Re: [8023-POEP] 2P v 4P and safety

[Maurice Reintjes <maurice.reintjes@xxxxxxxxxxxxx>]

Hi Steve:  All very good thoughts
and points, particularly about the flying applications.
How about  Active
Balanced Current (ABC) instead of Active Current Balance (ACB)?
The acronym seems to have a better cadence...

Best Regards

Maurice Reintjes
MindspeedTM
Hillsboro, Oregon,USA
Office Phone (503)-914-5370
Mobile (503)-701-0797

Steve Robbins <srobbins@xxxxxxxxxxx> Sent by: owner-stds-802-3-poep@xxxxxxxx 05/23/2005 09:50 AM Please respond to Steve Robbins

To:        STDS-802-3-POEP@xxxxxxxxxxxxxxxxx         cc:                 Subject:        [8023-POEP] 2P v 4P and safety

PoE Plus Team,
 
Here are some thought I had over the weekend
regarding 2P vs. 4P, and safety.  I checked with Mike, and he wrote
that it's okay to post this on the reflector and get a converstation going.
 
1.  There are basically two roadblocks
to higher power:
    a.  The limited amount
of current that can be carried on a single wire (not a pair of wires).
         We must
assume that sometimes a wire will break, or a connector pin will go bad,
         so all
the current will be on one wire instead of a pair.  This fault condition
must define
         the limit
on how high I_CUT can go, for safety.
    b.  The affects of heating
and current imbalance on the magnetics.
 
2.  To make sure we get it right in
terms of safety, I propose:
    a.  The objective regarding
the goal of higher power should be amended to include
         a few words
about safety.
    b.  Let's look for a worst-case
scenario to keep in mind while we work on the standard.
         The best
one I've thought of so far is the airline industry.  Assume PoE is
used onboard
         a jet so
that passengers can surf the net or watch DVDs in flight, without the batteries
         in their
laptops dying.  How hot do you think the FAA will allow the wires
to get?  How
         much current
can a single conductor (in a bundle) carry before it reaches that temp?
 
3.  I thought about Clays presentation
a lot over the weekend.  It seems to me that reaching
     higher power (>30W)
will require two things:
    a.  Current imbalance
sensing to detect broken wires, so we can increase I_CUT beyond
         what a
single conductor can handle, without compromising safety.
    b.  Active current balancing
to keep the magnetics happy.  Although this won't solve the
         heating
problem.
 
4.  Maybe it's time for an acronym here.
 How about Active Current Balance (ACB) and Current Imbalance
     Sensing (CIS).  They
seem to go together, so "ACB/CIS"?
 
5.  Compare the complexity of a 2P and
a 4P system that carry equal power.  The 2P system
     would need about twice
the current per conductor as the 4P system.  So, while the current
on
     4P system might just
be low enough to avoid the need for ACB/CIS, the 2P system would almost
     certainly need these
features.  So the complexity difference between 2P and 4P may not
be as
     big as previously thought,
at least in some medium power range.  For much higher power, 4P
     would require ACB/CIS
as well, but 2P becomes infeasable because of safety.
 
6.  Suppose PoE Plus allows both 2P
and 4P.  How do we define this without getting into several
     different types of PSE
and PD, with all the complexity of hooking them together?  I think
the
     answer is simple:  We
define a 2P system, and then say that it's legal to have one or two of
these
     on the same cable.  So,
two independant 2P systems make a 4P system.  I think this is really
the
     only way to go, but of
course I'm open to other ideas.
 
 
Steve Robbins