The single most common thermal question asked of semiconductor component suppliers is "What is Theta-JA for this device?" The only completely truthful answer, however unsatisfying, is always "It depends." This presentation will tackle the issue using the physical/mathematical concept of linear superposition. We will see not only why the simplistic question regarding Theta-JA is virtually always meaningless, but point to better questions to be asked in its place, and in particular, provide a methodology for answering the possibly most significant question of all, namely "How do other heat sources in this system affect the thermal performance of the device I thought I cared about?" Linear superposition says, in short, that you can make predictions about the operating temperature of devices of interest within your system, by considering each heat source in the system individually, and then summing up the responses. Thus, once we have our arms around the concept of linear superposition, we will also find that some other important questions have comprehensible answers, for instance, "What are my real design margins for avoiding thermal runaway?" and "Why do I get worse thermal performance when I put 10 packages, 1/10th the size of the previous generation, into the same space?"
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