Re: Re[2]: [SI-LIST] : power supply filtering and bypassing

Istvan NOVAK (istvan.novak@worldnet.att.net)
Sun, 8 Mar 1998 12:55:26 -0500

Hi Ray,

This discussion points toward the importance of losses for decoupling
inductors. Fortunately most decoupling inductors today are made purpously
lossy with a loss characteristic matching the decrease of mu at higher
frequencies. TDK used to show plots on their website with the inductive and
resistive parts of the impedance of cored decoupling inductors.

Regarding the simulation of frequency-dependent decoupling inductors, one
would need a tool which would take both the inductance and series resistance
as a tabulated frequency dependent input. I hope that when the w-line
element takes tabulated inputs, HSPICE with w-line element transmission
lines will be a good tool for that. Hopefully setting the capacitance of
the transmission line to zero (or to very low values) will not upset the
program. It would be interesting to hear Dmitri's comment on this issue.

Thanks

Istvan Novak, SI Engineer
SUN Microsystems
-----Original Message-----
From: Ray Anderson <raymonda@radium.Eng.Sun.COM>
To: si-list@silab.Eng.Sun.COM <si-list@silab.Eng.Sun.COM>
Date: Friday, March 06, 1998 12:58 PM
Subject: Re[2]: [SI-LIST] : power supply filtering and bypassing

>
> I just wanted to echo Ed's (ed_crean@smtplink.spectrian.com) comments
regarding
> power supply filtering add a couple amplifiying remarks.
>
> In many cases designers have used the small 3 terminal integrated filters
> that combine a couple of capacitors and a ferrite inductor into a single
> unit (typically from TDK, Murata and others) and have been suprised that
> the addition of the filter exacerbates the problem rather than solving it.
>
> If you look at the frequency response curves in the vendor's catalog you
> see a nice, well behaved, low-pass characteristic. The gotcha' is that
these
> parts are characterized in a 50 ohm system. Indeed, in a 50 ohm system
they are
> well behaved low pass filters. However, when applied as power supply
filters
> this assumption falls apart. In that application they are being driven
from
> a very low impedance source (typically on the order of tens of milliohms
or less).
> When driven from a very low impedance then nice low pass response degrades
to
> a very peaky response somewhere near the anticipated roll-off frequency.
>
> I have seen cases with peaking on the order of 10 dB and more. The net
result is
> that any noise energy on the input of the filter that you are trying to
> squash can actually appear on the load side of the filter with a much
higher
> amplitude than on the input side.
>
>
> This phenomena can be modeled quite nicely with spice and analyticaly if
you
> take the time to work through the transfer function of the LPF.
>
> A typical fix is to make sure a small amount of resistance is in series
> with the input of the filter to effectively raise the driving impedance
> that it sees. One needs to make sure that this doesn't introduce excessive
> DC voltage drop when the load is drawing max current. This typically works
> well for low current loads such as isolated Vcc pins for PLL's and the
like.
>
> And as Ed mentioned in an earlier post, this issue is also directly
relevant
> to other power supply filtering schemes composed of discrete parts. The
same
> caveats apply.
>
> -Ray Anderson
>
> Staff SI Engineer
> Sun Microsystems Inc.
> raymonda@radium.eng.sun.com