Editing Thermal design power (section)

== Ambiguities of the thermal design power parameter==

As some authors and users have observed, the thermal design power (TDP) rating is an ambiguous parameter.<ref>{{Cite web |title=Thermal design power |url=https://linuxreviews.org/Thermal_design_power |access-date=2025-01-15 |website=LinuxReviews |language=en}}</ref><ref>{{Cite web |last=Tarara |first=Arne |title=TDP and ACP for energy estimation in processors |url=https://www.green-coding.io/blog/tdp-and-acp/ |access-date=2025-01-15 |website=www.green-coding.io |language=en}}</ref><ref>{{Cite web |last= |first= |title=Cooling for modern CPUs |url=https://www.bequiet.com/en/insidebequiet/5017 |access-date=2025-01-15 |website=be quiet! |language=en}}</ref><ref>{{Cite web |title=Noctua’s Standardised Performance Rating (NSPR) and compatibility classification for CPU coolers |url=https://noctua.at/en/noctua-standardised-performance-rating |access-date=2025-01-15 |website=noctua.at}}</ref><ref name="TDP_Definitions">{{Cite web |title=AMD Ryzen TDP Explained: Deep-Dive on TDP Definitions & What Cooler Manufacturers Think {{!}} GamersNexus |url=https://gamersnexus.net/guides/3525-amd-ryzen-tdp-explained-deep-dive-cooler-manufacturer-opinions |access-date=2025-01-15 |website=gamersnexus.net |language=en}}</ref><ref name="Cooling_and_Efficiency">{{Cite web |last=Lagergren |first=Evan |date=2024-09-19 |title=Power Draw, Cooling, and Efficiency: AMD Ryzen 9000 Series Processors |url=https://www.pugetsystems.com/labs/articles/power-draw-cooling-and-efficiency-amd-ryzen-9000-series-processors/?srsltid=AfmBOooQlLsZaxtOMnCD84eLvA6PdlSYr4kN7tOi-1UTDxXuihqvq--b |access-date=2025-01-15 |website=Puget Systems |language=en-US}} </ref> In fact, different manufacturers define the TDP using different calculation methods and different operating conditions, keeping these details almost undisclosed (with very few exceptions). This makes highly problematic (if not impossible) to reasonably compare similar devices made by different manufacturers based on their TDP, and to optimize the design of a cooling system in terms of both heat management and cost. 

=== Thermal management fundamentals ===

To better understand the problem we must remember the basic concepts underlying [[Thermal management (electronics)|thermal management]] and [[computer cooling]]. <ref name=Cooling_and_Efficiency /> Let’s consider the [[thermal conduction]] path from the [[CPU]] case to the ambient air through a [[Heat sink]], with:

:Pd ([[Watt]]) = thermal [[Electric power|power]] generated by a [[CPU]] and to be dissipated into the ambient through a suitable [[Heat sink]]. It corresponds to the total power drain from the direct current supply rails of the [[CPU]].
:Rca ([[°C]]/W) = [[thermal resistance]] of the [[heat sink]], between the case of the [[CPU]] and the ambient air.
:Tc ([[°C]]) 	= maximum allowed temperature of the [[CPU]]'s case (ensuring full performances).
:Ta ([[°C]]) 	= maximum expected ambient temperature at the inlet of the [[heat sink]] fan.

All these parameters are linked together by the following [[equation]]:

:<math>(Tc-Ta)=Pd \cdot Rca </math>

Hence, once we know the thermal power to be dissipated (Pd), the maximum allowed case temperature (Tc) of the [[CPU]] and the maximum expected ambient temperature (Ta) of the air entering the cooling fans, we can determine the fundamental characteristics of the required [[heat sink]], i.e. its thermal resistance Rca, as:

:<math> Rca=\frac {(Tc-Ta)}{Pd} </math>  

This equation can be rearranged by writing 

:<math> Pd=\frac{(Tc-Ta)}{ Rca} </math> 

where in Pd can replaced by the thermal design power (TDP).

Note that the heat dissipation path going from the [[CPU]] to the ambient air flowing through the printed circuit of the motherboard has a thermal resistance that is orders of magnitude greater than that of the [[Heat sink]], therefore it can be neglected in these computations.

=== Issues when dealing with the thermal design power (TDP) ===
Once all the input data is known, the previous formula allows to choose a [[CPU]]’s [[heat sink]] with a suitable thermal resistance Rca between case and ambient air, sufficient to keep the maximum case temperature at or below a predefined value Tc.  

On the contrary, when dealing with the Thermal Design Power (TDP), ambiguities arise because the [[CPU]] manufacturers usually do not disclose the exact conditions under which this parameter has been defined. The maximum acceptable case temperature Tc to get the rated performances is usually missing, as well as the corresponding ambient temperature Ta, and, last but not least, details about the specific computational test workload.  

