[SI-LIST] Re: Ferrite bead question
- From: Joel Brown <joel@xxxxxxxxxx>
- To: Curt McNamara <CurtM@xxxxxxxxxxx>
- Date: Mon, 10 Apr 2006 08:03:37 -0700
Curt,
Good answer, I was thinking along those same lines, but I could not find
any information to support it. Manufacturers of ferrite beads nor IC
manufacturers mention this concept. Nor could I find out how much
current the device (or any device in general) consumes at the
fundamental or harmonic switching frequencies.
It seems manufacturers are willing to characterize the I/O
characteristics of a device through IBIS or HSPICE models but there is
no such modeling techniques for power which would be useful for
designing the bypass capacitors / ferrite beads.
Joel
Curt McNamara wrote:
> 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.
> Senior Electrical Engineer
> Logic Product Development
> 411 Washington Ave. N Suite 101
> Minneapolis, MN 55401
> Tel 612-436-5178
> Fax 612-672-0443
> curtm@xxxxxxxxxxx
> www.logicpd.com
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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 -----
> 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
>> DC
>> power lines. Sometimes a particular P/N is specified and sometimes
>>
> just
>
>> an impedance is given.
>> Wouldn't I want to choose a ferrite bead that has the highest
>>
> impedance
>
>> possible at the frequencies of interest, and that the current rating
>>
> and
>
>> DC resistance are appropriate? Would the frequencies of interest be
>>
> the
>
>> fundamental switching frequency of the part and possibly the 3rd and
>>
> 5th
>
>> harmonics?
>> Thanks - Joel
>>
>>
>>
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