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Thermal

發布時間：2023/12/16 编程问答 27 豆豆

生活随笔收集整理的這篇文章主要介紹了 Thermal 小編覺得挺不錯的,現在分享給大家,幫大家做個參考.

1. 幾個概念

1、結溫（Tj），指的是IC的結點（junction）的溫度，一般來說最高溫度為125℃，由晶體硅工藝決定。非常值得注意的是，這里所說的125℃其實更多的指的是芯片正常工作時的溫度（如Timing約束等），但是，即使超過125℃，如135℃，一般芯片也不會有損傷的。

2、環溫（Ta），指的是環境（ambient）的溫度。

3、殼溫（Tc），指的是IC表面外殼（case）的溫度。

4、功耗（PD），耗散功率，單位是W。

5、熱阻（Rθ），溫升除以PD，單位是℃/W。

6、熱阻θJA，The intent of θJA measurements is solely for a thermal performance comparison of one package to another in a standardized environment. This methodology is not meant to and will not predict the performance of a?package in an application-specific environment.

JESD51-2明確說，θJA只是用來對比封裝性能，不應該也不能用來估算結溫。但是，實際上利用θJA來計算結溫在某些情況下還是可取的（下文1.2.1小節例子）。

1.1 Psi（ψ）和Theta（θ）的區別

ψ：The temperature difference between point X to point Y if the total?heat is known. The heat flowing along the specific path point X to?point Y is NOT known.
-- This parameter typically serves for estimation of Junction?temperature (TJ) at known total dissipated power (PD) inside the?package when a temperature is measured at package perimeter (Lead, Exposed Pad, Board, etc.)

-- 熱特征參數ψ指的是不確定熱傳播路徑的情況，ψjt經常用來計算結溫（和PCB強相關）。
θ：The amount of heat that flows from point X to point Y if the?temperatures at X and Y points are known (connected by the?thermal resistance). The path the heat flows is known and it is?completely determined by the resistance.

-- 熱阻θ指的是確定re傳播路徑的情況，θja經常用來計算結溫，而θjc是不能用來計算結溫的（結果會偏大非常多）。

1.2、測試ψ和θ的方法

1.2.1、θJA測量方法：JESD51-2A

?芯片廠商一般會要求封裝廠仿真得到熱阻和熱特征參數（環溫：85℃），封裝廠可以根據JEDEC標準的Test Board（如2s2p PCB板子）或者芯片廠商指定的PCB進行仿真；另外還可以決定是否需要加上散熱片進行仿真。

?當封裝廠直接采用芯片廠商提供的評估板PCB進行熱仿真的話，芯片廠商是可以利用θJA來估算結溫的。

1.2.2、θJC測量方法：JESD51-14

θJC在仿真或實驗的時候，必須保證芯片的Case由溫度固定不變的散熱片導熱（這也就是為什么實際情況下θJC無法用來估算結溫的原因）。

2.計算結溫的方法

2.1.沒有散熱片Heat Sink的情形

沒有散熱片的時候，應該利用ψjt來計算結溫：

2.2.有散熱片Heat Sink的情形

Ψjt should not be used when application of a heat sink is intended. Instead, following equation ?should be used.

2.3.幾種錯誤的計算方式

2.3.1.通過θja計算結溫

Tj = Ta +?θja*Power

TI文檔：This is a misapplication of the RθJA thermal parameter because RθJA is a variable function of not just the package, but of many other system level characteristics such as the design and layout of the printed circuit board (PCB) on which the part is mounted. In effect, the test board is a heat sink that is soldered to the leads of the device. Changing the design or configuration of the test board changes the efficiency of the heat sink and therefore the measured RθJA. In fact, in still-air JEDEC-defined RθJA measurements, almost 70%–95% of the power generated by the chip is dissipated from the test board, not from the surfaces of the package. Because a system board rarely approximates the test coupon used to determine RθJA, application of RθJA using Equation 1 results in extremely erroneous values.

盡管TI文檔上說，利用θJA來計算結溫的時候會因為PCB等因素的影響而不準確，但是，只要PCB等因素差不多，還是可以利用該熱阻來估算結溫的，并非完全不可取。

2.3.2.通過θjc計算結溫

Tj = Tc?+?θjc*Power

TI文檔：The fallacy here is that only a very small percentage of heat energy in a typical plastic package is convected and radiated off the top surface of the package. Many models have shown 60%-95% of thermal energy from a chip is actually convected and radiated off the PCB to which the package is attached. If one assumes the entire power is dissipated by the top surface, the junction temperature calculated by Equation 4 is higher than reality.?