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RE: insuring the stability of power delivery


See below


>-----Original Message-----
>From: Yair Darshan [mailto:YairD@xxxxxxxxxxxxxx]
>Sent: Thursday, May 31, 2001 10:18 PM
>To: 'Dave Dwelley'; Lynch, Brian; 'Yair Darshan'; 'Rick Brooks';
>Subject: RE: insuring the stability of power delivery
>See my comments below.
>> -----Original Message-----
>> From:	Dave Dwelley [SMTP:ddwelley@xxxxxxxxxx]
>> Sent:	ה, מאי 31, 2001 11:52 PM
>> To:	Lynch, Brian; 'Yair Darshan'; 'Rick Brooks';
>> stds-802-3-pwrviamdi@xxxxxxxx
>> Subject:	RE: insuring the stability of power delivery
>> This is a new wrinkle! I'm not a distributed power supply 
>designer, but 
>> I'll take a stab at this anyway. Shout me down if I don't 
>know what I'm 
>> talking about.
>> At 11:02 AM 5/31/2001 -0400, Lynch, Brian wrote:
>> >In our work with power systems, we have found that to insure
>> >stability of a system, the closed loop output impedance of the
>> >sourcing supply must be lower than the closed loop input impedance
>> >if the load power supply over the frequency band and voltages
>> >of interest. This concept is also discussed and taught at various
>> >universities. VPI and MIT are two I have direct knowledge of.
>> I see this as two problems - startup and steady state.
>> In startup, the output impedance of the PSE power supply is 
>what it is,
>> and 
>> the input impedance of the PD appears in series with 
>whatever is doing the
>> inrush limiting, be it in the PSE or the PD. The PSE power 
>supply isn't 
>> likely to oscillate in this case. The line voltage is 
>another matter - 
>> presumably if the limiter was in the PSE, the line voltage 
>could oscillate
>> (by Brian's theory);
>	[Yair Darshan]  I do not agree to this assumption. If 
>the limiter is
>in the PSE, the PD input voltage can go to zero for some time 
>and then ramp
>	This is not an oscillation it is a normal short circuit 
>when a voltage source is applied to a discharged capacitor.
>	With other simple implementation of the isolating switch, you
>prevent this condition. In any case this condition is not a problem. 
>	The case I present here was tested and confirmed in lab and also
>with simulations. I think (and we need to check with Brian) that he had
>implementation problem and not a conceptual problem. 

[Brian Lynch] I agree that during startup (Inrush in the PSE; when the
input drops to zero when the PD switch closes and then increases as 
the PD bulk charges) that this is not an oscillation. The problems I saw
with the technique were that to insure startup:
	1) The PD cap size needs to be limited 
	2) The DC/DC must be held off until the cap is charged
	3) The startup/charging time needs to be limited
	4) The PD needs energy storage to hold on the switch when the
input goes away.

My thoughts are that even though we can put limits on these parameters in
the spec, they are four parameters that are not needed if inrush limiting
is put in the PD.

The only conceptual issue I see with inrush limiting in the PSE is
that if the PD is not capacitive at its input, there could be oscillation 
on the cable. A non capacitive input is unlikely, but possible if the PD 
does not have a DC/DC converter input stage.

>>  if it was in the PD, the line would stay quiet, but 
>> the node inside the PD between the inrush-limiting pass 
>device and the 
>> input of the PD supply could oscillate. The PD designer can 
>beat this by 
>> holding the switcher inactive until the input cap has 
>reached its final 
>> value, or by balancing impedances properly.
>	[Yair Darshan]  Again, in my opinion you are discussing 
>a problem
>that is not exist. Can any body send a detailed circuit 
>showing a problem ? 
{Brian Lynch] In my simulations, I could put in components which
would make the system fail to start. Only by putting in extra 
constraints on component values could I guarantee startup.