows an exemplary switching current during a double pulse test. In typical characterization tests, the times t2 and t3 are constant and the voltage and current switching conditions are set with the DC-link voltage and t1. In the modified double pulse test, we sweep the diode conduction time t2 in 100ns time steps while keeping the other times constant. The process is repeated with different target switching current, set by t1, as well as DC-link voltage and IGBT temperature settings to investigate the full operating envelope.

<figure><img src="image_8.png" /><figcaption>Fig. 7: Double pulse test circuit</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 8: Switching current of the DUT during double pulse test</figcaption></figure>

Furthermore, for the test to be successful, one requires a high inductance load coil, to enable reasonable conduction periods t1 for the IGBT, and specially designed drivers which will acknowledge very short gate time pulses.
## Test results manufacturer #1

The device under test was a 4.5kV IGBT module in a standard 190x140mm package from a first power device manufacturer named manufacturer #1. Fig. 9 shows an exemplary turn-on waveform with 10A load current. Until $t=0s$ the $V_{ceAUX}$, which is the freewheel diode voltage, is still falling. When the diode voltage settles to approx. 0V, the conduction phase is starting until the IGBT finally turns-on at $t=1.4\mu s$ which leads to a diode turn-off with excessive $dV/dt$.

The high $dV/dt$ section does not necessarily raise up to the full DC-link voltage. Therefore, the maximum instantaneous $dV/dt$ value is calculated. With the waveform in Fig. 9 the calculated average 10%-90% $dV/dt$ is only 7kV/µs while the maximum instantaneous $dV/dt$ reaches 58kV/µs. It is important to distinguish between norm-following definitions of $dV/dt$ (like 10% to 90% of the prevailing link voltage) and instantaneous voltage slope, which is computed out of time derivatives of the measured signals after suitable denoising. Of course, very fast sampling (2.5GS/s) and precision probes are required. For EMI reasons experience has shown us that it is the instantaneous and not the classical norm related definition the one that is relevant, and this is the focus in this paper. Also, automotive OEMs are using maximum instantaneous $dV/dt$ limits to avoid EMI and motor insulation issues.

<figure><img src="image_10.png" /><figcaption>Fig. 9: Exemplary waveform of manufacturer #1 module with 1.4µs diode conduction time, 2500V DC-link voltage and 10A load current</figcaption></figure>

Fig. 10 shows the evaluated maximum instantaneous $dV/dt$ against the actual diode conduction time for three DC-link voltage levels, and the highest DC-link voltage for two different junction temperatures. The highest $dV/dt$ values are achieved for a diode conduction time between 1.5-4µs. A large dependency on the DC-link

voltage can be observed where the maximum $dV/dt$ values range between 40-58kV/µs. Also noticeable is that the maximum $dV/dt$ is reached at smaller diode conduction time in case of higher DC-link voltage. At high temperature almost the same maximum $dV/dt$ is reached as in the 25°C case.

<figure><img src="image_11.png" /><figcaption>Fig. 10: Maximum instantaneous diode $dV/dt$ test results for manufacturer #1. Voltage and temperature dependency for 10A conduction current</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>Fig. 11: Maximum instantaneous diode $dV/dt$ test results for manufacturer #1. Current dependency at 25°C junction temperature and 2500V DC-link</figcaption></figure>

The dependency on load current level is shown in Fig. 11. Values from 5-20A have been tested. The impact of the load current level on the maximum $dV/dt$ is not too large with values between 55-63kV/µs. At higher load current the maximum $dV/dt$ is typically reached at lower diode conduction time. Reason for this is the higher minority carrier injection rate into the diode base at higher load current [9]. When a critical carrier charge is reached, the high $dV/dt$ is generated in a diode snap-off event.
## Test results manufacturer #2

For comparison, the tests were repeated with equivalent 4.5kV power devices from a second manufacturer named manufacturer #2. Fig. 12 shows the worst case with an instantaneous dV/dt of 145kV/µs which is even more than double compared to the device from manufacturer #1.

