I'm popping MOSFETS....linear derating factor involved?

[qrk]

I can't find anything more than 5mV of noise or voltage variation
anywhere with the scope. I tried grounding the probe to both the main
supply ground and at op-amp gnd (and measuring around). Even when
that noise on the input to the op-amp, the cap on its output seems to
slow everything down enough to not let the noise affect the MOSET's
gate voltage.

My (possibly) exceeding the total dissipation rating of the FET isn't
a possible cause for the popping of FETs too? Still don't know if my
calculations and assumptions about linear derating factors are right.

John

You can get oscillations at pretty high freqs (>50 MHz) that will
cause problems. A ferrite bead or small valued resistor in series with
the gate lead can help some oscillation problems. Make sure you use a
scope that can see above 100 MHz.

 

[John]

IME devices need to be torqued down with appropriate hardware (eg
Thanks Terry. I used just screws and flat washers for my prototyping
but will be adding belleville washers when I put it all together. No
torque wrench available but compressing the washers to 75% or so
should ensure proper pressure? Assuming that the washers are strong
enough, that is.

I was actually considering using nylocks when I had a different heat
sink (the current one is threaded for the screw) but stopped when I
realized how hot it would get.

I was hoping to use clips but my recent tests have shown that screw
mounting a TO-247 case held its case temp 14C lower than using clips
(even doubled up clips). This is with Aavid's UltraStick compound
which changes to practically water (filling all the nooks and
crannies) when it hits 60C or so.

John

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[John]

And Tc means case temperature (normally!). But I didn't stop to think

I'm running at just above 27V and 4.6A per TO-247 IRFP2907. Pretty
good safety margin for the current limit.

John

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[John]

You can get oscillations at pretty high freqs (>50 MHz) that will
Hmm...I have a 1K resistor from the op-amp output to the gate and
0.01uF cap from op-amp output to GND. I *think* that slows down the
amp enough to prevent oscillations at that high a freq., but it's easy
to add a bead too. Thanks!

John

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[John]

Viewing the schematic, I can see no fault in the sharing method,
Darn good point.
I can easily raise the source to 9V and test again. The gate voltage
was about 3.6V at full power and the two gates were being driven to
within 0.1V of each other (IIRC). Checking the Vds drop across the
FETs and the sense resistors showed a pretty good match (but I've
forgotten the numbers). I'll definitely be looking closer at those
numbers for the next round of testing though, thanks.



I'm beginning to agree with that, at least for my situation. I got a
14C decrease using screws instead of clips for these TO-247 cased
FETs....that's huge IMHO.

John

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[Terry Given]

Hmm...I have a 1K resistor from the op-amp output to the gate and
0.01uF cap from op-amp output to GND. I *think* that slows down the
amp enough to prevent oscillations at that high a freq., but it's easy
to add a bead too. Thanks!

John

the MOSFET can oscillate, all by its self. There is a nice Siliconix app
note (in the mospower apps manual) that does a Routh-Hurwitz stability
analysis on a MOSFET, showing why it oscillates, and how to stop it.

Cheers
Terry

 

[Terry Given]

Thanks Terry. I used just screws and flat washers for my prototyping
but will be adding belleville washers when I put it all together. No
torque wrench available but compressing the washers to 75% or so
should ensure proper pressure? Assuming that the washers are strong
enough, that is.

I use decent bolts (not cheese) and apply rated torque.

I was actually considering using nylocks when I had a different heat
sink (the current one is threaded for the screw) but stopped when I
realized how hot it would get.

I was hoping to use clips but my recent tests have shown that screw
mounting a TO-247 case held its case temp 14C lower than using clips
(even doubled up clips). This is with Aavid's UltraStick compound
which changes to practically water (filling all the nooks and
crannies) when it hits 60C or so.

John

Just use gruntier spring clips

unless you have enough compliance in your spring, those reflow-type
thermal interfaces require you to re-torque the fastener. I've done that
with 600V 1200A IGBTs - heat to 80C for a while, let cool then re-torque.

whats the surface finish like on your heatsink? have you checked the
failed untis, to ensure an even spread of goop (a lack thereof indicates
a bent heatsink)


Cheers
Terry

 

[John]

the MOSFET can oscillate, all by its self. There is a nice Siliconix app
I looked for that manual but couldn't find it even using searches for
"stability", "Routh-Hurwitz", "analysis", "MOSPOWER", "applications"
and "manual". May not be available anymore?

I'll check the web though, thanks!

John
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[John]

I use decent bolts (not cheese) and apply rated torque.

