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RE: insuring the stability of power delivery

for continuity, I'll reply at the bottom

>-----Original Message-----
>From: R karam [mailto:rkaram@xxxxxxxxx]
>Sent: Thursday, May 31, 2001 12:15 PM
>To: Lynch, Brian; 'Yair Darshan'; 'Rick Brooks';
>Subject: RE: insuring the stability of power delivery
>Comments on Brian's Input
>Adding cable to the mix: The closed loop output impedance of the PSE 
>plus the maximum cable impedance (R and L) must be lower than the 
>closed loop input impedance of the PD plus the maximum cable impedance 
>(L and R)=> Since we do not know the cable length to insure system 
>stability, the closed loop output impedance of the PSE plus the cable 
>impedance must be less than the PD's closed loop input impedance.
>Notice that I have said "closed loop" impedance. This means that the 
>feedback loops of the PSE power supply and PD DC/DC converter plus the 
>load of the PD are included in the calculations. Not an easy task for 
>an open standard.
>[rkaram]  this is either too simple or a mess.  my guess is 
>that in Front of the
>DC/DC we have some Choke and ceramic caps, these are meant for 
>emi control.
>so looking into the PD the AC impedance can be inductive or 
>capacitive based
>on where the choke's resonance is (as its parasitic 
>capacitance and the caps on 
>each side come into play with the inductance) so if we are 
>after some really low
>Z at very low frequencies, then we would need a huge cap at 
>the pd input?
>of course now to make this more fun, if what Yair mentioned 
>(negative Z)
>looking into the DC there goes another twist now...
>a lot to munch on...
[Brian Lynch] You are right that there is probably an LC filter
on the input of the PD for switching frequency as well as high 
frequency filtering. There is also a low frequency aspect of the
input impedance which is partially a function of the closed loop
frequency response of the DC/DC converter and the load. As such,
you cannot state the impedance as "capacitive" or "inductive". You 
simply have to plot the impedance as a function of frequency while
the equipment is operating. You can calculate the impedance, or 
measure it. 

Since PDs can be so varied, I recommend we concentrate on 
determining the closed loop output impedance of a typical 
range of PSE power supplies, and then add the cable (R&L) 
and 3 to 5dB of margin and call that the lower limit for the PD. 

I agree that this is a lot to comprehend, but this is how 
distributed power systems are designed. Any time a DC/DC converter
is powered by a power supply, the same stability criteria has to be met.