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[802.3_4PPOE] FW: [802.3_4PPOE] Backfeed ad hoc - 3P detection



hopefully my e-mail is delivered correctly now with just the measurement slides …

 

 

Joris Lemahieu

Application Engineer

ON Semiconductor

 

 

From: Joris Lemahieu
Sent: Wednesday, May 09, 2018 8:40 PM
To: 'Lennart Yseboodt' <lennart.yseboodt@xxxxxxxxxxx>; STDS-802-3-4PPOE@xxxxxxxxxxxxxxxxx
Subject: RE: [802.3_4PPOE] Backfeed ad hoc - 3P detection

 

Hi Lennart, all,

 

I’ve made some measurements as well.

 

As I already expected:

“When extending the existing ‘symmetrical’ backfeed voltage specification to an ‘asymmetrical’ 3P reflected voltage specification,

a significantly larger reflected voltage value can be expected even with PDs using real diode bridges.”

 

However, I was surprised to see how quickly it can become close to the existing symmetrical limit under 3-pair condition and potentially fail then.

See last two slides of my updated presentation.

 

 

Best Regards,

 

Joris Lemahieu

Application Engineer

ON Semiconductor

 

+32 55 33 29 49

 

 

 

From: Lennart Yseboodt [mailto:lennart.yseboodt@xxxxxxxxxxx]
Sent: Wednesday, May 09, 2018 11:30 AM
To: STDS-802-3-4PPOE@xxxxxxxxxxxxxxxxx; Joris Lemahieu <Joris.Lemahieu@xxxxxxxxxx>
Subject: Re: [802.3_4PPOE] Backfeed ad hoc - 3P detection

 

Hi Joris, all,

 

I've made a measurement on a PD with a schottky diode bridge we have here.

The diode type is the Vishay 30BQ100, a 3A rated diode.

 

The reflected voltage under 3-pair condition is much higher (factor 100x to 500x) than under 2-pair condition.

The good news is that even under high-temperate conditions that bridge meets our backfeed spec, even under 3-pair conditions.

 

At no point do I measure a voltage higher than about 820mV.

 

But... if the intent of the backfeed spec was to limit the current to 28uA, that goes out the window.

With an Rload of 150 Ohms in stead of 100K, we get 70uA at no load condition, and we go into 2mA+ currents at elevated temperature.

 

Specification wise we are OK because the requirement ONLY holds at 100KOhm load resistance.

 

Measurements attached.

 

Kind regards,

 

Lennart

 

 

On Mon, 2018-05-07 at 21:45 +0000, Joris Lemahieu wrote:

Hi all,

 

I believe we are on the right track with the updates represented in the ‘yseboodt_01_0518_backfeed_baseline’.

The only concern I still have is with a “3-Pair PSE” trying to detect a single-signature PD (just for a single port system).

I have the impression the (schottky) diode reverse leakage has not always been taken into account properly:

either just neglected or represented as being a resistor (without additional current source in parallel).

See my presentation in attachment.

My concern is that a PD with real (schottky) diode bridges might become non-compliant to the 3bt standard

and that a “3-Pair PSE” would be allowed not being able to detect a PDs with real (schottky) diode bridges anymore,

due to hard to meet leakage current requirements.

As such I do not have any problem with that, but do we really want to go that far?

 

Next to my presentation, I also attached a spice schematic file for those that would like make simulations.

FYI, the name “3-Pair PSE” is short for “a 4-pair capable PSE providing power in 2-pair mode, whereby two pairs are connected to the positive VPSE, and one pair is connected to the negative VPSE“.

 

 

Best Regards,

 

Joris Lemahieu

Application Engineer

ON Semiconductor

 

+32 55 33 29 49

 

 


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Attachment: Lemahieu_BfdAdHoc_3P detection_v2 - meas.pdf
Description: Lemahieu_BfdAdHoc_3P detection_v2 - meas.pdf