SCHURTER Applicaiton Note: Power over Ethernet0 pages

Power over Ethernet

Application Note

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Overcurrent protection for Power
over Ethernet (PoE)
Transmitting both power and data over a single cable is an ingenious and simple idea
– a “plug & play” solution. IEEE is working on a new standard called PoE++, which will
define powering devices with up to 60 watts. As the amount of current increases, so
does the danger of fire. Thus it is essential to protect electronics from overcurrents, and
SCHURTER offers just the right fuses to provide this protection.
Data networks based on the Ethernet standard
are in widespread use around the world. Just a
few examples of devices tied into these
networks include sensors, IP phones, card
readers, remote-controlled webcams, point of
sales (POS) terminals and small hubs and
servers. Since the Ethernet standard was
introduced, data rates have continually
increased and today are reaching speeds up to
100 Gigabits per second. Twisted pair cables
guarantee secure data transmission even over
long distances. Ethernet cable is also
increasingly being used to supply devices with
power in addtition to data over a single cable.
This eliminates separate power cables and
power supplies making devices standalone
without the need for connection to the wall
outlet or grid. There is also no need for a
certified technician that might otherwise be
required to install mains power. Ethernet power
is less than 50V and lines can be up to 100
meters long. This makes it possible to install
loads where power cables are not desirable or
permitted. In addition, thanks to smart power
management, this type of power transmission
can save electricity by turning off devices that
are in use.

IEEE 802.3af-2003 limits the power
consumption of a powered device (PD) to
12.95 W or 360 mA. Here the power sourcing
equipment (PSE) must supply 15.4 W or 400
mA. The input voltage is at least 44 VDC and a
maximum of 57 VDC (see Fig. 1).
PSE

PD
4

4

5

F

1
TX

2

RX

F

1
RX

2
SIGNAL

3
48 V

Overcurrent protection

5
POWER

6

DC/DC
Converter

3
TX

6
SIGNAL

7

7

8

8
POWER

Fig. 1: Standard configuration of PoE and
PoE+: two separate conductor pairs for
power and signals. Chip fuses (F) protect
the electronics simply and securely from
overcurrents (Source: SCHURTER AG)
With IEEE 802.3at-2009, a PSE can supply
30 W or 600 mA and a PD can consume up to
21.9 W of power. Here the power is fed over all
four cable pairs. The input voltage is at least 50
VDC and a maximum of 57 VDC (see Fig. 2).
PSE

PD
4

F
F

5
1

TX

48 V

RX

F

5
POWER

2
3

48 V

4

6

F

1
2

SIGNAL + POWER

RX
DC/DC
Converter

3
6

TX

SIGNAL + POWER
7

IP phones – simple to connect and supply
with power using Ethernet cable (Source:
shutterstock)

Standard IEEE 802.3
The standard for combining the transmission
of data and power has been around since
2003. There are two different types of
standards:

7

8

8
POWER

Fig. 2: Maximum power with PoE++ is up
to 90 W over four conductor pairs: 2x
power + signal and 2x power (Source:
SCHURTER AG)

Circuit Protection

IEEE is working on a new standard called
PoE++. It specifies power up to 60 W with
expansion capability up to 90 W.
More power, however, also means higher risk
due to the associated temperature rise.
Suitable components must therefore be
selected with great care.

The higher the current, the faster malfunctions
can lead to fires. Suitable devices to protect
against overcurrents must be installed in both
PSEs and PDs (see Figs. 1 and 2).
The simplest, most cost-effective and reliable
solution for protection against overcurrent is a
chip fuse. Midget fuse is a known standard for
UL & CSA size 10x38mm. This would not be the
appropriate term to use to describe small. In
addition, chip fuses take very little space. The
USF 0603 chip fuse from SCHURTER is the
ideal fuse for PoE applications. This thin-film
USF 0603 fuse measures just 1.6 mm long, 0.8
mm wide and only 0.6 mm high. It offers
overcurrent protection with super-quick-acting
characteristics in a range of rated currents from
500 mA to 5 A. Rated voltage is between 32
and 63 VDC. It stands out due to a breaking
capacity of 50 A at 32 V over the entire range of
rated currents. Even today, such breaking
capacity is unmatched in fuses of this size.
Further, its voltage drop of a low 65 mV makes it
unique. In addition, thanks to a design using
planar technology, it has no impedance
discontinuity and thereby guarantees better
signal quality and higher data rates. The USF
0603 has a permissible ambient temperature of
90 °C and is approved to UL 248-1. Very legible
lettering enables the visual identification of the
rated current. It is supplied in packaging well
suited for automated production according to
EIA-RS481 and IEC 60286-3 with either 5000
or 15,000 units per roll.
Additional fuses suited for overcurrent
protection in PoE applications on offer: the
USFF 1206 and USF 1206 (see Table 1). All
fuses mentioned are approved to CURus.

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