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Changing 'default' FSB speed with Tualatin?



 
 
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  #71  
Old July 30th 04, 07:36 AM
David Maynard
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P2B wrote:



David Maynard wrote:
[snip]

Which means, at your 'maximum load', you're running at about 74% of
what it would take, measured as current, to exceed the maximum temp
rating; assuming these numbers resemble reality in any way, ambient
doesn't increase, and the creek don't rise g



You forgot "if the Good Lord's willing". Johnny is spinning in his grave
:-)


hehe. Yes, well, part of the 'art' to plagiarism is to scramble it up a bit

  #72  
Old August 4th 04, 07:26 AM
P2B
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David Maynard wrote:

P2B wrote:



David Maynard wrote:

P2B wrote:

David Maynard wrote:

P2B wrote:

David Maynard wrote:

P2B wrote:

David Maynard wrote:

P2B wrote:

David Maynard wrote:

P2B wrote:


It really depends on the board - my P2B systems have run
7x24 for over 2 years with no sign of problems. The FETs and
caps don't even get hot (no extra cooling or sinks), just
don't touch the clock chip at 150Mhz FSB!

Keep in mind the Tualatin only draws slightly more power
than a Katmai P3 600 - so if the board was *properly*
designed to power a Katmai, it should handle a Tualatin.




It isn't (processor) 'power', per see, that's the problem
with the regulators, it's the *current*.

35 watts at 2 volts is 17.5 amps but at 1.5 volts it's 23.3
amps, or 33% more than it was 'designed for'.




I agree with your calculations, but my understanding is it's
overheating that causes the demise of these components over
the medium term - and I would expect temperature to be
proportional to power dissipated, i.e. 35 watts in both cases.




Well, yes, it's the 'power' but not the processor's power. It's
the current through the FETs, which causes voltage drop across
them (due to their RDSon resistance), which is power.

Say it's RDSon is .02 ('typical' for my BH6 FET with 5V gate).
That would be (P = I^2 x R) 23.2^2 x .02 = 10.8578 watts
(average).




Ah, that explains it :-)

Asus used several different Vcore FETs on the various flavours
of P2B I have here, but after trawling for datasheets I've
learned the 'wimpiest' one they used has .015 ohm RDSon (typical
@ Vgs=4v, Id=20A), maximum continuous drain current of 45A (peak
180A), and maximum channel dissipation of 75W (at room
temperature). At a case temperature of 100C, they are rated for
~30W.

Even using worst-case numbers (RDSon max=.025 ohms, Vcore=1.4V,
CPU=35W) yields an average dissipation of only 15.625W - so it's
little wonder P2B series boards can run overclocked Tualatins
without raising a sweat :-)


You need to be careful reading specs and while you didn't say
what the part number was there's no way that FET can handle 75W
free air, or soldered to a dinky PCB heat tab. I'll bet that
rating is the 'theoretical' maximum with an 'infinite' heatsink,
or some such 'in-your-dreams' criteria.




Point taken.

75W is the absolute maximum rating for the surface mount version
with a component case temperature of 25C (only parameter
specified). There's no mention of heatsinking requirements, I
assume you are supposed to figure that out from the handy-dandy
case temperature vs. channel dissipation de-rating graph - which
is where I obtained the 30W @ 100C figure.




Ah. OK, the 'case temp' spec. That one is even more 'subtle'
because, unless you've tried it, there's no clue as to just how
dern near impossible keeping the CASE at 25C is. Note, that's on
the 'wrong' side of any thermal compound (or solder in the case of
a PCB mount) you might employ to sink it to something.

Given there's no reason to think the FETs will be asked to
dissipate more than about 16W average, I'm fairly confident they
aren't at risk - but feel free to peruse the datasheet to see if
you agree:

http://www.alldatasheet.co.kr/datash...HI/2SK2885.pdf




Well, it seems to me that you're looking at the 'watts' vs 'temp'
and, whether consciously or not, imagining the case will BE at that
temp when those watts are dissipated, but that isn't what the spec
is saying. It's saying, IF you KEEP the CASE at this temp (good
luck, how?) then it can dissipate those watts. (Specifying a
'fixed', independent, non varying case temp is equivalent to saying
it's on an infinite heatsink, btw)

That's basically a junction to case thermal spec,
I.E. C/W = [max_junction_temp-case_temp]/Watts; e.g. [150-25]/75=
1.67C/W;
and not an 'as used in the real world' temperature spec, and
certainly not the power handling capacity when simply solder
mounted onto a PCB heat tab.

