[802.3af] RE: Startup and PD input cap
Yair, Dave, All:
Technically, I don't challenge Yair's calculations, nor Dave's corroboration
of those calculations. I do have concerns regarding many of the same system
issues as Dave raises, as well as one other. That other is as follows:
Granted, we agreed in St. Louis to only specify single-port power-up time.
I do NOT take that to mean that most PSE vendors - or most PD users - will
be willing to wait until their port is serviced in what is now being
proposed as a round-robin one-at-a-time fashion, from a multiport PSE power
control IC. To slow things to a 4-second average, 8-second worst-case PD
power-up is probably not our intention, no matter how precisely we pick at
the St. Louis wording. Even the 8-second timing given above is way too
generous, for it is necessary to keep in mind that the ~ 1 Joule figure
which Dave has given us is based on very low duty cycle (virtually
single-pulse events). Eight such events in succession would be likely to
grossly overheat any reasonable IC package, and the contents thereof.
From: Dave Dwelley [SMTP:ddwelley@xxxxxxxxxx]
Sent: Friday, June 15, 2001 9:23
To: Yair Darshan; 'Karl Nakamura'; 'Donald S. Stewart'; 'R
karam'; 'Rick Brooks'; 'Lynch, Brian'; 'Peter Schwartz';
'scott_burton@xxxxxxxxx'; 'Steve Carlson'; 'rk@xxxxxxxxxxxxxxx';
'henryhinrichs@xxxxxxxxxxxx'; 'Jetzt, John J'
Subject: Re: Startup and PD input cap
Your numbers look good. I've been off-line for awhile, but I'm back
least briefly. Here's how I see it (technical first, political
The 1J number is pretty rough - it's taken off the SOA curves from a
of typical, SO8 package FETs.* It does seem to be about right (or
little conservative) for the type of packages that a PSE chip might
packaged in. Since we removed the requirement that multiple ports
simultaneously at St. Louis, you're correct in saying that the "N"
in your equation is 1, not 8. My old 50uF number (actually 77uF) was
on N=8 - no longer necessary.
Energy in the cap = 0.5*C*V^2
Max cap for 1J = 2/V^2 = 2/57^2 = 615uF, same as Yair's number.
It's pretty clear that we can ramp this cap up in well under a
leaving lots of time for detection and classification. Note also
not spec'ing ramp time to 57V (in the worst case) - only to 44V. If
is 400mA and C=470u+20%, then:
ramp up time = C*V/I = 564u*44/0.4 = 62ms
Total energy (this time to 57V, which is what the FET could see
energy = 0.5*564u*57^2 = 0.92J, under 1J (barely).
470u + 20% is OK from a thermal point of view if N=1. What is the
of a typical 470u cap?
* Note that I've seen several SOA curves which appear to be VERY
conservative, based on a single time constant model. They suggest
FET is a constant power device below 100ms, not a constant energy
don't believe them.
Now I'm going to take off my technical hat and put on my system
I still think it's a mistake to allow unlimited-inrush PDs! There
several complications that such devices bring up, like memory in the
UVLO circuit, long short circuit timeouts in the PSE, possible large
on the wire when the PD UVLO comes on, and very large peak currents
PD end of the wire (and the PD end RJ45 jack) when the PD UVLO comes
(before the PSE current limit circuit kicks in). I agree that the
in the phone is cheaper this way (slightly, and even less down the
but I think the corresponding drawbacks make the spec weaker, and
creative interpretation by marginal PD vendors that will cause
interoperability problems and may hinder widespread acceptance of
If we mandate inrush control in the PD in all cases, nearly every
the above problems goes away, and interoperability is virtually
least with regards to power!). There is additional cost in the PD,
isn't much... and it the incremental cost will only drop. It's the
thing to do - even though it now has no impact on my ability to
This is the last time I'm going to plead for this - if the consensus
that the cost savings in the PD is worth the hassle of PSE inrush,
on the bus.
At 10:13 PM 6/14/2001 +0200, Yair Darshan wrote:
>I would like to have your comments for the following summary of the
>calculation procedure for setting the max. PD input cap to be
handled by the
>Target: To make it possible to define more than
50uF as the
>point in which the responsibility for inrush current limiting is
>the PSE to the PD.
>Incentive: 1. Low cost PD power supply implementations
>100KHZ. for 10-12W power supply, max. 470uF is needed. for 5W power
> is needed.
> Caps lower that 50uF requires high
>switching power supply (around 500KHZ) which costs much more.
> 1.1 50-60% of the applications are
>are 10-12Watts. It means that around 95% of the applications will
> (Data based on PD power requirement
>during the last 6 month)
> 2. In order to meets system stability
>discussed over the reflector during the last 3 weeks, we need to
>L/C ratio at the PD power
> supply input. Stability criteria
>L/(ESR*C)<< Zin, L inductance, C=Capacitance of the
> EMI filter, ESR is the equivalent
>resistance of the Cap.(There are additional stability criteria,
>one concerns the EMI filter
> connected to negative resistance
>means that we need to allow low inductance for a given Cap size or
> given inductor size. In order to
>EMI filter we need the inductor to have 10-500uH (pending on
>switching frequency and EMI
> requirements) therefor we need Larger
> Although (2) can be achieved when the
>limiting is in the PD, It will be cost effective to the system to
>larger cap in the PD allowing
> the PSE to be responsible to limit the
>current pending that it will allow the integrated chip in the PSE.
>Vport= Port voltage
>Iport = Port current limit level
>N= Number of active ports per device(active port=at startup mode).
>Tc= The time that the port is in current limit situation = The PD
>Emax= Max energy aloud on the device.
>1. Assuming Emax=0.5*N*Vport*Iport*Tc, Energy dissipated on the
>during startup mode
>For the following max. values:
>Emax=1Joule (as per Dave data)
>Assuming that in 8 port device only one port is performing the
>(we can control the timing) and there is a cooling time until the
>will be in startup mode.
>N=1. (Remember that it is non repetitive operation.)
>2. Tc max = Emax/(0.5*N*Vport*Iport) =
>4. Cin max=Ip*Tc/Vp=0.5A*70.16mSec/57V = 615uF
> >From eq. 4 we can have 615uF instead of 50uF.
>Since the above numbers are worst case calculation, we have the
>The PSE can be set to 0.4A min. (The calculation in eq-2 was for
>Tc max can be set to 50mSecmin (The result of eq-2 was 70.16mS)
>According to the above margin Cin max would be: Cin
>Therfore we have 615uF/350uF ==> 75% margin.
>In addition, the above calculations assumes repetitive operation
>not the case for startup, hence much larger margin, with no effect
>supply loss, cable loss etc.
>We can utilize the numbers that we have used for the normal
>and use them as a private case for the startup mode:
>During startup, the PSE will limit its output current to:
>1. Ip min=0.4 Ipmax=0.5
>2. Time duration: 50mSec min. 70mSec max.
>3. Period: 1 sec min. ( To allow low average power in order to
>enough cooling time. It is similar to the timings of the normal
>Under the above numbers the PD will be specified as follows.
>1. Up to 350uF at PD input, PD designer have the following
>2. Ip=0.4A min for 50mSec min.
>For caps greater than 350uF, the PD designer will take care of
>inrush current to be 0.4A max (i.e. < 0.4A)
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