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Re: [802.3_4PPOE] Are diode bridges really needed (2).pptx

Hi all--

  I come to this discussion with energy efficiency as my core interest.
I am also taking the point of view that PoE should be much more successful
in future than in the past in terms of number of ports shipped, number of
devices in use, range of types of devices powered, and number of buildings
that have PoE at all.  That is, existing/legacy installations could be a small
part of the future deployment.

  I am concerned by power overheads on each port that different technical
solutions may have, as they undermine the efficiency advantages that DC
can have.  The simplest way to think about this is what they ultimately cost
the customer in both up-front and operating costs -- at current electricity
prices.  For electricity, simplest to use $0.10/kWh.  This is close to the average
U.S. retail rate (which varies widely across the country and within many utility rates),
is simple to communicate, and enables easy conversion to other rates or currencies.
There are 8766 hours/year so each average watt of extra power costs $0.88.
Assuming a 5 year life of the supply, this is $4.40 per port.  This suggests it is
worth the consumer spending up to $4.40 extra per port to save money at
current prices.

  Obviously there are many complexities.  Supplies likely last more than five
years, but then some ports will likely go unused so these balance each other
out to a great degree.  Some power losses may be absolute per port and
constant; others may be proportional to the current flowing through the port
and so considerably less than the peak on average.  Component costs get
multiplied when they get to final customer cost.

  Backward compatibility and avoiding need for adapters are obviously
serious issues, though there are occasionally reasons to not embrace them.

  I encourage everyone to add this $4.40/W cost to their discussion of
alternatives so we can put the energy consequences of them into proper

  I would hope that the final standard does not require a significant amount
of power loss through a PoE port.  Whether it allows for designs that have
high power loss is another matter.



On Wed, Feb 12, 2014 at 4:11 PM, Picard, Jean <jean_picard@xxxxxx> wrote:

Hi Everyone,


I think there are good arguments on both sides, I’d like to give some information that may be useful to know about.



This is about efficiency, which depends on selecting a typical low cost schottky diode bridge versus selecting an expensive active (FET) bridge.


In the case of 51W PD, a low cost bridge (schottky rectifiers) will introduce close to 0.9 W x 2, which is a real concern for PD equipment makers, who struggle with thermal and efficiency.


If now we look at the whole PSE-PD-cable ecosystem, this has high impact on system efficiency, which partly defeats the purpose of driving 4P.




It is true that using an active bridge could eliminate a high portion of that loss, typically probably 75% of it, but it then becomes a quite expensive solution.


On the other hand, a single rectifier solution can be based on any of these two options:

1.       single schottky diode:  can already remove 50% of that loss with negligible cost,

2.       one-FET solution: eliminate about 90% of the loss with just one FET (much simpler and lower cost) instead a full FET bridge (4 FET).


And we all know how cost-sensitive the PD equipment market is.



I fully understand the concerns mentioned below, and I’m sure they are valid concerns, but I think that efficiency and cost should also be part of the discussion.








Bruce Nordman
Lawrence Berkeley National Laboratory
m: 510-501-7943