[SI-LIST] Re: Inductance of Via
- From: Neeraj Pendse <cnepsc@xxxxxxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Mon, 11 Jun 2001 09:16:16 -0700
To Yibing:
I checked up this formula from Dr. Johnson's book - and I feel that it should
be
L = 5.08 * h * [ln(4h/d) - 1]
If you make this correction, it is the formula for AC inductance (surface
currents only) of a straight, round conductor with diameter d and height h, all
distance units are inches. I tried simulating several vias in a 3D field
solver, and I realized that at AC, the inductance of a via is the same whether
it is filled or not. Also that it follows the above formula (with a -1) far
better than the formula in Dr. Johnson's book. (which has a +1). You might
want to try this formula, and check your results with a 3-D field solver/RLC
parameter extractor.
I disagree with Michael's statement that it does not make sense to talk about
the inductance of the via in isolation. While I agree to the fact that the
entire current "loop" must be considered when you are minimizing the inductance
- the concept of inductance is not limited to "loops", bet can also be applied
to discreet pieces of a loop. When all significant mutual inductances are
accounted for, the total inductance of the loop can be easily realized from
the pieces. This is the concept of "partial inductance", and in my opinion it
is a very powerful method of evaluating interconnects when you want to do a
lumped element analysis.
Regards,
Neeraj.
"Tsuk, Michael" wrote:
> Yibing Tang wrote:
>
> > I use a formula to calculate the inductance of through hole via,
> > L=5.08h[ln(4h/d)+1]
> > However, I find that it is not suitable for small ratio of h to d.
> >From this
> > formula,if decrease the ratio, I could get very lower inductance, even
> > negative.
>
> It makes no sense to talk about the inductance of a via in isolation.
> None at all. Inductance is a quality of a loop of current; without
> knowing where your return path is, you can't get a meaningful value for
> the total inductance.
>
> In any case, it seems that your formula isn't applicable to vias; this
> was discussed back in March. See:
>
> http://www.qsl.net/wb6tpu/si-list/0117.html
>
> > My question is how far I can go to low the inductance.
>
> You have two main choices:
>
> 1. Bring the return path vias closer to the signal via
> 2. Shorten the length of the via
>
> The effect of the diameter is relatively weak.
>
> --
> Michael Tsuk
> Compaq AlphaServer Product Development
> (508) 467-4621
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