They don't give 'free air' numbers but, for an example, here's a
similar type device that does give free air and 'minimum footprint'
for surface mount thermal resistance numbers.
http://www.semiconductors.com/acroba...P55N03LT_6.pdf


The TO-220AB version is 60C/W in free air and the surface mount is
50C/W. For it's max junction of 175C, and an ambient of 25C, that
comes to 2.5 Watts free air and a whopping 3 Watts on a minimum
footprint (meaning no extra thermal tab area) surface mount. Now,
before you reel in shock and say that one's no where near yours,
it's junction to case resistance is 1.45C/W vs 1.67C/W for yours.
And, using the 'case temp' methodology, if you keep the *case* at
25C (good luck, how?), that comes to [175-25]/1.45 = 103.45 watts
(@175C junction temp, 25C more than yours allows). Which is why it
lists "103W" was the "total power dissipation" (at Tmb=25C where
'mb' stands for "mounting base," which is 'the case'). If, however,
we use the same 150C junction temp yours is limited to it's still
86 watts (at 25C CASE temp): more than the 75 watts yours is rated at.

It's always a shocker the first time a home hobbyist discovers his
"75 Watt Power FET" is ready to cook eggs with only a couple of
watts being dissipated (woops, guess I need a heatsink =:O)).

Now, your Vcore FET isn't dissipating into free air, and most
likely has a motherboard thermal tab, but those numbers illustrate
just how important the thermal sinking is and how the 'case temp'
spec can lead one to over-estimate the power handling capability.




I think I followed most of that - and thanks for your effort - but
I'm glad I don't design this stuff for a living :-)




Hehe. It's always a contradictory set of conditions and requirements:
like "Free Lunch: $1.00"

I taped a temperature sensor on top one of the FETs, and it peaked
at 95C after running a Tualatin 1.0A @ 1.5Ghz on 1.4Vcore full bore
for 10 minutes, then eventually dropped to 65C at idle.

Based on the above dissipation calculations, and the temperature
derating graph from the datasheet, it still seems to me the FET is
only dissipating about 50% of it's maximum under load. Agree?




You're still looking at that curve cockeyed, as if knowing the temp
'means' something, but your FET is not mounted on a perfect,
infinite, heatsink, as if required for that curve to exist. And so,
seeing 'xC' doesn't mean a thing about how much of it's 'rating' is
being used, depending on what you 'mean' by 'rating' (current? total
power? junction temp?) You can't even tell how much power is being
dissipated because you don't know the thermal resistance of whatever
'heatsinking' it has.




Be gentle with me, 'cos I'm still missing something....

What does the case temperature vs. channel dissipation curve tell us?



It tells you how much power it can dissipate --IF-- the case is kept
at that temperature (not that it WILL be at that temperature). It's a
statement of the 'ideal'. It's nothing more than the junction to case
thermal resistance plotted on a power/temp graph.


Assuming the case temperature *is* 100C,



That IS the 'given' (well, in the way you're using it).

does it mean the channel *is* dissipating ~30W,



No.

or does it mean anything *under* ~30W is safe at that temperature? (My
assumption).



That is correct.

So if you measure your FET case temp, as you did, and see it at 65C then
the power being dissipated could be ANYTHING from the curve all the way
down to 0. Which is why I said measuring case temp doesn't TELL you
anything about how much power is being dissipated or 'what percentage'
of 'what?' it's operating at.

Or something else entirely?

I don't understand why you say we need to know the thermal resistance
of the sink to determine how much power is dissipated.



Well, you said it yourself, the power being dissipated can be "anything
*under*" the measured temp/power point. Ok, since it's "anything
*under*" then what is it?

Didn't we previously agreed it's equal to drain current squared x RDSon?



Except you don't know what the drain current is from the temp/power
graph either.

- and we have a number for RDSon (from FET datasheet) and



a reasonable guess as to drain


current (from CPU datasheet).



Which has nothing to do with the FET's power/temp graph. The issue here
was whether knowing it's at 65C means anything vs the power/temp graph.
It doesn't, except you can be sure it isn't above that point.

That graph tells you the BEST you could POSSIBLY get; with 'possibly
get' being with a 'perfect' heatsink. But since you know for dern sure
you don't have anything resembling perfection then all you know is it's
going to be something less than that point, but just what is unknown
without more data.


I can see it now... you will try to explain again, I'll finally get it
- then my FETs will blow up :-)



Hehe. Let's hope for number 2 without number 3 (number 1 was a given)


Thanks. We're now at number 2, and I have a hunch we won't get to number
3 despite not knowing the heatsink parameters. Time will tell :-)

 




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