Thermal design power
The thermal design power (TDP), sometimes called thermal design point, refers to the maximum amount of power the cooling system in a computer is required to dissipate. The TDP is typically not the most power the chip could ever draw, such as by a power virus, but rather the maximum power that it would draw when running "real applications". 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 underestimated 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.
For example, a laptop's CPU cooling system may be designed for a 20 watt TDP, which means that it can dissipate up to 20 watts of heat without exceeding the maximum junction temperature for the computer chip. It can do this using an active cooling method (e.g. forced convection) such as a fan or any of the three passive cooling methods, convection, thermal radiation or conduction. Typically, a combination of methods is used.
Since safety margins and the definition of what constitutes a real application vary among manufacturers, TDP values between different manufacturers 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 dynamic power consumed by a switching circuit is approximately proportional to the square of the voltage:
(where C is capacitance, f is frequency and V is voltage).
- Testing Thermal Throttling in Pentium 4 CPUs with Northwood and Prescott cores, By Stanislav Garmatyuk , March 26, 2004, iXBT Labs
- Enhanced Intel SpeedStep Technology for the Intel Pentium M Processor - White Paper, Intel Corporation, March 2004
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