Easiest way to implement PoE (Power over Ethernet) on PCB with RJ45 connector

[zphaze]

Hi,

I'm a bit unsure about what I have to do to implement PoE on a board that receives ethernet signals (2 differential pairs Rx/Tx). I then want to "inject" power on according to IEEE 802.3af mode B and route those signals to a RJ45 connector. So that at the connector I have RX, TX and PoE as output on a single RJ45 connector.

Can't I just use a buck/boost and transform e.g. 24V into 48V and then route this to the RJ45 and bingo?

It seems there are ICs such as the LTC4263-1, referred to as "PSE" chips, that do... what? Does it act as a buck or..? Sorry I'm just confused.

Thanks!

[abeyer]

Can't I just use a buck/boost and transform e.g. 24V into 48V and then route this to the RJ45 and bingo?

No, not if you're going for 802.3af support. First, that would break gigabit connections, which use the same pairs as mode B for data. Second, you wouldn't be 802.3af compliant if you always applied power: the supply is supposed to check that the attached device supports POE before applying power (via some specific sense resistor value, iirc.)

There are lots of "passive" POE injectors that are not 802.3af that do similar to what you're proposing, but they aren't compliant with 802.3af, nor any standard afaik.

[fchk]

Silvertel has you covered.

Powered Device:
https://silvertel.com/ag9700/

Power Source:
https://silvertel.com/ag6100/

[mikeselectricstuff]

You need to use something to do at least the minimum protocol for safety - this ensures that a PoE device is at the far end of the cable. Without his you can blow out termination resistors of a non PoE- aware device, and damage non-ethernet devices that happen to use RJ45s
 
The protocol initially applies a low voltage to detect  26K resistor in the device, and only then applies the 48-57v supply. It also switches off the supply and restarts detection if power draw drops below a certain value to detect unplug/replug events

[ejeffrey]

The LTC4263-1 is basically a fancy power switch.  It implememts the PoE protocol and switches on the 48-57V power but it isn't a buck converter.  You would need a separate DC power supply.  PSE devices are usually network switches with multiple power capable ports.  They use a large common power supply for all ports and individual controllers to do the negotiation and switching.

Note that power is applied to the center taps on the Ethernet isolation transformers not directly to the pins.  This allows the same wires to be used for power and data.

[NiHaoMike]

If there's a microcontroller in the design, it could be as simple as a MOSFET to switch the low side and some resistors to set the sensing voltage and allow the microcontroller to detect voltage and current.