THE REALITIES OF
POWER-OVER-ETHERNET DESIGN
Many technical articles, white papers
and application notes have already been written to theoretically explain the integration
of Power over Ethernet (PoE) into an application. However, the existing literature only
provides an overview of the IEEE 802.3af standard (for PoE), and of the technique used to
extract power from a CAT5e cable. The following practical guide treats the question of how
to develop a PoE powered device (PD).
The first block in the device diagram (Figure 1) is "Polarity Protection," also
called the "Auto-polarity Circuit." It is required because the IEEE
specification allows for the power to be injected onto the Cat5e cable in a number of
different ways.
"Alternative A" (Figure 2) shows the injection of power via the data line wires.
Because power is injected via the central pins of the transformers, the positive potential
can lie either on the TX wire pair or RX wire pair; or a crossover cable could be used.
Therefore the connected PD must be able to detect the polarity and respond accordingly.
For this, a simple bridge rectifier will do the job, and this is the solution recommended
by the (IEEE) standard.
"Alternative B" (Figure 3) allows for a power connection via the spare wire
pairs in 10Base-T and 100Base-T networks. The IEEE specification states that the supply
voltage must be connected to wire pairs 4 and 5, with wire pairs 7 and 8 set to
"0" potential. To protect the device against a polarity reversal, 2 simple
diodes on the Vin2 input suffice, as dictated by the standard.
For 1000Base-T networks, the wiring connections are arranged somewhat differently. In this
case, all 4 wire pairs are used. If the end device were connected as described above
within such network, it would cause a short circuit between 2 of the wire pairs and thus
risk destroying not only the end device, but also the switch / hubs / midspans (PSE) used
to feed the device. The circuit diagram in Figure 4 thus shows how to correctly connect
the PD within a 1000Base-T network.
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The second building block is the identification circuit (Signature Resistance and Current
Classification). To ensure that the power source (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 25kW. The maximum input capacitance must be less than
150nF. Also note that the signature resistance will need to dissipate approximately 130mW
(25kOhms at 57V).
The "current classification" is used to inform the PSE of the maximum power used
by the PD. After a valid signature identification, the PSE will increase its output
voltage to between 14.5V - 20.5V and measure the current - thus determining the power
class of the connected PD (see table, "PD Power Classification").
The third building block is the under-voltage cutoff (or control) stage. Because the
downstream DC/DC converter must not be in operation during signature identification, this
circuit meanwhile drops the downstream voltage. According to the IEEE standard, the
control stage must cut off the downstream circuit when the PSE output voltage drops below
30V, and then bring it back on line once supply voltage climbs back above 35V (42V/350mA
out of the PSE for a line resistance of 20Ohms).
The fourth block is the DC/DC converter. It must convert the input voltage of 36V - 57V to
the typical 3.3V, 5V or 12V used by the circuits it feeds. The easiest way to accomplish
this is to use a simple voltage regulator that is capable of performing such conversion
under maximum load conditions.
As you can see, implementing a PoE design from scratch is no easy task. To help designers
avoid the many potential pitfalls, SILVER TELECOM offers a series of PoE modules that
integrate all of the required functionality described above. When using these modules,
only a bare minimum of external circuitry is needed for implementation. The table below
shows a selection of the various PoE modules available from SILVER TELECOM.
I look forward to receiving your inquiries!
Ondrej Gavura, EXT 953
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