Power-over-Ethernet (PoE) Application Notes
PoE - the Cost-Effective Means of Power Supply
In addition to data communications between the two devices, Power-
over-Ethernet (PoE) enables an Ethernet Switch/Hub to power a
remote device over an existing LAN cable. Without PoE, any remote
device needs to be powered independently. Typically, device power is
derived from an AC-DC adapter or at board level inside the device.
This adds cost and requires locating the device near a source of AC
power.
Device installation is further complicated by different safety codes,
AC voltages and wall plugs around the world. PoE avoids these
problems by supplying low-voltage DC power directly to the
Powered
Device
(PD) from the
Power Sourcing Equipment
(PSE) that is located
in the Ethernet Switcher or mid-span Hub and eliminates the
complexity of AC power sources.
To ensure that all equipment is protected and devices operate
properly when connected, the IEEE established a universal standard
(IEEE802.3af). This specifies the power requirements of the PSE and
the PD, and the communication protocol between the two -
regardless of manufacturer or configuration.
Traditional Power-over-Ethernet
The 12.95W power limit was adequate for IP telephony when
IEEE802.3af was initially approved in 2003. Network cameras, were
initially the most prevalent application enabled by the Power-over-
Ethernet standard.
However, the popularization of WLAN applications triggered
developers’ and marketers’ inventiveness for creating more power-
hungry derivative devices, such as Video IP Phones, Multi-channel
WLAN Access Points (like the ones in the IEEE802.11n upcoming
MIMO standard) and Pan-Tilt-Zoom IP Cameras.
All these devices typically require between 13W and 30W - more
than the normally available 13W.
Enhancing Power-over-Ethernet
In December, 2004, the IEEE802 established a PoEPlus group to
study the market needs and possible technical solutions to address
higher power applications while maintaining backward compatibility
with the original standard. In September, 2005, the group set and
approved, as binding, objectives for the IEEE802.3at Power-over-
Ethernet Enhancements. One of the main objectives was providing
maximum power to PDs as allowed within practical limits of at least
30W.
Converter Topology Choices:
Flyback Vs. Forward
Flyback
or
forward
converter topologies are normally used in a
powered device DC-DC converter, depending on the PD’s voltage and
current requirements.
Flyback Topology
(See Figure 1)
Benefit: Lowest cost for <6A output currents.
When to Use: Typically for output voltages greater than 2.5V.
Recommended for multi-output, voltages (>12V) and for applications
requiring lowest cost.
Advantages:
• Output isolated from input.
• No output inductor required.
Disadvantages:
Higher output ripple current results in higher output capacitor cost
and medium efficiency due to higher peak and rms currents.
Input
+
Vin
v
L
_
Vx
+
Output
+
C
Vo
_
_
Figure 1. Transformer Flyback Topology
COOPER [ COOPER BUSSMANN, INC. ]