You use a belleville washer (with a flat washer), "bottom it out" and
continue to tighten until the rated torque is reached?

Hmm...maybe I need to make the investment and get a decent torque
wrench for this. More toys is always a good thing!


LOL, for half a second I was saying to myself "Hmm...I've never heard
of a company named Gruntier selling spring clips".

Any recommendations for these guys? The ones I have are rated for
18lbs, I can't find the Aavid MAX08 27lb. clips for sale in quantities
less than 2500. But, I'm guessing that the doubled-up 18lb springhs
reached at least that 27lb. rating (MUCH harder to tighten them both
down) so I'm not sure how much better they'd do.

I'm thinking that one of my tests needs to be with plain ol' silicone
grease, just to get the case temperature as a benchmark for the other
tests.


That's what I've done for screw-mounting in my prototype (since no
belleville washers were used) but you shouldn't need to do it when
using springs, should you? The springs I had were "pulled up" the
thickness of the entire TO-247 body and would surely push back down
the couple of mils needed to squeeze out any excess compound (when it
reflowed)?

It's the "ready-to-use" milled finish straight from the manufacturer
which I buff with 000 steel wool and rinse thoroughly. It was
starting to become mirror-like.

I did take a look at the mounting surface of the FETs and sink after
they blew and so far all of them have had a good even layer of goop.
I'm guessing, but it sure looked as thin or thinner than a sheet of
paper.

John

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[Terry Given]

I looked for that manual but couldn't find it even using searches for
"stability", "Routh-Hurwitz", "analysis", "MOSPOWER", "applications"
and "manual". May not be available anymore?

I'll check the web though, thanks!

John

Siliconix MOSPOWER Applications, Ed. R.P. Severns, Siliconix, ISBN
0-930519-00-0

one of the reasons I download and archive everything I see....

besides, web pages have a 6-month half-life.

Cheers
Terry

 

[Terry Given]

You use a belleville washer (with a flat washer), "bottom it out" and
continue to tighten until the rated torque is reached?

no, spring washers should never be flattened out. pick a belleville that
gives 50-75% compression at the rated torque of the bolt.

Hmm...maybe I need to make the investment and get a decent torque
wrench for this. More toys is always a good thing!

cant argue with that one.

LOL, for half a second I was saying to myself "Hmm...I've never heard
of a company named Gruntier selling spring clips".

antipodean slang.

Any recommendations for these guys? The ones I have are rated for
18lbs, I can't find the Aavid MAX08 27lb. clips for sale in quantities
less than 2500. But, I'm guessing that the doubled-up 18lb springhs
reached at least that 27lb. rating (MUCH harder to tighten them both
down) so I'm not sure how much better they'd do.

pass. I never picked the nice ones we stuck in our 2500W dc-dc
converters, but they were pretty stiff.

I'm thinking that one of my tests needs to be with plain ol' silicone
grease, just to get the case temperature as a benchmark for the other
tests.

I rather like John Larkins approach of a well-anodised
heatsink/insulator. sometimes less is more

That's what I've done for screw-mounting in my prototype (since no
belleville washers were used) but you shouldn't need to do it when
using springs, should you? The springs I had were "pulled up" the
thickness of the entire TO-247 body and would surely push back down
the couple of mils needed to squeeze out any excess compound (when it
reflowed)?

AIUI this is especially true from split-ring type spring washers, but
they tend to relax after being flattened out, so the original spring
force is not restored.

It's the "ready-to-use" milled finish straight from the manufacturer
which I buff with 000 steel wool and rinse thoroughly. It was
starting to become mirror-like.

I did take a look at the mounting surface of the FETs and sink after
they blew and so far all of them have had a good even layer of goop.
I'm guessing, but it sure looked as thin or thinner than a sheet of
paper.

John

thats a good sign then. I only ask because of a nightmarish 2 weeks a
decade or so ago - crux of story is mech eng threw out off-tool heatsink
sample, a tech fished it out of the bin to sell for scrap, another tech
used it to build a prototype, prototype mysteriously failed, quite
repeatably, for 2 weeks.

nowadays I always check surface finish and coplanarity - usually with a
combination of eyeometry and a straight edge.

Cheers
Terry

 

[John]

Siliconix MOSPOWER Applications, Ed. R.P. Severns, Siliconix, ISBN
Thanks! Founds lots of sources, several for only a couple of dollars.

John

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[John]

You use a belleville washer (with a flat washer), "bottom it out" and
Ahhh...OK, that makes sense.


You read my mind. I'm pretty sure I'll be going the hard anodized
route for this one (instead of using insulating pads) but I'll still
need a grease of some type between the FETs and the anodized sink.


