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RE: insuring the stability of power delivery




Yair et al,
 I think there's problems with specifying that the PSE will supply 500mA for
100mS max.  This causes significant heating.
An alternative is to say that the PSE will supply the full rated current
(i.e., 450mA for 50ms every 1000ms per the May meeting) for PD voltage
greater than, say, 30v.

Bruce Inn


> -----Original Message-----
> From:	Yair Darshan [SMTP:YairD@xxxxxxxxxxxxxx]
> Sent:	Monday, June 04, 2001 5:39 PM
> To:	Schwartz, Peter; G?rard Vergnaud; Yair Darshan
> Cc:	stds-802-3-pwrviamdi@xxxxxxxx
> Subject:	RE: insuring the stability of power delivery
> 
> 
> Peter,
> The time limit for inrush current is needed when the inrush current limit
> is
> set to the max. value say 0.4-0.5A.
> In this case with 470uF, the charging time+ power supply startup time is
> 100mSec max. 
> 
> For much lower current , more time is needed as per your calculation, and
> I
> don't see a problem with this since the time limit is a must for the max.
> peak current 
> in order to limit power loss and allow integration of Mosfet in the chip
> etc. 
> 
> In other words, we could specify that during startup the PSE will supply
> 0.5A max. for 100mSec max. and after 100mSec the power will be limited to
> the normal powering mode parameters. Your case is much below the normal
> powering mode parameters, thus it could be for infinite time.
> 
> I hope that I understand you correctly.
> 
> 
> Yair.
> 
> 
> > -----Original Message-----
> > From:	Schwartz, Peter [SMTP:Peter.Schwartz@xxxxxxxxxx]
> > Sent:	,  04, 2001 6:06 PM
> > To:	G?rard Vergnaud; Yair Darshan
> > Cc:	stds-802-3-pwrviamdi@xxxxxxxx
> > Subject:	RE: insuring the stability of power delivery
> > 
> > Gerard, Yair, and all:
> > 
> > My only concern in this message string is with Item #3 (Charge-Up time
> for
> > a
> > 470uF capacitor).  If foldback current limiting is to be allowed during
> > start-up as a means of reducing peak MOSFET power dissipation, I would
> > recommend lengthening the time allowed for charging a 500uF nominal
> > capacitor to:
> > 
> > T = (C x V)/I = (470uf x 1.2)(44V)/0.10A = 248ms = 250ms maximum.
> > 
> > Comments are appreciated...
> > 
> > Peter Schwartz
> > Applications Engineer
> > Micrel Semiconductor
> > Phone:	408.435.2460
> > FAX:	408.456.0490
> > peter.schwartz@xxxxxxxxxx
> > 
> > 	-----Original Message-----
> > 	From:	Grard Vergnaud [SMTP:gerard.vergnaud@xxxxxxxxxxxxxxxxxx]
> > 	Sent:	Sunday, June 03, 2001 14:04
> > 	To:	Yair Darshan
> > 	Cc:	stds-802-3-pwrviamdi@xxxxxxxx
> > 	Subject:	Re: insuring the stability of power delivery
> > 
> > 	Yair and all, 
> > 
> > 	I closely agree with the Yair statement : 
> > 	                1) The PD cap size needs to be limited 
> > 	                 I think, as Yair, that the only need is to limit
> > the PD input cap 
> > 	                 with a value less than 470F. With this value I
> > agree, we 
> > 	                 will cover more than 95% of the applications. If
> > any PD 
> > 	                 need more than 470F, we have to specify the inrush
> > 
> > 	                 current limiting will have to be taken into acoount
> > at the PD side. 
> > 
> > 	                2) The DC/DC must be held off until the cap is
> > charged 
> > 	                Sure, DC/DC converter must not start before the cap
> > get a 
> > 	                sufficient charge level. But, as Yair said, this
> > function is most 
> > 	                of time provided by PWM chip. 
> > 
> > 	                3) The startup/charging time needs to be limited 
> > 	                I completely agree with Yair statement, I'm thinking
> > 
> > 	                that with an input cap less than 470F the start-up
> > time 
> > 	                for a PD should easily less than 100ms. 
