July 16, 2006
Power over Ethernet: A practical guide
a practical guide to help take the information from the many articles
that explain powered devices and design a PoE-enabled powered device.
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 <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 @
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
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
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
The "Current Classification" or "Class Circuitry" is used to inform
the PSE of the maximum power used by the PD. This is useful for power
management in larger switch/hubs. After a valid Signature the PSE will
increase it output voltage between 14.5V and 20.5V and measure the
current. Table 1 shows the different Class ranges available, this is
optional and providing the measure current is ≤4mA the PSE will default
to Class 0. Class 4 has been reserved and may be used in the future.
|| Measured Current (mA)
|| PD Power Max (W)
||0 to 4
||0.44 to 12.95
||9 to 12
||0.44 to 3.84
||17 to 20
||3.84 to 6.49
||26 to 30
||6.49 to 12.95
||36 to 44
|| 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 <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 heatsinking, failure to do this may result in a reduced
Another important consideration when designing the DC/DC
converter stage is Electromagnetic Interference (EMI) and the PCB
layout will affect this. Here are some guidelines for reducing noise:
* Position the power components close together, to minimize power loops.
* Keep tracks with high dv/dt as small as possible, to minimize radiation.
* Keep high impedance tracks away from those with high dv/dt.
* Keep the track to the FET gate as short as possible.
* Maximize 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 standardized on their approach and at
this point in time the IEEE 802.3at standard is still a work in
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. email@example.com