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

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From: jrbarnes@lexmark.com
Date: Tue Apr 11 2000 - 13:53:23 PDT


Matt,
That is strange, to put an inductor between the bulk capacitors on the output of
the switching regulator and the power plane that it is supposed to feed.

For DC-DC converters, such as for converting +5V to the core voltage of an x86
microprocessor, it is fairly common to put an inductor/ferrite bead between +5V
and the bulk capacitors that feed the high side FET's:
* http://www.fairchildsemi.com/an/AN/AN42.pdf page 5.
* http://www.fairchildsemi.com/an/AN/AN50.pdf page 5.

This inductor helps keep noise from the DC-DC converter from getting onto the
+5V rail. It also reduces the inrush current into the bulk capacitors when
power is first turned on. Excessive inrush currents can degrade tantalum and
aluminum electrolytic capacitors, greatly reducing their life time.

R. Kenneth Keenan is/was a big advocate of using a ferrite bead on the supply
voltage for cards that plug into backplanes. His idea was to form pi-filters
between capacitors on the backplane and capacitors on the cards, again to reduce
noise getting out through the supply voltage net. He discussed this scheme in
great detail in his books:
* Digital Design for Interference Specifications (The Keenan Corporation,
1983).
* Decoupling and Layout of Digital Printed Circuits (The Keenan Corporation,
1985).

                                         John Barnes Advisory Engineer
                                         Lexmark International

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