MOSFET damage

[Nathan Hunsperger]

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

 

[John Woodgate]

I read in sci.electronics.design that Nathan Hunsperger

) 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.

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.

 

[Pooh Bear]

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...

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

 

[Fritz Schlunder]

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

By the sounds of your explanation there is nothing wrong or unexpected with
your results or the MOSFET. I assume you are using a digital multimeter
with a 10 Megohm input resistance on the voltage scale you are using.

If that is true, then take a look at the datasheet's drain-source leakage
specification. It claims a maximum leakage current of 20uA at 16V
drain-source with 25 deg. C junction temp. Ohms law says the 4.5V you
measured across your meter's internal 10 Mohm resistance suggests a current
of 450nA. Your meter was likely functioning as a pull up resistor, albeit
10Mohm. Is it possible your particular MOSFET sample under your conditions
had a leakage current of 450nA (with 12V-4.5V=7.5V drain-source)? I would
say so. The maximum spec isn't by any means a typical spec, and is probably
quite a bit larger than typical. The other devices you tested probably have
different actual leakage, hence the different results, but that doesn't mean
they are broken either.

It doesn't really make much sense to be measuring voltages on high impedance
nodes anyway. When the MOSFET is off the drain is close to floating
(assuming no load is attached to the drain), though not totally (pulled to
source potential with a small amount of leakage). When you attached your
meter to 12V and the drain you made a high impedance voltage divider. When
you attached your meter from drain to gnd, you effectively put the 10Mohm
meter resistance across the leakage "resistor", so you no longer had a
voltage divider, just two resistors pulling to ground. Hence the zero volt
reading.

 

[Terry Given]

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

Hi Nathan,

Others have covered the MOSFET readings, and Graham mentioned the die
attachment. But your thought process *is* correct - connecting the
wobbly bit to a large lump of copper is less than desirable from an emi
standpoint, and is often inconvenient electrically too. Some v*cor power
supply modules use single-ended or push-pull converters with FETs to the
positive, rather than negative, DC rail. Although the gate drive needs
to be level shifted, it allows the FET to be directly mounted on a DCB
substrate (ie heatsink) without injecting a huge amount of noise through
the drain-substrate capacitance, as would occur with the conventional
implementation. I gutted one once, and it was a marvel of production
engineering. The Faraday shields around the cores were almost works of art.


But wait, there's more. Copper and Silicon have different CTEs, and so
change size at different rates as the temperature changes, just like a
bimetallic strip. This then stresses the die-tab interface, which is
usually a solder joint [I assume this is true for TO220]. These stresses
tend to make the solder joint deteriorate - and the greater the
temperature change, the worse the degradation. Luckily the degaraded
connection increases the temperature rise, thereby exacerbating the
problem, eventually leading to thermal-fatigue induced runaway failure -
this is a leading cause of failure among high-power IGBT modules, and I
just read an interesting paper on its occurrence in TO-247 packages too.

As if thats not bad enough, the copper tab does a great job of
transmitting forces to the silicon (ie glass). Over-tightening of a bolt
can deform the copper tab, placing a direct mechanical strain on the
die, degrading lifetime. Another interesting thing that can happen with
tight bolts is the copper tab bends at the bolt interface, actually
lifting the die off the heatsink. 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. We changed to a nut, bolt and
belleville spring washer, tightened with a preset torque wrench, and the
failure rate dropped to zero, over 17,000 units. We also removed the
"nylok" self-locking nut from the smps transistor, because the inset
nylon washer melted at about 95C - in some units it ended up as a puddle
on the pcb, and the nut would spin freely. We used a washer, a
belleville spring washer and a nut. Since then I have found really neat
all-metal self-locking nuts - basically a spring washer is crimped into
the end of the nut. Spring clips are even better, as they provide a much
more controlled force over a wider area, and are a lot easier to assemble.

Cheers
Terry

 

[John Woodgate]

I read in sci.electronics.design that Terry Given <[email protected]>

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.

 

[John Woodgate]

I read in sci.electronics.design that Terry Given <[email protected]>

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.

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.

 

[Ian Stirling]

I read in sci.electronics.design that Terry Given <[email protected]>


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.

 

[Terry Given]

Mechanically, it's pretty glass-like.

Which was entirely the point - thump it and it will break.

Cheers
Terry