Telwin Force 125 inverter welder keeps blowing its IGBT - Page 1
Forum section: electronics / repair

author: ComfyChair
author_rank: Contributor
timestamp: December 09, 2024, 04:01:47 pm
content: Hi,I'm trying to repair my modest inverter welder, a Telwin Force 125.While I was using it, I heard a "pop" an it was gone. I found the schematic for it (attached for reference) and started my repair adventure.Once I opened it up I found the AC input resistor (R2 - 47R 7W-) completely charred. It goes without saying that it measured open circuit. Then, also the IGBT (Q1) was completely shorted. I mean all three terminals connected together.What I did was replacing IGBT and resistor. I probed around the IGBT section a bit and found no issues.With the new IGBT the welder turned on for ~10s then BANG! it was gone again...Not wanting to burn a 3€ IGBT again, I investigated further in the control circuitry. What follows is the list of things I tried:I was suspecting a EOS on the IGBT gate so I measured the zeners near Q1 (D8 & D9) they measure fineI started suspecting the peak current control circuitry so I measured all the resistors, transistors and diodes along the path from TA+ and TA- to U2. Again all looks goodI was suspecting something wrong with the supply to U2 and the opams so I powered the welder without the IGBT. VCC reads 22VI thought U2 was not switching, keeping Q1 in some strange state, but I must admit the design is quite nice. There is C5 that is protecting Q1 from any DC voltage and R9 keeps G & E shorted. I measured anyway there with my meter and see the voltage going up and down.Am I missing something?I have to admit that for the time being, I'm just using my meter. I do not have a differential probe for my oscilloscope and all the signals are ~100V above ground level.My last suspect is Q6. Can it be that perhaps the sink transistor is busted/weak for some reason and Q1 is not fully turned off?I was wondering wether I could arrange a poor man's differential probe by subtracting two channels of my scope and measuring the gate voltage over time. The common mode voltage is high (~100V as I said) while the gate voltage should go from 0 to ~20V. With my 8-bit scope set to 20V/div I could discriminate ~1V superimposed on a 100V common mode-signal. Will I be able to tell if something is wrong with the driver? Any other ideas?

author: ComfyChair
author_rank: Contributor
timestamp: December 12, 2024, 10:48:07 am
content: I was wondering wether i could power the board with 24V (i.e. via TP2) instead ov mains voltages so that I can attach the oscilloscope directly. What do you think?

author: ComfyChair
author_rank: Contributor
timestamp: December 22, 2024, 07:49:45 pm
content: Continuing my journey through the repair I decided to apply power to the 24V rail from an external supply, instead of mucking around a device directly conencted to mains voltage.So I connected 4 wires near Q6: 2 for power and 2 to probe the voltage before (purple trace in osciloscope photo) and after the capacitor C5 (yellow trace).When I turned on the supply I saw that Q6 was indeed switching but after the capacitor there was no voltage. When I added an external capacitor, also the yellow trace started switching.A capacitance meter on C5 reads nothing so it's completely gone!Even with C5 open, the IGBT is still turning on somehow. This is rather strangeI do not have a 1205 50V 1uF cap, can I replace it with a leaded capacitor of similar rating? I have a 1uF 400V leaded polypropilene cap. Will it work?Am I missing something else?

author: ComfyChair
author_rank: Contributor
timestamp: December 27, 2024, 02:37:31 pm
content: Here we are for the latest episode in this installment. I managed to solder in two 2.2uF 25V 0603 capacitors in series. Everything is working now!

author: Swake
author_rank: Frequent Contributor
timestamp: December 28, 2024, 12:05:57 pm
content: Nice repairThanks for the documentary