Like the Adafruit Ethernet FeatherWing, but with Power over Ethernet built-in!

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This projects aims to create a WIZ5500 / Power over Ethernet FeatherWing, fully isolated, in the same size and compatible with the official Adafruit Ethernet FeatherWing (which does not have PoE).

This idea was originally suggested to my by @professor after I released my #wESP32 and has been waiting in the wings for quite a while now. The Adafruit Feather contest made the time seem right for it to come to fruition.

I think it was originally in September of 2018 that @Prof. Fartsparkle suggested to me right here on to make a project like this as a "successor" to the #wESP32.  Since at the time the wESP32 wasn't even released yet, that seemed a bit premature to me. :)  But I liked the idea enough to do a good brainstorming session.  I don't see it as a successor so much as a nice addition to the PoE products that are available to makers.

There are a couple of potential snags in making a PoE FeatherWing.  The first is that the Feather system didn't intend for FeatherWings to back power the main board.  That doesn't mean that nobody has done this tough. :)  I think many Feather boards now include a diode to prevent back powering the USB port, and for those there is no issue.  There are some that don't include this circuitry though, so the problem needs to be managed carefully.

I intend to do this by putting ~4.5V on the USB pin, instead of 5V.  Since the flyback converter of the PoE supply has a diode rectifier on the output, it will just not provide power if there's a voltage higher than its own output on the USB pin already.  That's the theory, let's hope it works in practice.

The second potential snag is of course size.  Feather boards are tiny, and Ethernet jacks and flyback transformers are not.  I have some confidence it can be done though.  If necessary, I can put parts on both sides of the PCB, but I'm going to try to avoid it to ease manufacturing (although my CM keeps telling me it's a non-issue for them).

The plan at this point is to use the WIZ5500 just as it is used on the Adafruit Ethernet FeatherWing, so it is fully compatible with all the existing software out there.  If space permits (haha) I may add a 24AA02E48 chip to fix the annoying issue that WIZ5500 chips don't come with a MAC address built-in.

For the PoE side of things, this will have to shrink significantly from what I'm currently using on the wESP32.  To that end, I'm going to build the design around the TI TPS23758 instead of the Silicon Labs Si3404A.  The chip is slightly bigger, but it implements primary side regulation using the flyback transformer's auxiliary winding, making it possible to drop the huge opto-coupler and all the circuitry around the secondary side shunt regulator.  I'll also be using a 5W instead of 13W flyback transformer as I have done on the #LiFePO4wered/ESP32, since Feather systems don't tend to be power hungry.

  • Rough floorplan

    Patrick Van Oosterwijck11/11/2019 at 21:09 0 comments

    I got the project far enough to come up with a rough floorplan:

    It took quite a bit of fiddling with combining loose parts into resistor and capacitor arrays, reducing resistors to 0402 size and throwing out optional parts, but I think I have enough confidence to say now that this might be possible!

    The design will be isolated, but things are just too tight to be able to keep recommended clearances and make any claims about the isolation voltage.  I will have to go to smaller via sizes and trace widths and clearances than I usually do, but that's alright.  Most board houses nowadays have no issue with 4 mil traces/spacing and 0.2 mm via drills.

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Prof. Fartsparkle wrote 11/08/2019 at 22:51 point

Oh hey that's great! Glad you got around to it :)

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Patrick Van Oosterwijck wrote 11/08/2019 at 23:18 point

Right! It only took more than a year. ;)

  Are you sure? yes | no

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