Thermal Design Power (TDP), also known asthermal design point, is the maximum amount ofheat that a computer component (like aCPU,GPU orsystem on a chip) can generate and that itscooling system is designed todissipate during normal operation at a non-turbo clock rate (base frequency).
Some sources state that the peakpower rating for a microprocessor is usually 1.5 times the TDP rating.[1]
| ACP | TDP |
|---|---|
| 40 W | 60 W |
| 55 W | 79 W |
| 75 W | 115 W |
| 105 W | 137 W |
Theaverage CPU power (ACP) is the power consumption ofcentral processing units, especiallyserver processors, under "average" daily usage as defined byAdvanced Micro Devices (AMD) for use in its line of processors based on theK10 microarchitecture (Opteron 8300 and 2300 series processors). Intel's thermal design power (TDP), used for Pentium and Core 2 processors, measures the energy consumption under high workload; it is numerically somewhat higher than the "average" ACP rating of the same processor.
According to AMD the ACP rating includes the power consumption when running several benchmarks, includingTPC-C,SPECcpu2006,SPECjbb2005 and STREAM Benchmark[3] (memory bandwidth),[4][5][6] which AMD said is an appropriate method of power consumption measurement fordata centers and server-intensive workload environments. AMD said that the ACP and TDP values of the processors will both be stated and do not replace one another.Barcelona and later server processors have the two power figures.
The TDP of a CPU has been underestimated in some cases, leading to certain real applications (typically strenuous, such as video encoding or games) causing the CPU to exceed its specified TDP and resulting in overloading the computer's cooling system. In this case, CPUs either cause a system failure (a "therm-trip") or throttle their speed down.[7] Most modern processors will cause a therm-trip only upon a catastrophic cooling failure, such as a no longer operational fan or an incorrectly mounted heat sink.
For example, alaptop's CPU cooling system may be designed for a 20 W TDP, which means that it can dissipate up to 20 watts of heat without exceeding the maximumjunction temperature for the laptop's CPU. A cooling system can do this using an active cooling method (e.g. conduction coupled with forced convection) such as aheat sink with afan, or any of the two passive cooling methods:thermal radiation orconduction. Typically, a combination of these methods is used.
Since safety margins and the definition of what constitutes a real application vary among manufacturers, TDP values between different manufacturers cannot be accurately compared (a processor with a TDP of, for example, 100 W will almost certainly use more power at full load than processors with a fraction of said TDP, and very probably more than processors with lower TDP from the same manufacturer, but it may or may not use more power than a processor from a different manufacturer with a not excessively lower TDP, such as 90 W). Additionally, TDPs are often specified for families of processors, with the low-end models usually using significantly less power than those at the high end of the family.
Until around 2006AMD used to report the maximum power draw of its processors as TDP.Intel changed this practice with the introduction of itsConroe family of processors.[8] Intel calculates a specified chip's TDP according to the amount of power the computer's fan and heatsink need to be able to dissipate while the chip is under sustained load. Actual power usage can be higher or (much) lower than TDP, but the figure is intended to give guidance to engineers designing cooling solutions for their products.[9] In particular, Intel's measurement also does not fully take into accountIntel Turbo Boost due to the default time limits, while AMD does becauseAMD Turbo Core always tries to push for the maximum power.[10]
TDP specifications for some processors may allow them to work under multiple different power levels, depending on the usage scenario, available cooling capacities and desired power consumption. Technologies that provide such variable TDPs includeIntel'sconfigurable TDP (cTDP) andscenario design power (SDP), andAMD'sTDP power cap.
Configurable TDP (cTDP), also known asprogrammable TDP orTDP power cap, is an operating mode of later generations of Intel mobile processors (as of January 2014[update]) and AMD processors (as of June 2012[update]) that allows adjustments in their TDP values. By modifying the processor behavior and its performance levels, power consumption of a processor can be changed altering its TDP at the same time. That way, a processor can operate at higher or lower performance levels, depending on the available cooling capacities and desired power consumption.[11]: 69–72 [12][13]
cTDP typically provide (but are not limited to) three operating modes:[11]: 71–72
For example, some of themobile Haswell processors support cTDP up, cTDP down, or both modes.[14] As another example, some of the AMDOpteron processors andKaveriAPUs can be configured for lower TDP values.[13] IBM'sPOWER8 processor implements a similar power capping functionality through its embeddedon-chip controller (OCC).[15]
Intel introducedscenario design power (SDP) for some low power Y-series processors since 2013.[16][17] It is described as "an additional thermal reference point meant to represent thermally relevant device usage in real-world environmental scenarios."[18][promotional source?] As a power rating, SDP is not an additional power state of a processor; it states the average power consumption of a processor using a certain mix of benchmark programs to simulate "real-world" scenarios.[16][19][20]
As some authors and users have observed, the Thermal Design Power (TDP) rating is an ambiguous parameter.[21][22][23][24][25][26] 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.
To better understand the problem we must remember the basic concepts underlyingThermal management andComputer cooling.[26] Let’s consider thethermal conduction path from theCPU case to the ambient air through aHeat sink, with:
All these parameters are linked together by the followingequation:
Hence, once we know the thermal power to be dissipated (Pd), the maximum allowed case temperature (Tc) of theCPU and the maximum expected ambient temperature (Ta) of the air entering the cooling fans, we can determine the fundamental characteristics of the requiredHeat sink, i.e. its thermal resistance Rca, as:
This equation can be rearranged by writing
where in Pd can replaced by the Thermal Design Power (TDP).
Note that the heat dissipation path going from theCPU to the ambient air flowing through the printed circuit of the motherboard has a thermal resistance that is orders of magnitude greater than the one of theHeat sink, therefore it can be neglected in these computations.
Once all the input data is known, the previous formula allows to choose aCPU’sHeat 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 theCPU 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, anIntel’s general support page states briefly that the TDP refers to"the power consumption under the maximum theoretical load".[27] Here they also inform that starting from the 12th generation of theirCPUs the termThermal Design Power (TDP) has been replaced withProcessor Base Power (PBP) .[28] In a support page dedicated to theCore i7-7700 processor,Intel defines the TDP asthe maximum amount of heat that a processor can produce when running real life applications,[29] without telling what these "real life applications" are. Another example: in a 2011 white paper where theXeon processors are compared withAMD’s competing devices,Intel defines TDP asthe upper point of the thermal profile measured at maximum case temperature, but without specifying what this temperature should be (nor the computing load).[30]It is important to note that all these definitions imply that theCPU is running at the base clock rate (non-turbo).
In conclusion:
In October 2019, the GamersNexus Hardware Guides[25][34] showed a table with case and ambient temperature values that they got directly fromAMD, describing the TPDs of someRyzen 5, 7 and 9CPUs. The formula relating all these parameters, given byAMD, is the usual
The declared TPDs of these devices range from 65 W to 105 W; the ambient temperature considered byAMD 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.
