[SI-LIST] Re: Parallel Inductor in Capacitor !!!
- From: Rohit MISHRA <rohit.mishra@xxxxxx>
- To: "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
- Date: Mon, 16 May 2011 16:42:32 +0800
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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