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Document Number: 81257For technical questions, please contact: optocoupler.answers@vishay.comwww.vishay.com Rev. 1.2, 25-Apr-

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Content	Document Number: 81257For technical questions, please contact: optocoupler.answers@vishay.comwww.vishay.com Rev. 1.2, 25-Apr-081039 Thermal Characteristics of Optocouplers Application Note Vishay Semiconductors INTRODUCTION Behavior of any semiconductor device depends on the temperature of its die. Hence, electrical parameters are given at a specified temperature. To sustain the performance of a component and to avoid failure, the temperature has to be limited by managing the heat transfer between the chip and the ambient atmosphere. It is a good design practice to make sure that the rated junction temperature of a phototriac optocoupler is not exceeded even if the device may not fall into what many designers consider the “power device” category. This is true for two main reasons. The first is to increase the overall long-term reliability of the phototriac. The operating temperature of any solid-state device is inversely proportional to its long-term viability. Consequently, it is recommended to 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 RON. There are different ways of performing thermal calculations. •Using a component thermal derating number •Using a graph of allowable power versus temperature •Using a thermal model Use of a thermal derating number (given in power/degrees) is the simplest approach to take. Manufacturers are very conservative when deriving this number. Hence, this approach does not provide the most accurate results. Use of a graph of allowable power versus temperature is very similar the first approach, but instead of a simple number, the designer follows a graph of allowable power versus temperature similar to the one in figure 1. Again, this is a very conservative approach and should allow for a very reliable design, but it does not provide the most accurate results. Fig. 1 - Allowable Dissipated Power A more comprehensive of performing thermal calculation is to use a thermal model. Thermal models have been created for some Vishay optocouplers containing multiple dice, including phototriacs, to make these calculations as simple and as accurate as possible. MULTIPLE DICE OPTOCOUPLER THERMAL MODEL This document will demonstrate a simplified resistive model. When used correctly, this will give results that provide “engineering accuracy” for practical thermal calculations. The simplified electrical analogous model for any optocoupler is provided in figure 2. Fig. 2 - Multiple Dice Optocoupler Thermal Model/Resistor Network Where: 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. 0 100 200 300 400 Phototransistor IR-diode Coupled device 020406080100 Tamb - Ambient Temperature (°C) P tot - Total Po w er Dissipation (m W ) 9611701 TA θCA TC TJDTJE TB θEC θEB θDC θDB θBA θDE TA Package Network 19978
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Following Datasheets	thermal-3 (1 pages) thermal-4 (6 pages) Thermal-Behavior-Microelectronic-systems (10 pages) Thermal-Resistance-Pin-Fin_Heatsinks (8 pages) Thermal-Solutions-Brochure (8 pages) thermal-system_mg (1 pages) thermal (9 pages) thermalapplet-1 (9 pages) thermalbond4949a (4 pages) thermalbond4949b (5 pages)
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