Enabling devices for a Power over Ethernet world

Enabling devices for a Power over Ethernet world
Enabling devices for a Power over Ethernet world
Author: Paul Lee, Director of Engineering, C&D Technologies, Power Electronics
Division, Milton Keynes, UK
Power over Ethernet (PoE) brings a host of benefits to the design, implementation and
long-term usability of wired Ethernet local area networks (LANs). Cost, flexibility and
even safety are all enhanced. PoE overcomes the major limitation that system designers
often encounter whereby they must locate powered network devices within close
proximity to AC power outlets.
With PoE, both data and power at a safe nominal 48VDC are carried over the same
Ethernet cable. If network devices can be configured to run from 48V, the need for
devices on the network to be supplied with intrinsically unsafe AC power from a
separate power cable connected to a building’s AC ring main is eliminated. The
freedom this gives to position devices where they are needed rather than where power
cords dictate, and the long-term flexibility to move devices around to suit the
changing operational needs of the business, are both highly desirable benefits for any
organisation. As an example, networked security cameras benefit from PoE as they
normally need to be sited high up on walls, away from AC outlets. Typical devices
used on a powered Ethernet such as VoiP phones, cameras and Wi-Fi access points
may need to be specially designed or adapted to run from Ethernet 48VDC. DC/DC
converters in the network devices will normally be required to drop and isolate the
48V supply down to a lower voltage such as 5V, suitable for the device circuitry. To
take full advantage of PoE, the network devices will also be required to handle the
handshaking process that allows the network hub to recognise a device as being PoE
enabled and understand and manage its specific power needs.
The basics of a PoE network
There are three main elements to a PoE system:
Power sourcing equipment (PSE) – Connected to the Ethernet switch and often
itself powered by an uninterruptible power supply (UPS) providing 48V, the PSE
feeds data to all devices on the network, and data plus power to all PoE compatible
devices. Although the nominal output is 48VDC, the allowed range is 44 to 57VDC.
Powered device (PD) – The range of PoE powered devices is continually growing as
the adoption of, and market for PoE continues to expand rapidly. Current types of
enabled devices include VoiP phones, EPOS systems and Wi-Fi access points. The
PD gives a ‘signature’ to the PSE to indicate that it is PoE enabled and how much
power it requires. The management and conversion of the incoming power is
normally handled by a DC/DC converter in the PD, described in more detail later in
this article.
Cable – Ever since CAT5 cable was invented for the purpose of carrying data, the
spare conductors have been utilised to additionally carry power to the other end of the
cable. The PoE standard IEEE802.3af defines the detail of how this is done and
provides the platform for industry to adopt a single approach for the design and
implementation of PoE.
Transferring power to the PD
A standard CAT5 Ethernet cable has four twisted pairs, only two of which are
required for the transmission of data. With PoE, there is a high degree of flexibility
with the IEEE802.3af standard allowing DC power of either polarity to be transferred
using either the two spare pairs or superimposed on the two data pairs. Fully
compliant PoE Devices must be able to accept power from either option.
Currently, the IEEE standard allows nominal 48V and approximately 15W of power
to be transferred over a single powered Ethernet cable with a minimum of about 13W
available to the powered device. The restricted available power does limit the types of
peripheral devices that can be used with PoE. However, future changes to the
standard are being proposed which will allow for much higher power levels and
consequently a wider variety of devices to be operated via Cat5 cables on the
Ethernet. PoE-enabled laptops for example might be charged directly from network
connections without cumbersome AC adapters.
In order to prevent damage to existing Ethernet equipment, which may not be
compatible, the IEEE802.3af standard requires that a ‘discovery process’ be
instigated by the PSE. This examines the cables looking for PoE enabled devices. It
checks for the presence of a 25KOhm parallel resistor in the remote devices and only
if this is found is the full 48V applied. The supply is current-limited to prevent any
damage to the equipment and cables should a fault condition occur. Before
connection, the PD also signals back to the PSE what current it expects to draw in
four categories. If the powered device does not draw a minimum current - which may
occur for example when the device is unplugged from the network - then the PSE
removes the power from that particular cable and is then aware that the power is
available for other devices. The PSE might also remove power from selected noncritical devices on a power outage, to maximise the run time of a UPS, maintaining
more critical security devices.
Power conversion
Isolated DC/DC converters are usually required within PoE enabled devices to
transform the 48V supply to a lower voltage appropriate for the PD. IEEE802.3af
defines stringent low-noise, start-up and isolation requirements for the converters to
be used in PoE applications. As an example, the C&D Technologies NPH10 range is
ideally suited. C&D also offers a full interface solution in its NMPD product which
integrates the data isolation transformers, PoE handshaking and DC/DC conversion
with customer-specified output voltage. The SIP module provides the full PoEcompliant data and power interface to the Ethernet line.
The 48V Ethernet power source is also defined by the IEEE standard, (actually 44 to
57VDC) at the hub. However, with significant voltage drops along the Ethernet
cables, the PoE standard allows the minimum voltage at the powered devices to be as
low as 30V. The nominal 48V can be derived from an AC/DC converter but also from
an existing system 48V. In this case another high power isolated DC/DC converter is
required in the PSE. Although not defined by the PoE standards, this DC/DC
converter, designed to be incorporated into the PSE, should ideally possess a number
of characteristics and features to make it more suited to the task. As an example,
C&D Technologies’ HHS04-520 has been specifically designed with PoE in mind.
As well as complying with the requirements of the standard, it has extremely high
power density enabling it to manage and deliver 200W in a very small package. This
is important as many non-PoE hubs may be re-engineered to make them suitable for
use on a powered network. Engineers will often be tasked with squeezing any PoE
related circuitry into the existing enclosure - so clearly the smaller the converter the
The HHS04 operates from a nominal 48V input and delivers an isolated 52.5V and
200W at its output with a high efficiency of 92% at full load. IEEE802.3af specifies
an open frame design that allows for better thermal management of the product when
fitted in the end system. It also decreases component stress levels to provide increased
reliability and module longevity. C&D also has a 400W part in development, the
HHS08, for larger PoE systems.
Other useful features of a DC/DC converter that engineers may wish to consider
when designing PoE enabled devices include over-temperature protection, remote
sense and remote on/off and output trimming.
Next generation DC/DC converters for PoE enabled network devices are likely to
include functions such as device to PSE handshaking and Ethernet data isolation
within more integrated packages. This will help simplify the task of developing PoE
devices by decreasing the amount of electronics design work required and reducing
the PCB real-estate needed within the device to give PoE compatibility.
PoE is undoubtedly going to change the way that many pieces of electrical equipment
are powered. The benefits and flexibility that the approach provides are huge and
make it impossible to ignore for businesses of all types. For companies building new
premises or those refitting their existing locations, it is well worth them considering
becoming early adopters of PoE.
Benefit breakdown
The key benefits of a PoE network compared to a network with power supplied by
conventional AC power cords are:
Flexibility to install devices regardless of proximity to AC mains and move
them around to suit changing workplace requirements.
- Safety as no mains voltage is present.
- Low cost of deployment through reduced amount of wiring which need not be
installed by certified electricians.
- Enhanced operational support through simple network management protocol
(SNMP) that allows control of devices remotely to perform functions such as
shut down and resetting.
- Simplified international deployment as manufacturers do not need to satisfy
the different mains power requirements of various countries.
Continuous power is guaranteed, as using a UPS ensures that the entire PoE network is
unaffected by a mains power failure.
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