engineering

Thermal design power

Thermal design power is a engineering topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Thermal design power rather than just read about it. In short: Thermal design power (TDP), also known as thermal design point, is the maximum amount of heat that a computer component (CPU, GPU, or system on chip) can generate and that its cooling system is designed to dissipate during normal operation at a non-turbo clock rate (base frequency). Some sources state that the peak power rating for a microprocessor is usually 1.5 times the TDP rating.

Key takeaways

Thermal design power belongs to engineering; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Thermal design power to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Thermal design power from memory before moving on to harder problems.

Reference excerpt

Thermal design power (TDP), also known as thermal design point, is the maximum amount of heat that a computer component (CPU, GPU, or system on chip) can generate and that its cooling system is designed to dissipate during normal operation at a non-turbo clock rate (base frequency). Some sources state that the peak power rating for a microprocessor is usually 1.5 times the TDP rating. Graphics processing units are known to have even larger discrepancies between peak and TDP.

Calculation

The average CPU power (ACP) is the power consumption of central processing units, especially server processors, under "average" daily usage as defined by Advanced Micro Devices (AMD) for use in its line of processors based on the K10 microarchitecture (Opteron 8300 and 2300 series processors). Intel's thermal design power (TDP), used for Pentium and Core 2 processors, measures the energy consumption under high workload; it is numerically somewhat higher than the "average" ACP rating of the same processor. According to AMD the ACP rating includes the power consumption when running several benchmarks, including TPC-C, SPECcpu2006, SPECjbb2005 and STREAM Benchmark (memory bandwidth), which AMD said is an appropriate method of power consumption measurement for data centers and server-intensive workload environments. AMD said that the ACP and TDP values of the processors will both be stated and do not replace one another. Barcelona and later server processors have the two power figures. The TDP of a CPU has been underestimated in some cases, leading to certain real applications (typically strenuous, such as video encoding or games) causing the CPU to exceed its specified TDP and resulting in overloading the computer's cooling system. In this case, CPUs either cause a system failure (a "therm-trip") or throttle their speed down. Most modern processors will cause a therm-trip only upon a catastrophic cooling failure, such as a no longer operational fan or an incorrectly mounted heat sink. 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. conduction coupled with forced convection) such as a heat sink with a fan, or any of the two passive cooling methods: 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 (a processor with a TDP of, for example, 100 W will almost certainly use more power at full load than processors with a fraction of said TDP, and very probably more than processors with lower TDP from the same manufacturer, but it may or may not use more power than a processor from a different manufacturer with a not excessively lower TDP, such as 90 W). 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. Until around 2006 AMD used to report the maximum power draw of its processors as TDP. Intel changed this practice with the introduction of its Conroe family of processors. Intel calculates a specified chip's TDP according to the amount of power the computer's fan and heatsink need to be able to dissipate while the chip is under sustained load. Actual power usage can be higher or (much) lower than TDP, but the figure is intended to give guidance to engineers designing cooling solutions for their products. In particular, Intel's measurement also does not fully take into account Intel Turbo Boost due to the default time limits, while AMD does because AMD Turbo Core always tries to push for the maximum power.

Multiple TDPs TDP specifications for some processors may allow them to work under multiple different power levels, depending on the usage scenario, available cooling capacities and desired power consumption. Technologies that provide such variable TDPs include Intel's configurable TDP (cTDP) and scenario design power (SDP), and AMD's TDP power cap. Configurable TDP (cTDP), also known as programmable TDP or TDP power cap, is an operating mode of later generations of Intel mobile processors (as of January 2014) and AMD processors (as of June 2012) that allows 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 and desired power consumption. cTDP typically provide (but are not limited to) three operating modes:

Nominal TDP – 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 modes. As another example, some of the AMD Opteron processors and Kaveri APUs can be configured for lower TDP values. IBM's POWER8 processor implements a similar power capping functionality through its embedded on-chip controller (OCC). Intel introduced scenario design power (SDP) for some low power Y-series processors in 2013. It is described as "an additional thermal reference point meant to represent thermally relevant device usage in real-world environmental scenarios." As a power rating, SDP is not an additional power state of a processor; it states the average power consumption of a processor using a certain mix of benchmark programs to simulate "real-world" scenarios.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Thermal design power

Start with the simplest possible case. Write down what Thermal design power claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Thermal design power before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Thermal design power ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Thermal design power

In research: Thermal design power appears in engineering research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Thermal design power in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Thermal design power is common in secondary-school and first-year university syllabi. It links to neighbouring topics Computer engineering, Heat transfer, so understanding it makes those chapters shorter.
In everyday life: Look for Thermal design power outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Thermal design power in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Thermal design power means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Thermal design power out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Thermal design power in simple terms?

Why does Thermal design power matter?

Because it connects several engineering ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Thermal design power?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Thermal design power.