[SI-LIST] Re: Ferrite bead question
- From: "Curt McNamara" <CurtM@xxxxxxxxxxx>
- To: <zhang_kun@xxxxxxxxxx>, <joel@xxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Mon, 10 Apr 2006 08:43:44 -0500
These are very good points to which I would add another: current needs
to be supplied to a device at its switching frequency. If you have
placed a ferrite with significant impedance between the device and the
power planes, then all the current is taken from local capacitors. The
implication of such an arrangement is that the device generates noise
which you are trying to localize and keep from the power planes.
However, as current is taken from the local cap, it needs to be replaced
from the plane by current flowing through the ferrite. Since there is
significant impedance at this frequency, the bucket drains faster than
it can be refilled. As a result you can't use a ferrite with significant
impedance at the switching frequency. In general ferrites are selected
to be useful at high harmonic frequencies.
Curt
Curt McNamara. P.E.=20
Senior Electrical Engineer=20
Logic Product Development=20
411 Washington Ave. N Suite 101=20
Minneapolis, MN 55401=20
Tel 612-436-5178=20
Fax 612-672-0443=20
curtm@xxxxxxxxxxx=20
www.logicpd.com=20
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-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Zhangkun
Sent: Sunday, April 09, 2006 11:00 PM
To: joel@xxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Ferrite bead question
Dear Joel
As to my experience, it is not enough to select bead based on the P/N.
The decoupling circuit should be considered very carefully.
In several cases, we use beads and the noise become larger. The
following element should be considered:
1.The resonance between bead and capacitor.
2.The frequency span of the noise source.
A simulation is suggested. It is very simple.
Hope this is helpful
Best Regards
Zhangkun
2006.4.10
----- Original Message -----=20
From: "Joel Brown" <joel@xxxxxxxxxx>
To: <si-list@xxxxxxxxxxxxx>
Sent: Saturday, April 08, 2006 2:33 AM
Subject: [SI-LIST] Ferrite bead question
> I often see recommendations in application notes for ferrite beads on=20
> DC
> power lines. Sometimes a particular P/N is specified and sometimes
just=20
> an impedance is given.
> Wouldn't I want to choose a ferrite bead that has the highest
impedance=20
> possible at the frequencies of interest, and that the current rating
and=20
> DC resistance are appropriate? Would the frequencies of interest be
the=20
> fundamental switching frequency of the part and possibly the 3rd and
5th=20
> harmonics?
> Thanks - Joel
>=20
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