A Quick Power Over Ethernet Review

Quote of the Day

Until lions have their historians, tales of the hunt shall always glorify the hunter.

— West African proverb. A variation on Winston Churchill's "History is written by the victors" quote.

Introduction

Figure 1: Hookup for a 30W PoE Type 2 System.

I have been asked to write some requirements for an optical product that is powered using Power Over Ethernet (PoE). It has been a few years since I have worked on a PoE-based design, I thought it would be useful to review the standard and ensure that I still understand it. This is a good exercise in basic electrical design and will also illustrate how to design circuits using Mathcad utility functions that I have written over the years.

My objective in this post is to show how a useful amount of power (25.5 W at the load) can be transferred over an Ethernet cable. I will be avoiding discussions on the protocol details associated with PoE because that would result in an enormous post.

Background

Definitions

Power over Ethernet (PoE)

PoE is an IEEE standard for sending power and data over the same category 5e Ethernet cable, which contains four wire-pairs (i.e. 8 wires total). PoE is enormously popular because only one cable is required to network an Ethernet-fed device, which greatly reduces the cost and complexity of networking remote devices, like cameras. For the version of PoE discussed in this post, power is transmitted over two  wire-pairs by applying a DC voltage between each pair (see Figure 1). Superimposing DC on the wire-pairs does not interfere with data transmission because Ethernet uses differential signalling.

Type 2 PoE

Type 2 PoE is an IEEE standard (802.3at) for transferring as much as 25.5 W over an Ethernet cable. The standard is also known as  "PoE+".

Power Supplying Device (PSE)

A PSE is a device that provides power on an Ethernet cable.

Powered Device (PD)

A PD is a device powered by a PSE.

PoE Basics

Here are the key points about a type 2 PoE system discussed in this post:

• The source power is limited to 30 W.
• The wire temperature is assumed to be no more than 50°C.
• All design work will assume category 5e cable, which means 24 AWG wire.
• I will be using two of the four Ethernet pairs for power transmission.
• The category 5e cable length is limited to 100 m.
  100 m is also the maximum reach for data transfers on Ethernet. This means that you can use PoE on any Ethernet network.

Analysis

Modeling Resistance of Annealed Copper Wire

Figure 2 shows my linear interpolation of some annealed copper wire resistance data that I found years ago. I believe it was from an old Bell Telephone, but I do not recall the original source – certainly something I googled. I have scanned the original table into an Excel workbook.

Figure 2: Linear Interpolation of Copper Resistance Data.

One-Way Cable Resistance

Figure 3 shows how to compute the resistance of a 100 m long, 24 AWG, category 5e wire at 50 °C using the functions shown in Figure 2. My calculations show the maximum wire resistance is ~10 Ω, which does not include connector losses. The standard actually assumes 12.5 Ω, which will provide a reasonable amount of margin.

Figure 3: One Resistance of a Category 5e Cable Wire.

Figure 4 shows the basic circuit I am working with here.

Figure 4: Circuit Model for PoE (Source).

PoE+ Analysis

My intent here is compute a few of the key product parameters of my PoE+ driven system, like maximum input power and internal heat generation (shown shaded green in Figure 5).  To perform my analysis, I need to state a few PoE+ characteristics:

• The source power, P_Source, is limited to 30W.
• The load power, P_Load, is limited to 25.5 W.
• The minimum source voltage, V_Min, is specified as 50 V.

Figure 5: PoE+ Analysis.

Conclusion

This exercise was a useful refresher exercise as to how PoE delivers power over category 5e cables. My application requires 15.3 W, so the 25.5 W capability of PoE+ will provide enough power for my application plus some reserve power in case we want to add more features later.