I have seen numerous designs that ahve made it into production even though the electronics are being operated above the suggested die temperature. Sometimes it is a simple product that has a price point of under $5 so the engineer didn’t care or have time to check. Yet other times it is because an engineer completed a beautiful power budget and thermal calculations towards the end of a project, and then had to fix a last-minute problem in pre-production without the time to go back and re-check thermal calculations. But no matter the reason, some parts just run too hot. How hot is too hot? My new Element14 article explains a calculation!

….Power Dissipated in the part

This is my favorite point to consider because it is pure circuit analysis and one can actually calculate a quantitative number. The key here is to be sure to find the worst-case power consumption of the part in question. In the simple case of our LM317 linear regulator, let’s say it’s dropping a 5V rail to create a 3.3V rail and will need to supply 500mA of current at most. Therefore we can calculate full-on power dissipation as:

P = (Voltage In – Voltage Out) * (I) or (5V – 3.3V) * 0.5A = 0.85 Watts

Of course not all parts are that straightforward, and someswitchers may require the use of SPICE to determine the dissipated power (LTspice is my favorite for this kind of simulation). But in the end, a figure in Watts is what you want to have…..

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