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"The board gets fairly warm, likely due to the overhead of the PoE+ power conversion (which consumes 6-7W on its own!"

7 W of losses? That's either wrong or insane for DC/DC regulation. Rough figures, pi4CM (5W) + NVMe (2W) = 7W. So regulation of 50% efficiency?

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I found the same to be the case on the official Pi 4 PoE+ HAT (the new 2021 version)[1].

[1] https://www.jeffgeerling.com/blog/2021/review-raspberry-pis-...

PoE can't do "purely" DC/DC regulation. They require (in any reasonable setup) a transformer for isolation on both ends, which lowers efficiency, especially at lower end. Now with that said you can definitely get decent efficiency with PoE, it's just that most people don't care and so it's not as big of a focus.

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