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Posted on 01 March 2019

Exploring the Behaviour of Parallel IGBT Modules

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Tested under extreme SOA conditions

The extreme ruggedness of high voltage SPT-IGBT modules has enabled the investigation of new limits in IGBT SOA performance when operating in parallel. In this article, a set of results is presented where two 3300V/1200A SPT-IGBT modules were tested in parallel under extreme RBSOA conditions with a forced temperature difference of up to 100°C. Current and voltage waveforms were recorded and the associated redistribution mechanisms during the dynamic avalanche and Switching-Self-Clamping-Mode SSCM are discussed.

By Arnost Kopta, Ulrich Schlapbach, and Munaf Rahimo

 

The power electronics community upholds a long wish list of improvements targeted at the power semiconductor device electrical performance. Despite the fact that the IGBT offers the user a wide range of attractive electrical characteristics, further improvements and superior performance is continuously required. Parallel operation of IGBTs represents an important topic in many systems, due to the inherent nature of MOS controlled devices to operate in parallel in order to achieve the required average current as demanded by power electronic applications. The circuit layout and spread of device parameters will always result in an imbalance in current sharing between the devices under static and dynamic conditions. Therefore, the device and circuit designer has to deal with a number of issues depending on the application to make the system reliable. The effect of the circuit and device parameters on the dynamic performance has been thoroughly investigated in the past. The negative effects of the gate drive design, stray inductances, temperature variations and the spread of device parameters have been shown to be the main cause of unbalanced current sharing in paralleled IGBT modules. The majority of previous investigations were carried out under moderate switching conditions and for lower voltage rated IGBTs. This work, however, focuses on high voltage IGBTs modules with paralleling effects under high stress levels and different temperatures.

Recently, it was demonstrated that the performance of HiPak modules employing the highly rugged HV-SPT IGBT chips is capable of attaining new levels of turn-off RBSOA [1]. These characteristics of the SPT-IGBT range have allowed us for the first time to study the effect of externally induced temperature variations on the current mismatch between IGBT modules under extreme RBSOA conditions. Those include dynamic avalanche and an operational mode referred to as Switching Self-Clamping Mode (SSCM), characterized when the overshoot voltage reaches levels close to that of the static breakdown voltage [2]. The benchmark module results presented will provide a new outlook for high voltage system designers aiming for an all-around performance improvement in future high voltage applications.

3300V SPT-IGBT parallel operation during dynamic avalanche and SSCM

A number of tests were carried out in order to take a closer look at the current sharing between paralleled IGBTs under SOA conditions. In the beginning, a single 3300V/1200A SPT-IGBT module with 3 separate “legs” (sections) in parallel was tested at 4000A and 2.65kV DC link voltage. No clamps or snubbers were used in the test fixture. The current was recorded for each leg and is shown in Figure 1. The waveforms show very uniform current sharing between the three legs even as the module enters the dynamic avalanche and SSCM stages of operation. A variation of around only 1% was observed between the current in each leg of the module. This is a negligible figure and thus no current de-rating is needed under SOA conditions. This is a consequence of using lower turn-off gate resistance values (RG) than those required by conventional technologies resulting in shorter turnoff delay times, thus improving current sharing between individual IGBT chips within the module.

3300V-1200A IGBT module RBSOA at 125°C VDC=2650V, Ic=4000A, RG=1.5O, LS=280nH

Temperature induced current mismatch in 3300V modules during dynamic avalanche and SSCM

The main investigation was carried out on two 3300V/1200A SPTIGBT modules in the test set-up shown in Figure 2. The RBSOA tests were carried out at high currents while operating with a low gate resistance value. Also, neither clamps nor snubbers were used in the test fixture. The two modules were capable of turning off more than 6000A at a DC-link voltage of 2600V in spite of the temperature induced current mismatch and associated redistribution mechanisms. By forcing large temperature variations between the two parallel modules, the current redistributions were magnified, and therefore, can be better analyzed for exploring the limits of device operation under such extreme conditions. This work was especially focused on current mismatch effects and redistribution mechanisms during dynamic avalanche and Switching-Self-Clamping-Mode (SSCM). By maintaining one module at a fixed temperature of 125°C, and varying the second module temperature between 25°C and 125°C, a full set of turn-off results were obtained for the current sharing mechanisms. The modules showed excellent ruggedness and capability of withstanding both dynamic avalanche and SSCM in spite of a forced temperature difference of up to 100°C.

Measurement set-up for turn-off of parallel-connected IGBT modules

Figures 3, 4, and 5 show RBSOA turn-off waveforms at 6400A and 2600V for three cases, where the second module was tested at 125°C, 105°C and 25°C, respectively.

