Thermal Design Power

# Thermal Design Power

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Encyclopedia
The thermal design power (TDP), sometimes called thermal design point, refers to the maximum amount of power
Electric power
Electric power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is the watt.-Circuits:Electric power, like mechanical power, is represented by the letter P in electrical equations...

the cooling system
Computer cooling
Computer cooling is required to remove the waste heat produced by computer components, to keep components within their safe operating temperature limits.Various cooling methods help to improve processor performance or reduce the noise of cooling fans....

in a computer is required to dissipate
Dissipation
In physics, dissipation embodies the concept of a dynamical system where important mechanical models, such as waves or oscillations, lose energy over time, typically from friction or turbulence. The lost energy converts into heat, which raises the temperature of the system. Such systems are called...

. For example, a laptop
Laptop
A laptop, also called a notebook, is a personal computer for mobile use. A laptop integrates most of the typical components of a desktop computer, including a display, a keyboard, a pointing device and speakers into a single unit...

's CPU
Microprocessor
A microprocessor incorporates the functions of a computer's central processing unit on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and...

cooling system may be designed for a 20 watt
Watt
The watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...

TDP, which means that it can dissipate up to 20 watts of heat
Heat
In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...

without exceeding the maximum junction temperature
Junction temperature
Junction temperature is the highest temperature of the actual semiconductor in an electronic device. In operation it is higher than case temperature and the temperature of the part's exterior...

for the computer chip. It can do this using an active cooling method such as a fan
Computer fan
A computer fan is any fan inside, or attached to, a computer case used for cooling purposes, and may refer to fans that draw cooler air into the case from the outside, expel warm air from inside, or move air across a heatsink to cool a particular component...

or any of the three passive cooling methods, convection
Convection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....

Thermal radiation is electromagnetic radiation generated by the thermal motion of charged particles in matter. All matter with a temperature greater than absolute zero emits thermal radiation....

or conduction. Typically, a combination of methods is used. The TDP is typically not the most power the chip could ever draw, such as by a power virus
Power virus
A power virus is a malicious computer program that executes specific machine code in order to reach the maximum CPU power dissipation...

, but rather the maximum power that it would draw when running "real applications
Application software
Application software, also known as an application or an "app", is computer software designed to help the user to perform specific tasks. Examples include enterprise software, accounting software, office suites, graphics software and media players. Many application programs deal principally with...

". This ensures the computer will be able to handle essentially all applications without exceeding its thermal envelope, or requiring a cooling system for the maximum theoretical power (which would cost more but in favor of extra headroom for processing power).

In some cases the TDP has been under-estimated such that in real applications (typically strenuous, such as video encoding or games) the CPU has exceeded the TDP. In this case, the CPU will either cause a system failure (a "therm-trip") or throttle its speed down. Most modern CPUs will only cause a therm-trip on a catastrophic cooling failure such as a stuck fan or a loose heatsink.

Since safety margins and the definition of what constitutes a real application vary among manufacturers, TDP values between different manufacturers
Integrated device manufacturer
An integrated device manufacturer is a semiconductor company which designs, manufactures, and sells integrated circuit products. As a classification, IDM is often used to differentiate between a company which handles semiconductor manufacturing in-house, and a fabless semiconductor company, which...

cannot be accurately compared. While a processor with a TDP of 100 W will almost certainly use more power at full load than a processor with a 10 W TDP, it may or may not use more power than a processor from a different manufacturer that has a 90 W TDP. 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.

The power consumed by a CPU is approximately proportional to the square of the voltage according to Ohm's law:
(where C is capacitance, f is frequency and V is voltage).