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.

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

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.

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.


Ahhh...OK, that makes sense.



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 :)


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.



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.


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

Are you sure nothing's oscillating?

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

-- remove SPAMMENOT for e-mail responses --
One thing i can tell you for sure, running your devices even near power

Perhaps the mod below.

12V
. |
. 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 | |
. | | | | | | |
. +-----R3--|--+----|---+---R5----+ |
. | | | | |
. | | | R7 |
. | | | | |
. | '--R4---+-------------+-------+
| _____________
SPCO o | |-------LOAD+
Switch /------------|Voltage sense|
o/ o |_____________|-------LOAD-
| |
Vref--+ +--LOW-LVL GND

If the voltage across LOAD+/- is below a certain level
then the bottom ends of the R3's are switched to Vref.
Could be a relay, or solid state.

I wasn't able to figure out how the 208mV figure was derived but
[snip]

I wasn't able to figure out how the 208mV figure was derived but
[snip]

Abs Max opamp +ve input voltage is when the pot is
right at the top and is..... 1.25*R4/(R2+R4),
where R2= 49.9k and R4= 10k.

Or simply power the reference from the load.

I was thinking that instead of biasing the op-amp inputs with the
output of the comparator that I could essentially switch Vref (to
the servo loops) on/off by using the output of the comparator as
the source for the voltage reference.

I notice that this thing is manually adjusted. So why not add an
on/off switch?

You'd still want some kind of effective pull-down to prevent connector
surges through load application of dV/dT, through crss. The 1K
resistors currently isolate the gates - perhaps a pnp transistor in
there somewhere would do it.