MOSFET damage

N

Nathan Hunsperger

Guest
Hello,

I've noticed a very peculiar problem with a MOSFET in one of my
designs. For reference, I am using an IRL3715, and I should be
well within the rated voltage, current, and switching times. I
have the source connected to GND, the gate to my uC (with a large
value pull-down), and the drain goes to a connector so I can sink
current through a bunch of LEDs.

Now onto the weird part. When the gate is at 5V, I show 0V difference
between GND and the connector, and 12V difference between 12V and
the connector. This tells me that the MOSFET is switched on like it
should be.

However, when the gate is grounded, I show 0V difference between
GND and the connector, but a 4.5V difference between 12V and the
connector. While the 0V between GND and the connector can be
indicative of an open (which should be the case), where exactly is
this 4.5V comming from? Changing the polarization of my multimeter
reads -4.5V. Placing an LED+resistor from 12V to the connector
does not light up, so the 4.5V is extremeley weak.

Additionally, my 5V rail is nowhere near this area of the PCB, and
I have 3 other channels that opperate as I would expect (0V between
the connector independent of which power supply I measure against).

Obviously, this MOSFET is damaged, and I will be replacing it, but
I'm curious as to why this happened. All assembly took place on
an anti-static mat, with me carefully grounded to it. Admitantly
though, I know I touched the drain several times while not grounded,
but I would have touched all the channel's drains checking for heat.
And, from my understanding, it is only supopsed to be the gate that
is extremely sensitive to ESD.

And, on a side note, why did component MFGs attach the heat tabs
of TO220s to the drain? I know you can get full-packs, and have
isolation, but why was this done in the first place? Drain is
always going to be hot with respect to GND, and I can't think of
any reason to have your heatsinks at a different potential than
your conducting chassis...

Looking forward to an explanation,
Nathan
 
Nathan Hunsperger wrote:

And, on a side note, why did component MFGs attach the heat tabs
of TO220s to the drain? I know you can get full-packs, and have
isolation, but why was this done in the first place? Drain is
always going to be hot with respect to GND, and I can't think of
any reason to have your heatsinks at a different potential than
your conducting chassis...
Click to expand...
The die is bonded to the heatsink. The die substrate is usually the drain
( or collector for bipolars ).

As for your other question, I suggest you look at some currents as well as
voltage. A voltage reading can be meaningless ( leakage currents etc ).


Graham
 
I read in sci.electronics.design that Nathan Hunsperger
<njh@soda.csua.berkeley.edu> wrote (in <ctgsn8$13ne$1@agate.berkeley.edu
) about 'MOSFET damage', on Sat, 29 Jan 2005:

Obviously, this MOSFET is damaged, and I will be replacing it, but I'm
curious as to why this happened.
Click to expand...
No, everything is very probably OK. Your voltmeter is simply responding
to a **very** small leakage current through the FET. If you connect a
few LEDs to the connector you will find everything works. Almost
certainly.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
 
I read in sci.electronics.design that Terry Given <my_name@ieee.org>
wrote (in <BV2Ld.13078$mo2.1015836@news.xtra.co.nz>) about 'MOSFET
damage', on Sun, 30 Jan 2005:
As if thats not bad enough, the copper tab does a great job of
transmitting forces to the silicon (ie glass).
Click to expand...
I'm not sure what you mean by that, but silicon isn't glass. Nor is
silica (silicon dioxide); almost all glass is metallic silicate and/or
borate.

--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
 
John Woodgate <jmw@jmwa.demon.contraspam.yuk> wrote:
I read in sci.electronics.design that Terry Given <my_name@ieee.org
wrote (in <BV2Ld.13078$mo2.1015836@news.xtra.co.nz>) about 'MOSFET
damage', on Sun, 30 Jan 2005:
As if thats not bad enough, the copper tab does a great job of
transmitting forces to the silicon (ie glass).

I'm not sure what you mean by that, but silicon isn't glass. Nor is
silica (silicon dioxide); almost all glass is metallic silicate and/or
borate.
Click to expand...
Mechanically, it's pretty glass-like.
 
Ian Stirling wrote:
John Woodgate <jmw@jmwa.demon.contraspam.yuk> wrote:

I read in sci.electronics.design that Terry Given <my_name@ieee.org
wrote (in <BV2Ld.13078$mo2.1015836@news.xtra.co.nz>) about 'MOSFET
damage', on Sun, 30 Jan 2005:

As if thats not bad enough, the copper tab does a great job of
transmitting forces to the silicon (ie glass).

I'm not sure what you mean by that, but silicon isn't glass. Nor is
silica (silicon dioxide); almost all glass is metallic silicate and/or
borate.


Mechanically, it's pretty glass-like.
Click to expand...
Which was entirely the point - thump it and it will break.

Cheers
Terry
 
I read in sci.electronics.design that Terry Given <my_name@ieee.org>
wrote (in <BV2Ld.13078$mo2.1015836@news.xtra.co.nz>) about 'MOSFET
damage', on Sun, 30 Jan 2005:

Pop-riveting applies a fairly
uncontrolled force with a very rapid risetime, and can easily break the
die. motorola AN1040 covers this subject in detail, is on-line and a
must to read.

[anecdotal evidence] A product I "inherited" once had a 30% failure rate
(from 600pcs) on the motorola/on-semi 7805 regulator - but nobody in
production thought that was at all unusual - the official explanation
was "motorola makes lousy regulators." Yeah right - one look at the pcb
showed the device was pop-riveted on.
Click to expand...
This probably explains why two successive staple guns I bought failed
after about 50 staples. I had a full refund, otherwise I would have
changed the triac and, of course, bolted the new one to the heat sink.

I agree that spring clamps are even safer than bolts, BUT you need very
strong springs to ensure good thermal contact.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
 
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