Thermal design power

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"Thermal design power", or "TDP", is a marketing term used by AMD and Intel to indicate the power usage and cooling requirements of their processor products. It can be used to get a ball-park indication of the actual power draw and heat generated by a computer processor. The actual power drawn by a CPU under sustained all-core load can be much higher than the supposed TDP.

TDP is also used by CPU cooler manufacturers to indicate how much heat output from a CPU a cooler can handle. How CPU cooler manufacturers decide the TDP rating used in their marketing material varies, one cooler claiming to handle 95W TDP could be better or worse than another cooler with the same TDP rating.

A Meaningful TDP Rating

The German computer case and cooler manufacturer Be Quiet! measures the TDP of their CPU coolers using a hot-plate. They will list a CPU cooler product as having a 250W TDP capacity if they test a designed and manufactured product and find that it can dissipate a sustained load of 250W pumped into a hotplate below it at a room temperature of 20C.

Be Quiet's approach is how you would expect a honest person with a clue what he's doing to to measure TDP. They measure amount of power put in which, in the case of computer processors, is equal to the amount of energy leaving the processor in the form of heat, and use that figure to produce a TDP rating. Energy can not be created or disappear (the law of conservation of energy), it can only change form. A CPU which is drawing 65W of power will generate 65W heat.

How fast heat dissipates from a CPU cooler will vary with room temperature. That is something those who make CPU coolers should care about and specify. CPU manufacturers do not, a CPU will produce the same amount of heat regardless of room temperature.

Intel and AMD could test how much power their CPUs draw under a sustained full all-core load and cite that figure as a processors TDP. They don't. Their numbers are, instead, almost entirely made-up. That is specially true when it comes to AMD processors.

Comparing how one would expect Intel and AMD to measure and specify TDP with the way they actually specify TDP reveals that TDP has very little to do with the scientific method and everything to do with marketing propaganda. That they use the term "Thermal design power" to begin with is a clear sign of decpetion: They could and should specify "Max power draw" or "Sustained power draw under full utilization".

Intel Story

Intel story as published on their website, last updated November 20th, 2019 as of writing, is conflicting.

"TDP stands for Thermal Design Power, in watts, and refers to the power consumption under the maximum theoretical load. Power consumption is less than TDP under lower loads. The TDP is the maximum power that one should be designing the system for. This ensures operation to published specs under the maximum theoretical workload."

Thermal Design Power (TDP) in Intel® Processors
last updated November 20th, 2019

"Power consumption under the maximum theoretical load" is a clear and understandable description. It would be great if it was that simple. However, the very same page has a FAQ which states:

"What is the maximum power consumption for my processor?

Under a steady workload at published frequency, it is TDP. However, during turbo or certain workload types such as Intel® Advanced Vector Extensions (Intel® AVX) it can exceed the maximum TDP but only for a limited time, or

Until the processor hits a thermal throttle temperature, or
Until the processor hits a power delivery limit."

Thermal Design Power (TDP) in Intel® Processors
last updated November 20th, 2019

Put simply: Intel TDP is an indication of a Intel CPU's power-draw during a all-core workload when it is running at its specified base clock. No modern Intel CPU does that unless you change BIOS settings and force it to run at its base clock. Intel is admitting that actual power draw at boost ("turbo") clocks is higher than the rated TDP.

AMD Story

Searching for TDP on amd.com yields no results describing what they mean by "TDP". Nothing.

However, there is a long statement from a "AMD_Robert" on plebbit posted August 17th, 2017 which you can read if you want a laugh and/or headache:

"TDP is about thermal watts, not electrical watts. These are not the same.

TDP is the final product in a formula that specifies to cooler vendors what thermal resistance is acceptable for a cooler to enable the manufacturer-specified performance of a CPU.

Thermal resistance for heatsinks is rated in a unit called θca ("Theta C A"), which represents degrees Celsius per watt.

Specifically, θca represents thermal resistance between the CPU heatspreader and the ambient environment.

The lower the θca, the better the cooler is.

The θca rating is an operand in an equation that also includes optimal CPU temp and optimal case ambient temp at the "inlet" to the heatsink. That formula establishes the TDP.

Here's the TDP formula:

TDP (Watts) = (tCase°C - tAmbient°C)/(HSF ϴca)

  • tCase°C: Optimal temperature for the die/heatspreader junction to achieve rated performance.
  • tAmbient°C: Optimal temperature at the HSF fan inlet to achieve rated performance.
  • HSF ϴca (°C/W): The minimum °C per Watt rating of the heatsink to achieve rated performance.

Using the established TDP formula, we can compute for the 180W 1950X:

(56° – 32°)/0.133 = 180W TDP

  • tCase°C: 56°C optimal temperature for the processor lid.
  • tAmbient°C: 32°C optimal ambient temperature for the case at HSF inlet.
  • HSF ϴca (°C/W): 0.133 ϴca
    • 0.133 ϴca is the objective AMD specification for cooler thermal performance to achieve rated CPU performance.

In other words, we recommend a 0.133 ϴca cooler for Threadripper and a 56C optimal CPU temp for the chip to operate as described on the box. Any cooler that meets or beats 0.133 ϴca can make this possible. But notice that power consumption isn't part of this formula at all. "

"AMD_Robert" statement on AMD TDP at plebbit
posted August 17th, 2017

AMD story starts off with a statement which simply does not hold water. The very first sentence, "TDP is about thermal watts, not electrical watts. These are not the same.", is hogwash. 1W energy in the form of electricity is the same as 1W energy in the form of heat just like the weight of 1 kilo sugar is the same as the weight of 1 kilo flour.

The last sentence in the quote does hold water: "power consumption isn't part of this (..) at all". Power consumption should not only be "part of this", it should be the only part, because it is very measurable, concrete and easy to use when comparing one part with another. Yet they choose to completely ignore it.

And it gets worse. We left out the last two paragraphs in the plebbit statement. Here is the first and only one of those worth including:

"Notice also that this formula allows you to poke things around: a lower ϴca ("better cooler") allows for a higher optimal CPU temp. Or a higher ϴca cooler can be offset by running a chillier ambient environment. If you tinker with the numbers, you now see how it's possible for all sorts of case and cooler designs to achieve the same outcome for users. That's the formula everyone unknowingly tinkers with when they increase airflow, or buy a beefy heatsink."

"AMD_Robert" statement on AMD TDP at plebbit
posted August 17th, 2017

Put simply: AMD is using a "formula" which does not include power consumption and allows them to "poke things around" to get whatever TDP numbers they want to rate their CPU and APU products with.

In Bullet Summary

  • Intel's TDP numbers are dishonest and misleading.
  • AMD's TDP numbers are made using a meaningless "formula" capable of producing any desired TDP as a result.
  • TDP numbers from Intel and AMD can not be compared to each other or anything else for that matter.
  • "TDP" should be seen as a rough indication of a CPU's power draw or a CPU coolers capacity to dissipate heat.

If you are wondering what kind of CPU cooler would be ideal for a given CPU and you are unsure about the TDP numbers for the cooler and the CPU you have: Bigger is better. A CPU cooler rated for 140W TDP will cool a CPU with a 65W TDP just fine and do it more quietly than a CPU cooler at 65W. Bigger coolers are more expensive, there is a premium which is typically worth it; the fan(s) on a small cooler will need to spin at a much higher RPM than the fans on a grossly oversized cooler.


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