光耦的熱特性

通過管理芯片和周圍空氣之間的熱傳遞，維持光耦特性，避免失效。

　　任何半導體設備的動作依靠其模型溫度，這就是為什么電子參數要按照特定溫度給出。為維持光耦特性，避免失效，通過管理芯片和周圍空氣之間的熱傳遞限制溫度。不應該超過設計規(guī)定的連接溫度，即使光耦也許沒有被歸入“功率器件”的種類。這么做有以下兩個原因：

　　首先，全面增加光耦長期可靠性，因為任何固態(tài)設備的工作溫度都與其長期可靠性成反比。因此應該是器件工作在最低的實際工作溫度下。其次，某些參數與設備的問題緊密相連，這些隨溫度而變得參數包括漏電流、觸發(fā)電流、CTR、驟回電壓和電阻。

　　進行熱計算的三個主要方法是通過使用器件降額值、隨溫度變化功率圖或溫度模型。最簡單的方法是使用熱降額值（假定用功率/度）。然而，制造商非常保守的得到這個數字，所以這個方法不能提供最精確的結果。

　　隨溫度變化功率圖與第一種方法非常相似，但是用簡單的數字代替，依照隨溫度變化功率圖（圖1）。并且，這是一個非常保守的方法，應該非常顧及可靠的設計，但是它也不能提供最精確的結果。

　　進行熱計算更全面的方法是使用熱模型。一些光耦的熱模型已經建立，用于大多數簡單精確的計算。

　　英文原文：

　　Thermal characteristics of optocouplers

　　Sustain an optocoupler's performance and avoid failure by managing the heat transfer between the chip and the ambient atmosphere.

　　By Roshanak Aflatouni and Bob Gee, Vishay Intertechnology -- EDN, 10/18/2007

　　The behavior of any semiconductor device is dependent on the temperature of its die, which is why electrical parameters are given at a specified temperature. To sustain an optocoupler's performance and to avoid failure, the temperature is limited by managing the heat transfer between the chip and the ambient atmosphere. You should not exceed the device's rated junction temperature, even if an optocoupler may not fall into what you consider the power device category. This is true for two main reasons.

　　The first is to increase the overall long-term reliability of the optocouplers, as the operating temperature of any solid-state device is inversely proportional to its long-term viability. Consequently, you should operate a device at the lowest practical operating junction temperature. Secondly, certain parameters are closely tied to the operating temperature of the device; these temperature-dependent parameters include leakage current, trigger current, CTR, snapback voltage, and on-resistance.

　　The three main ways of performing thermal calculations are by using a component derating number, or a graph of allowable power versus temperature, or a thermal model. The simplest approach is to use a thermal derating number (given in power/degrees). However, manufacturers are very conservative when deriving this number, so this approach does not provide you with the most accurate results.

　　A graph of allowable power versus temperature is very similar to the first approach, but instead of a simple number, you follow a graph of allowable power versus temperature (Figure 1). Again, this is a very conservative approach and should allow for a very reliable design, but it does not provide you with the most accurate results.

　　A more comprehensive method for performing thermal calculation is to use a thermal model. Thermal models have been created for some optocouplers containing multiple dice —including phototriacs — for the most simple and accurate calculations.

　　Multiple Dice Optocoupler Thermal Model

　　This article demonstrates a simplified resistive model. When used correctly, this model produces results that provide engineering accuracy for practical thermal calculations. Figure 2 provides the simplified electrical analogous model for any optocoupler.

　　θCA = Thermal resistance, case to ambient, external to the package.

　　θDC = Thermal resistance, detector to case

　　θEC = Thermal resistance, emitter to case

　　θDB = Thermal resistance, detector junction to board

　　θDE = Thermal resistance, detector to emitter die

　　θEB = Thermal resistance, emitter junction to board

　　θBA = Thermal resistance, board to ambient, external to the package

　　TJE = Emitter junction temperature

　　TJD = Detector junction temperature

　　TC = Case temperature (top center)

　　TA = Ambient temperature

　　TB = Board temperature

　　Thermal resistances and specified junction temperatures for a particular device are provided in select datasheets.

　　Thermal Energy Transfer

　　There are three mechanisms by which thermal energy (heat) is transported: conduction, radiation, and convection. Heat conduction is the transfer of heat from warm areas to cooler ones, and effectively occurs by diffusion. Heat radiation (as opposed to particle radiation) is the transfer of internal energy in the form of electromagnetic waves. Heat convection is the transfer of heat from a solid surface to a moving liquid or gas.

　　All three methods occur in optocouplers. However, for most products in most environments, the majority (~ 75 %)

of heat leaving the package exits through the lead frame and into the board. This occurs because θBA is a conductive phenomenon with a much lower thermal resistance than the convective and radiative phenomena associated with θCA (θCA is typically an order of magnitude larger than other thermal resistances). Because very little heat leaves through the top of the package (heat convection), junction-to-case temperatures (θDC and θEC) are negligible in most environments.