[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