IGBT failure symptoms

[deaninkc]

I have an inverter TIG welder that all of a sudden started working at maximum current. Tried using it and burn a hole right through a 1/8" piece of aluminum and my welding table . None of the controls affect it. It's about 20 years old and has analog circuitry to generate the PWM. The pin on the board driving the IGBT has obviously gotten hot but I've tested most of the components and they seem to be OK. Is it common for IGBT's to permanently latch when they fail? Could this also cause high current through the gate causing the burnt spot around the pin to the IGBTs? The IGBT's are down in the bottom of the unit and I have to take the whole machine apart to get at it so I have not been able to identify the specific IGBT's or the layout. The machine is an Italian FIMER unit and the schematics are not available. Any ideas if I am on the right track?

 

[FvM]

In case of IGBT failure your welder won't source current any more. It's most likely a problem of power control circuit. The good news is, it's possibly a small defect. The bad news, without a schematic or profound knowledge of electronic circuits, it may be hard to find though.

The pin on the board driving the IGBT has obviously gotten hot

???

 

[deaninkc]

I've got a lot of experience with microprocessors and digital circuits, I designed my own fuel injection system and use it in my race car but I don't have much experience with analog systems. I'm creating a schematic for it and have all the components identified except for a transformer that drives the IGBT (or SCR or MOSFET). The transformer is an inch square and has 6 pins and does not have any marking except for the pin numbers. Attached is a picture of the board. https://obrazki.elektroda.pl/6449539800_1353966798.jpg

 

[FvM]

O.K., I see what you mean with "pin gotten hot". It's somewhat more, the PCB is slightly burned. If I understand the background of your photo right, you have schematics of the overall wiring, except for the PCB. That's imporatnt information to understand the function of each pin and might allow a first guess what's probably burned. A good photo from the PCB bottom side would allow to extract a principle schematic diagram.

 

[deaninkc]

Unfortunately the schematics they sent me was for a similar but later model. It is more digital, uses DSP's and has a display. Mine has switches and pots. The board is pretty simple,single layer and has all though pin components. When you hold it to the light you can see all the tracks. I'll post the schematics when I get them done.

 

[FvM]

It would be particularly interesting to know what's connected to the burned pins.

 

[iop95]

Try to replace with new all electrolitic capacitors especialy from aux. power supply and drivers.

 

[deaninkc]

Creating a schematic has been a bit more work than I had hoped. I'm still working on it. I did finally take the welder apart so I could get in to the power control section which was at the bottom tucked under the secondary transformer. Come to find out it does not have IGBT's, it has 2 SCR's driven by a pulse transformer circuit using op-amps, none of which I am familiar with. It takes the mains through the SCR's and uses phase control to vary the current into a bridge which feeds the primary on the transformer. The SCR's are CS35-06g04's which I can't find but I did find CS35-08g04's which seem to be the same except for 800v instead of 600v. Unfortunately they are $40 each so I'm a little reluctant to just swap them out to see if that fixes it.

Dean