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Old 19th June 2009, 10:30 AM   #1
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Default Blown mosfets

Ok, i have a Powerbass Xa3000D here, which i'm repairing for a friend.
All 20 power mosfets blew.
I replaced all 20, powered it up (before resinking it) and it worked (turned on and played tones, gains worked etc....).

So i re-sinked it up, then powered it up again, and bang, a fet went pop, so i de-sunk it again, and changed the fet (hoping it was a poor solder joint or the like).

When i powered it up the second time it worked fine again, great.
But when i put it back into the heatsink again it blew the fet once more....

All 20 fets are 2 sets of 10 parralleled, which are joined to the SMPS control board by a set of 8 transistors.

What would cause the fet to only blow when they are back on the sink? I assume another component blew when the fets went, any ideas???
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Old 19th June 2009, 12:05 PM   #2
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Are you certain that that sil-pads/mica/insulators under the FET's is in good shape? nothing shorting?

It might be a good idea to check the tab of each FET against ground with your DMM,to rule out any shorts,etc.
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Old 19th June 2009, 01:18 PM   #3
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That is the problem, sorry, i am getting ~+14v DC across the fets to the ground.
The mica pads are in sound shape.
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Old 19th June 2009, 03:02 PM   #4
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If you blew the same FET, it's likely that you have a broken connection in the gate drive circuit for that FET or a gate resistor that's out of tolerance. It's possible that the foil trace is torn and it breaks the connection when you clamp it to the sink.
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Old 19th June 2009, 04:07 PM   #5
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I checked the traces from one to another and there is perfect conductivity ( also very very very little resistance, between the fets.)
Where will these gate resistors be???
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Old 19th June 2009, 04:11 PM   #6
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by agithegreat
Where will these gate resistors be???
the gate resistors provide best damping when they are attached directly to the gate leg with a VERY short lead.
They are there to create resistance between the inductance of traces and leads from the capacitance seen at the gate. If this RL circuit is not damped the FET may oscillate.
Each FET needs it's own gate resistor.
If you are measuring continuity (near zero ohms) gate to gate then the designer has forgotten to place the gate resistors.

Car audio quality - what quality?
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Old 19th June 2009, 04:22 PM   #7
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Ah yes, these........ By the left hand legs. Click the image to open in full size.
I read 4 ohms across every one.

So gate drive circuit?
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Old 19th June 2009, 06:07 PM   #8
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If you read 4 ohms across each one of the gate resistors, you likely have a lot of shorted FETs. What is the resistance from leg 1-2 (no power applied)?
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Old 19th June 2009, 06:41 PM   #9
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you can see RP9 is bad!!!
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Old 19th June 2009, 06:56 PM   #10
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Ok, excuse my stupidity i have just gone back and i was measuring resistance of the trace, not the resistor, lol!
I got ~24 ohm across all resistors, except one was 68ohm, and another (the one which keeps blowing) says 100ohm. Which is pin 1, 2 sorry perry? The 2 pins other than the gate?
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