Changing 'default' FSB speed with Tualatin?

On Mon, 26 Jul 2004 20:59:16 -0400, P2B wrote:
I've
seen pictures of Abit BX boards that died spectacularly shortly after
being asked to run a Tualatin.

yes BH6 first revision & trying to OC 1,4 Tuallies or a bit lower ones
but too high final frequency by people who do not know much about
electronics mostly ...

--
Regards, SPAJKY ®
& visit my site @ http://www.spajky.vze.com
"Tualatin OC-ed / BX-Slot1 / inaudible setup!"
E-mail AntiSpam: remove ##

Spajky wrote:
[snip]
You can reduce Vtt further - BX chipset runs just fine with Vtt=1.25v.
My systems run Vcore=1.4v, Vtt=1.3v.


I know (I´ve tested with 1,22 Vtt) but dont´t wanna go lower, since
time to time I test even some old Mendocino ones on my board
occasionally ...

Older processors will usually run at stock speed with low Vtt, but they
won't overclock - that's why my Vtt mod is a jumper, one position is
standard Vtt, the other position adds a resistor for Tualatin Vtt levels.

David Maynard wrote:

P2B wrote:



David Maynard wrote:

P2B wrote:



Spajky wrote:

On Fri, 23 Jul 2004 21:40:06 +1200, "~misfit~"
wrote:


I got a Slot-T today and fiited it into my only MSI 6163 Pro 440BX
mobo of
the three I have that *doesn't* have bulging caps. At default (14
x 100) I
got 110 marks! That is good huh? BX does indeed rock!






nice & Ok!


. I don't think I'll even
try 115Mhz FSB (the next available setting) as I don't think it
will give a
great improvement in CPU marks and will put my PCI/AGP busses even
further
out-of-spec. Well, maybe I will. :-) 1.6Ghz sounds good to me.
Maybe I'll
need to raise vcore a bit though....






do not exagerate & leave it like this ... not to overheat mosfets &
caps; problems will start to manifest after a year or so if not
replaced caps & voltage regulators with stronger ones!





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 :-)

P2B wrote:



David Maynard wrote:

P2B wrote:



David Maynard wrote:

P2B wrote:



Spajky wrote:

On Fri, 23 Jul 2004 21:40:06 +1200, "~misfit~"
wrote:


I got a Slot-T today and fiited it into my only MSI 6163 Pro
440BX mobo of
the three I have that *doesn't* have bulging caps. At default (14
x 100) I
got 110 marks! That is good huh? BX does indeed rock!







nice & Ok!


. I don't think I'll even
try 115Mhz FSB (the next available setting) as I don't think it
will give a
great improvement in CPU marks and will put my PCI/AGP busses
even further
out-of-spec. Well, maybe I will. :-) 1.6Ghz sounds good to me.
Maybe I'll
need to raise vcore a bit though....







do not exagerate & leave it like this ... not to overheat mosfets &
caps; problems will start to manifest after a year or so if not
replaced caps & voltage regulators with stronger ones!






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.

David Maynard wrote:

P2B wrote:



David Maynard wrote:

P2B wrote:



David Maynard wrote:

P2B wrote:



Spajky wrote:

On Fri, 23 Jul 2004 21:40:06 +1200, "~misfit~"
wrote:


I got a Slot-T today and fiited it into my only MSI 6163 Pro
440BX mobo of
the three I have that *doesn't* have bulging caps. At default
(14 x 100) I
got 110 marks! That is good huh? BX does indeed rock!








nice & Ok!


. I don't think I'll even
try 115Mhz FSB (the next available setting) as I don't think it
will give a
great improvement in CPU marks and will put my PCI/AGP busses
even further
out-of-spec. Well, maybe I will. :-) 1.6Ghz sounds good to me.
Maybe I'll
need to raise vcore a bit though....








do not exagerate & leave it like this ... not to overheat mosfets &
caps; problems will start to manifest after a year or so if not
replaced caps & voltage regulators with stronger ones!







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.

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

My maxed-out P2Bs will probably die eventually, but they've been running
7x24 for over 2 years and my money is on the capacitors quitting before
the FETs :-)

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.

My maxed-out P2Bs will probably die eventually, but they've been running
7x24 for over 2 years and my money is on the capacitors quitting before
the FETs :-)

Possible.

On Tue, 27 Jul 2004 18:33:24 -0400, P2B wrote:

time to time I test even some old Mendocino ones on my board
occasionally ...

Older processors will usually run at stock speed with low Vtt, but they
won't overclock - that's why my Vtt mod is a jumper, ..

clever! :-)

it can be done automatically (idea!)
using AF36pin on s.370 or apropriate slot1 pin on MoBo which on
Tuallies is DETECT (a 3-state Hi-Z output for telling the MoBo which
CPU is inserted) while on older ones is Vss = Gnd !
That signal there tells Tuallie ready MoBo what Vtt to use (if Cu_Mine
is inserted /or PPGA one which do not work on Slot-T or Tuallie ready
MoBo´s IMHO/ Cpu shorts it to Gnd level! ... did not measure it but
IMHO is max Vcore value (positive)...

so ... you need than IMHO just maybe an apropriate resistor (if
needed) from there to a B-ase terminal of a NPN Ge-rmanium transistor
connected its E to Gnd & C to control pin of a voltage regulator.

