POWER OVER ETHERNET (POE) enabled devices
and their electronic components are designed
for operation within specified current and voltage
ratings. If these ratings are exceeded due to
short-circuit or voltage transients, components
may sustain permanent damage and the equipment
may fail. Overcurrent and overvoltage
protection devices are used to help protect both
power-sourcing equipment (PSE) and powereddevice
(PD) equipment.
A growing number of PoE applications—ranging
from smart signs, vending machines, building
access control, and time and attendance systems
to phone and PDA chargers and electronic
musical instruments—has created a demand
for more reliable and flexible overcurrent- and
overvoltage-protection devices. These devices
are required in order to:
• protect the PSE from damage caused by shorts
in the Ethernet cable or PD
• protect the PD from faults in the PSE
• protect both the PSE and PD from overvoltage
shorts/transients
In many cases, single-use fuses help to
provide overcurrent protection in PoE applications.
Polymeric positive temperature coefficient
(PPTC) devices, installed in series with electronic
components, also provide a reliable, resettable
method of interrupting current flow. Solid-state
thyristor overvoltage-protection devices may also
be installed in parallel with these components
to switch rapidly from a high to a low impedance
state in response to an overvoltage surge.
The fuse is generally considered one of the
simplest and lowest-cost solutions. However,
many equipment manufacturers find it easy to
justify the cost of resettable PPTC device protection
if it helps protect against overcurrent
damage caused by electrical short, overloaded
circuit, or customer misuse. PPTC devices don’t
generally require replacement after a fault event,
and they allow the circuit to return to normal
operating conditions after the power is removed
and the overcurrent condition is eliminated.
In applications where resettable functionality
isn’t desired, high-current, surface-mount fuses
that provide clean blow characteristics and physically
contain the fusing event within the package
can be used to meet the overcurrent-protection
requirements of the IEEE 802.3af standard. It’s
important to note that single-use fuses must be
tolerant of the current spikes and fluctuations
associated with PoE applications.
Overvoltages caused by switching or lightning
transients can damage PoE-enabled equipment.
However, a variety of methods is available for
protection. There are two major categories
of overvoltage protection devices—clamping
devices and foldback, or “crowbar,” devices.
Clamping devices, such as metal oxide varistors
(MOVs) and diodes, allow voltages up to
a specified clamping level to pass through to
the load during operation. Foldback devices,
such as gas discharge tubes and thyristor
surge suppressors, operate as shunt devices
in response to a surge that exceeds the breakover
voltage.
Foldback devices hold an advantage over
clamping devices. In the foldback state, very
little voltage appears across the load while it
conducts harmful surges away from the load,
whereas clamping devices remain at the clamping
voltage. Therefore, the power dissipated
in the foldback device is much lower than in a
clamping device.
For many PoE applications, the thyristor surge
suppressor is the preferred solution. The thyristor
“folds back” the overvoltage transient to a
lower voltage level than the transient voltage
suppressor (TVS) diode and has lower peak and
average voltage let-through values than the TVS
diode. This results in less overvoltage and power
stresses passed through to the PoE equipment.
In addition, the thyristor’s low on-state voltage
allows for smaller form-factor devices—as
compared with a TVS diode of comparable
energy-handling capability—conserving valuable
printed-circuit-board real estate. The relatively
low capacitance of the thyristor also allows its
use on high-data-rate circuits.
See Associated Figure