[SI-LIST] Re: Damped voltage oscillation

Andy,

Thanks for the reminder that the wording was not accurate.  In fact the
effectiveness of filtering with resistor (instead of ferrite bead) is lower
in the frequency range where the inductive reactance would be higher than
the resistor value.  To get the highest attenuation at very low frequencies,
it might be a good idea to add a series resistor to the ferrite bead such
that their total resistance is still acceptable in terms of voltage droop.

Regards,
Istvan


----- Original Message -----
From: "Ingraham, Andrew" <a.ingraham@xxxxxxxx>
To: "silists" <si-list@xxxxxxxxxxxxx>
Sent: Monday, June 23, 2003 9:48 PM
Subject: [SI-LIST] Re: Damped voltage oscillation


> > Changing the inductor to a resistor helps to dampen the oscillation and
> > ringing.  However, the effectiveness of filtering at low frequencies
also
> > goes down.
>
> Actually, changing to a resistor improves the effectiveness of the low
> frequency filtering, which is why a resistor may sometimes be a better
> choice.  But it also degrades the voltage stability or regulation at low
> frequencies, which can increase the low frequency ripple and/or voltage
> droop seen on the filtered side, caused by the load current.
>
> Ringing can happen even if the LC resonant frequency is nowhere near the
> oscillator's frequency or a harmonic of it.  All it takes is spikes in the
> current (at any suitably low frequency), and a high-Q LC filter.  The
> ferrite bead, or resistor, reduces the Q.
>
> Regards,
> Andy
>
>
>
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