Electrical – How to implement PoE with WIZnet W5500

ethernetpoe

I want to implement a PoE solution and I started to design the board based on WIZnet's W5500 reference designs. I am using the design with the external transformer.

My questions are:

  1. How to adapt the schematic to use 2 pairs for PoE and be compatible with both modes of PoE? In DFRobot's Ethernet shield (see schematic at bottom of page), the pairs were filtered with an C-R-C network, but, correlating with WIZnet's schematic, should these components support 2kV? Also, for me it is not clear if DFRobot's design works in both modes of PoE.

  2. I've seen RJ45 jacks with integrated magnetics and WIZnet gives a separate reference design which uses this type of jack. Can that design be adjusted for PoE? How?

  3. I've found many types of jacks which integrates PoE magnetics such as:

MagJack SI-52008-F

Wurth 7499210121A

Why do some jacks, like Wurth, integrate diode bridges when PoE is DC? Also, see Silvertel's PoE 9600 module which integrates diode bridges.

  1. In the schematic from DFRobot, a PoE PD interface and regulator circuit is used. Can't I just take the 48 V DC and use that voltage in a simple DC-DC converter? Why does such an interface is required?

  2. Would it be better to use a jack with metallic enclosure, than a plastic one?

  3. Would the PoE design work if I used a lower voltage, 5-12 V, instead of 48V?

  4. Relating to the WIZnet designs, why does the center tap on the TX transformer, on the chip side, needs to be connected to 3.3V?

Thank you!

UPDATE
Thanks to your answers I managed to obtain a working design.

Best Answer

The diode bridge looks to be reverse polarity protection when using the jack to power your circuit. The jacks with these diodes in it can not be used to provide power to a remote device.

Additionally the W5500 seems to require independent center taps to match the reference designs which limits the selection further. We found the following to fit the requirements but it is so far untested:

https://www.digikey.com/product-detail/en/abracon-llc/ARJ-199/535-13149-ND/5430434

As far as power - you should use a regulated supply. 48 is part of the POE "standard" but some devices do not conform to that. You would need to figure out what device you plan to use if you plan to use a lower voltage. That said, lower voltage means higher current for the same power output.

The actual standard involves some sensing and ability to adjust or remove power under certain circumstances but will require more circuitry to implement.

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