> > 	  
> > 	  
> > 
> > 	Yair Darshan a crit : 
> > 
> > 		Brian and all, 
> > 		see my comments below. 
> > 		Thanks 
> > 		Yair. 
> > 
> > 	
> > =================================================================== 
> > 		        [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. 
> > 
> > 		        Yair Darshan: The problems you saw when the inrush
> > current is in the 
> > 		PSE are actually not a real problems and I try to explain
> > why: 
> > 		        Correct, the input cap needs to be limited. 
> > 		        There is no need for higher cap than 470uF for more
> > than 95% of the 
> > 		applications that we see using the Power over MDI
> > technology. Standard 
> > 		switching power supply working at 100Khz and supporting
> > 12-15W can work well 
> > 		with 470uF at their input with large margin. 
> > 		        This statement is tested and verified in the field
> > many times. 
> > 		        Other applications that needs more than 470uF will
> > be supported by 
> > 		the PD inrush current limiter. 
> > 		        The point is that there is no reason to increase the
> > PD cost for for 
> > 		95% of the application while we have the hardware to do this
> > function in the 
> > 		PSE for protection, current sensing switching the port to
> > off etc. 
> > 
> > 		                2) The DC/DC must be held off until the cap
> > is charged 
> > 		        Correct, however, you need to do it anyway if you
> > want to increase 
> > 		your design reliability by preventing the power supply from
> > turning on at 
> > 		low voltage and drawing much more current that it needs.
> > I=P/Vin,  P is 
> > 		constant.... 
> > 		        You need this function to ensure startup and working
> > on the stable 
> > 		region as I showed in earlier presentations. 
> > 		        Most of the commercial PWM controllers have the UVLO
> > function any 
> > 		way for the above reasons. 
> > 
> > 		                3) The startup/charging time needs to be
> > limited 
> > 
> > 		        Why it is a problem? In light of May discussion it
> > seems that we 
> > 		want to decrease the time for single port insertion and this
> > is one of the 
> > 		components that sets this time. 
> > 		        If for special application we want to have more
> > time, its fine, add 
> > 		the inrush current at the PD, however it is not the problem 
> > 		        of 95% of the applications. 
> > 		        In standard DC/DC supply for 12-15W with 470uF max
> > at its input the 
> > 		total charging time is around 50mSec and the power supply
> > startup time is 
> > 		10-30mSec thus the PD can be fully functional within 100mSec
> > max. 
> > 
> > 		                4) The PD needs energy storage to hold on
> > the switch when 
> > 		the 
> > 		        Correct, add 0.1uF between the gate and source of
> > the Isolating 
> > 		mosfet switch and utilize the high impedance of Mosfet gate 
> > 		        to have long memory effect. (I have circuit that is
> > working and 
> > 		tested in the lab, I can send it to you if you want) 
> > 		        (The inrush current limiter that you want to add to
> > the PD contains 
> > 		much more components than the needed to solve the storage
> > time issue) 
> > 
> > 		        Brian: 
> > 		        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. 
> > 
> > 		        We have limits all the time, and limits on the
> > design are needed 
> > 		even if the inrush current limiter is in the PD. 
> > 		        And the reasons are: 
> > 
> > 		                1) The PD cap size needs to be limited : 
> > 
> > 		        The inrush current limit set point is needed to be
> > calculated 
> > 		according to the PD power supply input capacitor along with
> > the the PD power 
> > 		supply normal operating current right after startup.
> > Otherwise you will have 
> > 		startup problem. 
> > 		        You can argue this statement by saying, that in your
> > application or 
> > 		model you haven't noticed that problem, and it could be true
> > 
> > 		        for a specified sets of parameters... but not for
> > any set of 
> > 		parameters. 
> > 		        The bottom line is that you move the definitions and
> > limitations to 
> > 		the PD box instead of to the System definitions. 
> > 		        I prefer the system definitions and not complicating
> > the PD in order 
> > 		to make it simple and low cost. 
