Good comment, Heath.
From: "Stewart, Heath" <Heath.Stewart@xxxxxxxxxx>
Reply-To: "Stewart, Heath" <Heath.Stewart@xxxxxxxxxx>
Date: Tuesday, May 15, 2018 at 5:58 PM
To: "STDS-802-3-4PPOE@xxxxxxxxxxxxxxxxx" <STDS-802-3-4PPOE@xxxxxxxxxxxxxxxxx>
Subject: Re: [802.3_4PPOE] Backfeed ad hoc - 3P detection
We have been chewing on this issue quite a bit. Our main consternation stems from a concern that we do not want to compromise detection and/or connection check compliance at the PD.
The argument today is, if a given technology, eg active bridge or Schottky bridge, can meet IEEE compliance, its use should not be precluded.
When reviewing PD shalls and PICS it has become clear that the PD signature requirements can be interpreted as two-pair (one P, one N) tests only. This opens the door for sub-optimal implementations
of otherwise valid technologies to introduce interoperability issues.
In short, we would be open to compromise on backfeed specifications if the attached text changes were part of the proposed set of changes.
These changes explicitly require PD signature shalls to be met in in both 2-pair and “3-pair” configurations.
Consider, af/at PDs are not subject to 3-pair detection criteria. However, if an af/at PD is mis-binned as a single or dual signature PD it will still receive its full power allocation. However,
if a SS Class 5-8 PD is mis-binned as a dual signature PD it can claim only Class 4 power. Thus bt PDs need to meet signature criteria in both 2 and 3-pair detection environments.
Cheers,
Heath Stewart
Design Center Manager
PoE/PoDL Products
Cell (805) 895-0499
Office (805) 560-7658
Websites analog.com, linear.com
Linear Technology is now part of Analog Devices. Learn
more.
From: Lennart Yseboodt [mailto:lennart.yseboodt@xxxxxxxxxxx]
Sent: Tuesday, May 15, 2018 5:00 AM
To: STDS-802-3-4PPOE@xxxxxxxxxxxxxxxxx
Subject: Re: [802.3_4PPOE] Backfeed ad hoc - 3P detection
Hi Shiyong,
Your measurements are very similar to what I'm seeing.
The voltage goes up by a large amount when a second positive pair is connected.
Since we've only sampled one bridge of one type, all these results are statistically insignificant to derive wheter there really
is an issue or not.
What we can say is that a 3-pair backfeed test is completely different from a 2-pair backfeed test, as it was originally written.
Lennart
On Tue, 2018-05-15 at 08:00 +0000, Fushiyong wrote:
Hi Joris,
Do you have test data for 21V and 30V?
Attached please see my test data Which shows that the Vbfd (0V~57V)of Schottky diodes bridge have no problem under 3p.
Thanks.
Best Regards
Shiyong
发件人: Joris Lemahieu [mailto:Joris.Lemahieu@xxxxxxxxxx]
发送时间: 2018年5月15日 5:34
收件人: STDS-802-3-4PPOE@xxxxxxxxxxxxxxxxx
主题: [802.3_4PPOE] Backfeed ad hoc - 3P detection
Hi all,
please find attached my updated presentation from today.
Note that the 3P reflected voltage at 10V is larger than the 2P backfeed voltage at 57V (see measurements).
That would indicate that the new 3P reflected voltage requirement (even) at 10.1V would be harder to meet than the current 2P backfeed voltage at 57V.
Why then even push this further to 20.5V and still holding on to the very low 2.8Vrefl threshold.
You can never convince me that +0.028mA is the very critical threshold for classification to work under 3P.
For class signature 0/1/2 the PD current can change ±1.5mA from nominal and at the PSE side it can even change ±2.5mA from nominal.
Existing (single-signature) Type1 and Type2 PD’s were never designed to support 3-pair detection and 3-pair classification, only real 2-pair.
Some of those T1, T2 PD’s will already turn on their bottom mosfet even in the detection range if supplied by 3-pair.
For the success of the 3bt standard, it is important 4-pair capable PSE can successfully detect and classify as many ‘unexpected’ PD implementations as possible.
Now it seems like all PSEs will suffer and that PSEs are not able to act anymore. This is not the case.
If the 4-pair capable PSE can switch the positive pair, there will not be an issue during detection and classification.
If the PSE detection circuit is restricted to the first quadrant (due to series diode D2 in figure 145-20), there will not be an issue during detection and classification even if the PD would reflect the full PD voltage.
There are still things a 4-pair cable PSE that does not switch the positive pair and that does not have series diode D2 could do to keep the margin on the classification current even if the PD would reflect the full PD voltage:
· the PSE detection source can be disabled (made Hi-Z) on the secondary alternative.
Nowhere in the 145B.1 CC_DET_SEQ timing diagrams Det is active on the secondary alternative when Class is active on the primary alternative !
· the PSE could adjust the voltage of the voltage source on the secondary alternative.
If the PSE really wants to keep the additional current at (or below) +0.028mA during classification it could adjust the voltage source value on the secondary alternative to:
§ 2.8V (or less) below the Vpse on the primary alternative for Zsource=100kOhm
§ 1.26V (or less) below the Vpse on the primary alternative for Zsource=45kOhm
Best Regards,
Joris Lemahieu
Application Engineer
ON Semiconductor
+32 55 33 29 49
From: Joris Lemahieu [mailto:Joris.Lemahieu@xxxxxxxxxx]
Sent: Monday, May 14, 2018 3:51 PM
To: STDS-802-3-4PPOE@xxxxxxxxxxxxxxxxx
Subject: Re: [802.3_4PPOE] Backfeed ad hoc - 3P detection
Hi Yair,
these measurements clearly indicate:
1. The backfeed voltage specification has always been a symmetrical 2P test
2. If the symmetrical 2P backfeed voltage is extended to an asymmetrical 3P reflected voltage, a larger threshold then 2.8V would need to be taken into account for higher operating voltages.
3. There is no such thing as a 28uA backfeed current requirement for resistances lower than 100kOhm.
4. A 4-pair capable PSE that does not switch the positive conductor of the other mode during detection will influence the detection of a single-signature PD.
Best Regards,
Joris Lemahieu
Application Engineer
ON Semiconductor
+32 55 33 29 49
From: Yair Darshan [mailto:YDarshan@xxxxxxxxxxxxx]
Sent: Sunday, May 13, 2018 2:17 PM
To: Joris Lemahieu <Joris.Lemahieu@xxxxxxxxxx>; STDS-802-3-4PPOE@xxxxxxxxxxxxxxxxx
Subject: RE: [802.3_4PPOE] Backfeed ad hoc - 3P detection
Hi Joris,
What do you suggest as a result of these measurements?
Yair
From: Joris Lemahieu [mailto:Joris.Lemahieu@xxxxxxxxxx]
Sent: Wednesday, May 9, 2018 10:01 PM
To: STDS-802-3-4PPOE@xxxxxxxxxxxxxxxxx
Subject: [802.3_4PPOE] FW: [802.3_4PPOE] Backfeed ad hoc - 3P detection
EXTERNAL EMAIL
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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