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Hi,
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???
thanks,
- Rick