[SI-LIST] Re: resend - Specctraquest model: mounted inductance
- From: "Bart Bouma" <bart.bouma@xxxxxxxxx>
- To: Larry.Smith@xxxxxxx
- Date: Tue, 17 Jun 2003 11:56:27 +0200
Larry,
I guess I was a bit crude with my statement. I'll explain.
The ESL-value is of course of interest for the end-user, for instance as a
first selection of which capacitor to use. However, this too is not as
straight as one might think since the measurements done for extracting the
ESL-values are not standardized: fixture type, use of a shorting block and
compensate for this, or not, not using a shorting block. Result: one
finds a large variation on ESL-values for capacitors with the same
case-size, but I'm sure you know this.
And I will continue using the ESL-values we extract, with a description of
the measurement method, as a starting point for the (intrinsic) inductance
of Spice-, SQ- and other models. But I think, at least this is what I
learned in the past weeks, as long it is undefined on what inductance we
talk about and on how it is measured, just an undefined bare ESL value or
this ESL-value put into a model is just an inductance value measured in
that particular undefined situation, and therefore less of interest for
the end user when carrying out board simulations, because it would result
in unreliable or untrue results.
Therefor, I hope that measurements on a standardized set of pcb's with
known geometry (see discussion I had with Istvan) will help to define a
value for inductance that is usable in board simulations.
" A power distribution system is likely to be very low
(less than 1 Ohm) in impedance. Capacitors mounted on such a structure
will behave quite differently than capacitors mounted on a 50 Ohm
structure. "
I assume that you are refering to the effect you mentioned in a former
mail, when planes have distances to the capacitor of aprox. 4-5 mils and a
stong coupling between the capacitor and the plane exists, and that the
low impedance of the network itself doesn't influence the cap's behaviour.
Mentioning this effect I wonder, what happens when using the X2Y as a
decoupler. X2Y's outer electrodes are ground electrodes, so coupling
between the X2Y capacitor and a plane will be different compared to a
standard capacitor. Will this be an advantage or a disadvange?
One could say that this coupling now takes place inside the part itself,
for the X2Y has multiple ground planes inside with distances of less than
1 mil upto a few mils to the "hot" electrodes, resulting in the low
inductance property of the X2Y.
regards, Bart
Bart Bouma
http://www.yageo.com
Bart - Be careful of extreme statements... I would not say that "a
bare capacitor ESL value is not of interest," and that "inductance
values listed in these models are of no value for the end user."
Istvan, Nick and I are saying that you have to be very careful about
where the inductance values used in power distribution simulation came
from.
"Partial" self and mutual inductances are not unique. They have to be
interpreted in the context of a loop. You could have several different
self and mutual inductances for each element of a loop and they may all
be self consistent and correct in the context of a loop. This means
that when you go around the loop, you get the correct loop inductance.
I don't know how to get an exact inductance to represent the intrinsic
inductance of a capacitor. I don't believe that such a thing exists.
It is however possible to use a fixture that is very similar to your
product environment and obtain a sufficiently accurate inductance for
the capacitor. A power distribution system is likely to be very low
(less than 1 Ohm) in impedance. Capacitors mounted on such a structure
will behave quite differently than capacitors mounted on a 50 Ohm
structure. By using a fixture that is similar to your product
environment, it is possible to obtain an intrinsic inductance value
for a capacitor that is useful for SQPI simulation and will produce
good model to hardware correlation results in a complete power
distribution system.
It is even possible to obtain the parameters for a distributed
transmission line model for the capacitor which will obtain even better
model to hardware correlation, as discussed in our papers. This
however is outside the scope of what SQPI is presently offering.
regards,
Larry Smith
Sun Microsystems
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> Cc: si-list@xxxxxxxxxxxxx, si-list-bounce@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: resend - Specctraquest model: mounted inductance
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> From: "Bart Bouma" <bart.bouma@xxxxxxxxx>
> Date: Mon, 16 Jun 2003 12:11:14 +0200
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>
> Istvan,
> thanks for the detailed answer.
> After the discussions during the past weeks on this subject (thanks
also
> go to Larry and Nick), I am aware that just a bare capacitor ESL value
is
> not of interest and useful for modern designs.
> Question: what's the added value of having e.g. a Specctraquest model
with
> values for intrinsic and mounted inductance then?
> That's where it all started about, maybe I'm wrong, but I believe that
Sun
> was involved in defining these models. Are these models superseded ?
> Without knowing the configuration the inductance values listed in these
> models are of no value for the end user. Please correct me when I'm
wrong.
>
> The tips you gave for the capacitor manufacturers at the bottom of your
> answer will be helpful to give better defined "ESL"-values: we will
study
> these and see what's possible to realize.
> Here some comments and remarks:
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