When Tuallie is inserted gives to that transistor positive voltage so
can almost short a voltage regulator control pin & voila, there you
have around 1,3Vtt for Cpu ... :-)
..... if older Cpu is inserted there is no voltage to do that, so this
additional circuit is like not there & you have a 1,5Vtt ...

.... damn, I´m clever to! ... :-)))))))

(PS.: I am not planning to try cause I do not need it, but you can ..)
--
Regards, SPAJKY ®
& visit my site @ http://www.spajky.vze.com
"Tualatin OC-ed / BX-Slot1 / inaudible setup!"
E-mail AntiSpam: remove ##

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 :-)

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?


My maxed-out P2Bs will probably die eventually, but they've been
running 7x24 for over 2 years and my money is on the capacitors
quitting before the FETs :-)


Possible.

Spajky wrote:

On Tue, 27 Jul 2004 18:33:24 -0400, P2B wrote:


time to time I test even some old Mendocino ones on my board
occasionally ...

Older processors will usually run at stock speed with low Vtt, but they
won't overclock - that's why my Vtt mod is a jumper, ..


clever! :-)

it can be done automatically (idea!)
using AF36pin on s.370 or apropriate slot1 pin on MoBo which on
Tuallies is DETECT (a 3-state Hi-Z output for telling the MoBo which
CPU is inserted) while on older ones is Vss = Gnd !
That signal there tells Tuallie ready MoBo what Vtt to use (if Cu_Mine
is inserted /or PPGA one which do not work on Slot-T or Tuallie ready
MoBo´s IMHO/ Cpu shorts it to Gnd level! ... did not measure it but
IMHO is max Vcore value (positive)...

so ... you need than IMHO just maybe an apropriate resistor (if
needed) from there to a B-ase terminal of a NPN Ge-rmanium transistor
connected its E to Gnd & C to control pin of a voltage regulator.

When Tuallie is inserted gives to that transistor positive voltage so
can almost short a voltage regulator control pin & voila, there you
have around 1,3Vtt for Cpu ... :-)
.... if older Cpu is inserted there is no voltage to do that, so this
additional circuit is like not there & you have a 1,5Vtt ...

... damn, I´m clever to! ... :-)))))))

(PS.: I am not planning to try cause I do not need it, but you can ..)

Yep, that's pretty much how the Intel Universal S370 Design Guide tells
you to design a motherboard for all S370 processors - but I decided not
to bother because changing the jumper when you change CPUs is easier.

P2B

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.

What you CAN say is it seems to be within it's temperature limit of 150C
with 'whatever' power it's dissipating but you don't really know how many
'watts' is producing that temp and so not how many 'more' would push it
over. That wouldn't answer the peak current question, either, btw

Well, let's see if we can 'guess' some things. Stop grant is 15.37 amps
(but then you're not at the 'right' Vcore), and maybe we can guess that's
the majority of the 'idle' time. That would be 15.37^2* x .015 (typical
RDSon) for 3.54 Watts and you're seeing 65C there. What is ambient? 30C?
That would be (65-30)/3.54 for 9.89C/Watt (must be getting some PCB
heatsinking to lower that down from 50C/Watt).

If we take the tualatin's max current spec and the 'typical' RDSon of .015
we'd get 8.14 watts. If we 'guess' that our 'idle' C/W number is right
that's 8.14 x 9.89 for 80.50C, plus the assumed 30C ambient, for 110.5C.
Well, we're significantly off (16%) but we're in the ball park (it gets
closer if the ambient around the FET is higher) and it suggests that the
tualatin probably isn't pulling the spec sheet 'max' current; which we'd
expect anyway as yours is, hopefully, more 'typical' and you're also
running Vcore a bit low (although clocking it faster than the max spec'd
1.4 gig. I can account for about 7% from these two). Not to mention we made
some 'guesses' (such as my wild stab in the dark about your case ambient)
and who knows just how precise your temp measurement is. Which one is
closer is debatable but I'd trust the idle-stop grant number before I'd
take a 'max' current spec as 'typical' (for the reasons given plus it's
independent of clock speed).

BUT, if you were a designer you'd have to design for the max because, sure
as shootin, SOME of them WOULD pull that much. (Which is how you get the
manufacturer not willing to say it works while 'hotshots' tell you theirs
works 'just fine')

Anyway, let's say our 'idle' thermal resistance is 'close'. That would mean
your 'max load' test, since you got 95C instead of 110.5, was closer to
20.9 amps actual current draw (through the .015 RDSon) rather than the
spec'd 'max'. I.E. (95-30)/9.89 = 6.57 watts dissipated and sqrt(6.57/.015)
= 20.9 amps

Soooooo. What amps would it take to hit the 150C? Using the same equation
we get 28.4 amps will heat'er up to 150C.

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

Btw, if someone else were running 1.7V Vcore to get the same overclock
that's a 21.4% increase, for 25 amps, and that's getting close enough to
the 28.4 'cooking point', considering how much of a 'guess' these number
are, that I can see it possibly failing. 1.8V Vcore would be 26.9 amps.

It does illustrate the significance of too high a Vcore on the regulator
FET. Using 1.7 vs your 1.4 raises the FET temp an estimated 30C.