Mma welding system ( heat problems)

hi again .

I have another one to fix ,
So in previous ones i usually find faulty MOSFETS or system that works but soldering stops after a moment of usage but there is an output anyways even if its weak .
This time it seems that the MOSFETS are good , but there is no soldering at all there is no output voltage .
One thing i noticed in the normally working units is that when i shut down the machine i hear a sound that lasts for a tiny moment ( like something is discharging ) followed by a relay click , in this one i don't notice that .

BTW the system i got contains 3 PCBs , the 2 containing mosfets and dual diodes are hold together by an insulating plastic stick .

1 : consists of 6 470 uF caps , and a power relay
2: contains 3 transformers , a bridge rectifier , hyperfast dual diodes ( 6 facing 6) all of them connected to heat sink .
3 : MOSFETS 6 facing 6 , two daughter boards , one of them containing a PWM integrated circuit , this board has the main output connection for the soldering process.

I hope i made a clear description of the system ; what could be causing that in your opinion .

For me i guess its from the PWM chip maybe ??

Do you have any way to check the drive to the mosfet gates? An oscilloscope would be great, but even a DVM would help.

What do you find at the gate-driver pin?

John

jpanhalt said:

Do you have any way to check the drive to the mosfet gates? An oscilloscope would be great, but even a DVM would help.

What do you find at the gate-driver pin?

John

Click to expand...

I have a Voltmeter , and what is the gate driver please ?? is it an ic ?

Go to your post #7. Look at the mosfets. When they are positioned as shown, the pins at the bottom are named, left to right, Gate, Drain, Source. The gate is what turns the mosfet on. The Source is (should be) at a lower potential than the Drain. Here's a picture from the datasheet:

Referring back to the picture you posted, there is a resistor to each gate. The other end of that resistor goes to a gate drive. That is, the IC, transistor, or transformer that provides voltage to turn on the mosfet. You reference a "PWM chip" in post #22. The PWM chip often includes the gate driver. Regardless of what the driver is, if you measure the voltage at the drive resistor relative to the Source pin, it should be about 8 to 12 V peak to peak, Your voltmeter probably won's show that, but it should show some voltage. If there is no voltage at the gate, then the mosfet won't turn on. Trace the circuit from the gate resistor just described to the PWM chip. Try to draw it out and post the picture here.

John

I will draw a schematic to give you an idea on how the mosfets are related to the PWM daughter board .

OK here is a schematic , the part at the center is a coil that is connected to the board with the SMPS controller :



SMPS controller : KA3525A

OK. Now, as I understand it, you are trying to figure out why there is no voltage at the output. If that is wrong, please correct.

Test the pwm voltage/signal to the daughter board. If you have a frequency scale, use it. Otherwise, use the DC scale. It won't show peak-to-peak, but it should show something. If that works, then check each of the four gate drives (the pins connected to the gate resisters) relative to ground or mosfet source.

John

Yes that it exactly .

But right now i am planing to check that daughter board alone because its hard to check when the unit is assembled , do you have any idea on how to check this ic alone :

Here is a datasheet : https://pdf1.alldatasheet.com/datasheet-pdf/view/53198/FAIRCHILD/KA3525A.html

What do your tests show? Is Shutdown (pin10) high or low? It should be low to operate.

John

Actually i didn't check the pcb yet .
It would be a lot easier for me to test that IC alone out of circuit , because its hard to perform the tests with the unit is assembled , the daughter board becomes hard to reach .

Ok jpanhalt .

I have removed the daughter board i checked every thing in it , all good , but the PWM chip , i don't know how to check it alone .
Any hints ???

That Fairchild chip is apparently the same as the SG3525 chip. You may find more information on testing the latter chip. There has to be an output to the mosfets gates. Do you have access to an oscilloscope?

John

No unfortunatly , just a DMM but its good quality one

If it can be done am ready .

You should see voltage and frequency at the PWN pins (#11 and #14).

John

jpanhalt said:

You should see voltage and frequency at the PWN pins (#11 and #14).

John

Click to expand...

Hi john am back on topic and i just want to make a small description of the unit i got .

The unit i got is a 200A welder : it has a SMPS card containing inputs for mains and 6 470 uF electrolytic caps and two power rectifiers , this card has two connections , 1 for rectified mains going to the PWM motherboard that has also an output for another card containing 3 transformers (EER43x15 21:4) that are finally delivering current to the outputs of the welder .
The second connection is for the(24 v dc ) relay , but this one is an input that comes from the PWM motherboard it self .

Any ideas on what could be stopping the relay from being energized ??