Thread Links Date Links
Thread Prev Thread Next Thread Index Date Prev Date Next Date Index

Re: insuring the stability of power delivery

Title: insuring the stability of power delivery
    Your questions are all good. But I believe this over laps some what with the classification process. (Or maybe I am just sensitive because of a real world situation that I have seen.)
    Recently I have been made aware of a situation at an end user site. The PD, (and I use this term loosely to put it in terms of the groups discussion), equipment was specified as watt load of X watts by Manufacturer A. The power engineering was done for Manufacturer B's equipment. It was determined that a system of Y watts was required to hand X*Z loads. (The system actually had 2*Y capacity) Yet in reality during the start up phase of the system the real requirement for power delivery was 4*Y.
    Unofficial investigation showed that indeed Manufacturer A load specification was as stated, except during startup. During start up  the system exhibited a large capacitive characteristics.
And we all know that caps take time to charge, and draw decreasing currents as they do charge.
So initially they look like short circuits. In all other ways the PD works very well.
    Unspoken here is the reaction of the end user.
    This puts me firmly on the side of specifying the behavior of both the PSE and the PD during the start up mode. In addition, (now that I am thinking about this, I was not in St. Louis), should there be some specification for startup during the classification process? (Now I am getting away from the issue.) The point being that System Engineering of the whole Ethernet link could possibly create a system which would run perfectly once it is up. But because of the lack of power up specification would not start. Modeling both the PSE and PD during start up mode would prevent this from occurring.
    Another point. I fully understand the engineering reasons for increasing the cap at the PD. But in the above example this clearly did not help during the start up phase.
    So I have a couple of questions.
    1)    Do we need perfect stability? Or is it possible that some instability can be acceptable? ( I am defining stability as the change in voltage or current over time.) I would guess that getting perfect stability is not possible. But some low bandwidth instability would be possible without impacting system performance. But where would this instability begin to impact the signaling?
    2)    Is there a trade off between instability and cap size, which will allow a minimum size cap to be used?
    Thank you for your input.
David Kohl
----- Original Message -----
Sent: Wednesday, May 30, 2001 7:01 PM
Subject: insuring the stability of power delivery

I just wanted to voice one concern that I have regarding power ramp up, and continuous power modes.
I was reminded of this issue when I was simulating the current limits in the PSE and PD and watching
oscillations on the cable during startup. Yair's Pspice circuit does not have these problems
due to the fact that there are Op-amps in the current limit feedback loops (i.e. band limited).

It seems to me that we do not yet have a defined and specified behavior that will insure that the DTE power will be
delivered in a stable way.
How do we guarantee that a PD from vendor "A" will not oscillate when connected to a PSE from vendor "B"?
The PD as an electrical load needs to be dominated by a capacitive reactance.
I think that we all agree, but this has not yet been specified.

During power ramp up, either the PSE or PD current limit will be in effect, so we need to specify the behavior
during power ramp up.

At the present time, what behavior is specified to insure that the PSE feedback loops,
either to regulate the output voltage, or to limit the current (protection circuits),
will not oscillate with the longest UTP cable or the PD?

It seems that we need to address these issues or we do not have a complete standard.

Some ideas:
1) specify the maximum loop bandwidths of the PSE and PD loops so that any such system will always
behave as a lumped circuit. In other words, spec the loop bandwidth so that it is about 6 to 10
times smaller than 1/lambda of the maximum length UTP cable.
We do not want sudden load changes to cause oscillation or ringing, for instance.
Slew rate helps, but it is a large signal behavior only.

2) specify a minimum phase margin into a "standard" PD load (whatever that is)

3) specify a maximum gain at the UTP bandwidth at the longest cable.

4) both the PSE and PD need to have a specified behavior.

Any other ideas or discussion on this topic?
How about you power guys???

- Rick