No they don't... they produce a current that is dependent upon their temperature. When you heat silicon you change it's characteristics... Vj the diode junction voltage is very temperature dependent but not linearly accurate. When they reach a certain Tj (usually in the 100c range) the junction cascades and they begin to conduct heavily and this sudden change in current is what triggers the thermal protections. This is brought out to a pin so we can look at it, but in fact the tolerances are very bad, the temperature which is calculated from voltage across a resistor is likely to be plus or minus 5c of actual.
Originally Posted by Mario F.
No you don't have a 20c safety margin at Tcase... In fact bringing TCase up to max with an inadequate cooling solution might already have the inner parts of the cores over tjMax.
The Case Thermal Specification (Tcase) is the most important. And it is the one Intel publishes. This is usually set at around 20 c below TJMax and is very dependent on TDP. Tcase is the base for the usual 20-25c range to TJMax you hear OCers always talking about. They want their CPUs under load to say here or below it. Not because there's any risk of damage or degradation, but because these are fully safe operational values that will guarantee system stability. But if you exceed Tcase, you have still ~20 degrees to go before you hit TJMax. The thing is, regardless of the effects this can have on system stability (including the processor own functions), this will not damage or degrade your CPU.
Once again, on these CPU chips you are only applying a cooling solution to *one side* of the chip. From the side bonded to the heat spreader to the opposite side you can have a linear increase in temperature of several degrees... A heat gradient.
You get the idea of a color gradient, yeah? Black to bright red as a constant linear fade? That happens with heat in these chips.
I suppose that's why Intel pubishes it under "Absolute Maximum Ratings"...
TJMax is not the point of damage. The point of damage is set somewhere above this within a secure range (maybe 10 c or 20 c above that). It would not make sense to set the point at which the CPU starts throttling or asserting where it is already receiving damage. It must be done before that. TJMax is no different than any other junction temperature that establishes a safe absolute maximum value.
Yes, it might be safe to hit TjMax for a few seconds, beyond that the risk of damage increases exponentially. No joke... that TjMax = 100c means just that... 101c and you're likely to cook the chip. The themal safeties are as close to that as they can get to prevent false trips. They do note exist as a feature... they exist as a last ditch effort to save the chip *because* your cooling solution is inadeqate.
You need to rethink that... as I've been trying to explain --against some interesting resistance-- the measured temperatures on the surface of the chip or it's cores does not fully reflect the actual junction temperatures within the chip... Because you have a large heat sink drawing off heat from one side of the chip, the side opposite that chip can easily be considerably hotter than where case meats fins.
TJMax is as such still within Intel's operational limits, but considered as the absolute maximum. Now, I may have to search for this if you ask me to show up the citation, but Intel is very clear in their datasheet when they say that any operation below absolute maximum values guarantees your CPU a long-life and incurs in no degradation.
So no, you will not damage or degrade the life of your CPU by coming between Tcase and TJMax.
TcaseMax is actually telling you when the innards are about to hit TjMax, with their recommended cooling solutions... It's not some safety margin that you can safely enter and re-enter with no concern for damage... it is the "red line" at which things start to go wrong.