From: Bob Lewandowski ([email protected])
Date: Fri Feb 23 2001 - 13:02:49 PST
The potential issue is the proximity of the inductor to a piece of metal, such
as a circuit board ground plane, other traces, or just the mounting pads. It's
a well known effect that you can change the value of an inductor by bringing a
significant sized piece of non-ferrous metal near it. Brass slugs were used for
tuning inductors in the "olden days". The tuning range of this effect is small
and depends on the construction of the inductor and the proximity of the metal
to the high magnetic flux areas. I believe the effect is due to eddy currents
induced in the metal producing a 'countering' field that reduces the flux
linking the turns in the inductor. This effect also reduces the Q of the
inductor due to the induced eddy current losses in the metal. The so called
"Shorted Turn" effect.
There is also the effect of adjacent dielectric materials. These cause a second
order effect by increasing the parasitic inter-winding capacitance, which will
lower the self resonant frequency, and if the coil is being operated close to
the self resonant frequency, will change the apparent inductance.
The bottom line is to position the inductor so the magnetic flux linking the
turns is least influenced by the circuit board and traces on it, and adjacent
components, particularly other inductors. Keep the inductor as much in "free
space" as is physically possible. Sometimes it not obvious how to do that with
inductors whose innards are hidden from view. "Solder and See" is a frequently
a good approach.
---Bob Lewandowski
Vixel Corp.
Steve Rogers wrote:
> My original message may not convey my requirement. I shall have another
> attempt
>
> I believe that certain chip inductors are constructed such that depending on
> which-way-up you mount them will change their inductance and Q slightly?
> Unless you are 'doing your own thing' then generally a chip inductor (lets
> say an 0805 package) will only be able to be mounted onto correctly sized
> pads in 4 ways. Assume you are looking at a chip inductor mounted on a PCB
> you could de-solder it and 'turn it over' (what used to be the top of the
> inductor becomes the bottom and the bottom you can now see) this gives two
> ways to place it. Also if you name one end of the inductor (a) and the other
> (b) then name one of the pads (x) and the other (y) clearly you could
> connect the inductor such that:-
>
> (a) connects to (x)
> (b) connects to (y)
>
> or....
>
> (a) connects to (y)
> (b) connects to (x)
>
> This gives another two options making the total of four.
>
> Now! what I want to know is.......
>
> Will the inductor, when mounted in any of the four ways cited above have the
> same Q and Inductance regardless of the way it is mounted? If the design of
> the inductor has symmetry (round toroid say) we expect this to be the case.
> However I believe that chip inductors (Multilayer, thin film etc) might not
> have symmetry?
>
> I hope I have now clarified what I am trying to find out???
>
> Can anyone now help?
>
> Again, all comments and suggestions are welcome
>
> Regards.
>
> Steve Rogers
>
>
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