Here you see a V-I graph including many curves achieved under different junction temperatures of an IGBT. As it can be seen there is a cross point where all curves intersect each other. Does it make sense? If yes, what does the cross point mean and what is its concept?
The Cross point is very important. Below the crosspoint you have thermal stability of paralleled devices. Above the crosspoint you may not depending on your heat sink system. Low crosspoint is better. Many devices have a crosspoint that exceeds their rating which is a disaster.
It doesn't have any special meaning, but at that point (about VCE=1.25 V in the example), the current that flows is independent of temperature. The temperature independent occurs because the VCE is a combination of a PNP junction voltage and the VDS voltage of the internal structures in the IGBT. The PNP junction voltage has a negative temperature coefficient and the VDS has a positive coefficient. At the crossover point, these tempos are equal and opposite.
It may be easier to consider the current. The point above is at about 200 A.
At current lower than 200 A, the VCE would be observed to decrease as temperature increases. At a current of 200 A, VCE would be independent of temperature, and at higher currents, VCE would increase as temperature increases.
If you had multiple IGBTs in parallel (so sharing the same VCE), and the overall VCE was lower than that 1.25 V threshold, then if one IGBT got hotter than the others, it would tend to take more than its 'fair' share of the total current. Thus its power dissipation would be higher and it would continue to get even hotter. In extreme cases it could get dangerously hot (this is called thermal runaway). Thermal runaway can be mitigated by tightly thermally coupling the IGBTs together.
The other disadvantage of this is that if you were sensing the current by monitoring current on only one of the IGBTs, you would get an incorrect result.
Note that this could only happen if the total current was less than 200 A * N_IGBTs.
At currents >> 200 A, while all the IGBTs would get hotter, they would share better.
