Power over Ethernet: A practical guide

Figure 1: Building Blocks of a Powered Device

The first block is "Polarity Protection" or "Auto-polarity Circuit". This is required as the IEEE specification allows the power to be injected onto the Cat5e cable in a number of ways. "Alternative A" shown in Figure 2, injects and extracts the power using the center tap of the data transformers (Medium Dependant Interface or MDI). The PSE can apply the positive to the centre tap of the TX pair transformer or the RX pair transformer (or a crossover cable could be used). Therefore the PD must be able to handle the unknown polarity and operate normally. A simple bridge rectifier will do the job and the IEEE specification allows for such a component to be used in the PD's input.

Figure 2: Endpoint PSE, Alternate A

The other alternatives methods detailed in the IEEE specification are shown in Figure 3 and Figure 4. Were the power is supplied by the PSE over the Power Interface (PI), or the spare pairs in 10BASE-T and 100BASE-T networks.

Figure 3: Endpoint PSE, Alternate B

Figure 4: Midspan PSE, Alternate B

In Figure 3 and Figure 4 the IEEE specification states that the PSE positive must be connected to 4 & 5 and the negative connected to 7 & 8. So if the polarity is fixed does the Vin2 input to the PD require a bridge rectifier? It doesn't need a full bridge rectifier but it would be worth putting two diodes in-line to match the way the Signature circuit responds to either input method.

The IEEE specification details 10BASE-T and 100BASE-T networks, but only makes references to 1000BASE-T networks. 1000BASE-T network topology differs from 10BASE-T and 100BASE-T networks in that it uses all four pairs within the cable to transfer data. If a powered device using one of the methods shown above is connected to a 1000BASE-T network, then two of the data pairs will be shorted. Figure 5 shows how to configure the PD to work with a 1000BASE-T network.

Figure 5: 1000BASE-T Configuration