[SI-LIST] Re: Capacitor ESR
- From: Krina Kothari <krinakothari@xxxxxxxxx>
- To: mappiani@xxxxxxxxxxxx, "Si-List (E-mail)" <si-list@xxxxxxxxxxxxx>
- Date: Tue, 22 Jul 2003 20:53:40 -0700 (PDT)
Mario,
There is lot of device physics involved in modeling
capacitors. Just to be precise on Si-list I would say
that as far as tantalum caps are concerned, we just
use RLC model. ESR which stands for equivalent series
resistance is a function of frequency and this
particular values varies a lot based on the
measurement technique used.
To accurately model a capacitor it is very essential
that we characterize capacitors in-house and try
extracting parameters of interest from the mesaured
results. To model a cap we usually use ESR at
resonant frequency which we can easily extract from
measured results. Try simulating model using Hspice
and you should definitely get good model to hardware
correlation. Usually I have seen that for X7r and X5r
caps we can get real good model to hardware
correlation, but for bulk caps or tantalum caps model
correlation is pretty close, but not as good as X7r or
X5r. I am not sure how widely these caps are used as
ceramic caps take over at high frequencies and at low
frequency people now tend to use polymer caps. Also
with polymer caps I am not very satisfied with
modeling results as compared to X7r or X5r. I get
pretty close results, but not exactly overlapping the
measured results. Can somebody give me a little input
on type of model to use or can guide me to some papers
which discusses modeling of polymer/tantalum caps?
I also agree with you Mario, that lot of spec sheets
for tantalum caps show ESR at 100 KHz (max) because
that's the maximum ESR shown by the supplier for a
particular value of capacitor at 100 KHz. For e.g.
33uf capacitor shows ESR of 4 ohms at 100 KHz (max),
which means that maximum ESR achieved is 4 ohms at 100
KHz, we can get low ESR as well, but it is guaranteed
that we will never see beyond 4 ohms at 100 KHz.
Please correct me if anybody feels that this is an
incorrect statement.
Thanks,
Krina Kothari
--- mappiani@xxxxxxxxxxxx wrote:
> Is a cap's ESR value specified at it's resonant
> freq? I seem to remember
> seeing some spec sheet for tantalum caps specifying
> an ESR value at 100KHz.
>
> I was trying to model up a power input filter and
> for the capacitors and I
> realized that I didn't really understood where the
> ESR number came from. I
> wanted to use the ESR at my frequency of interest
> but that didn't seem
> right for a cap model. I get that ESL is tied to the
> package size and
> pretty well fixed but I 'm not sure about what ESR
> value to use.
>
> Thanks for any input.
>
> MArio
>
>
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