> > 
> > 		                2) The DC/DC must be held off until the cap
> > is charged 
> > 
> > 		         In many power topologies and/or applications, you
> > need to do it 
> > 		regardless if the inrush limiter is in the PD. 
> > 
> > 		                3) The startup/charging time needs to be
> > limited 
> > 
> > 		        If you need long startup for some reason you can add
> > the hardware to 
> > 		the PD to get it. However it is not a drawback for 95% of
> > the applications. 
> > 
> > 		        Brian: 
> > 		        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. 
> > 
> > 		        95% of the applications are with capacitive input
> > since they have 
> > 		DC/DC. 
> > 		        Why we should care for the 5%. 
> > 		        In any case we cant spec. for 100% of the
> > applications (the known 
> > 		today and the future) since it will not be a cost effective
> > solutions. 
> > 
> > 	
> > =================================================================== 
> > 
> > 		        {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. 
> > 
> > 		        Isn't it a normal design procedure that any design
> > will fail with 
> > 		some set of numbers and will work perfectly with other set
> > of numbers? I am 
> > 		sure that if you put the inrush current limit in the PD, it
> > will fail for a 
> > 		some set of numbers. 
> > 
> > 		        From the experience that we gathered from the field,
> > for the case 
> > 		that the inrush current was in the PSE, the PD designers
> > find that it is 
> > 		easy to design the PD power supply if they know the energy
> > source parameters 
> > 		during startup and during normal operating mode and the only
> > limitations 
> > 		that they have on the PSE and PD where: 
> > 		        PSE side: 
> > 		        1. Max. available power during normal operating
> > mode. 
> > 		        2. Voltage range 
> > 		        3. Max. average current at the lowest line possible.
> > 
> > 		        4. Peak current and time from the PSE during
> > startup. 
> > 		        PD side: 
> > 		        4. Max. PD input cap 
> > 		        5. Turn on and turn off voltages 
> > 
> > 		        Very simple and it works always. 
> > 
> > 		        I agree that for very large PD input capacitor the
> > Inrush current 
> > 		limit should be in the PD, and we already agree to this
> > concept in St Louis. 
> > 		We need to close the value of this capacitor in which it
> > consider to be 
> > 		large capacitor. 
> > 		  
> > 
> > 	
> > ====================================================================== 
> > 
> > 		> -----Original Message----- 
> > 		> From: Lynch, Brian [SMTP:brian_lynch@xxxxxx] 
> > 		> Sent: ?, ???? 01, 2001 4:51 PM 
> > 		> To:   'Yair Darshan'; 'Dave Dwelley'; Lynch, Brian; 'Rick
> > Brooks'; 
> > 		> stds-802-3-pwrviamdi@xxxxxxxx 
> > 		> Subject:      RE: insuring the stability of power delivery
> > 
> > 		> 
> > 		> All, 
> > 		> 
> > 		> See below 
> > 		> 
> > 		> Brian 
> > 		> 
> > 		> >-----Original Message----- 
> > 		> >From: Yair Darshan [ mailto:YairD@xxxxxxxxxxxxxx
> > <mailto:YairD@xxxxxxxxxxxxxx> ] 
> > 		> >Sent: Thursday, May 31, 2001 10:18 PM 
> > 		> >To: 'Dave Dwelley'; Lynch, Brian; 'Yair Darshan'; 'Rick
> > Brooks'; 
> > 		> >stds-802-3-pwrviamdi@xxxxxxxx 
> > 		> >Subject: RE: insuring the stability of power delivery 
> > 		> > 
> > 		> > 
> > 		> >Dave, 
> > 		> >See my comments below. 
> > 		> >Yair. 
> > 		> > 
> > 		> > 
> > 		> >> -----Original Message----- 
> > 		> >> From:      Dave Dwelley [SMTP:ddwelley@xxxxxxxxxx] 
> > 		> >> Sent:      ?, ??? 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 
> > 		> >again. 
> > 		> >     This is not an oscillation it is a normal short
> > circuit 
> > 		> >condition 
> > 		> >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.
> > 
> > 		>
> > ====================================================================== 
> > 		> >
> >