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RE: insuring the stability of power delivery

Roger and all,
See my comments below.

> -----Original Message-----
> From:	R karam [SMTP:rkaram@xxxxxxxxx]
> Sent:	ה, מאי 31, 2001 6:15 PM
> To:	Lynch, Brian; 'Yair Darshan'; 'Rick Brooks';
> stds-802-3-pwrviamdi@xxxxxxxx
> 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.
	[Yair Darshan]  It is not a simple task. See my previous two emails.
>   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?
	[Yair Darshan]  Now you understand what I have meant when I said
there is a system issues here regarding the 50uF limitation at the PD. 
		                 The problem raised here is one of them and
the reason that I was favor of larger caps at the PD input is that it allows
	                       even if you need some inductance at the PD
power supply for EMI filtering or you have some inductance in the cable.
				     The Idea is to have  low L/C ratio.
> of course now to make this more fun, if what Yair mentioned (negative Z)
> looking into the DC there goes another twist now...
	[Yair Darshan]  Correct, it adds more complex but it can be solved
by defining the total equivalent PD input closed loop impedance
	                    to be positive when it is added to the PSE
output impedance. (I think now it looks more complicated ....)

> a lot to munch on...
> roger