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New Ethernet systems distribute dc power with data

Posted: 24 Nov 2004     Print Version  Bookmark and Share


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New Ethernet

systems distribute

DC power with


The universal network is coming to an

RJ-45 jack near you, carrying power with

your packets!

Power-over-Ethernet (PoE) is a power-distribution

technique recently approved as IEEE standard 802.3af.

It allows that ubiquitous and universal network called

the Ethernet to carry DC power along with your data

packets. The recent ratification of this IEEE standard

appears to be the first instance in which a universal

power jack or plug has been defined for worldwide use.

It means that local AC power is no longer needed for all

network-attached devices that need continuous power--

such as IP phones, wireless access nodes, and web-

surveillance cameras. It also means that the devices do

not have to be placed near wall outlets, and means that

power cables can be eliminated.

In PoE systems, a client device that receives power over

an existing Ethernet network is called a powered device

(PD). The device that delivers power to the PD is called

power-sourcing equipment (PSE). Power consumption for

the PD is limited to 12.95W, and PSE outputs are limited

to 15.4W per RJ-45 port. Each PD can expect to draw a

maximum continuous current of 350mA, assuming the

Ethernet link cable and physical layer device (PHY) trans-

formers are well balanced.

To accommodate the voltage drop that takes place along a

CAT-5 Ethernet link (up to 100m long), the IEEE

standard specifies different power ratings for PD and PSE.

The longer links exhibit a significant drop, which obliges

the PSE to output more than the nominal 48VDC to

maximize power available to the PD. Therefore, voltages

as high as 57VDC can be seen anywhere along an

Ethernet link.

Most PoE networks can be implemented with an endpoint

or midspan PSE. An endpoint PSE integrates an Ethernet

switch and power source in a single device, and is situated

at the other end of the Ethernet link. This PSE type is the

most convenient way to implement a PoE network, as

power is already inline at the endpoint of an Ethernet link.

Such Ethernet switches are sometimes referred to as

having "inline power" (see Figure 1). Endpoint PSEs are

ideal for new infrastructure deployments.

For existing Ethernet networks that cannot justify such a

complete overhaul, power can be injected into the

Ethernet link using the midspan PSE method. Midspan

PSE provides power over the "spare pairs" in the CAT-5

cable--an approach that can be cost effective if only a

few Ethernet devices need power. Such a case is the 4 to

24 ports in a local area that are part of a system

comprising a larger multiport network (Figure 2).

Endpoint PSE differs from midspan PSE in its option to

deliver power either combined with the signal over the

same pair of wires, or over the spare pairs. In general, a

PSE must be able to provide power over the signal pairs

or the spare pairs, but not both.


Figure 1. For endpoint PSE and PD devices, power is delivered over the signal pairs.






RJ-45 RJ-45









Simple as they seem, such systems entail considerable

design effort. They must include safeguards to ensure

backward compatibility with devices that do not expect

to see 48VDC on their Ethernet connection. The IEEE

802.3af standard covers backwards compatibility and,

by including optional features for powering Ethernet

networks, it also looks forward. This article covers what

a designer should know in developing products destined

to operate in new or existing systems--i.e., products

expected to migrate towards Gigabit Ethernet or


What about Gigabit Ethernet?

Gigabit Ethernet works with endpoint PSEs, but not

midspan PSEs, because it uses all four pairs within the

CAT-5 cable for data transport. In contrast, 10BASE-T

and 100BASE-TX use only two pairs for data (wires 1-2

and 3-6), leaving the spare pairs (wires 4-5 and 7-8)

available for midspan power injection. To provide inline

power for Gigabit Ethernet, therefore, endpoint PSE

switches are required.

CAT-3 cable is supported by the IEEE 802.3af standard

because it was originally used with 10BASE-T systems.

To maximize signal integrity in new deployments,

however, we recommend use of the highest rating of

Ethernet cabling available (CAT-5e or CAT-6). This is

because cabling infrastructures typically represent a ten-

year investment. Gigabit Ethernet (1000BASE-T, specifi-

cally) requires CAT-5 cabling, but some applications

using CAT-5 and Gigabit Ethernet switches have proven

marginal. Consequently, the latest 1000BASE-TX

standard requires CAT-6, while the original 1000BASE-T

standard requires CAT-5.

Detection of PDs

When connected to its Ethernet links, the PSE must

detect whether each of the Ethernet devices requires

power. The PDs must therefore exhibit characteristics

beyond those of a legacy Ethernet device. To accomplish

this detection, the PSE makes V-I measurements while

probing the signal wires with a current-limited voltage of

2.7V to 10.1V. Table 1 lists the criteria a PD must have for


Figure 2. For midspan PSE and PD devices, power is delivered over the spare pairs.








RJ-45 RJ-45













Parameter Conditions (V) Minimum Maximum

V-I slope (at any chord of 1V or greater) 2.7 to 10.1 23.75k 26.25k

Voltage offset -- -- 1.9V

Current offset -- -- 105A

Input capacitance 2.7 to 10.1 0.055F 0.125F

Input inductance 2.7 to 10.1 -- 1005H

Table 1. For a valid-PD signature, all criteria below must be detected by a midspan or

endpoint PSE.

detection as a valid PD. The 1.9V series offset allowed is

a consequence of the diode bridges typically used to

control voltage polarity. Two such bridges per PD are

required, as the PD must be backward compatible with

midspan PSE applications (Figure 3). The 105A current

offset is typically due to leakage within the PD. Table 2

lists another set of criteria, for which any detection fails

an Ethernet device by making it an invalid PD.

