Thermal design power

The thermal design power (TDP), sometimes called thermal design point, refers to the maximum amount of heat generated by the CPU, which the cooling system in a computer is required to dissipate in typical operation.

The TDP is typically not the largest amount of heat the CPU could ever generate (peak power), such as by running a power virus, but rather the maximum amount of heat that it would generate 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).

Some sources state that the peak power for a microprocessor is usually 1.5 times the TDP rating. However, the TDP is a conventional figure while its measurement methodology has been the subject of controversy. In particular, until around 2006 AMD used to report the maximum power draw of its processors as TDP, but Intel changed this practice with the introduction of its Conroe family of processors. A similar but more recent controversy has involved the power TDP measurements of some Ivy Bridge Y-series processors, with which Intel has introduced a new metric they call scenario design power (SDP).

Overview
In some cases the TDP has been underestimated such that in real applications (typically strenuous, such as video encoding or games) the CPU exceeds the TDP, overloading the computer's cooling system. In this case, the CPU will either cause a system failure (a "therm-trip") or throttle its speed down. Most modern processors 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 W TDP, which means that it can dissipate up to 20 watts of heat without exceeding the maximum junction temperature for the laptop's CPU. A cooling system 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 these 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. For example, 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 from the same manufacturer, 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:
 * $$P = C V^2 f$$

where $C$ is capacitance, $f$ is frequency, and $V$ is voltage.

Variable TDP

 * Configurable TDP (cTDP)
 * cTDP (also known as programmable TDP) is an operating mode of later generations of Intel's mobile processors, allowing adjustments in their TDP values. By modifying the processor behavior and its performance levels, power consumption of a processor can be changed altering its TDP at the same time.  That way, a processor can operate at higher or lower performance levels, depending on the available cooling capacities.


 * With cTDP, three operating modes are available:
 * Nominal TDP – this is the processor's rated frequency and TDP.
 * cTDP down – when a cooler or quieter mode of operation is desired, this mode specifies a lower TDP and lower guaranteed frequency versus the nominal mode.
 * cTDP up – when extra cooling is available, this mode specifies a higher TDP and higher guaranteed frequency versus the nominal mode.


 * For example, some of the mobile Haswell processors support cTDP up, cTDP down, or both.


 * Scenario Design Power (SDP)
 * SDP is an operating mode of certain later generations of Intel's mobile processors, allowing them to work at much lower power levels when compared to similar mobile processors not featuring the SDP. They can still exceed SDP and operate at higher power levels under certain workloads and with appropriate cooling capacities provided; for example, they can reach TDP power levels.


 * For example, Y-series (extreme-low power) mobile Haswell processors support SDP.