Hi all,
Chad, it is not that Lennart is confirming something and the rest are reluctant to admit it.
-The fact is Lennart test doesn’t prove that there is no problem: He test one combination of PSE and PD where in the PSE the range for valid was set to its maximum
range which is not the case in all PSEs.
The only proof to show a problem is to calculate the worst case within the spec boundaries which is:
If at the PD we have 25K*0.95=23.75K and in parallel we have 45K, we will have 45K*23.75K/(45K+23.75K)=15.54K<19K which is invalid signature i.e. the detection was polluted (as I explained in my previous mail).
So how you can ignore it?
-The fact is that some people are very
cautious and some are jumping to conclusions because they too eager to approve
PDs with problems without verifying that all is good. In this case the detection range. Nothing will be good if we will exclude backfeed from 3-pair mode in the detection range so please don’t jump to judge people motivation.
-The fact that we
have been producing 4P systems for years and there are millions of ports in the field already and all PSEs tied the positive pair together which means we have millions of 3P ports already in the field and these ports get hooked to all kinds of PDs: ones
with standard silicon diodes, ones with schottky diodes, ones with discrete ideal bridges, and ones with controller based ideal diode bridges etc. doesn’t prove that you didn’t have issues or you will not have issues. Why?
Because:
- You didn’t look for issues since you were not aware of the potential issue in 3-pair mode. 3-pair mode was detected as an issue just during the last plug fest with pre 802.3bt implementations.
- If there were issues, they were treated locally by PSE vendors by changing the valid/invalid thresholds without knowing the real reason.
- There are many ideal diode bridges that under 3-pair, when started with 2-pair, operates well and meet backfeed. Only if they start with 4-pair and then move to 2-pair, then they will
have the backfeed issue too. These means no issues with detection and classification since backfeed was met!
- And more arguments to indicate that your conclusion is scientifically convincing.
You ask: “Does anyone have actual field failures to report?”
Yes, we do. Over the years we got reports of marginal valid/invalid signature and jumping between classes due to inaccuracy issues and we couldn’t put the finger on the root cause since we too where not aware of the 3-mode effect. Now I
start to suspect that it was due to failure to meet backfeed which makes sense to me for sur in detection since we can prove it clearly by calculation.
My opinion:
Backfeed should be met up to 20.5V (21V) in 3-pair mode.
Regards
Yair
From: Chad Jones (cmjones) [mailto:cmjones@xxxxxxxxx]
Sent: Tuesday, May 15, 2018 6:01 PM
To: STDS-802-3-4PPOE@xxxxxxxxxxxxxxxxx
Subject: Re: [802.3_4PPOE] Backfeed ad hoc - 3P detection
EXTERNAL EMAIL
I appreciate that everyone is running off and confirming what I think we already know, it’s just that some are reluctant to admit it.
Lennart points out that measuring one or two of something is statistically insignificant. He’s right, but it does make us feel better.
BUT, we do have statistically significant data. Many of the members on this email reflector have been producing 4P systems for years. There are millions of ports in the field already. I’m not aware of a single system that isolates the positive
rails on the PSE such that they are truly probing via 2P, meaning we have millions of 3P ports already in the field. And these ports get hooked to all kinds of PDs: ones with standard silicon diodes, ones with schottky diodes, ones with discrete ideal bridges,
and ones with controller based ideal diode bridges.
Does anyone have actual field failures to report? I’ve heard Chris Bullock report one and we know that was a poor choice for Vth on a second source FET. Aside from that one, I’m not aware of any other reports of detection or classification
problems from the millions of ports already deployed.
Let’s try to focus on getting consensus on text so that we can finish our work in Pittsburgh and get this standard ratified. Reminder that I’ve promised to be in the bar/lounge area of the hotel Monday night for any one that wants to come
discuss this prior to the meeting.
Thanks,
Chad Jones
Tech Lead, Cisco Systems
Chair, IEEE P802.3bt 4PPoE Task Force
Principal, NFPA 70 CMP3
From: Lennart Yseboodt <lennartyseboodt@xxxxxxxxx>
Reply-To: Lennart Yseboodt <lennartyseboodt@xxxxxxxxx>
Date: Tuesday, May 15, 2018 at 9:27 AM
To: 4PPOE Reflector <STDS-802-3-4PPOE@xxxxxxxxxxxxxxxxx>
Subject: Re: [802.3_4PPOE] Backfeed ad hoc - 3P detection
I checked if this schottky leakage can cause issues with detection.
Attached are measurements of two devices:
- an active bridge controller with 8*NMOS. These transistors turn on above 30V, thus during detection the body diodes serve as the rectifier
- a schottky diode bridge
Three measurements are performed:
- true 2-pair
- 3-pair (2xpositive + 1x negative)
- 3-pair + 45K on the idle mode
Result: detection behaves the same in all cases
Kind regards,
Lennart
On Mon, 2018-05-14 at 21:33 +0000, Joris Lemahieu wrote:
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:
- The backfeed voltage specification has always been a symmetrical 2P test
- 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.
- There is no such thing as a 28uA backfeed current requirement for resistances lower than 100kOhm.
- 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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