RE: [SI-LIST] : Bulk Caps -> Inductor? -> Plane

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From: Tom Dagostino (tom_dagostino@mentorg.com)
Date: Tue Apr 11 2000 - 14:13:36 PDT


We used to do this kind of thing all the time in oscilloscope designs. The
purpose is to keep noise generated on the board from poluting the power
distribution system. The LC network acts as a low pass filter. There
should be C on both sides of the L. You will not really draw the current
spikes through the inductor, there will be some average current passing
through it charging the C on the board side of the L. The distributed C on
the board will be charged from that bulk C, thus the sizing of that bulk C
is important.

Tom Dagostino
ICX Modeling Group
tom_dagostino@mentor.com
503-685-1613

-----Original Message-----
From: owner-si-list@silab.eng.sun.com
[mailto:owner-si-list@silab.eng.sun.com]On Behalf Of mjs
Sent: Tuesday, April 11, 2000 11:59 AM
To: si-list@silab.eng.sun.com
Subject: [SI-LIST] : Bulk Caps -> Inductor? -> Plane

I am not sure if the list accepts attachments, but here goes.
If the attachment is stripped by the listserve, please find
it visible at www.enteract.com/~mjs/induct.gif (16k)

--

The attached gif show a snip from a schematic currently in layout.

The transformer shows the secondary of the SMPS, connected to a whole mess of bulk capacitance, especially on +5V net.

Please note L202 series inductor inserted between the bulk capacitance and the connection to the power plane. This worries me most. There seems to be no valid reason to stuff series inductance between the bulk and the plane. Further, all the local ceramic IC pin bypass will also have to suck current through that same inductor. Seems like a terrible idea to me. Comments welcome.

Matt Stanik Sr. PCB Design Eng. Charles Industries, Ltd.

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