<figure><img src="image_13.png" /><figcaption>Fig. 12: Exemplary waveform of manufacturer #2 module with 1.5µs diode conduction time, 2000V DC-link voltage and 20A load current</figcaption></figure>

The voltage and current dependencies can be seen in Fig. 13 and Fig. 14. Conversely to manufacturer #1 we see that the maximum dV/dt is reached at longer diode conduction times in case of higher DC-link voltage. The basic sensitivity on DC-link voltage and load current is however very similar to manufacturer #1.

<figure><img src="image_14.png" /><figcaption>Fig. 13: Maximum instantaneous diode dV/dt test results for manufacturer #2. Voltage dependency for 5A conduction current</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>Fig. 14: Maximum instantaneous diode dV/dt test results for manufacturer #2. Current dependency at 25°C junction temperature and 2000V DC-link</figcaption></figure>

The effect of excessive dV/dt generation with short diode conduction time was observed in power modules from two different manufacturers. Accordingly, it can be assumed that it is a generic issue in medium voltage IGBT modules which needs to be considered.

## SiC MOSFET voltage slopes

Larger SiC power modules are emerging in the market especially for solar applications where the converter efficiency is critical or for auxiliary power systems on board of rolling stock, where the size of magnetics is decisive.

A high power SiC module with 2kV blocking voltage was tested [10] and the instantaneous dV/dt of the body diode recovery phase evaluated. The 2kV SiC module was chosen for comparison because of availability. Presently there are no 4.5kV blocking voltage SiC modules commercially available. The highest voltage class is currently 3.3kV, targeting mainly traction applications.

Fig. 15 shows the resulting dV/dt versus the switched load current for 1500V DC-link voltage. The switching speed of SiC devices can be very well controlled with the gate resistor in standard voltage source gate drivers. Two different values of the turn-on gate resistance ($R_{gon}$) were tested to show the controllability of the diode dV/dt. The SiC module reaches the maximum instantaneous dV/dt at high load current and ambient temperature. With low $R_{gon}$ it reaches 74kV/µs during diode turn-off, while with high $R_{gon}$ it is reduced to 45kV/µs.
<figure><img src="image_16.png" /><figcaption>Fig. 15: Maximum instantaneous diode dV/dt test results for 2kV blocking voltage SiC power module at 1500V DC-link</figcaption></figure>

For a SiC converter a high level of dV/dt is expected and the whole converter system is optimized to withstand these conditions and reach electromagnetic compliance. In a medium voltage IGBT converter however it is not initially expected to reach excessive dV/dt levels which even exceed the ones from SiC devices. Hence, it might not be capable to withstand these conditions without errors in the converter system.

## Comparison of Si IGBT and SiC module

The presented results of the modified double pulse test with 4.5kV Si IGBT modules and 2kV SiC MOSFET module are summarized in Fig. 16.

<figure><img src="image_17.png" /><figcaption>Fig. 16: Absolute maximum instantaneous diode dV/dt from 4.5kV IGBT modules and 2kV SiC module. Dashed lines show standard 10%-90% dV/dt</figcaption></figure>

It shows the absolute maximum values for each device type which were reached during the test series. For comparison the standard 10%-90% dV/dt values are shown as dashed lines. The device from manufacturer #1 is still in the same range as the SiC module, depending on the driving strength of the SiC module. However, the dV/dt generated from the 2nd manufacturer's device is about 2x larger. This justifies the title claim: "Silicon IGBT modules with voltage slopes higher than SiC modules".

