About Mosfet

Hi Audiogonians, Merry Christmas.

I got some questions about MOSFET.

First I would like to know how can one damages a MOSFET? Can wrong connection from powersupply kill it? Next up is how does a damaged MOSFET sounds like and how can one knows which one is dead if there are more than one in a poweramp? Finally, if there such a thing as life span of a MOSFET?

Equipment-Adcom GFA5500. Problem-One channel sounds like a weak reception radio. Cause-could be wrong connection. Can someone help me to trobleshoot this?
Some answers:
>>First I would like to know how can one damages a MOSFET?
Several ways: put more current thru it than it is rated for, put more voltage on its terminals than it is rated for, put a high surge voltage on its gate during power-on, for example, open the lid of your power amp & without adequate ground shielding for yourself touch something in the amp, create a spark that could fry the circuit (& yourself!). For now, this is my short list.

>>Can wrong connection from powersupply kill it?
Yes, it's possible. However, the MOSFET must have been installed in the factory. It must has passed QC where they would have caught this wrong connection & fixed it before shipping.
IFFFFFF somebody else or you have tinkered with the MOSFET then it *could* be soldered in there incorrectly.
However, if the MOSFET connections are flipped it is unlikely to turn on as the most of the MOSFETs used are enhancement devices which need a positive gate-source or source-gate voltage to conduct. It *seems* that a bad connection is unlikely but........possible.

>>Next up is how does a damaged MOSFET sounds
You should hear lots of distortion like a voice over a crackling radio or a bad AM reception.
I personally doubt that a bad MOSFET will put out any sound 'cuz when there is a catastrophic failure in MOSFETs, these devices fail totally.
MOSFET aging is gradual like many other devices but in my experience failure is total i.e. it's a 0 or 1 situation.

>> how can one knows which one is dead if there are more than one in a poweramp?
This could be risky business as the power supply caps store a lot of energy EVEN when the power amp is disconnected from the wall. Be VERY careful of putting your hand in there - you could create a spark that could fry you or a good part of your hand!
Get a tech/Adcom to trouble-shoot this for you. You'll need a voltmeter to check MOSFET output voltages, bias voltages, etc. You'll also *might* need a schematic.

>>Finally, if there such a thing as life span of a MOSFET?
Yup! there most certainly is! Life of a power MOSFET is usually pretty damn long - 10 years is not uncommon with the multiple on/off cycles where the devices heats up from stone cold. Today's MOSFETs might be hardier.

I think that it might be some other component in the amp such as an aging electrolytic circuit cap or one or more of the power supply caps.
Sounds more like the driver circuit than the output devices. Usually a dead mosfet is a dead mosfet. If it is shorted, it would be blowing your rail fuses. I would leave this to a trained tech.
Sorry if this is obvious, but, did you check the fuses?

I feel that it cannot be the fuses. If the fuse is blown, then there'll be zero (or no) output. Rainchild reports "Problem-One channel sounds like a weak reception radio".
Bombaywalla, recently I blew a fuse on one channel of a stereo amp. The channel with the blown fuse continued to play music, albeit scratchy and distorted. After reversing speakers and cables, changed sources and eliminating the pre from the system, no luck. Then I checked the fuses. One new fuse, no problems.
P.S. it was an all Mosfet amp.

Incredible indeed!
I didn't think that a blown fuse would allow any power to that channel. Must be the way that the fuse was routed into the circuit. Many times amps have fuses but they are not in the direct signal path to avoid spoiling the sonics. In such cases the fuses do their job but not like a traditional fuse which might be in series with the circuit path & once blown would create an open circuit.

Ok, Rainchild, check them fuses!
Most-likely you have one or more of the output device(s) failure.
Thanks guys,

Oohlala, Bombaywalla knows so much, how I wish I could be like you. I'd checked and yes, I got a 8A blown fuse. I'll replace it tomorrow and see how it goes. Thanks Unsound. Like an AM radio with bad reception. Is that what you've experienced?
Hey guys,

Bad news, I changed the dead fuse, amp was ok for a few mins and spark starts coming out and now the Mosfet of that faulty channel ready blown. Sent the damaged channel PCB to a nearby technician. Ask me some questions. He thinks it is because I didn't screw back all the screws that fasten the mosfets onto the heatsink. That's the price of laziness.
Nothing to wonder.
One channel was already going out due to the late action of fuse anyway. Hence the distortion on one channel you were experiencing.
Just next time make sure you fasten tight to the heatsink no-matter bipolar or FET so the tech won't have to make-up the reason.