[SI-LIST] : approximations for partial self inductance

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From: Eric Bogatin (eric@gigatest.com)
Date: Tue Mar 13 2001 - 12:21:36 PST


Itzhak-

you asked the question about the difference in the approximations
for the partial self inductance of a via that were given by
myself and Howard Johnson. I wanted to provide some
clarification. You wrote:

(4) While calculating vias inductance, I've encountered 2 similar
but
different equations for this parameter. One is given by Mr. H.
Johnson
in his famous book (page 259), as follows:

L=5d*{ln(2d/r)+1}nH.

The other is given by Mr. Bogatin in one of his articles, and is:

L=5d*{ln(2d/r)-3/4}nH.

Can somwone explain the reason for the difference, or who is
right? The
difference starts to be quite critical when dealing with u-Vias!

The approximation is for the partial self inductance of a round,
solid rod, of radius, r and length d. The length is in units of
inches, while the inductance is in units of nH.

This is the approximation that was originally derived by Fred
Grover, in his classic book, Inductance Calculations", in 1946. I
just re-checked the one I offered, and it is correctly reproduced
above. It is listed on page 35, eq 7, of his book. I think it has
since been reprinted as a Dover Book.

Keep in mind two things when using this approximation: 1st, it is
an approximation. Grover says it is good to about 2%. I have
found good agreement to better than 5% for wire bond structures.
Approximations are wonderful tools to assist you in exploring
design space, run in a spread sheet and play what-if trade offs.
They give you good answers and let you see the geometry and
materials trade offs. However, they are APPROXIMATIONS. You
should never use an approximation in a situation where the
accuracy of the answer may cost you significant time and expense.
You should be using a 3D field solver that you have confidence
in. One of the second order effects in this approximation, for
example, is that it includes the "internal" self inductance. As
the skin depth gets to be comparable to the geometrical cross
section, the partial self inductance will decrease and reach a
constant value when all the current is in the outer surface.

The second thing to keep in mind when using this approximation is
that it is for the PARTIAL self inductance of the via, under the
assumptions of uniform current flow down the long axis. If you
are using it in a situation where the length of the structure is
comparable to the diameter, ie, d ~ 2r, the current distribution
through the structure may not be even close to parallel to the
long axis. Further, the actual loop inductance, which is what
matters in a real circuit, is probably dominated by other
elements than this small, squat element. The partial self
inductance may depend strongly on the proximity of other
conductors and how it affects the current flow through this via.
If you are in a regime where worrying about the presence of the
-3/4 term is important, you probably want to use a 3D field
solver before any design signoff. A good 3D solver will calculate
the actual current distribution through the via structure and the
rest of the current path.

I hope this helps.

If anyone is interested, I have various application notes related
to approximations to inductance and general principles related to
inductance posted on our web page. These are listed as app notes
with index numbers: 33, 32, 29, 25, and 9. You can find them
under application notes at www.gigatest.com

As always, comments are welcome.

--eric

From: Itzhak Hirshtal [mailto:hirshtal@is.elta.co.il]
Sent: Monday, March 12, 2001 09:33
To: si-list
Subject: [SI-LIST] : Inductance and Decoupling

Hello, all

I've recently started to calculate the de-coupling needed for
efficiently supplying the spike currents needed by high-speed
devices.
During this task, I've encountered several ambiguities and
results that
I would like to share with you and perhaps hear some (useful)
feedback
from you.

(1) I tried to evaluate the situation for one high-pin-count
device with
several buses connected to it (essentially a bus bridge). Even
calculating for just one synchronous bus (with 144 bits overall)
I
arrived to the result that a few Amps (maybe even 5) are drawn
when all
or most of this bus bits change state. I wonder what will be the
result
if I would calculate for an additional bus (assuming it's
synchronous
with the first). And what about the internal changes? They might
be
contributing even more than the external bus! (e.g., the Motorola
PowerPC HW manual states that 90% of the power consumption of
this
device is drawn internally, not externally).

(2) I've also tried to calculate the inductance of the decoupling
capacitors connections to the device. Even assuming a 40-mil wide
50-mil
long trace right above a reference plane for the connection I
have app.
L=150-200pH. If I can't connect at least one of the capacitor
pads so
short I might have to do a direct connection via to a reference
plane. I
calculated this to have more than L=1nH!

(3) I assumed the calculated peak currents change at a rate
equivalent
to the rise time of the device's output buffers. I don't know if
it's
true, but this seems to me the most logical thing to do. Even if
I take
it to be 2ns (1 ns is closer to worst-case, I believe), I get
the
result that I need 40 to 50 low-ESL decoupling capacitors for the
case
where L=1nH. Only if I succeed to connect the capacitors directly
and
close enough to both GND and VDD pins (L=150-200pH) do I get the
result
that it is sufficient to use 4-6 decoupling capacitors.

(4) While calculating vias inductance, I've encountered 2 similar
but
different equations for this parameter. One is given by Mr. H.
Johnson
in his famous book (page 259), as follows:

L=5d*{ln(2d/r)+1}nH.

The other is given by Mr. Bogatin in one of his articles, and is:

L=5d*{ln(2d/r)-3/4}nH.

Can somwone explain the reason for the difference, or who is
right? The
difference starts to be quite critical when dealing with u-Vias!

Thanks for anyone who makes the effort to read this email.

- --
Itzhak Hirshtal
Elta Electronics
POB 330 Ashdod
Israel 77102
Tel: 972-8-8572841
Fax: 972-8-8572978
email: hirshtal@is.elta.co.il

**************************************
Eric Bogatin
CTO, Giga Test Labs
v: 913-393-1305
f: 913-393-1306
e: eric@GigaTest.com
corporate office:
408-524-2700
134 S. Wolfe Rd Sunnyvale, CA 94086
web: www.gigatest.com <http://www.gigatest.com>
**************************************



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