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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

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switch’s pulse transformers, to minimize the length of

high current traces as well as RFI pick-up.

Ground & Power Planes - As the Chipset PoE

solution is a mixed-signal (analog and digital)

circuitry, special care must be taken when routing

the ground and power signals lines. Ground planes

are crucial for proper operation and should be

designed in accordance with the following

guidelines:

•

Separate analog and digital grounds, with

a gap of at least 40 mils.

•

Earth ground is used to tie in the metal

frame of the RJ-45 connectors. This

ground is to be routed separately and

connected to the switch’s metal

chassis/enclosure

•

Only a single connection point is to be

used between the digital and analog

grounds, in order to prevent ground loop

currents. This connection is to be

accomplished near the current loop

senses resistors R9, R11

•

The traces from Vmain input to the port

output, PoE switch (Q4) and sense

resistors must be designed to carry 1A

continuous current

Peripheral Components –

•

To prevent hot spots on the peripheral

components minimum distance of 5mm

should be maintained between the

powering Zener diodes D1 and D3, the

linear regulator U1, sense resistors R9 and

R11 (placed together) and current limit

MOSFET Q4. Other methods may be used

in order to dissipate the heat, such as

thermal copper planes at the top and

bottom layers with heat transfer vias

between them

•

Filtering capacitors for Vcc (U4 pins 5, 15)

are to be located close to these pins

•

The input and output capacitors of the

linear regulator should be placed close to

the regulator pins

•

The trace connecting the power MOSFET

and the sense resistors should be as short

as possible and the current loop sampling

trace should be connected as close as

possible to the sense resistors

Specifying PoE Power Supply_____

An intelligent selection of the power supply to be

used in conjunction with the 1-port system may avoid

the use of additional filtering circuitry (e.g. Front End

Common Mode filter) for standard regulatory

approval.

Electromagnetic compatibility requirements –

Typical Power over Ethernet (PoE) technology

systems are classified in most applications as

Information Technology Equipment (I.T.E).

The I.T.E should comply with three tests in order to

get a standard approval:

•

Conducted disturbance at mains ports (AC

input)

•

Radiated disturbance at a measuring

distance of 10m

•

Conducted common mode disturbance at

telecommunication ports (output port

containing Ethernet+Power)

A relatively new requirement of EN55022

that is mandatory as of August 1st, 2007

In most cases, the PSE side of a PoE system

incorporates a main AC to DC Switching Mode

Power Supply (SMPS) required for delivery of a DC

power to the PoE ports.

This power supply is most likely to be the largest

source for common mode noise injection on the

output ports wires, as its noisy output voltage is

super-imposed on the telecommunication wires by

the PoE control circuitry. The common mode noise is

generated by the high dv/dt and dI/dt switching rates

in power MOSFETs and high speed rectifying

diodes, common in SMPS designs.

As the system designer is usually responsible for

specifying this power supply, it is important to add

the standard requirements to the power supply

specifications and specify a specific reference test

setup to be used for the tests.

This reference test setup should be as similar as

possible to the final product, which means using the

same box material (plastic or metal) and the same

distance and polarity between the SMPS and PoE

circuitry.