Simple circuit design tutorial for PoE applications

Lastly, it addresses designing PDs to operate from alternate power sources, such as AC power adapters. Since PoE enabled PDs will become consumer commodity items, the simplicity, cost-effectiveness and performance of the circuits presented in this paper make them attractive for most PD applications.

The current specification and what it mandates
The specification that describes the delivery of power over CAT-5 Ethernet cabling is an amendment to part three of the main IEEE Ethernet specification. Specifically it is: IEEE 802.3af Part 3: Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method and Physical Layer Specifications Amendment: Data Terminal Equipment (DTE) Power via Media Dependent Interface (MDI). The first piece of information provided by the spec, that is relevant to PD circuit design, describes how the PSE can provide the power to the Ethernet cabling system (see Figure 1). The system on the left hand side of Figure 1 uses a PoE enabled switch/router to provide power--through the patch panel--to the PDs.

The system on the right hand side uses an older switch that does not provide power, so the power must be injected somewhere between the switch and the patch panel. Since the power is added at the starting “end” of the enabled switch, it is referred to as an “endpoint” system, versus a midspan, which injects the power somewhere in between the switch and the patch panel.

Figure 1. PSE power injection schemes: Endpoint (left) vs. Midspan (right)

Essentially, midspan PSEs are merely a way to allow PoE to be added to older systems without replacing the switch hubs (with newer, PoE enabled units). The current specification only allows power on two of the four wire pairs in the CAT-5 cable. As a general practice, Endpoint PSEs place their power onto the data pair of wires in the CAT-5 cable (see Figure 2), while midspan PSEs are restricted to using the spare pair of CAT-5 wires (see Figure 3 and 802.3af, pages 29 and 30). The specification also allows three different power connection options (802.3af, page 31), which determine the number and orientation of diodes found on a PD front-end.

Figure 2. Endpoint PoE configuration with PSE injecting power onto data pair

Figure 3. Midspan PoE configuration with PSE injecting power onto spare pair