I changed all MOSFETs in parallel and they burnt again. I AM SO TIRED, IVE WASTED MORE THAN 20$ for this UPS:'( The gate Resistance is 24ohms, I will try increasing it and check again.

 

I changed all MOSFETs in parallel and they burnt again. I AM SO TIRED, IVE WASTED MORE THAN 20$ for this UPS:'( The gate Resistance is 24ohms, I will try increasing it and check again.

If the FETs are not turning on adequately that could be the problem. So now another question: Did this UPS EVER perform correctly, and deliver at least 100 watts into some load for a few minutes without burning up?
Some manufacturers repackage returned items if they seem to work, but the checkout is very short. That is another possibility.
One more thought is that if you are intending to use it as a backup for a laptop power source, it might be more effective to just use the included battery pack from the UPS directly. That is not a trivial option but possibly a possibility.

 

If the FETs keep burning out then something else is wrong. Don't change values from the original design yet, it's unlikely in a commercial product that the original design is so bad that it will die right away. It is more likely that something else has broken in the circuit which is causing the fets to burn out. The fets are the symptom, you haven't found the actual problem yet. Keep looking for other bad parts or connections.

 

it had 2 originally, and one open space to add another FET. all diodes and capacitors are fine.

How did you test the diodes and caps? In circuit? Did you use a tester designed specifically for diodes and caps?

What type and spec. are the caps? Can you post a clear, close picture of the board?

 

If the FETs keep burning out then something else is wrong. Don't change values from the original design yet, it's unlikely in a commercial product that the original design is so bad that it will die right away. It is more likely that something else has broken in the circuit which is causing the fets to burn out. The fets are the symptom, you haven't found the actual problem yet. Keep looking for other bad parts or connections.

This is what I was saying in post #3.
And still, we do not know that the performance was ever correct, only guessing that it was.
Finding another part that has failed will be a bit of a challenge because of the compact construction of most UPS devices. In addition there is a custom control IC running the whole show and if it has a problem then there is probably not a replacement available. It may be that some of the spike suppression components have failed and are causing way too much current to flow.
And if the problem is indeed a parasitic oscillation, tracing the cause will take a lot of skill and some good test equipment.
Does anyone have access to a circuit diagram for this UPS?? THAT would help with fixing it.

 

@MisterBill2 gives good advice, I would just like to add that you can check the more basic components such as diodes and even capacitors, at least visually. Just about every multi meter will have a diode test feature, and a swollen or leaking capacitor is a sure sign of trouble. The power stage of one of my audio amplifiers (also a SMPS like your UPS) burnt out all the mosfets when a single small diode went bad. For some designs all it takes is one key part to go bad then everything else falls like dominoes.

 

OK. something strange happened. I increased gate Resistance and then made another Circuit board for the MOSFETs to be in parallel, they are working perfectly generating less heat than the others(I increased from 2 to 4 Power FETs ), which is strange, because all of the FETs now get a little warm instead of heating up. It looks like all the load was being pushed to Only ONE MOSFET, but I don't know why .

 

It's using a PIC16F72 Microcontroller.

 

OK. something strange happened. I increased gate Resistance and then made another Circuit board for the MOSFETs to be in parallel, they are working perfectly generating less heat than the others(I increased from 2 to 4 Power FETs ), which is strange, because all of the FETs now get a little warm instead of heating up. It looks like all the load was being pushed to Only ONE MOSFET, but I don't know why .

Not only did you reduce the load on the FETs by half, but in addition, by making a separate circuit board and connecting it you probably corrected some other problem, possibly the parasitic oscillation at some very high frequency, or some failed connection that your modification repaired. But the good point is that you have it working again.

 

Is it that MOSFETs do not share load equally when in parallel? or is it low voltage?

 

And another question. Do Inverter MOSFETs switch at 50hz or 100hz? Or at what frequency?

 

And another question. Do Inverter MOSFETs switch at 50hz or 100hz? Or at what frequency?

There are several different schemes for producing an alternating 120 volt AC output. Many of the simplest UPS packages us a driven inverter with the primary input fed directly from the battery supply. The drive is an on/off signal with a dead time in between, so that it is sort of equivalent to a sine wave effective power. Some others use a much higher PWM signal to come closer to an effective sine-wave equivalent. Those devices are switching at a much higher frequency. Probably your lower price system uses a driven inverter version.
But there are other schemes arriving all the time and there may be some others. If your UPS has a large transformer with heavy leads connecting to the power FETs then it is some sort of inverter configuration.

 

Thanks @MisterBill2 .