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RE: current limiting of 802.3 power

While there may be good reasons to limit current at the source for safety
reasons, it may be useful to make the distinction between "regulating" the
current in the system  using PTCs vs. using them to "protect" against
overcurrent.  The PTCs are not meant to be used as a current-regulating
device, but rather as an overcurrent protection device.  Under excess
current conditions, a PTC device will go high in resistance, effectively
cutting off all but a few mA of current through the circuit.  It does not
provide a regulated or limited output at, say, 350mA.  

Regarding the use of PTCs for circuit protection, devices such as Raychem's
PolySwitch devices are being used in the IT and computer industry worldwide
for port protection against power faults and port short circuits.  They are
designed to meet or exceed the current limiting requirements for compliance
with standards such as UL 1950 and IEC 60950.  Under faults of several Amps,
they can trip within ms.  While they do exhibit a thermal derating effect
(ie, their carry current decreases with increasing temperature), this is
true of the board traces, and other current-carrying components in the
system.  As long as the appropriate PolySwitch device is selected, taking
into consideration the maximum normal operating current and temperature
conditions, the PTC devices can provide effective resettable circuit
protection and help designers comply with safety standards such as

Lisa Leo
Raychem Circuit Protection
Division of Tyco Electronics

> -----Original Message-----
> From:	Walker, Ed [SMTP:ed_walker@xxxxxx]
> Sent:	Thursday, July 20, 2000 7:42 AM
> To:	'Mike_S_McCormack@xxxxxxxxxxx'; stds-802-3-pwrviamdi@xxxxxxxx
> Cc:	Hemmah, Steve; Pavey, Trevor
> Subject:	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. 
> 		Ed Walker 
> 	Technical Staff 
> 	Analog Field Applications 
> 	Texas Instruments Inc. 
> 		7001 Weston Parkway 
> 	#100 MS4038 
> 	Cary, North Carolina 27513 
> 		919-677-7061 
> 	919-677-7030 fax 
> 	ed_walker@xxxxxx <mailto:ed_walker@xxxxxx> 
> -----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