Turn-off of parallel-connected modules with no temperature difference at 125°C

As expected, the waveforms show a clear trend in increased current mismatch with increasing temperature difference between the modules. We will now briefly analyze the waveforms in Figure 5 with the help of Figure 6 by including also the gate voltage waveform. The temperature mismatch was in this case 100°C, showing a number of distinct phases with substantial current redistributions between the two modules.

Turn-off of parallel-connected modules with a temperature difference of 25°C.

The current mismatch during the switching transient will depend on the following temperature dependent parameters: MOS-channel injection (threshold voltage and transconductance), excess carrier concentration in the N-base (lifetime, mobility and anode emitter efficiency) and avalanche generation (critical electric field). Prior to the switching transient (at t<0μs), the cooler module carries a significantly higher current due to the positive temperature coefficient of the IGBT chips. In order to explain the current redistribution mechanism during the switching transients, the following observations are important: Firstly, during conduction, the hot IGBT has a higher plasma concentration in the N-base than the cold one in spite of its lower current density. This can be explained by the lower mobility, the higher lifetime and the higher anode emitter efficiency of the hot IGBT. Secondly, the initial voltage rise dVCE/dt will depend on the cathode current density JC and the initially stored carrier plasma concentration. For a given plasma concentration, dVCE/dt will increase with increasing JC. On the other hand, for a given JC, dVCE/dt will decrease with increasing plasma concentration in the N-base. In parallel operating IGBTs, VCE has to be the same across both devices and therefore, the current will always redistribute into the slowest IGBT (the one, which has the lowest dVCE/dt when switched alone).

Turn-off of parallel-connected modules with a large temperature difference of 100°C

During stage 1, the gate-drive begins to discharge the IGBT gate and VGE starts dropping. However, both IGBTs are still in the active (onstate) region. During stage 2, the gate voltage drops further and the MOS-channel of both IGBTs enter the saturation region and VCE starts rising. In this stage, the stored charge in the IGBT will determine the further course of the switching transient. Coming into stage 2, the hot IGBT has both a higher plasma concentration and a lower current density, which means that it is the slower one in terms of dVCE/dt. As discussed above, the current therefore has to commutate from the cold to the hot module. At the beginning of stage 3, VGE drops below the threshold voltage of the cold IGBT. In this stage, dVCE/dt is still determined by the hot module, which is limited by the higher stored plasma concentration. The channel injection in the cold IGBT comes to an end, and as a consequence, dynamic avalanche sets in. This slows the cold IGBT down and as a result, the current starts again to redistribute from the hot to the cold module. At this point, the voltage waveform presents no evidence that the cold module is in dynamic avalanche because the total voltage rise is still determined by the hot module. In stage 4, the hot module also enters into dynamic avalanche. Avalanche generation will, however, be higher in the cold IGBT due to the temperature dependence of the critical electrical field. The high avalanche generation will therefore slow down the cold IGBT, which will once more force the current to redistribute from the hot to the cold module. Up till the end of stage 4, the total current in the two modules remains at 6000A. During stage 5, the total current starts dropping. Due to the fact that the cold IGBT originally had a lower stored plasma concentration than the hot one, the electric field has in this stage penetrated deeper into the N-base and consumed most of the stored charge. Therefore the current in the cold module starts dropping and reaches zero in the beginning of stage 6. All the current now flows in the hot module, which still has a significant stored charge, whereas the cold module has reached its static off-state. In stage 7, the hot module will also run out of charge. Throughout this stage, there is still a substantial energy stored in the stray inductance, which does not allow the current to drop to zero. Instead, the hot module enters into SSCM and dissipates this stored energy. Finally, in stage 8 the current in the hot module has reached zero and the turn-off transient is completed.

Different stages during turn-off of parallel-connected modules with a large temperature difference of 100°C

These results show that the modules are perfectly capable of withstanding such extreme conditions and reveal exceptionally stable performance under all operating conditions within the limits of device capability. These results will help designers optimise their systems in order to make use of this recently acquired high SOA capability especially for high voltage applications.

 

References:

1) M. Rahimo et al., “2.5kV-6.5kV Industry Standard IGBT Modules Setting a New Benchmark in SOA Capability,” Proc. PCIM’04, NURNBERG, GERMANY, 2004.
2) U. Schlapbach et al., “Switching-Self-Clamping-Mode “SSCM” for Over-voltage Protection in High Voltage IGBT Applications,” Proc. PCIM’05, NURNBERG, GERMANY, 2005.

 

 

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