5 Things You Need To Know When Selecting Ethernet Magnetics

October 11, 2016 // By EDN Europe
By Adam Chidley, Avnet Abacus
Magnetics are Not Optional

Magnetics are part of the Ethernet specification for xBASE-T networks, and as such, they are required for all Ethernet enabled equipment. The magnetics protect against faults and transients, including rejection of common mode signals between the transceiver IC and the cable. These signals are commonly caused by electromagnetic interference (EMI), either from noise picked up by the cable or from slight impedance mismatches. The magnetics also provide galvanic isolation from Ethernet cables, and offset any DC biasing caused by connected nodes having been powered from different sources.

The properties that Ethernet magnetics need to have are specified in the standard – for example, a transformer for a 100BASE-T connection is required to have an inductance value of at least 350µH when a DC bias of 8mA is applied. Since these attributes are fixed, it may seem like Ethernet magnetics components are all the same. In fact, there are different types of solution available that suit different applications.

Discretes and Integrated Magnetics Both Have Pros and Cons

Specifying your own Ethernet transformer module to sit between your device’s PHY chip and the RJ45 jack you’re using means you can boost electrostatic discharge (ESD) protection since the placement and layout is up to you – in this case, the distance between the components in the Ethernet front end should be maximised. Discrete transformer modules are also marginally cheaper than the integrated alternative, which will appeal to some high volume applications. On the other hand, more design work is obviously involved.

The alternative is connector jacks with integrated magnetics, called Integrated Connector Modules (ICMs). ICMs are usually slightly more expensive than going the discrete route, but they are very quick and easy to design-in and provide assurance that the connection will comply with Ethernet standards. They are very compact, helping save valuable board area if space is tight by combining as many as 10 discrete components. Component count is lower, which has an impact on assembly costs, but do note that any fails during production test incur higher rework costs than discrete modules would.

The Bel Magnetics S558-5999-Q2-F is a dual channel 1000BASE-T LAN magnetics module while the M8-F (also pictured bottom left) is single channel

How to Compare Discrete Magnetics Modules

So you’ve decided to use a discrete magnetics module – how do you select one? Checking the literature for the PHY chip in your design will often reveal the manufacturer’s recommendations based on specific modules they have tested alongside their own chip. This is an excellent place to start!

Beyond that, when comparing transformer modules there are several figures which can be considered to get an idea of the performance of individual parts. First, have a look at the common mode rejection figures, which indicate the modules’ ability to filter out EMI from the received signals. Some Ethernet transformer modules also include a common mode choke for additional filtering, which may be desirable. Another figure of merit for comparison is the amount of crosstalk between transmit and receive channels; effective shielding between the channels should help keep this to a minimum. If your