It is a common mistake to mix the terms TDP (to make it clear, thermal watts) and consumption(electric watts). In this article, we will explain exactly what TDP is to which manufacturers refer in their processors, how to calculate it, how to use it, and why it is very important.
TDP (which literally means Thermal Design Power) is strictly the measure of thermic output of an ASIC (or, using other words, the thermal heat generated by the processor), which defines the necessary thermal solution to achieve its nominal performance.
How to calculate TDP
The formula to calculate TDP is the following:
TDP (watts) = (tCase°C – tAmbient°C)/(HSF Θca)
- tCase°C: The maximum temperature to the union between IHS and the processor’s die to achieve the nominal performance.
- tAmbient°C: The maximum temperature of the ambient at the entrance of the sink fan to achieve the nominal performance.
- HSF-Θca (°C/W): The minimum value of temperature per watt of the sink fan to achieve the nominal performance.
With these data in hand, let’s see an example with the new processor AMD Ryzen 7 1700, which as a 65 watts TDP.
(72.3 – 42)/0.4972 = 64.96W TDP
- tCase°C: 72,3ºC of optimal temperature, stablished by AMD.
- tAmbient°C: 42ºC, also established by AMD.
- HSF-Θca (°C/W): 0.4972 Θca. This is an AMD specification for the sink fan thermal performance, so that the processor can work at its full potential. For example, the AMD Wraith sink fan meets this Θca, and most of the sink fans that are sold separately on the market far exceed it.
What is its use and how it is used
We already know what TDP is and that we mustn’t confuse it with the power consumption of a processor. Now, keeping the example of the AMD Ryzen 7 1700 processor, if the intelligent algorithms that manage the Precission Boost and XFR technologies detect that the temperature conditions meet the required values, the processor will convert the existing margin in gross yield.
In other words – and this works in the same way with Intel – if the processor temperature allows it, the processor will function at full capacity, while if the value exceeds the limits, the processor will intelligently reduce its performance to decrease the TDP and return to acceptable values. The user can notice this on the performance of the equipment, which visibly gets lower when the temperature is high, which is why we insist so much that it is essential to install a good heatsink for the processor in our equipment.
A more technical example
Let’s look at a more technical example, for those who like it. Let’s assume a scenario in which all cores and threads are working at full capacity and that temperature allows it; according to what we just explained, we know that the processor is working at its full potential.
According to the processor white paper (technical documentation) from our example, the Ryzen 7 1700, the electric limit for the consume of this processor are 90 watts (differently from the 65 thermal watts, see the difference?) (Note: the power consumption limit for the AM4 socket is bigger, it reaches 128 watts). The processor’s die thermal conductivity, its IHS, HSF and the welding allow the processor to amortize the tCase implications of the peak consumption values over time, or in other words, it allows the processor to increment its performance as long as it is still above the values defined by the TDP formula that we saw in the beginning. The Precision Boost and/or the XFR will be responsible for leveling the performance according to the values of 72.3 tCaseºC or the 90 watts of power consumption (whichever happens first).
Surely, now that you know how to correctly calculate TDP and what is its real use, you will have much more in mind the thermal solutions (Heat sinks) that are available on the market, and you will not be conformed with the standard ones that usually come with the processors (even if the new AMD are much better than the older ones, they still offer a very low margin). Remember that a low processor temperature will effectively reduce the TDP, giving it the processor more margin to make use of the Precision Boost and XFR, in case it is an AMD.