## EMI aspects of high dV/dt

The electromagnetic interference (EMI) between the different components of a power converter needs to be considered during the design to allow a reliable converter operation. The dV/dt generated by the power device is injecting common mode currents into the system ground through parasitic capacitances between the switching node and ground [11]. One current path is e.g. through the power module capacitance between the module terminals and baseplate. With large power modules the capacitance can reach approximately 1nF. In liquid cooled power converter systems, the baseplate is typically conductively coupled to ground through a grounded cold plate. This allows the common mode current to distribute within the converter system. With 1nF module capacitance and 100kV/µs a high bandwidth current pulse with an amplitude of 100A is generated:

$$I_{CMpeak} = C \frac{dV}{dt} = 1nF \frac{100kV}{\mu s} = 100A \quad (2)$$

This current pulse will propagate through the power converter system and can interfere with gate driver and controller electronic circuits. In worst case the interference can command an unwanted gate driver turn-on which leads to a short-circuit and eventual destruction of the power converter.

It is recommended to carefully investigate the short diode conduction characteristic of medium voltage IGBTs. If the short diode conduction events can't be avoided in the converter operation, the converter electronics must be EMI hardened to withstand these events without functional errors. These events occur at very specific time correlations between different switching events or at the zero current transition of the phase current. Therefore, it might not occur at the initial commissioning of the converter but later at specific operating conditions at the customer. Late debugging at the customer site
will result in high effort, cost and loss of
reputation. Therefore, an early investigation of
the presented phenomenon is recommended.

Conclusion

Medium voltage IGBT modules, which typically
show an order of magnitude slower switching
transitions compared to SiC power modules, can
generate under certain operating conditions
excessive dV/dt which is higher than in SiC
modules.

Two application scenarios were shown in which
the freewheel diode is conducting a very low
current for just a few microseconds. The
following turn-on of the opposite IGBT results in
excessive dV/dt at the diode and following at the
phase output which can create EMI issues in the
converter electronics. The phenomenon was
investigated in detail with a modified double
pulse test which allows a fine sweep of the diode
conduction time. Dependencies on DC-link
voltage, load current and junction temperature
were discussed. A comparison of the maximum
instantaneous dV/dt in diode recovery with a 2kV
SiC module was shown. The IGBT module
reaches in the short diode conduction state higher
dV/dt slopes than the SiC power module. Finally,
some EMI considerations show the importance of
this topic which should be considered early in the
converter design.

References

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[2]. P. Losee, M.-J. Kell, F. Carastro, J. Mari, M. Menzel, T. Schuetz, T. Zoels: Soft Recovery Diodes with Snappy Behavior, 17th European Conference on Power Electronics and Applications, Geneva, Switzerland, 2015.

[3]. Infineon's New 4.5kV IGBT Modules Increase Energy Efficiency and Reduce Switching Losses, Market News INFIMM201105.043, May 17, 2011

[4]. S. Honda, T. Minato, K. Shimizu, A. Furukawa, Y. Terasaki, K. Hatade, Y. Takata: Multidirectional development of IGBTs and diodes: low loss and tough but gentle (user-friendly) power devices, 4th International Seminar on Power Semiconductors (ISPS), 2018

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[6]. M. Rahimo, A. Kopta: The Field Charge Extraction (FCE) Diode A Novel Technology for Soft Recovery High Voltage Diodes, ISPSD Santa Barbara USA, May 2005

[7]. Mohan, Undeland, Robbins.: Power Electronics: Converters, Applications and Designs, Wiley, 3rd edition, p. 420

[8]. D. Levett, Z. Zheng and T. Frank: Double Pulse Testing: The How, What and Why, Bodo's Power Systems, April 2020, pp 46-51

[9]. Lutz J., Semiconductor Power Devices, Springer 2011, Ch. 3.4

[10]. J. Mari, T. Schuetz, X. Dong, Y. Shen, M. Kirner and L. Findanno, The Design of a 2kV 1700A SiC MOSFET Dual Module, *PCIM Europe 2024*; Nuremberg, Germany, 2024

[11]. M. Garrett: Common Mode Current Challenges and Opportunities in Electric Powertrain Testing, AIAA/IEEE Electric Aircraft Technologies Symposium (EATS) June 12, 2023