RE: [SI-LIST] : Bead-choking the power supply--everywhere!

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From: Mark Gill (gillmf@nortelnetworks.com)
Date: Thu Sep 14 2000 - 04:04:16 PDT


Boris -

It does sound like overkill - shotgun approach, and touches on something
I've seen in the past - blindly applying rules. Certain
situations/applications require certain approaches. I have seen the single
capacitor and ferrite bead combination work well when implemented correctly,
and make things far worse when not. I have also seen plenty of situations
where nothing was done, and the systems pass class B. The trick in
implementing the concept (if you so choose) is in the parasitics. But a pi
filter approach seems excessive. Any expert worth her/his salt should be
able to explain how the fix resolves a concern. If you see waiving arms,
lookout!

Regards,

Mark Gill, P.E.
EMC/Safety/NEBS Design
C-MAC Engineering Design Services
RTP, NC, USA
(P) 919-991-7513

> -----Original Message-----
> From: Boris Yost [SMTP:yost@rainbowdisplays.com]
> Sent: Wednesday, September 13, 2000 2:59 PM
> To: si-list
> Subject: [SI-LIST] : Bead-choking the power supply--everywhere!
>
> Dear list members:
>
> I'm looking at a design right now--portions of which were inspired
> by an
> "EMI expert". Specifically, instead of ordinary decoupling practice,
> there
> are filters with a capacitor on each side of a ferrite
> bead...individually...for each and every device on the board. Almost all
> of
> the devices have "CMOS" outputs--the output goes to the high rail. Almost
> all of the devices are fully static logic. The system is a digital video
> display, it has one input, and the board output doesn't leave the box.
> 80%
> of the board, and everything doing real work, operates synchronously to
> the
> input clock.
> Is this "decoupling" scheme overkill, useful, or a disaster waiting
> to
> happen?
>
> Best regards,
> Boris Yost
>
> P.S. Order ferrite beads! Now! They will soon be on-allocation!
>
>
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