Power classification of PDs

The driver that first started the movement to combine

power with Ethernet networks was the voice-over IP

(VoIP) telephone. Because so many other Ethernet devices

are able to use this convenient source of power (RFID

readers, chargers for PDAs, mobile phones, or even

laptops), the IEEE 802.3af standard includes an optional

feature called power classification. This allows the PSE to

manage its power budget more closely. Table 3 lists the

different power classes for which a PD can be provisioned,

and their corresponding classification signatures.

To implement the optional power classification method,

the PSE applies a probing voltage of 14.5V to 20.5V. In

response, the PD exhibits a signature (classification

current), indicating back to the PSE the maximum power

the PD can draw. That information enables the PSE

switch to manage the maximum power it delivers to the

connected PDs at any given time.

By selecting a proper PSE controller IC, you could

implement another feature that is outside of the IEEE

802.3af standard: a hard limit on the PSE's output power

per port. Unless the deployment administrator can

guarantee that no PD will ever be swapped out for one

that dissipates more power, the switch's expected power

budget can occasionally be exceeded. In that case, the

PSE will refuse to power the port unless the PD power

classification is met.

Another feature handy in emergencies would be an ability

for the PSA to prioritize which ports receive power first,

or which ports should be disconnected first when the UPS

or backup generator begins to run out of energy. The

switch could then maintain power for the most important

Ethernet ports. Such ports might include E911 telephones,

badge readers for access, certain surveillance cameras or

access points, or other revenue-generating data circuits.


Table 2. Detection of any criterion below by the midspan or endpoint PSE indicates that

Ethernet device is an invalid PD.

Parameter Conditions (V) Range of Values

V-I slope 2.7 to 10.1 Either >45k or <12k

Input capacitance 2.7 to 10.1 >105F

Figure 3. A PD operating with Gigabit Ethernet must be backward compatible with midspan PSE applications, and therefore receive power from an

endpoint PSE switch.






























The presence of such fail-safe features within the PSE

controller IC, either hardwired or software-configurable,

can help manage the power budget during emergencies.

Consequently, look for a software-configurable PSE

controller IC.

Detecting disconnected PDs

After a PSE applies power to a PD, it must monitor the

PD for a "maintain power" signature in accordance with

the IEEE 802.3af standard. The PSE must also detect

whether the PD has been disconnected. The standard

defines both AC and DC methods for detecting a PD

disconnect. Consider, for example, that a PD has been

disconnected and a legacy Ethernet device immediately

plugged into the same RJ-45 jack on the switch. If

48VDC power is not quickly disconnected after the PD is

removed, the legacy device may be damaged.

AC-impedance measurements performed on a PD are

generally more accurate than pure DC-resistance

measurements. A small, common-mode AC voltage is

sent down the Ethernet link simultaneously with the data

signals and 48VDC. You then measure the AC current

and calculate the resulting port impedance, which (if the

PD has not been disconnected) should be less than

26.25k. The frequency for this AC voltage must be

between 1MHz and 100MHz. For the many other details

pertaining to DC and AC methods of disconnect

detection, the designer should consult the IEEE 802.3af

standard. Regardless of the method chosen, the

measurement must be made quickly and the power

removed quickly thereafter.

Advanced features in silicon

Among the multiport PSE silicon chips now available, the

most common are PSE controllers that control inline

power to four ports. Look for I2CTM-compatible, serial-

interface capability with programmable registers that

provide the option of use with an MCU. Some of the

advanced features residing in various operating modes are

now important for emergency reasons, an importance that

was magnified after 9/11/01.

Operational modes offered by the MAX5935, for instance,

include automatic, semiautomatic, manual, shutdown, and

debug mode. Automatic mode allows the device to operate

without software supervision. Semiautomatic mode (on

request) continuously detects and classifies a device

connected to a port, but does not apply power to that port

until directed by software. Manual mode, useful in system

diagnostics, allows complete control of the device by

software. Shutdown mode terminates all activity and turns

off power to the ports. Finally, debug mode allows detailed

system diagnostics by fine-stepping through the device

state machine.

Figure 3 is a simplified example of a PoE system design,

illustrating PSE and PD connections using Gigabit

Ethernet. Because Gigabit Ethernet does not work with

midspan power injection, the 100/10M Ethernet modes can

only connect to an endpoint PSE switch. (The MAX5940

PD-interface controller does not need a diode bridge, yet it

operates with one if required.) Today's PD-interface

controller ICs (such as the MAX5941 and MAX5942)

include a pulse-width modulation (PWM) controller, even

though the PD usually includes a DC/DC converter.


Purchase of I2

C components from Maxim Integrated Products, Inc., or one of its sublicensed Associated Companies, conveys a license under the Philips


C Patent Rights to use these components in an I2

C system, provided that the system conforms to the I2

C Standard Specification defined by Philips.

Class Conditions (V)

Classification Current


PD Power Range




14.5 to 20.5 0 to 4 0.44 to 12.95


(Reserved for future use)

-- -- --

1 14.5 to 20.5 9 to 12 0.44 to 3.84

2 14.5 to 20.5 17 to 20 3.84 to 6.49

3 14.5 to 20.5 26 to 30 6.49 to 12.95

Table 3. Five classes for PD power classification and their classification signatures.

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