POE Technical Article 1

Power over Ethernet - the reality of designing a Powered Device

There have been many technical articles, white papers and product application notes that
explain the theory on how to make a powered device. They give an overview of the IEEE 802.3af
standard for Power over Ethernet and explain how to extract the power from the CAT5e cable.
The following is a practical guide to help take that information and to design a PoE enabled powered
device.

By Tony Morgan, Silver Telecom Ltd

The Powered Device (PD) can be broken down into four basic building blocks as shown in Figure 1.

Figure 1: Building Blocks of a Powered Device (PD)

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 centre 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 requires a
bridge rectifier? It doesn't’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

The second building block is the Signature and Class circuitry. To ensure that the PSE does not apply 48V
to a non POE enabled device, the PSE will initially apply a low voltage (2.7V to 10.1V) and look for a
Signature Resistance of 25K Ohms. The PSE will expect that the Signature Resistance will be after some
form of Auto-polarity Circuit and will compensate for the DC offset in the Signature. The maximum input
capacitance of the PD must be less than 150nF. There are a number of PSE’s that don’t check this
parameter, but some do. It is important to remember that if the Signature Resistance is not switched out
when the full PSE voltage is applied, it will need to dissipate ~130mW (25K @ 57V).

Class	Measured Current (mA)	PD Power Max (W)	Comment
0	0 to 4	0.44 to 12.95	Default
1	9 to 12	0.44 to 3.84	Optional
2	17 to 20	3.84 to 6.49	Optional
3	26 to 30	6.49 to 12.95	Optional
4	36 to 44	Reserved	Not Allowed

Table 1: PD Power Classification

The third building block is the under-voltage lock-out or control stage. It is important that the DC/DC
converter does not operate when the PSE is validating the Signature and Current Classification. The
control stage must be ON when the PD input voltage = 35V, the PSE output voltage (Von) = 42V with
20 Ohms series resistance (cabling and connectors) at 350mA.

The IEEE specification does contradict itself with the Voff voltage. It states that control must be OFF
if the PSE output voltage = 30V with 20 Ohms series resistance, implying that it can go down to 23V. But
in the recommended PD power supply test procedure the specification states that the input current must
be less than 1.14mA at 30V. So to ensure that the PD complies with the specification set the control
switch threshold between 30V to 35V.

The forth and final building block is the DC/DC converter. A nominal 48V is not the most practical voltage
and most applications would require a lower voltage such as 3.3V, 5V or 12V. An effective way of
achieving this would be to use a DC/DC (Buck) converter. This converter must be capable of operating
normally over a wide input range 36V to 57V, under minimum to maximum load conditions.

A question often asked is “How much power is available?” The PSE will be capable of outputting 15.4W
(350mA @ 44V). But the IEEE 802.3af specification states that with 20 Ohms series resistance, the
maximum input power to the PD = 12.95W (350mA @ 37V). If the DC/DC converter is 80% efficient then
the available output power = 10.36W. This is something to be aware of this when working out the actual
power, under worse case conditions. The IEEE have set-up a new task force to progress POE further with
a higher power standard IEEE 802.3at. This is still in progress and the new standard is not expected to
be ratified in the near future.

Linked to the maximum available power will be heat. It is important to remember that even the most
efficient DC/DC converters will generate heat and this must be taking into account at the design stage.
Ensure that the enclosure is ventilated and the DC/DC has sufficient heat sinking, failure to do this may
result in a reduced power budget.

Another important consideration when designing the DC/DC converter stage is Electromagnetic
Interference (EMI) and the PCB layout will affect this. Here are a few guidelines to reducing noise:-

• Position the power components close together, to minimise power loops.
• Keep tracks with high dv/dt as small as possible, to minimise radiation.
• Keep a high impedance tracks away from those with high dv/dt.
• Keep the track to the FET gate as short as possible.
• Maximise the copper on power and ground track.

Another question that is frequently asked is “Does the PD need 1500V isolation?” The IEEE specification
states that “electrical isolation shall be in accordance with the isolation requirements between SELV
circuits and telecommunication network connections in subclause 6.2 of IEC 60950-1:2001”, so the answer
to the question is “Yes”. There are two approaches that can be used which will depend on the final
product. Either the DC/DC converter could have the isolation barrier built-in, or if the product completely
enclosed in a non-conductive material that could form the isolation barrier.

Power over Ethernet offers many benefits and can be used in a wide range of applications. There are
modular and silicon solutions available today which are designed specifically for the powered device and
under the existing specification IEEE 802.3af these can provide ~12W. Several PSE manufacturers have
developed equipment that offers high power, but they have not standardised on their approach and at this
point in time the IEEE 802.3at standard is still a work in progress.

About the author:

Tony Morgan is Senior Applications Engineer with Silver Telecom. Having worked at Mitel Semiconductor
(Zarlink) and TT Electronics, Tony has been with Silver Telecom since January 2004. Silver Telecom is a
leading developer of telephone interface, and power over ethernet solutions. Established in 1997, the
company provides solutions for developers of the latest ethernet and telecommunications equipment,
particularly within the growing areas of VoIP and computer telephony. The company continues to invest
heavily in product development, releasing innovative products specifically in their SLIC, DAA and PoE
(powered device) ranges with specific attention to low cost, small size and ease-of-use. Silver Telecom
is privately owned with headquarters in Newport, Wales, and an extensive sales network covering 6
continents.

For further information please contact

Tony Morgan
Silver Telecom Ltd
Imperial House
Imperial Park
Newport
Gwent
NP10 8UH
Tel: +44 1633 811833
Fax: +44 1633 811834
e: sales@silvertel.com

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