TDP 基础知识

Thermal Design Power 或者说 TDP，是一个以W（瓦）为单位的散热能量参数，它表达了一个处理器为避免过热而需要的散热量大小。比如说，一个12W TDP的芯片可能只需要一个很小的风扇或者被动热沉（被动吸热的物质，比如空气），而一个95W TDP的芯片则可能会需要一个更大的风扇或者一个专用的热沉（比如循环的冷却水）。

用户通常会在一个 CPU 或 GPU 的标准中看到 TDP 这个参数，但TDP能用于描述更多电子产品耗能的一个状况。TDP功耗通常作为电脑（台式）主板设计、笔记本电脑散热系统设计、大型电脑散热设计等散热／降耗设计的重要参考指标。

由于TDP是以能量来表述的，所以它能够表征一款产品与竞争产品能量消耗的对比情况，低的散热设计功耗TDP值通常意味着低的能耗，也就意味着电池寿命的延长。

TDP是有产品制造商提供的一个数值，制造商会根据自己的实验研究得出这个数值，但通常这些实验系统都会对自己的产品TDP值有所夸大，但我们仍有理由相信这一个数据，因为如果TDP值没有一个可信的精度，在后续的基于此芯片的产品开发中将会产生许多问题。

TDP 应用举例

如上所述，不同的芯片会有不同的TDP。最常见的一个例子是最新用于智能手机的ARM处理器和用于台式机的CPU的一个TDP对比。 www.zhangpeng.info

用于智能手机的高通 Snapdragon 处理器标识的TDP为0.5～1 W之间，而 酷睿 Core i5-2500 的TDP则会高达95W。 这是一个很大的区别，很典型的体现了不同芯片能耗与性能的一个对比情况。

新型的GPU甚至有更高的TDP值，有些芯片的TDP值超过了225W。这并不能说明这些芯片就会一直消耗这么多的能量，但在工程开发过程中，必须对散热部件进行充分考虑以保证芯片长时间处在这样的能量消耗状态下散热正常不会产生过热烧毁现象。

了解 TDP 知识对你有什么好处

你已经对TDP有了一个简单的了解，那么或许你要问TDP知识对你有什么用呢？ 虽然TDP经常被引用，但它却并非是对芯片能量消耗或者新能的一个精确计量，TDP更多的是一个工程设计的指导性数据。

同时，了解一款CPU的散热设计功耗能够更好的理解它的能力，最好的一个例证是 Intel 酷睿系列移动处理器的一个对比。通过下表你可以看到Core i5-2540M 的 TDP 为 35W， 而Core i5-2557M的TDP则为17W.

这也就是说，Core i5-2557M相对Core i5-2540M处理器的设计功耗更低，由于两款处理器架构相同，这也就是说i5-2557M性能相对会差些但能延长电池使用寿命，也就是说对于笔记本电脑来i5-2557M可能比较合适，而如果更多的关注性能则应选择i5-2540M更为理想。

相同的结论可以适用与台式机的处理器。 AMD Phenom II X4 980 处理器 TDP为125W，而i5-2500的TDP为95W，从台架实验数据来看，确实AMD X4 980 消耗更多的能量

总结

TDP是衡量处理器性能与能源利用效率的一个重要参数，虽然无法替代最终的实验数据，但知道TDP值（同时知道其架构和主时钟频率）仍然可以帮助了解一款CPU的性能表现。

Let me have a brief research on this interesting phenomenon. Is the temperature decreased rapidly during its way to basin?www.zhangpeng.info

In my opinion, this phenomenon causes by three reasons.

First, your hand-skin temperature is lower than your skin on head, so your skin will cheat you.

Second, the heat transfer efficiency is much higher in a forced convection than natural convection.

Third, the temperature decreased during its way to basin due to convection and radiation.

First, biologic reason.

Based on biology, your skin feels the temperature by compare the difference between your skin and surroundings. It’s to say, if your skin temperature is same to the surrounding, you may just have no feeling about the temperature. For example, you can’t feel hot if you skin temperature is the same as evioriment though it’s 70 degree now. So, when your hand-skin is cold than your skin on your head, it will feel warmer at a same temperature. And in cold days, our hands are usually colder than our head. This is one reason why you feel the temperature is higher by testing with your hands. But, after testing once time, your hand is warmed by hot water. That’s to say you will feel temperate goes down after wetted your hair.

Second, I’ll calculate the heat transfer in a forced convection.

According to Newton equation:

Q=h(Tw-Tf)
Where:
Q. Heat transfer during the whole process
h. Convection coefficient
Tw. Surface temperature
Tf. Fluid temperature

In a forced convection, ‘h’ can be much higher than a natural convection. Usually, ‘h’ in a forced convection can be 10 times in a natural convection.

When you put your hand under flowing water, the Q is high, in other words, the heat transfer is high. And the temperature of your skin increase rapidly. So you think the water temperature is suitable. But put your hand in basin, the ‘Q’ is lower; your skin may tell you the temperature is lower than the flowing water.

Third, the temperature decreased during its way to basin due to convection and radiation. It’s to say, the temperature is surely lower in the basin.

On its way to basin, there are convection and radiation. The convection is a forced convection. According the above discuss, result on temperature decrease is obviously.

And base on black body radiation equation:

Q=δ(T_f^4-T_0^4)
Where
Q. Heat transfer during the whole process
T_f. Temperature of water
T_0. Temperature of the environment

We know during its way to the basin, water will lose some heat because of radiation and the temperature decrease.

So, the real situation is that the temperature in your basin decreased when the water flow out of water pipe.

Above all, my conclusion about this interest phenomenon on some degree is misleading by your feeling but it’s a fact that the temperature is lower in your basin.

How to make this process better?

There are three ways to improve. First, warmer your hands before you prepare water. Second, turn down the valve to let the water flow slowly so the convection reduced. Third, just put you basin near outlet to decrease the heat transfer during its way to your basin.

There also a simple solution, remember next time when you want to wash your hair in winter, prepare your water hotter than perfect.