Induction cooker

[Kopernik]

Hi!
I have a problem to repair a induction cooker.
A IGBT transistor always dies in time.. like one month or two..
I change all four of them, but in one month one will die..
I measured all caps near IGBT, but they are all OK.
I don't know where to search for error..
Can somebody help me?

[HighVoltage]

Start measuring the current going through the IGBTs with a scope and see if they get overloaded.
May be something is wrong with the inductor and it is overloading the IGBT's ?

[bktemp]

Check the MKP capacitors. If they lose capacity the peak voltage at the IGBT increases.

[Kopernik]

MPK caps are OK. Will be better if I put one the same IGBT parallel to existing one?

[unitedatoms]

Looks like the RT1 is a thermistor, mount on lug. The thermal coupling looks too weak. May be you can improve thermal conductance between thermistors and IGBT body by using some clips to attach sensors closer to crystals with more paste around and also spot isolate the sensors from air flow.

It seems that overheating IGBTs is the reason. Could be drift of thermistors values.

Oh. The best would be adding few layers of shrink around thermistors to make them less cooled by air.

Edit: And in general this is kWatt range boards. The parts selection looks so flimsy (carbon resistors, non-solid electrolytic caps, too few ceramic/plastic caps in general. The fiberglass tubing is possibly cut too short on thermistors - cooling by air lowers the reading. May be the design was good at start, but iteration after iteration of going with cheaper parts made it fail exactly next day after warranty period.

I'd think that original wires for thermistors should be some thick insulation high temp silicon wires with fiberglass layer over all length (not half the length) and including metal part of lug being covered by fiberglass with double shrink tubing.

[HighVoltage]

They either die from heat (too much current) or voltage spikes higher than allowed.
Did you always replace the IGBTs with the same type and models?
What is the model number?
They are probably 400 V rated, same as the MPK capacitors?

You can connect IGBTs in parallel but that would only help, if the problem is heat/current related
But then there must be a reason for the extra heat/higher than normal current?

[Kopernik]

ok.. I'll try to rotate the thermistors and give a bit of thermal conductive paste.. transistors are H30R1602.. Vce could be up to 1600V so, I believe that voltage is not a problem.. Maybe is problem in coil that need more current and IGBT is overheating because of it?