[SI-LIST] Re: resend - Specctraquest model: mounted inductance
- From: "Bart Bouma" <bart.bouma@xxxxxxxxx>
- To: nick@xxxxxxxxxxxx
- Date: Thu, 12 Jun 2003 13:16:40 +0200
Nick,
thanks for response. It is very useful.
I'm not familiar with the terms self partial inductance etc.
But I think that we agree on folowing:
- such a wire has some kind of inductance (partial or self or else- )
- but it is quite meaningless.
More interesting, at least for me, ESL of capacitors.
It's good to know for us that an ESL value specified for MLCC's is quite
meaningless when it comes to simulation of circuits and multilayer boards
with planes.
But I think that it is still good as comparison to other components, it
gives at least an idea on possible improvements.
Condition: the measurements to extract ESL must be done in the same way
and with the same fixture, and this is certainly not the case.
you wrote:
" The ESL values published by capacitor manufacturers are not meaningful or
helpful unless the test fixture and test method are well defined.
If you want to determine the ESL of a capacitor for a particular
application (e.g.
decoupling), it is important to measure the ESL in a fixture where the
capacitor is mounted in a manner that emulates your application. Any
attempt
to extract an 'intrinsic' inductance, attributable to the capacitor only,
will result in a number that is of little use to anybody. "
But for a capacitor manufacturer it is impossible to give inductance
values for every application situation, in MHO the inductance will still
consist of two parts: the inductance of the part it self (call it
'intrinsic' and this should be supplied by the manufacturer) and an
inductance-value associated with mounting which must be extracted by the
simulation program used.
Measuring the inductance of a capacitor that is mounted in a de-embedded
and well defined fixture, like the ICM-fixture we have, would be
meaningful I think.
Or is the measurement described in the document comparing MLCC's with X2Y
(small FR_4 test board with microstrip line) a better way. More or less
simulating the practice (see link below)?
http://www.x2y.com/cube/x2y.nsf/(files)/X2YMLCC.pdf/$FILE/X2YMLCC.pdf
Nick, I like to have your comments on this, maybe (?) it will lead to a
standardized method of measuring inductance (or ESL) of decoupling
capacitors which can be used as a basis for simulations? Somewhere in the
simulation process you must assign a inductance property to the capacitor.
I don't know wether this is the right way, but maybe the measurements
described above can be a start.
As mentioned above, the loop-inductance then must be extracted by e.g. a
3D-solver from the given pcb-structure and geometry.
Makes this sense or is this too simple?
Any way, I'm willing to help in providing more usable ESL values for e.g.
multilayer ceramic capacitors, X2Y-capacitors and other types like 0306
(RG).
Bart Bouma
http://www.yageo.com
Bart,
Self-inductance is a property of current loops. A copper wire does not
have
a self-inductance. A thin copper wire has a well-defined self partial
inductance, but self partial inductance is pretty much meaningless unless
you also know the self and mutual partial inductances for the rest of the
loop. These quantities can't be measured directly, they can only be
calculated.
The quantity that you described (total magnetic flux generated by an
alternating current in a wire segment divided by the current) is not the
self inductance or the self partial inductance. I'm not sure whether this
quantity has a name, but it would have little value in any real
application.
It can't be measured and it can't readily be used to determine the
inductance of a loop containing that wire segment.
The ESL values published by capacitor manufacturers are not meaningful or
helpful unless the test fixture and test method are well defined. If you
want to determine the ESL of a capacitor for a particular application
(e.g.
decoupling), it is important to measure the ESL in a fixture where the
capacitor is mounted in a manner that emulates your application. Any
attempt
to extract an 'intrinsic' inductance, attributable to the capacitor only,
will result in a number that is of little use to anybody.
Nick
-----Original Message-----
From: Bart Bouma [mailto:bart.bouma@xxxxxxxxx]
Sent: Wednesday, June 11, 2003 2:56 AM
To: nick@xxxxxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx; si-list-bounce@xxxxxxxxxxxxx
Subject: Re: [SI-LIST] Re: resend - Specctraquest model: mounted
inductance
Hi Nick,
I think, but I might wrong, that Inductance is not only a property of
current loops:
e.g. a copper wire also has self-inductance:
imagine a piece of copper wire, and assume an ac-current flowing through
it.
This results in an alternating magnetic flux, which will induce a current
flowing in opposite direction in the wire which will try to cancel the
original current.
Is this not the definition of self-inductance? And it can be defined.
From this view a capacitor has self inductance too. Isn't it this my
intrinsic inductance?
I agree with you that in case of decoupling capacitors the current loop is
the determining factor for total inductance.
Bart Bouma
http://www.yageo.com
-----Original Message-----
My idea of intrinsic inductance is that this is the inductance of the
capacitor itself, and that this inductance is determined by the capacitor,
and solely the capacitor itself. So not by its environment like a
copper-plane. The intrinsic inductance - in my opinion - is determined by
the capacitor's mechanical properties like length, width, heigth, internal
structure (e.g. number of layers) etc.
Like the inductance one can calculate for a small rectangular piece of
copper with known dimensions. Independent wether it is mounted on a pcb or
floating in vacuum.
Bart,
You've identified the main problem with trying to assign a value of
inductance to a capacitor independent of its mounting. Inductance is a
property of current loops! The inductance of a small rectangular piece of
copper cannot be defined or measured independent of its environment.
For a small rectangular piece of copper, you could calculate a matrix of
self and mutual partial inductance parameters, but you would not be able
to
use this matrix to find an actual measurable inductance until you defined
the rest of the loop.
Nick
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