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Re: [802.3_4PPOE] Ilim/Tlim Ideas



Hi Pete,

Please see below my inputs.

Yair

 

From: Peter Johnson [mailto:peter_johnson@xxxxxxxxx]
Sent: Friday, April 27, 2018 12:12 AM
To: STDS-802-3-4PPOE@xxxxxxxxxxxxxxxxx
Subject: [802.3_4PPOE] Ilim/Tlim Ideas

 

EXTERNAL EMAIL

Lennart:

 

Finally spent a few more minutes thinking about Ilim/Tlim.

 

A “short circuit” condition is basically any condition that causes the PSE output to meet or exceed Ilim  (Ilim, a PSE-specific variable, of course).

Yair: Agree.

I agree, and pointed out in some rejected comments, that there is a fundamental inconsistency in text in the following 3 sentences:

 

From 145.2.10.8:

 

If a short circuit condition is detected on a pairset, power removal from that pairset shall begin within TLIM as defined in Table 145–16.

(I’m taking Tlim  to be a PSE-specific variable here that has a well-defined range.)

Yair: This text was meant to be valid for PSE operating voltage. So, I suggest to clarify it in this way.

 

If IPort-2P exceeds the PSE lowerbound template, the PSE output voltage may drop below VPort_PSE-2P min.

Yair: I believe that this text is confusing and redundant. I dont remember why we add it?

 

 

A PSE in a power on state may remove power from that pairset without regard to TLIM when the pairset voltage no longer meets the VPort_PSE-2P specification.

Yair: This text is OK if you clarify the 1st and address the 2nd one and it is very important for PSEs.

The final of these 3 sentences washes out any behavior implied by the first 2 sentences.

Yair: See above.

The practical truth is, a PSE limiting output current will drop outside the Vport_PSE-2P specification in the vast majority of cases.

Yair: Correct. This is because the distance between the nominal PSE voltage to the minimum PSE voltage is lower in percentage compared to ILIM-2P/ILIM_2P_min

Sentence #2 addresses this.   No doubt, this then puts strain on the PSE controller as it dissipates power.   That’s why sentence #3 exists.

Yair: Agree, so what if we delete sentence #2?

A further practical truth is that many PSE’s that hit their Ilim threshold will sustain power, perhaps with current foldback to preserve the PSE controller, until at least Tlim_min.   Other PSE’s will implement policy that when in current limit and voltage drops, power goes off immediately without regard to Tlim_min.    802.3at essentially enabled this duality and 802.3bt is continuing it.

 

I do see that sentence #3 here is completely redundant to the sentence in 145.2.10.1:

 

A PSE in a power on state may remove power from a pairset when the pairset voltage no longer meets the VPort_PSE-2P specification.

 

I would propose that we don’t need this sentence given the overlap.

Yair: For sure we dont need to sentence that say the same but I am not sure which is redundant. Need to think which to remove and which to keep.

--------------------------

Finally, I would propose setting Tlim_max in Table 145-16 to hard number 75 msec.   I don’t think this would break anything and it would eliminate the absurd case of a PSE operating permanently in a current limited state.

Yair: Make sense to me.

Regards,

Pete J

 

Peter Johnson

Sifos Technologies

1061 East St.

Tewksbury, MA 01876

www.sifos.com

 

Sifos is relocating on June 1, 2018 to:

1 Technology Drive

Suite 100

Andover, MA  01810

 

 

 


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