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Hi Steve and all, Please find attached below few comments on
the thoughts presented below: 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. ---------------- 1a. If a wire or a pin will break the data
path will not work due to the fact that the magnetic will immediately saturate.(Lets
assume that at the worst case all pairs are data pairs such in 1G) There fore assuming the above condition may
not be considered as a worst case that we have to handle since it is a case that
the system is not working and we don’t care what happens as long as we
are not causing damage. Regarding causing damage: If one conductor is break, we will have twice
the current going through the other conductor and if we keep the TLIM/TOVLD and
duty cycle than even for 2-4 times the current overload/Ilim numbers we probably
will not have damage issue due to the averaging factor over time. Connector pin according to its spec need
to meet 750mA at Regarding safety issue: There is no safety
issue here in terms of human safety. There is an issue of "Infrastructure
safety or equipment safety" in terms of damage however as mentioned above,
with TLIM/TOVLD and duty cycle limitation the equipment is kept un damaged and
"safe". 1b. Yes, this is one of the major issues
in 2P. -------------- 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? ------- 2a – We don’t have a mandate
to define safety (human safety) in our standard. We need to address
the reader to current safety standard.
We should address issues that have impact on keeping the equipment and infrastructure
undamaged. 2b – I agree that we should look for
the worst case HOWEVER the worst case scenarios should be real cases and not
corner or theoretical cases in order not to increase the cost of solution or
eliminating some implementation or support some applications. Regarding the example presented: I guess
that it is not the worst case since in aircraft designs there are a lot of
margin in the design (Aircraft…) so in my opinion it is not representing
the worst case. -------- 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. ------ 3a. The intent is not clear. Do you mean current
balance in 4P? If this is the case than Yes, we need to address this
issue and your suggestions sounds reasonable. 3b- Active and passive current balancing
to the magnetic was discussed in the AF and we didn’t want to force this
requirements in the standard due to the following reasons: -
The vendor should keep his system working under his equipment spec.
How he does it is implementation specific. -
Forcing a requirement for active/passive current balancing will
increase the port cost while in most cases it is not required (above some
cabling length there is no issue) Actually we address this issue in the
informative part of the standard and let the designer to decide how to handle
the worst case situation. In our new project doing active current
balancing will dramatically increase the port cost so the the 2P concept will
cost much higher that the 4P… In addition as you have mentioned, the heat
issue still exist. Bottom line: In my opinion: Current balancing
is not cost effective -------- 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 infeasible because of safety. --- 5. Agree. From complexity wise, the is no
difference between 2P and 4P high power and in my opinion 4P from system point
of view will cost less than 2P and will be more reliable by far. (Regarding safety: again
there is no safety issue here (the system may not work… but no safety
issue)) ---- 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 independent 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. ----- 6. Basically I agree with this concept
which is what the 4P concept (part of it) represents: Let's take two improved
AF power channels, combined them together to have twice or more the power by
having the 4P.. The problem that I have with part of your
suggestion is that it will allow at the PSE side, PSE that supports only 2P
high power and PSE's that support only 4P which leads to the undesired
environments were we have many different PSE's and PD's that will cause us
complex system. I suggest looking at the 4P as the only HP
concept that will address all PD types. By using this approach, the 4P design will
be efficient, and simple. And when it will be integrated, the
difference in chip level between 4P and 2P will be small enough to stay cost
effective. From system point of view (cost,
reliability, flexibility etc) 4P will represents better cost numbers than 2P. Yair From:
owner-stds-802-3-poep@xxxxxxxx [mailto:owner-stds-802-3-poep@xxxxxxxx] On Behalf Of Steve Robbins 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 |