[SI-LIST] Re: Gigabit Ethernet Magnetics

Martin,

The information contained in transformer vendors' data sheets is 
related to the IEEE 802.3 spec only, which is usually not a real 
problem (from the transformer point of view).

However, the right selection of the magnetics is also a key factor 
that will determine EMI performance of a product. The relevant 
information is not contained in any data sheets. Factors such as 
common-mode (CM) choke impedance, balance (CM-DiffMode conversion), 
inductance in center tap are key factors, but they are never 
specified in the data sheets. The performance is also in part a 
function of the system-level design, and therefore cannot be 
completely specified by the magnetics vendors. Magnetics that works 
excellent in one application may not be appropriate in a different 
system.

Price vs. performance is sometimes a tradeoff, but more expencive 
doesn't necessarily mean better. The EMI performance often does not 
depend on the magnetics schematic-level configuration, but these 
other factors.

Transformer's function is in the first place to provide high-voltage 
isolation. However, there are many PHYs that require center-tap 
connection to Vcc. Some do it to feed power to the Tx, some just to 
provide bias to the differential pair (but no power to Tx). These 
PHYs do not work properly without the transformers. However, some of 
them may work even without the transformer over short distance, e.g. 
if Ethernet is used only for board-to-board over a backplane 
communication. There may be special design considerations in that 
kind of application, but as long as Ethernet doesn't leave the 
chassis, the high voltage isolation is not required.

Is your question also related to a specific PHY? I recommend that 
you work with your PHY vendor to help you out.

Neven


--- In si-list@xxxxxxxxxxxxxxx, "SI List" <si-list@xxxx> wrote:
> Hello all!
> 
> I have two questions about 1000BASE-T Gigabit Ethernet Magnetics:
> 
> 1) Transformer configuration
> ----------------------------
> There are at least 3 configurations which are commonly used. Some 
use
> one, two or even three transformers per pair.
> 
> Is it a price/performance trade-off?
> 
> Which configuration should be used in which application? My 
application
> would be a Gigabit Ethernet switch. I couldn't find this 
information
> from the manufacturers.
> 
> 2) Transformerless Ethernet
> ---------------------------
> Why is a transformer actually required?
> 
> I have read information that AC coupling with a 0.1uF ceramic 
capacitor
> also works (as in Fiber Channel, Infiniband, etc.).
> 
> The only reason I can see is the 1500V isolation. Is this a 
regulatory
> requirement? Why is this requirement not imposed on Infiniband, 
etc.?
> 
> 
> I am new to this field. Any information is appreciated.
> 
> Is there a good book, or papers, or web pages which would be 
helpful to
> me?
> 
> Best regards,
>   Martin
> 
> -- 
> Enclustra GmbH                   Email: heimlicher // enclustra \\ 
com
> Martin Heimlicher                Phone: +41 43 3431943
> Wibichstrasse 10                 Fax:   +41 43 3431944
> CH-8037 Zurich, Switzerland      Mobile:+41 79 2798183
>  
> 
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