Ouch, I can just picture you guys absolutely pulling your hair out for
those 2 weeks.

I just tried mounting two of the blown FETs via screw and flat washer,
heating the sink up to phase-change the compound and then popping off
the FETs to check how well the compound spread. It is definitely a
much thinner layer of compound when screw mounted (almost
non-existent) and looks to be pretty even along the length of the
TO-247 body. Unless I can find clips with 100lb. of pressure, I'm
sticking with screws for this application (or a screw-mounted bar
pressing down on the FETs).

John

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[Terry Given]

Ahhh...OK, that makes sense.





You read my mind. I'm pretty sure I'll be going the hard anodized
route for this one (instead of using insulating pads) but I'll still
need a grease of some type between the FETs and the anodized sink.





Ouch, I can just picture you guys absolutely pulling your hair out for
those 2 weeks.

oh yes. it was just after we fixed a subtle gatedrive problem, so we
were pretty paranoid about blowups. Eventually the mech guy wandered
past as another tech was dismantling it, picked up the heatsink and said
"what the **** is this doing here". Half an hour later, we had the drive
up and running, and it stayed that way.

I just tried mounting two of the blown FETs via screw and flat washer,
heating the sink up to phase-change the compound and then popping off
the FETs to check how well the compound spread. It is definitely a
much thinner layer of compound when screw mounted (almost
non-existent) and looks to be pretty even along the length of the
TO-247 body. Unless I can find clips with 100lb. of pressure, I'm
sticking with screws for this application (or a screw-mounted bar
pressing down on the FETs).

John

the latter is a nice approach, a good compromise wrt parts count.

Cheers
Terry

 

[Winfield Hill]

John wrote...

Well, originally, no.
With my testing to confirm that the circuit was stable and no other
ratings were exceeded I was concentrating on the "other" stuff like
derating factors. Especially since the circuit is based on one you
gave me last year. But, here it is. I could be very wrong about my
asssumptions that exceeding the total power rating for the FET could
be my only problem.

. Vcc
5V
. |
. 1K
. |
. +------, +5V ,-----+----- LOAD+
. | | | LT1013 | |
. 1.25V POT <-+--R2--+-----|+\ D |
. Ref | | | | >--+--R6-- G |
. -+------+-, | | ,--|-/ | S |
. | | | | | === C1 | |
. | | | | | | | |
. +--|--R3--|--+----|---+---R5----+ |
. | | | | | |
. | | | | R7 |
. | | | | | |
. | | '--R4---+-------------+-----|-+--- LOAD-
. | | | |
. | | | |
. | | +5V ,-----+ |
. | | | | |
. | +--R2--+-----|+\ D |
. | | | >--+--R6-- G |
. | | ,--|-/ | S |
. | | | | === C1 | |
. | | | | | | |
. LOW-LVL GND+-----R3--|--+----|---+---R5----+ |
. | | | |
. | | R7 |
. | | | |
. '--R4---+-------------+-------+
|
HI-AMP GND

R2, R3 = 49.9K,1%
R6 = 1K,1%
R4, R5 = 10K,1%
R7 are 0.01ohm,1%,3W
C1 = 0.01uF monolithic ceramic
MOSFETs are IRFP2907 or IRF1405.

LOW-LVL GND and HI-AMP GND are brought back separately to the
supply's GND pin.

Prototype circuit is breadboarded with short leads, well decoupled
with a quiet linear power supply. After setting the pot, current
is steady to within 0.1A up to 30A (max for my tests). Scoping out
the power supply, inv. and noninv. inputs to op-amps and gate lead
to MOSFETs shows no more than 4-5mV of noise.

Thanks for that fine diagram. Before seeing your details I was
speculating that you were using higher-voltage MOSFETs, which
have a nasty tendency to RF oscillation when biased for linear
operation with Vds more than 10V (the oscillation is internal to
the MOSFET and often cannot be stopped even with a gate-source
bypass capacitor. I have not experienced this problem with low-
voltage parts like your IRFP2907 and IRF1405, but it is still a
possibility and, depending on wiring inductances, could mean that
your FET may be exposed to excessive RF gate voltages. However
these RF oscillations (10 to 30MHz) are easy to see on the scope.

I think it's more likely you have a power dissipation issue, and
are exceeding the MOSFET's junction-temperature spec. You can do
all the calculations you want, but the bottom line comes from a
complete set of careful thermocouple measurements. Watch out for
gradients across the MOSFET's case. Most of the power-supply big
boys use strong clips placed on the MOSFET's plastic housing to
evenly press the case against the heat sink, rather than screwing
the MOSFET down by the tab, which can lead to a slight tilt to the
case. Hmm, I wonder if this can lead to a stress on the silicon,
lowering the degradation and failure junction temperature?

