Terminations
Hi All:
I have inclosed a very common scheme for the termination of 100 Base
TX interface. You should note that the auto tranformers will conduct
all the D.C. current which will protect the
common mode chokes from saturation. This is good, however, because I
have been hearing from
various people that the D.C. current requirements might be up to 500mA,
I am some what concerned about the heat rise of the Auto transformer/
transformer module which could get fairly hot
with 500mA being pulled thru it. Their is also the question (which
doesn't bother me as much from a magnetics point of view, but more from
a system stand point) power loss in the resistive termination network
i.e. Bob Smith termination. May I suggest that the resistive network be
D.C.
isolated by connecting the center taps of the Autoformers / transformers
to the resistors via capacitors. Their is also the question of
imbalance around the center taps of the transformers/Autoformers,
imbalance in the cable, and imbalance in the connector pins. Under
worst case conditions and short cable lengths, this could add an
additional D.C. bias to the magnetics. This could create some problems
with transformers / Autoformers used in 100 baseTx applications; as
there is already a 350uH min. @ 8mA D.C. requirement for these type of
parts. This of course is aggrevated as the overall load current
increases. At any rate I would be glad to discuss.
Best Regards,
Steve Ellsworth
PHsch.PDF