Can I Fix This

[bigrockcandymountain]

I am looking for some advice and experience.  I have this old outback flex max 60 amp mppt charge controller.  It turns on fine and seems like it should work, but doesn't charge.  So I pulled it apart and found half of the 13 mosfets burned and a few resistors.  The rest of the hardware looks good.

Would this be caused by over voltage? If i solder in new mosfets and resistors, will it have a more than 50% chance of working?

It looks like new mosfets will be about $20 off ebay from china or $60 from digikey.  Is this worth it or am i throwing money away?  I would like the challenge and could use this controller.

Thanks
Derek

[bigrockcandymountain]

One more picture.

[mab]

well I'd give it a go.

overvoltage can kill mosfets yes. Bear in mind though that the component that drives the mosfet gates might be cooked too, particularly if the gate drive resistors have been fried - I'd probably replace that/them too.

I'm not sure if all those fets work as a single bank in parallel or not; if so, and you can find a spare n-channel 150v mosfet from you junk pile (like an old ups or something), you could try fitting that single spare (after removing all the dead ones) and doing a 'bench test'  with limited power applied to see if it comes to life before committing to buying new components. Actually you could use a <150v fet as long as you didn't exceed it's rated voltage.

[Mary B]

Swap them out and try it! Be careful solder that is probably a double sided board... If you can pull back and get a high rez pic of more of the board so we an see the drivers...

[bigrockcandymountain]

Here are a few more pictures.  Sorry i only have a smart phone camera. 

There are 6 mosfets that look burned.  Every other one.  The ones with the blue thing on the center pin look ok.  The 6 burned are parallel and go to the 2 big transformers.  Looks like 7 or 8 resistors are burned too.  And yes, i believe it is a 2 sided board. 

[bigrockcandymountain]

Alright, so a quick look at the board tells me this.  There are two a3120 optocouplers.  Each one drives 3 of the burned mosfets.  The only components obviously damaged are the 6 mosfets and the six 10 ohm resistors directly connected to the gate of said mosfets.

One other resistor directly connected to that gate circuit is also badly burned so i can't get a value for it. 

I ordered the mosfets and resistors.  I will start there.  This is my first real attempt at this kind of repair, so hopefully it works. I expect i might have to replace the drivers too.

I suspect it was may be lightning damage but it is hard to know for sure. 

Do you have any tricks for determining the value of a burned resistor? It is only the one that i am questioning. 

[mab]

Hmm, do you know where the burnt resistor 'goes'? I.e is it from gate to ground, gate to +ve terminal of a capacitor, or what? On the other bank of unfried fets can you find it's equivalent?

This sort of repair usually entails a certain amount of circuit tracing. If the traces are hard to follow i tend to use an audible continuity meter.

I'd guess the opto isolators are cooked if they were passing the current through the gate resistors; but the good news is the damage is unlikely to have spread 'upstream' of the opto's, which makes it more likely to be a successful fix.

[bigrockcandymountain]

Ok so i think i have it figured. It is 22 ohm and is parallel with a capacitor on the output of the opto.  Your advice to look at the other unburned side made the difference mab so thank you.  This forum is great.

It looks like for about $3 i can get opto isolators so i will do that. 

Now to wait on postage....