W5500 magnetics configuration with POE

Thread Starter

epic511

Joined Mar 16, 2019
3
Hello, I'm interested in implementing POE on the W5500. I found ARJM11C7-114-BA-EW2, which is the cheapest modular port on digikey that supports POE.

Most W5500 designs I see have the common mode choke before the main transformer, is having the choke after the transformer ( see photo below) an issue?

If this won't work, what is a cheap port that supports POE and the W5500?

1596661311010.png
 

MisterBill2

Joined Jan 23, 2018
7,040
I just read an article explaining POE and the circuit that you show is way less than 1% of the picture. The comunication needed to decide how much power is sent is fairly large and complex.
 

panic mode

Joined Oct 10, 2011
1,856
as far as i know that is only IF you need a lot of power (the only reason to manage it). in that case you can use one of methods (Class or LLDP) to state how much power you really need. but even without negotiation, power can be obtained upto some default level (12W or so), value varies depending upon type of cable. if that was not the case, POE device would never be able to power up to tell how much power it needs. also check:
http://www.usbmeter.com/Electronics_Kits_PoE_injector.htm
 

MisterBill2

Joined Jan 23, 2018
7,040
i think the common mode choke is moved to other side of transformer simply to minimize voltage drop on PoE
As the current increases the wire size for a choke must also increase to avoid the voltage drop, making the choke larger. So to avoid that the choke is moved. It reduces cost and improves efficiency at the same time.
 
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