PoE for Pi

This project is still fairly dormant, but the 3B+ has given it a little kick in the pants.

The 3B+ includes a 2x2 .1" header on the board that (ostensibly) has the center taps for the four magnetics. You just pass each pin through an AC pin on two bridge rectifiers that have common + and - outputs and there's your 48 VDC power input (there's more to it - you need to properly engage the 802.3af protocol to actually get that 48 VDC, but that's the job of the TPS23753A, of course).

The 5 volt output would be fed back to the Pi on the GPIO header (the same way Pi Power does it today), so the whole thing winds up being a Pi Hat.

To be fair, the Pi Foundation has already announced that this will be A Thing very soon, so there isn't a huge amount of point to continuing on, other than learning something and/or the typical "because it can be done" hacker mentality.

Turns out the TPS23753A is a better chip choice. It's an 802.3at PoE controller combined with a flyback DC-DC controller. It's exactly what we need.

Still TBD is the physical form factor. At the moment, I'm considering a fully externalized design. Two RJ-45 jacks are the Ethernet+power input and Ethernet data output, and a USB-A jack is the output power. You'd use a short CAT 5 cable to go from the data jack to the Raspberry Pi, and a short USB A to µB cable to power the Pi.

But another way to do it would be to make a Pi Hat form factor board. It would have the two RJ-45 jacks, with the data jack positioned to be directly above the Raspberry Pi Ethernet port, so a very short cable could go from one to the other. The output power could be fed directly to the GPIO +5 pins, just as with Pi Power.

I couldn't help brainstorming this a little today. I don't think there's a market for it, but in principle, you could make a PoE power splitter combined with a USB Ethernet interface and terminate the whole thing with an OTG jack. The result would be a PoE Pi Zero adapter.

The problem with this idea is that the thing would probably cost at least four times as much as a Pi Zero.

This schematic over at ON Semi answered one question about the design: how to pass the Ethernet signaling on to the device. The secondary side of the magnetics goes directly on to the output RJ45.

Most of the schematics for PoE adapters out there also show flyback power supplies rather than buck converters. They're more complicated, but they are better isolated, and the spec does call for isolation, so maybe that's the way to go. The good news is that flyback converters can run just fine on DC. In fact, when they're used in cheap wall warts, the incoming AC is usually just directly rectified and the flyback transformer provides the requisite galvanic isolation along with an optoisolator for feedback.