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Copyright © 2009

Microsemi

Rev. 1.3 2009-02-23

Analog Mixed Signal Group

2381 Morse Avenue, Irvine, CA 92614, USA; Phone (USA): (800) 713-4113, (ROW): (949) 221-7100 Fax: (949) 756-0308

9

When specifying and testing the power supply, it is

important to assure that test requirements are met

under all load conditions and line voltages.

Most switching power supplies designs will exhibit

the highest conducted noise at the maximum load

and the lowest line voltage.

However, other cases may also exist. As it is

impractical to go through all loads span, it is

recommended to test at full load, medium load and

minimum load.

The cables suggested for use are Cat 5 UTP cables

of at least 5m. (Both UTP and FTP cables shall be

tested for compliance in the final system tests).

Note that the standard deals with shielded and un-

shielded cables independently and the appropriate

test procedure shall be performed.

Resistive loads are selected to remove possible

noise injection from the PD side, leaving only the

main power supply as the source of noise.

For radiated disturbance using a proper SMPS

layout (short power patch and avoiding current

loops) may contribute to disturbance reduction.

For additional reduction if needed, full or partial

metal frame connected to mains earth may be used.

Immunity requirements –

An I.T.E should comply with EN55024 in order to get

a standard approval:

Most power supply manufacturers are familiar with

the standard requirements from the power supply

and know how to design the power supply to comply

with it. The requirements from the power supply's AC

input power port are:

•

Immunity to radio frequency continuous

conducted according to IEC61000-4-6

•

Voltage dips and voltage interruptions

according to IEC61000-4-11

•

Surges according to IEC61000-4-5

•

Fast transients according to IEC61000-4-4

•

The PoE circuitry may also be exposed

to immunity tests on the

telecommunication ports such as:

•

Radio frequency continuous conducted

according to IEC61000-4-6

•

Surge line to earth according to IEC61000-

4-5 (usually for port that may connect to

outdoor cables)

•

Fast transients according to IEC61000-4-4

and the PoE enclosure port exposed to:

•

Radio frequency electromagnetic field

immunity according to IEC61000-4-3

•

ESD electrostatic-discharge according to

IEC61000-4-2

To satisfy these tests requirements on the

telecommunication lines, "Y" capacitors to the

chassis (earth) will create a current path to earth.

It is a good practice to have two "Y" capacitors of

10nF/2000V each at the power supply output

positive and negative nodes to earth, to create a

current path for surge generated on the

telecommunication lines (respect to earth).

The system may benefit from two small "Y"

1nF/2000V capacitors, on the PoE positive and

negative output lines close to the output RJ45

connector, by increasing system immunity to ESD.