[SI-LIST] Re: Parallel Inductor in Capacitor !!!
- From: steve weir <weirsi@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Mon, 16 May 2011 02:21:33 -0700
Rohit, it's not really accurate to call the capacitances in the branches
modeled with parallel inductances, stray. Those branches are just model
elements that most likely do not each correspond to any particular
physical device feature(s), but which when taken together replicate the
device's expected or measured frequency dependent behavior.
Steve.
On 5/16/2011 1:42 AM, Rohit MISHRA wrote:
> Hi Andy/Steve,
>
> It makes sense that inductor is not parallel to basic capacitor instead stray
> capacitance is parallel to series inductance.
>
> Thanks a lot for clearing the doubts and explaining things in detail !!
>
> Rgds,
> Rohit
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
> Behalf Of Andrew Ingraham
> Sent: Friday, May 13, 2011 11:33 PM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Parallel Inductor in Capacitor !!!
>
>> What I am trying to ask is how come parallel inductor is possible
>> in parallel plate capacitor geometry ? All the conductor like pins&
>> plates come in series with capacitor so we should see only series
>> not parallel inductor, right ?
> Read and understand Steve's reply.
>
> But you can also step back and consider SOME OF the lumped elements in
> the model as representing real physical parts of the capacitor.
>
> Your basic capacitor, at the heart of the physical capacitor, is C1.
> Look closely at it. It has NO inductors in parallel with it. The
> only thing in parallel with it is R100, perhaps representing the
> leakage resistance of the dielectric, or maybe just to keep SPICE
> happy.
>
> The plates that make up the capacitor, and its leads (if any), have
> some inductance and DC and AC resistance. Those are represented by
> all of the other elements in series with C1. That includes L2, L5,
> L6, L7, and L8, all of which are in series with the basic capacitor,
> C1 ... not in parallel with it.
>
> Why are there five inductances in series with C1? We can't really say
> for sure (because the model was probably derived on the basis of
> curve-fitting the impedance to measurements). But consider the fact
> that even these lead inductances have stray capacitances across them
> and perhaps distributed along them.
>
> The impedance of an ideal inductance increases linearly, without
> bounds, as frequency goes up. The lead inductance probably does not,
> due in part to those stray capacitances. So the model (the RLC series
> circuit) needs to be modified to make the inductive reactance behave
> correctly, by splitting it into five portions and putting capacitances
> and resistors across some of them.
>
> L2 apparently represents that portion of the lead inductance that is
> not shunted by any stray capacitance.
>
> However, the further you look into the exact details of the model, the
> less likely each part represents an actual physical analog in the
> capacitor, the more likely they are there to make the total impedance
> curve fit what they measured.
>
> Consider it this way. It's not that the capacitor has an inductor in
> parallel with it. It doesn't. It's that the inductors which are in
> series with the capacitor (C1), need to have some additional
> capacitances added in parallel with them. I'm trying to answer the
> question you didn't ask, which is, why are there capacitors put in
> parallel with the lead inductance?
>
> Andy
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