CPU Power Limits – How to Reduce CPU Temperatures in a Fanless PC

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One of the questions that I get asked most often is how do you lower the CPU’s temperature in a fanless PC. There are multiple ways to do this, other than simply using a CPU with a lower TDP. One method in particular though is very simple and effective, without sacrificing a huge amount of performance. One method is to under-clock the CPU, but this method is not the most efficient. Directly limiting CPU frequencies will also prevent any boost advantages, and will sacrifice more performance than necessary. Another method is to under-volt the CPU. This method can be effective while minimizing performance loss, but it can also affect stability and cause unusual side effects that limit performance more than expected. I wouldn’t recommend under-volting to most people because finding a sweet spot can be complex and time consuming. It can be worth it though if you like tweaking and you know what you are doing.

The one method that I would recommend to just about anyone is to limit CPU power directly. The amount of power running through a CPU, measured in watts, can be directly correlated with the CPU’s temperature. By limiting power to the CPU, we are not directly limiting frequencies, so the PC can still take advantage of boost frequencies for one or two cores, when it fits within the power limits chosen. We are also not affecting system stability by not altering voltages. Single threaded performance is generally not greatly affected by limiting power, only multi-threaded performance. Limiting power has become increasingly important in recent years as Intel has been unable to move on from its 14nm++++++ transistor technology. The way that they’ve been able to keep adding cores and improving performance has included gradually allowing more and more power to flow to the CPU. Despite the consistent 65 W TDPs for Intel’s mainstream desktop chips, boost performance has evolved from pulling a modest amount of power above TDP to pulling massive amounts of power, well above double the TDP in many cases now. On the AMD side, advancements in efficiency by being able to continue shrinking the transistor size have allowed their Ryzen CPUs to see large performance gains without drastically increasing power use.

When loaded with instructions, the CPU will initially draw up to as much power as is listed for the short duration power limit. After a certain amount of time, here this time limit is known as the “Package Power Time Window,” the CPU’s power draw will fall to the long duration power limit for as long as it needs to complete the instructions. After the power draw falls below the long duration limit, it will be able to boost back up to the short duration limit again for a limited amount of time. The way I like to think of the long duration limit is that it resembles TDP, and is set as appropriate for the CPU cooler used. I think of the short duration limit as a Turbo Boost limit. It is set as appropriate for the power supply used. As long as the Package Power Time Window is around a minute or less, the short duration limit should not pose much trouble for CPU temperatures.

AMD does not use a short and long duration limit like Intel. Instead, this is just one power limit that the CPU will not exceed. AMD uses more complex algorithms than Intel to determine what frequencies and power levels to operate at, although it will not exceed PPT.

These settings allow you to use the CPU of your choice with the cooler of your choice, and have control over the CPU’s temperatures. Both Intel and AMD have lower TDP CPUs available such as Intel’s T series, and AMD’s GE series, but they are more or less the same as the higher TDP CPUs, with lower default power limits. In my mind they are redundant as long as we have these power limit settings available to modify. You won’t get any extra performance with an Intel i9-11900T over an i9-11900 with a 35-watt long duration power limit, and at least with the 11900, you can customize the power limit a little higher if your cooler allows for it.

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