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current limiting of 802.3 power

Title: current limiting of 802.3 power

You must limit the current at the source. Otherwise if someone were to short out the cable you could have quite a safety problem. The 350mA limit came from the limits of the PCB traces on the patch panels.

Current must be limited at the source.

I would be very leery of using PTC's. They are very slow, have a wide tolerance, are affected by temperature, and characteristics change over repeated use.

The control the committee wants to have over the power delivery dictates an active circuit that can:

  1. provide a switch in the power path to keep power off the port until discovery has been made.
  2. Slowly apply the power to the load, controlled ramp of current and voltage to prevent ringing due to the inductance of the cable length and to prevent surges being sourced from the PSE. Could effect other units power when connected in parallel if port to port isolation is not required.
  3. Look at the current, once applied, and make sure it stays above a minimum level to detect a device that has been removed and then disconnect power.
  4. Sense overcurrent conditions and provide protection.
  5. Possibly do the detection also, if a simple resistive or diode method is agreed upon.

To sense a minimum current you will already have a current sense circuit at the PSE. Utilizing that same circuit to control maximum output current is very reasonable.

-----Original Message-----
From:   Mike_S_McCormack@xxxxxxxxxxx [mailto:Mike_S_McCormack@xxxxxxxxxxx]
Sent:   Thursday, July 20, 2000 10:04 AM
To:     stds-802-3-pwrviamdi@xxxxxxxx
Subject:        RE: power delivery question from Liaison report

Just my two cents worth . . .

1) I believe, if memory serves me correctly, that the current imbalance that can
be tolerated in the signal transformers is very small and will be the driving
factor in the imbalance that can be allowed and therefore may require activity
to correct the problems found in the wiring, connectors, etc.  (series
resistors, etc.)

2) I feel that the proper place to set the current limit is in the design of the
PD.  It is not unreasonable to expect the designers to take into account their
power budget and live within it when developing the end device.  I believe that
putting a tight margin on the shut off current of the PSE would only be done at
a relatively high cost that really is not necessary.  It seems more reasonable
to me that the PD be designed not to exceed and that the PSE have some
reasonable hard limit above 350 ma where power is removed via a PTC or some
other device that can't quite hit a hard number like 350 ma.

The first item we can probably do some engineering on to determine the right
answer to imbalance, the second item is probably not so clear cut.  I think the
decision about how exact the cut off at 350 ma is and how exactly it is enforced
is something that is more opinion than science.

Mike McCormack