The big boys also use higher safety margins, e.g., four MOSFETs
instead of two.

One thing that raises my eyebrows is your low 5V supply voltage
for the LT1013, which limits the MOSFET gate voltage. According
to the IRFP2907 and IRF1405 datasheets these are not logic-level-
drive MOSFETs, and they may need a Vgs of nearly 5V to operate at
15A, which the LT1013 may not be able to deliver with Vcc = 5V.

 

[John Larkin]

One thing that raises my eyebrows is your low 5V supply voltage
for the LT1013, which limits the MOSFET gate voltage. According
to the IRFP2907 and IRF1405 datasheets these are not logic-level-
drive MOSFETs, and they may need a Vgs of nearly 5V to operate at
15A, which the LT1013 may not be able to deliver with Vcc = 5V.

So if all the opamps rail, fet differences again dominate, and one fet
could be doing most of the work.

John

 

[John]

I have not experienced this problem with low-
Whoa, never heard about those type of problems, thanks for the info.

I will be double-checking for oscillations as it's becoming clear to
my bull-headed brain that I can't assume just because I didn't see
oscillations early on that they aren't there.

If the wiring inductance is causing excessive gate voltage (and for
conversation's sake the inductance can't be reduced), would would be
the best way to stop that? Slow down the gate further, use the
previously recommnended ferrite beads, place TVS or zener's on the
gate?



Yea, measuring those temps is tricky....hate it.

I'm fairly confident of my case temperature measurements though as I
compared thermocouple readings taken from the "notches" in the sides
of the TO-247 case (where the metal thermal pad is exposed) to
readings I took when I drilled out the heat sink behind the thermal
pad. They were within 2 degrees-C of each other. I'm using the
"notch" temp measurements now....much easier to take.



Yea, may have to go that route. No room though for the supporting
circuitry though. <sigh> There's always something!



I think this might be critical.
I was hoping to use 5V because of some other circuitry that needed to
be added but when it was mentioned earlier (I've forgotten who) I
realized that this could be a big gate voltage problem. Vcc is now
9V.

Now I just need to wait for the new shipment of FETs (arriving
tomorrow) as I've blown all of mine up.

Thanks Win!
John

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[John]

One thing that raises my eyebrows is your low 5V supply voltage
It's funny how obvious a problem this is now....never saw it.

I haven't measured the gate voltage at the full 15A load but at about
5A it was 3.67V. Is this where I can use the forward transconductance
for the FET to figure out what the gate voltage might be at 15A? Or
is operating it at such high temperatures, and linear, throwing that
possibility out the window?

If I had any more FETs left I'd just hook them up....I gotta' wait 24
hours.

John
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[John Larkin]

It's funny how obvious a problem this is now....never saw it.

I haven't measured the gate voltage at the full 15A load but at about
5A it was 3.67V. Is this where I can use the forward transconductance
for the FET to figure out what the gate voltage might be at 15A?

Most fet data sheets post a *typical* transfer curve. But it will vary
a lot from part to part, and with temperature and drain volts.

Or
is operating it at such high temperatures, and linear, throwing that
possibility out the window?

It's like Mother Nature spends all her waking hours scheming to find
subtle bugs to plant in our hardware and software, and makes sure they
are disguised or tangled with other bugs, so that they're hard to
find.

John

 

[John]

Most fet data sheets post a *typical* transfer curve. But it will vary
I took a look at the LT1013 specs again and I believe the output can
only go to 4.0V-4.4V (60ohm load to no load) with a Vcc of 5V.
Assuming 4.0V max output, I checked the IRFP2907's transfer curves.

I had to extrapolate down to a Vgs of 4.0V but at 15A it appears that
I am wayyyyy to close to the drain-source current rating, even at a Tj
of 175C.

If I assume that the junction temp is cool (like when the load is
first turned on and I always seem to blow my FETs, in a few seconds),
the FET is only rated for 9A when I'm trying to push 15A thru it.
This is a problem!!!

I'm definitely going back to a Vcc of 9V.
And I'm soooo pissed off at myself. I KNEW this about the FET and
lower Vcc voltages for the op-amp affecting my max gate voltage. I
completely forgot about it for these latest tests. Ten expensive FETs
later, and with everyone's help, I'm finally beginning to see the
light again.

Of course, I could still be oscillating too.


<deep sigh> Amen brother.

John

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