Power Over Ethernet (PoE) Transformers

PoE Overview

In recent years, the emergence of “The Internet of Things” has lead to a massive rise in the number of devices connected to the Internet via Ethernet cabling.
Historically each of these devices also has a separate power supply to provide the local 5 or 12 voltage required to drive them
One rapidly emerging solution for this is Power Over Ethernet (or PoE for short).

There are two basic versions of PoE.
One version supplies the voltage on the Data Transmit and Receive pairs (TX and RX); i.e. combines the power and data.
The second version supplies the power on unused pairs of the cable
The first method is preferred for simplicity and also has the advantage of backward compatibility with existing wiring.

The advantages of such a system are many
   - Firstly the power is regulated and isolated at one central point.
   - Secondly the installation of PoE devices is simpler as only reliant on one cabling system.
   - Thirdly, any interruptions to the power supply can be handled by one central UPS system,
   - Fourthly, the design of the devices can be mechanically and electrically simpler, as they require only one external port to the outside world.

Furthermore, newly designed equipment can be designed with the transformers and circuitry needed to split the digital data and power signals in the device, and legacy equipment can still be used by employing a local “splitter” box, (similar in concept to ASDL splitters).
In all cases transformers are used to isolate the device and separate the signals.

Wurth manufacture an extensive range of transformers for PoE applications.
Here we will examine the Wurth Electronics part #7491199212. 
This is a four winding transformer with turn ratios of ;
N1 : N2 : N3 : N4
1.00 : 0.29 : 0.29 : 0.21
It comes in a Surface mount package.