Anyone repair inverters?

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pishta

I know I'm right....
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I know a few of you all work with this kind of stuff: I got a Xantrex 1750 Plus inverter that goes to FAULT whenever I put any load on it. It was in fault when it was given to me so I pulled the MOSFETS out and sure enough one of them was bad so I replaced it with a spare I had and I got it fired up to clear the fault. As soon as I plug something into it (from a heat gun to a 30W soldering iron) it trips to fault. When I was in there and probing the AC out, I must have touched something as I heard a crack like a .22 went off. It went to fault again and I blew another MOSFET but I had a 2nd spare so I put it back in an now it wont take any load, just goes right to FAULT when something is plugged in, but I do have 107V AC off the plugs (?). It clears if I power cycle it. I looked at the PCB for a blown capacitor but there is nothing burned on the board, no bulging caps, no arc track or any sign of a spark. Dont know what it was but scared the fool out of me. I replaced 2 of the 8 MOSFETS on the bottom right, did not check any of the others but other are 2 leg and some are 3 leg. None have any burn marks on them. Any idea where to start looking? As I said, nothing looks damaged on the board. My $12 LCD component tester really made checking stuff easy!
inverter2.jpg
 
Capacitors like to pop. Does your component checker test capacitors if you remove them from the circuit?
 
yes, but non are bulged or popped. I would have expected to see a blow out with that crack
I heard (seen it before, looked like half a firecracker) but they are all perfect looking. I also have a capacitor meter.
 
Do you have a scope? Try troubleshooting at the lowest input voltage that allows switching. Measure the gate and drain switching times. Excessive ringing or impedance mismatch usually pops mosfets.
A shorted turn on a transformer could be the cause. Any damper diodes in the circuit?
 
It's also possible that the PWM driver IC has blown. This would cause the fault when a load is applied. By the way, there are no "2 legged MOSFETs". Those are likely rectifiers. Also, don't try to run it with a load for very long without a heat sink on those MOSFETs. That might have been what popped the last one.
 
Here is an easy to read app note on inverters:

http://www.ti.com/lit/an/slaa602a/slaa602a.pdf

My experience is shorted windings is the probably cause of failure. That is the most expensive component and the item targeted to reduce costs. The cheapest transformer is always selected to save costs.
 
well, I pulled every MOSFET and diode pack in the edges and found another MOSFET had gone bad so I ordered 10 new ones ($2.40!) and will try one more time. I did locate the PWM driver chip and if the 8 new MOSFETS dont get it working, and possible a new PWM chip... its gonna get donated to anyone that wants to cover shipping. As far as I could test, the windings are not shorted between the poles.
 
VISUAL INSPECTION! You could have one shorted turn. The App Note explains inverter operation and theory well.
Don't apply a load until you look at switching signals with a scope.
 
VISUAL INSPECTION! You could have one shorted turn. The App Note explains inverter operation and theory well.
Don't apply a load until you look at switching signals with a scope.
Visual inspection showed no sign of even an arc or a blown component. Cant do jack with no output stage MOSFETs installed. Will wait until I get the new ones installed. Im not an ET so a lot of that theory stuff was over my head.
 
Works! Got 10 new MOSFETs and replaced all 8. Although it says 1750W, I tried a 1000W sawzall or an 800W die grinder and both tripped the fault, probably not designed to give that much so fast. It did light up a dual F40 shop light and a 90W laptop power supply, I really only needed it to do the laptop and maybe a 20V Dewalt battery charger. My little lighter socket one would start to melt if I put the old laptop on it. Had to put an A/C duct hose onto it. The little watt meter doesnt even register the 90W power supply...? Maybe its not pulling that much, but I did see it jump when I turned on the sawzall for a split second. It has a remote RJ11 (phone plug) switch on it too, so I can run a switch to the dash to turn it off. Killed a few batteries in my old truck with an inverter on over a weekend.
 
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