CUT50 Plasma cutter repair

This is somewhat a break from the normal type of post for me but I thought I would share by experiences of fixing my plasma cutter..

Firstly it went with a 'bang' and stopped working mid way through cutting a panel of a car body shell.


On inspection some of the mosfets (Toshiba K2837) had disintegrated, which appears from looking online to be a common problem. I ordered some more in (chinese variety as its all I could find) and we replaced the faulty ones after testing them. You can test mosfets in a number of ways (google will help here)
Once the mosfets had been replaced the unit powered up, I plugged a torch in and tried it - bang the breaker tripped.

I stripped it down again and did some googling and I found something listed here 

To summarise from the post within that thread:



Set the meter to the diode test position



Connect the test meter with gate (+) and source (-) Meter should read around 735. Reverse the meter connections and it should read infinity.

Next connect the meter with source (+) and drain (-), the meter should now read about 450. Reverse the meter connections and it should read infinity. (It might take a couple of seconds for the reading to settle)

If any of the readings show a short circuit this indicates there is a fault with any or all of the 3 MOSFETS and possibly the zener diode.







I had at least another 3 dead mosfets again.. I ordered some more chinese ones in and these ones were suspect - the legs were thin and fragile and there was no way they would take the current. I started looking around for something comparable available from uk suppliers and found this 

within this thread the IGBT IRG4PC50UPBF is mentioned - I ordered these Infineon parts from farnell in the UK - they are no longer manufactured. I decided to take a punt and replace the mosfets with these (mine needed 12 - as you need to replace all of them).

I also tested the switching diodes on the bottom board, all of these checked out ok, which is nice - however I had also ordered some of them in so I have spares if needed.

After a bit more searching I also found this https://www.youtube.com/watch?v=Yhza65nWBZY youtube video, it's quite interesting if you can ignore the soldering skills.


I checked the 12 gate resistors they were not 4.7Ohm as mentioned in the linked youtube video but 7 ohm - hardly much difference but I used the correct values, some of mine we looking suspect so I decided to replace them all (with 7 ohm still), as you can see in the photos I managed to lift some tracks slightly when desoldering components - this is all fixed now.

When I tested the zeners on the driver board one was shorted. - They weren't 3.3v as on the video but 5.6v - confirmed this by removing one of the working ones and added a 1k resistor in series and then connected across a psu and measured the voltage across the diode.
So mine were a 5.6v Zener in Mini MELF format (manufacturer part used was TZMC5V6-GS08, farnell order code 1612412)

This was an easy replacement (D12) using my hot air rework station, but I suspect easily done with a soldering iron and tweezers.

After replacing the gate resistors as they looked crap, a zener diode and all the mosfets with IGBTs I'm now back up and working - although I've only done a quick test so far.



If you are going to repair your own then the usual disclaimers apply around the high voltages present inside the device - and remember they are higher than the input voltage due to the HF arc.