How To Understand Thermal Design Power (TDP)


TDP (Thermal Design Power) describes the amount of heat a component can dissipate. ...it is a measure of the expected amount of heat the CPU or GPU can give off under heavy use expressed in terms of watts.

Author: Emman Jemuel Velos
Category: Hardware
Author: Emman Jemuel Velos
Category: Hardware



TDP measurements for specific chips are one of the least understood acronyms on the computer professional’s vocabulary. While TDP has always been associated with power consumption, it is not an entirely accurate description. So, what is TDP? How does it relate to the actual power consumption? Let’s find out.

How To Understand Thermal Design Power (TDP)

What is Thermal Design Power?

TDP stands for Thermal Design Power.  It describes the amount of heat a component can dissipate. To be more precise, it is a measure of the expected amount of heat the CPU or GPU can give off under heavy use expressed in terms of watts.

But aren’t watts a unit of power?

Yes, they are. But power can also be expressed as energy over time. Therefore, TDP is the rate of energy output per unit of time. For instance, a CPU with a TDP of 82 W means it is expected to have an output of 84 W, or 84 J of heat, every second during heavy use.

Understanding TDP

In building their PCs, most hardware shoppers take TDP as a component’s power consumption and use it as a basis in selecting a power supply unit (PSU). This should not be the case since the power consumption of the component is significantly higher than the heat it releases. Instead, the TDP should be used to select an appropriate cooling system.

Power Consumption and TDP

The amount of heat released during use is a function of power consumed. Power, in turn, is a function of electrical current. This means the more electrical current is drawn, the more power is consumed, and the more heat is released. The higher the TDP, the higher the expected performance. Look at this relationship in terms of CPU performance.

  • At lower TDP settings, the power consumed is low with less heat being generated. This results in poorer chip performance.
  • Standard TDP settings result in a better, more stable performance. More power is drawn, which also increases heat output at a controlled level. Most chips operating at this level typically range from 65 W to 95 W.
  • High TDP essentially guarantees a stable performance. High levels of power are consumed to ensure stability. In effect, more heat is dissipated. CPUs in this range are set at 130 W or higher.

TDP and Cooling Systems

TDP is meant for cooler vendors () as a measurement of acceptable thermal resistance to maintain a chip’s performance at a specified level.

It corresponds to a cooler’s minimum capacity to dispel the heat and maintain CPU performance. A cooler operating at 95 W can effectively support a CPU with a 95 W TDP at minimum.

Depending on specific use cases, it is recommended to select cooling systems that far exceed the CPU’s TDP rating. That is because there are some factors that can contribute to the amount of heat dissipated by the CPU. These include:

Clock Speed. TDP ratings correspond to the CPU’s maximum performance level at the standard clock speed. This does not specify the amount of heat it can generate when it exceeds standard clock speed. TDP settings for overclocked systems are way higher compared to the standard rating. In such cases, better cooling systems are required to maintain its heightened activity.

Use Case. As discussed earlier, some platforms require less CPU performance in order to limit power consumption. Laptops, notebooks, and tablet CPUs typically have lower TDP in order to extend battery life. Desktop processors are a different story since they focus on performance. CPU manufacturers, especially in newer generations, employ Configurable TDP mechanisms on their processors for adjustability. This allows the user to adjust TDP based on the user’s needs and the platform on which it is being used.

Size of Processor. Larger CPUs require more power to operate and naturally release more heat. Take for example two processors made for laptops, the Intel Celeron N4000, and the Core i7-8850H. With a size of 25 mm x 24 mm, the N4000 has a TDP of 6 W. In contrast, the Core i7-8850H has a size of 42 mm x 28 mm and a TDP of 45 W.

Intel vs AMD

There are currently no standards that exist when establishing TDP values for each processor. Due to this lack of a standard, chip manufacturing giants Intel and AMD have vastly different interpretations when it comes to TDP.

Generally speaking, many enthusiasts argue that Intel’s TDP values are way lower than what users experience. Anandtech explains in more detail why Intel’s numbers are always off.

They explain that Intel CPUs are rated at base frequencies. However, CPUs do not always operate at standard settings. Instead, they operate at boosted levels when under a heavy workload. For this reason, Intel CPUs draw more power and generate/expel more heat than what is prescribed. With the absence of coolers that can manage the heat levels being emitted, the CPU slows down to prevent damage. This results in poor performance.

On the other hand, most enthusiasts agree that AMD’s TDP numbers are more realistic. Their TDP ratings are based on maximum heat values. This gives a better approximation of the expected heat output when operating at boosted levels. Some even argue that stock coolers prove to be more than sufficient in managing the heat given off even with moderate overclocking.

T-Junction and Max Temps: What Heat Level is Tolerable?

In systems where high performance is required, too much heat is always a problem. Prolonged exposure to high levels of heat can damage a processor. When choosing an appropriate cooling system, it is helpful to determine what amount of heat is considered safe or tolerable. This information is provided by the chip manufacturers themselves.

Intel processors use the junction temperature (T-Junction) metric to indicate maximum junction temperature. When the core temperature reaches above the indicated T-Junction, the fail-safe is activated and the system crashes. For instance, the Core i7-8750H has a TDP of 45 W and a T-Junction of 100oC. Intel’s Ark Site contains this information and more about various Intel processors.

AMD is more straightforward with its usage of the term Max Temps to indicate maximum safe temperature. As an example, the AMD A12-9800 APU has a TDP of 65 W and a Max Temps of 90oC. This indicates that it needs a more powerful cooling system due to its slightly lower maximum allowable core temperature.

What Should I Do When It Gets Too Hot?

Both CPU manufacturers have installed a feature called the Configurable TDP-down. This causes the CPU to perform at a slightly lower level, which also brings down the TDP, effectively decreasing operating temperature. For example, Intel’s Core i7-8750H has a standard TDP of 45 W and a Configurable TDP-down of 35 W.

The TDP can be modified in the BIOS under Power Settings.

Summary

The Thermal Design Power is a metric used to determine the amount of heat expelled by the CPU while it is being used. While it is typically expressed in terms of watts, it is not to be confused with power consumption.

There is a strong correlation between power consumption and TDP, as well as the CPU’s performance. Higher-performing CPUs draw more power and tend to release more heat. In contrast, lower-performing CPUs, such as those in notebooks and laptops, consume less power and perform at a significantly lower level. The TDP should not be used as a guide to select a PSU. It should be used as a guide when choosing an appropriate cooling system. TDP determines the minimum level of activity a cooler should have to maintain CPU performance.