Power-over-Ethernet is there standard for 5 or 3.3 V ?

[tom_hanks]

Power-over-Ethernet

Hi Fellomates,
i need some information related to POE.

according to IEEE802.3af 48v can be used on pins 7 & 8..
but if i need 3.3v and 5v then is there any provision or standard is there..??

plz put some focus on present progress on POE..

TIA

 

[cypherpunks]

You would use a switching power supply to convert the 48V to 3.3/5 with high efficiency.
You can either use one of your own design, or use an encapsulated DC-DC converter module.
Since Ethernet cabling is relatively thin, it's not a good idea to try running 5V over it directly.
Using a high voltage at a low current lets you get a higher wattage out of the cable.
If you try running the same wattage with a high current at 5V, the cable will drop voltage like crazy.
Note that your DC-DC converter should be of the isolated type for safety reasons.

 

[alexz]

Re: Power-over-Ethernet

Is there a reference design or an application note regarding PoE and its components?

 

[cristi7521]

Re: Power-over-Ethernet

Some documentations:

hxxp://www.fairchildsemi.com/an/AB/AB-44.pdf
hxxp://focus.ti.com/lit/an/slyt085/slyt085.pdf
hxxp://www.transition.com/TransitionNetworks/Resources/EN/PDF/PowerOverEthernet.pdf

cristi

 

[dennis zhang]

Re: Power-over-Ethernet

Is there a reference design or an application note regarding PoE and its components?

Please explore the website of MAXIM, TI, NS as well as ST. I personally think TI play a major role in PoE field. Current af standard is matuar, but the higher power at standard is not finalized yet.

 

[kot_b]

Re: Power-over-Ethernet

See attachment. Could be something like this. 48V need in order to reduce current. So it is best to source 48V and have DC/DC on the other side.

 

[dennis zhang]

Re: Power-over-Ethernet

Many PD chips has already integrated DC/DC controller, even high side MOSFET.

 

[plusminus]

Re: Power-over-Ethernet

See **broken link removed** for a ic with PoE interface and SMPS.

Other solution is: http://www.linear.com/pc/productDetail.jsp?navId=H0,C1,C1003,C1006,C1125,P2282/ Here you have to design your own SMPS in. A good one could be the LM2576-5 for 5V.

Max power for current 802.3AF is ~15W. They are busy with a high power version for about 30W.