外殼溫度與環(huán)境溫度

　　當(dāng)你想要控制一個(gè)設(shè)備的溫度時(shí)，關(guān)于系統(tǒng)環(huán)境溫度的模糊性會(huì)導(dǎo)致降額曲線的不精確。測(cè)量外殼溫度而不是環(huán)境溫度可以簡(jiǎn)化和提高熱測(cè)試過程的準(zhǔn)確性。

　　環(huán)境溫度是指一個(gè)房間內(nèi)的溫度，或討論物體周圍的溫度。對(duì)于電子元件，環(huán)境溫度以及附近組件的功耗和元件自身的功耗是極端溫度的主要來源。極端溫度會(huì)損壞部件，應(yīng)盡可能避免。不幸的是，通常情況下，限制環(huán)境溫度是不可能的，因?yàn)樘囟ǖ膽?yīng)用程序中使用的設(shè)備。終端系統(tǒng)設(shè)計(jì)者唯一可用的選項(xiàng)是限制設(shè)備本身組件的功耗。這是通過在高環(huán)境溫度的應(yīng)用程序中限制流經(jīng)這些組件的電流（有效地降低電源功率限制）來實(shí)現(xiàn)的。這種降低的功率限制，被稱為“額”，通常是在數(shù)據(jù)表中指定使用“降額曲線”（見圖1）。

　　Figure 1 shows the derating curves for the Xgen power supply. Taking the XCC/XCV model operating at 230 Vac as an example， the power supply is rated to output 1000 watts from 0°C to 45°C ambient. Then from 45°C to 70°C， the power supply is then linearly derated to 600 watts. The power supply is not rated for operation at ambient temperatures higher than 70°C. If the power supply is operated within these limits， any harmful thermal conditions are avoided. However， the accuracy of the derating curve is limited by the accuracy of the ambient temperature measurement， and this raises a number of issues， which will be discussed in the next section.