For instance, an [[Intel]]’s general support page states briefly that the TDP refers to "the power consumption under the maximum theoretical load".<ref>{{Cite web |title=Thermal Design Power (TDP) in Intel&reg; Processors |url=https://www.intel.com/content/www/us/en/support/articles/000055611/processors.html |access-date=2025-01-15 |website=Intel |language=en}}</ref> Here they also inform that starting from the 12th generation of their [[CPU]]s the term ''thermal design power (TDP)'' has been replaced with ''processor base power (PBP)''.<ref>{{Cite web |last=Aufranc (CNXSoft) |first=Jean-Luc |date=2022-01-08 |title=TDP (Thermal Design Power) vs PBP (Processor Base Power) - Are there differences? - CNX Software |url=https://www.cnx-software.com/2022/01/08/tdp-vs-pbp-thermal-design-power-vs-pbp-processor-base-power-differences/ |access-date=2025-01-15 |website=CNX Software - Embedded Systems News |language=en-US}}</ref> In a support page dedicated to the [[Core i7]]-7700 processor, [[Intel]] defines the TDP as the maximum amount of heat that a processor can produce when running real life applications,<ref>{{Cite web |title=Does Thermal Design Power Mean Real Power Consumption? |url=https://www.intel.com/content/www/us/en/support/articles/000031072/processors/intel-core-processors.html |access-date=2025-01-15 |website=Intel |language=en}} </ref> without telling what these "real life applications" are. Another example: in a 2011 white paper where the [[Xeon]] processors are compared with [[AMD]]’s competing devices, [[Intel]] defines TDP as the upper point of the thermal profile measured at maximum case temperature, but without specifying what this temperature should be (nor the computing load). <ref>https://www.intel.com/content/dam/doc/white-paper/resources-xeon-measuring-processor-power-paper.pdf  Measuring Processor Power – TDP vs. ACP</ref>
It is important to note that all these definitions imply that the [[CPU]] is running at the base clock rate (non-turbo). 

In conclusion:

:*Comparing the TDP between devices of different manufacturers is not very meaningful. 

:*The selection of a [[heat sink]] may end up with overheating (and [[CPU]] reduced performances) or overcooling (oversized, expensive [[heat sink]] ), depending if one chooses a too high or a too low case temperature Tc (respectively with a too low or too high ambient temperature Ta), or if the [[CPU]] operates with different computational loads.

:*A possible approach to ensure a long life of a [[CPU]] is to ask the manufacturer the recommended maximum case temperature Tc and then to oversize the cooling system. For instance, a safety margin taking into account some turbo overclocking could consider a thermal power that is 1.5 times the rated TDP. In any case, the lower is the silicon [[junction temperature]], the longer will be the lifespan of the device, according to an acceleration factor very roughly expressed by means of the [[Arrhenius equation]].<ref> https://www.ti.com/lit/an/sprabx4b/sprabx4b.pdf?ts=1736495510813 Calculating Useful Lifetimes of Embedded Processors</ref><ref>{{Cite web |last=Wilcoxon |first=Ross |date=2017-08-18 |title=Does a 10°C Increase in Temperature Really Reduce the Life of Electronics by Half? |url=https://www.electronics-cooling.com/2017/08/10c-increase-temperature-really-reduce-life-electronics-half/ |access-date=2025-01-15 |website=Electronics Cooling |language=en-US}}</ref><ref> {{Cite web |last=Johnstone |first=Caitlin |date=2019-05-30 |title=Device Reliability - How Temperature Affects Mean Time to Failure |url=https://jetcool.com/post/semiconductor-lifetime-how-temperature-affects-mean-time-to-failure-device-reliability/ |access-date=2025-01-15 |website=JetCool Microconvective Liquid Cooling |language=en-US}}</ref>

=== Some disclosed details of [[AMD]]’s thermal design power (TDP)===

In October 2019, the GamersNexus hardware guides<ref name=TDP_Definitions /><ref>{{Cite AV media |url=https://www.youtube.com/watch?v=tL1F-qliSUk |title=AMD Ryzen TDP Deep-Dive & What Cooler Manufacturers Think of "TDP" |date=2019-10-14 |last=Gamers Nexus |access-date=2025-01-15 |via=YouTube}}</ref> showed a table with case and ambient temperature values that they got directly from [[AMD]], describing the TPDs of some [[Ryzen]] 5, 7 and 9 [[CPU]]s. The formula relating all these parameters, given by [[AMD]], is the usual 

:<math> TPD=(Tc-Ta)/Rca </math>  

The declared TPDs of these devices range from 65 W to 105 W; the ambient temperature considered by [[AMD]] is +42°[[C]], and the case temperatures range from +61.8 [[°C]] to +69.3[[°C]], while the case-to-ambient thermal resistances range from 0.189 to 0.420 [[°C]]/W.