[SI-LIST] Re: Inductance of Spiral Inductors
- From: "Ozgur Misman" <omism@xxxxxxxxx>
- To: SRogers@xxxxxxxxxxxxx
- Date: Tue, 20 Nov 2001 17:51:04 -0700
Hi Steve,
I was curious why one should not use a network analyzer to get the Q of
the inductor. A few years back when I was working on spiral circular
inductors ( or printed inductors in general). I always did network analyzer
( S-parameter ) measurements. Once you have the 1 port or 2 port
S-parameter data you can calculate the Q factor. It is a ratio of
imaginary part/real part of input impedance which you can easily deduce
from S-parameter data whether it is a 1-port or 2port inductor.
I have read lot of both journal / conference papers in the area that use
same S-arameter techniques to get the Q , and L of the printed inductors.
You seem to have worked extensively in this field. I just would like to
know very briefly why network measurement is not a good approach?
dont use a network analyser to get the Q of the spiral (if the
Q is high) I Wont bother giving you the what whens and whys but believe me
when I tell you it wont work. I suggest either a boonton meter (not much
chance you have one) so in the real world use a tracking generator, a
spectrum analyser and resonate your spiral with a high quality capacitor
Best Regards,
Ozgur Misman
Steve Rogers <SRogers@xxxxxxxxxxxxx>@freelists.org on 11/20/2001 10:22:10
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Subject: [SI-LIST] Re: Inductance of Spiral Inductors
Hi Rafael,
Heres some refs that may be of use for designing inductors
1. Inductance Calculations and working formulas ///Fredrick Grover
you may have some trouble getting a copy of this. This is the mother of
all
inductor references.
2. Radio Engineers handbook ///// Terman. You should be able to get a copy
of
this one. Much of the content is taken from Grover. This gives a number
of
expressions for spirals. If you dont have a copy of this book then get
one.
It may be old but it cuts the mustard.
3. Simple accurate expressions for planar spiral inductances /// Mohan,
Hershenson, Boyd and Lee. Get this off the net. This is and IEEE journal
paper Volume 34 #10 octobler 1999.;
5. Do a search for "C PATRICK YUE" he has written lots of stuff re spirals
on silicon. Also Stanford university are very much into planar spirals on
silicon. I suggest you have a dig on their site.
6. Free EM simulator analyses spiral inductor on silicon /// J Rautio. This
is a paper by James Rautio of sonnet. This paper is worth reading (the guy
knows his stuff)
7. Inductance Calculation techniques part 2 /// Marc T Thompson. This kind
gent has put a short paper on the web with lots of useful inductor
formulas.
There are a number here for spirals.
8 I almost forgot the classic book by Wadell. I think its called
Transmission line design handbook. Anyway this book is a real gold mine.
Get
one now.
All of this lot is worth a look. Most of it will give you a good stab at
getting a theoretical inductance prediction. Q is another story.
Not to put you off but I have done a lot of work in this area and it gets
pretty complicated. Heres some things to think about
(a) Lots of things affect the Q. Some less obvious than others.
(b) If you print the inductor, your currents are confined to a thin
rectangular strip conductor. The current distribution in this type of
conductor at high frequency has the majority of the current crowded towards
the edges of the strips. The effects are will described (if you like maths)
in Collin's micrwave engineering book using conformal mapping. The
phenomena
is skin effect. The problem here is that at high frequency you end up with
large conductor losses (I squared R) due to the high current density over a
small area)
(c) You have to put tracks next to each other to increase the mutual
inductance (is there another way?) by doing this you alter the current
distributions even more making the bad situation even worse (this is
proximity effect)!!!!! I would not dream of trying to sort this out with a
pen and paper (E-M solver needed - but!!!!!!! see later)
(d)The PCB traces you can fabricate in reality are not perfectly smooth at
their edges. This is a big big problem that is overlooked all the time. I
have made lots and lots of measurements and performed lots of simulations
using various E-M tools. The rough edges of real life conductors push the
losses up!!!!!! Wadells book covers the subject in some detail. The effect
seems to be at max a degradation by a factor of 2. You could try to model
the surface roughness (if you were barking mad) remember that you would
need
to have detailed mesh down to sub skin depth with a model size that would
encompass your spiral.
(E) What about dielectrics. If you are running a high ish frequency, you
will need to consider the tan delta of your substrate.
(F)Wheres the spiral going???? If its only ever going to run in free space
your fine. The reality of it is that it will be in some form of enclosure.
If you have any conductor near the spiral you will have great fun with eddy
current effects. In addition to this if theres a poor dielectric around the
E fields will give you grief as well!!!!
(g) Another one to watch out for is the coating nightmare. I made some
spirals once and they had 60/40 tin lead coating. This is not the best
approach to High Q inductor design given the conductivity of the coating. I
wouldnt like to model this either given that it will form a convex miniscus
on the rough surface of the conductor.
(h) Return current. Heres another good one that gets overlooked. You pump
current through your inductor, but it must return to the supply. Where is
this return? If your inductor is far from this return you have a good
antenna. Putting a ground plane under the inductor as a return will be a
problem. Eddy current will circulate in the ground plane, this will be an
image of the current in the spiral but in the opposite direction. This
image
current causes a negative mutual inductance which drops your spiral
inductance (Q therefore goes down - oops). In additon the eddy current
gives
I squared R loss in the ground plane. You might think you can make a
patterned ground shield, trust me these only work on silicon (terminate the
E field before it gets into lossy silicon)
---------------
So an E-M tool will give you the best stab at the inductance and Q for a
spiral. It will account for all but not the surface roughness. Your guess
on
that one!
E-M is how I would go if I lost the use of my test equipment. What I am
saying I think is use whatever method to design a basic spiral for around
the inductance you need. Then, build one and measure it. One final
warning..... dont use a network analyser to get the Q of the spiral (if the
Q is high) I Wont bother giving you the what whens and whys but believe me
when I tell you it wont work. I suggest either a boonton meter (not much
chance you have one) so in the real world use a tracking generator, a
spectrum analyser and resonate your spiral with a high quality capacitor
Good luck, let me know more about what exactly it is you are doing
Regards,
Steve Rogers (Purveyor of many fine and some really useless novel planar
inductors)
Good luck and I hope you have not lost all hope......
-----Original Message-----
From: Rafael Martinez [mailto:rmartinez@xxxxxx]
Sent: 20 November 2001 15:57
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Inductance of Spiral Inductors
Hi there,
I am a postgraduate student and I need to design a couple of PCB printed
spiral inductors for my project. To do that we have purchased Ansoft
Ensemble 8.0 full version 3D simulator included. So, I would like to know a
couple of things about this.
1.- As far as I know I need to get any sort of empirical formula to relate
the dimensions of my inductor with the value of the inductance. Does
anybody
know any reference or any website useful for me?
2.-The second step is to simulate the model... Does anybody know if with
the
Ansoft package there is any straight forward method to extract the value of
the inductance and the Q? I mean the kind of simulation I have to do, what
kind of analysis.
Thank you very much,
___________________________________________________________________
Rafael Martinez Catala Rossa Avenue, Bishopstown,
Postgraduate student Cork, Ireland
Cork Institute of Technology Fax: +353 21 4326625
Electronic Engineering Dep. Tel: +353 21 4326858
http://www.cit.ie
___________________________________________________________________
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