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Fanless design or not?

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This is a recurring problem for me. Do I need to use forced air cooling with fans or not? And although I always have this situation, I can never decide what to do, or to put it better I don't know how, on which criteria, should I decide about this.

What my designs usually have is a power hungry CPU (let's say around 15W give or take), several buck Point-of-Load converters and other ICs that do not normally need more than 1-2W. For example, for my last project I have made an estimation that it needs around 28W after the buck converters, which if calculated to the input voltage, after two stages of converters, I get around 40W. I also have a requirement that the circuit should work up to 55°C.

And now what do I do with this numbers?

For sure, I understand that my main concern should be the CPU. I should make sure that the Tj of it never exceeds the max value given by the manufacturer at my maximum ambient temperature. And based on the data given in the datasheet and my estimated power I can find a suitable heatsink that can adequately cool the CPU with natural convection. And if no heat sink with reasonable size can be found, then I can install some fans to help with cooling.

But the question is: is the CPU really the only concern?

What happens with all this thermal energy generated by the circuit? What if, when without forced air circulation, all this heat brings the whole system to an unstable point where the temperature of the components keeps increasing without end?

Is there any way to somehow estimate how much thermal energy could be removed from a PCB of specific dimensions with natural convection and/or estimate a limit over which fans are necessary?

I mean, for sure, the whole situation can be analyzed after the first prototypes come in, but in some cases at this point it will be too late to make big changes to the system.

What do you normally do? Are there any resources with relevant information?
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