Low Side Ignition with IGBT to fire ignition coil

[danipisca07]

Hello everybody.
I've printed a pcb to make an adapter for my OEM wiring harness (there isn't a PNP kit).

I have everything working correctly but after about 30 seconds of functioning the IGBT I have used to drive te coils seems to burn, stops working and starts buzzing (at which point I have disconnected everything to prevent damage to the ecu).

I have setup the coils like this:


And I have used this IGBT: STGB20NB41LZT4 (https://www.st.com/resource/en/datasheet/stgb20nb41lzt4.pdf)

Which I control using the IGN_LS1 output from the ecu.
The engine is a 4 cylinder but have 2 coils with a wasted spark.

I have setup dwell times around 5/6 ms which I know to work perfectly with other adapters and the same ecu on the same coils/engine.
The igbt doesn't seem to get hot (touching it by end) right away, but at some point it stops warking (spark not happening) and starts to heat up until burning hot and buzzing.
Something strange I noticed is that IGN_LS1 when active doesn't provide 12V (battery pos) but hovers around 6/7 V. Is that normal? Anyway that should be more than enough to saturate the IGBT.

Did I do something wrong?
Thank you beforehand to anyone that will try to help.

[SprinterTRD]

"Something strange I noticed is that IGN_LS1 when active doesn't provide 12V (battery pos) but hovers around 6/7 V. Is that normal? Anyway that should be more than enough to saturate the IGBT."

Yes this is normal and it is not connected to 12V. The output is a constant current one not a constant voltage one.

[danipisca07]

Thank you for the info sprinter, didn't know that, it make sense.
Anyway I still don't understand why the igbt "fryies".

This is what i'm using: https://www.st.com/resource/en/datasheet/stgb20nb41lzt4.pdf

From what I understand of it's characteristics it should enable correctly with a 40mA current...

[Stephen Dean]

When it is not getting a control signal, i.e. the ECU is not either supplying 0mA or 40mA, what is happening with the output? It may be that if it is not under control by the ECU then the output is floating, which leads to the coil constantly dwelling and cooking itself.