FreeLists / si-list / [SI-LIST] Re: Ferrite Bricks on power Supplies

[steve weir <weirsp@xxxxxxxxxx>]

Chris, I think one important response that you missed was Ray's caution 
about the inductive reactance of ferrites at low frequency.

Whether you need to do something to suppress the supply outputs or not 
depends.  Frequently the answer is yes.  You can suppress by any number of 
means:

1) Discrete inductors such as the ferrite blocks over etch to avoid 
resistive losses.
2) Inductance through a little etch manipulation.
3) High frequency shunt capacitance.  A relatively small X2Y capacitor in 
the circuit 1 configuration can do very nicely here for $0.05 to  $0.15.

This is just a filter design, so if you know what your VRM looks like 
emissions wise you can optimize.

I suggest yes, place the HF filtering as close to the VRM as possible.  The 
reason is that any bulk capacitors are large and make good radiators, so it 
is best to knock the fast stuff down before it hits those radio towers.

Steve.

At 09:08 AM 3/2/2004 +0000, Chris Chalmers wrote:

>         Thank you all for your responses.  I understand what you are
>  saying.  The output impedance of the switching regulator on the
>  board is pretty high at the frequencies where the ferrite will be
>  operating anyway so no major problem.  At these frequencies 10MHz+,
>  the bulk decoupling and other local decoupling will be suppling
>  the current to the load.  I have a two part question.
>
>Do I really need them?
>
>If I do need them should they be before the bulk decoupling?
>(will this not impede the bulk decoupling ability to supply
>  current if they are before it, not to mention the interaction
>  with the inductor of a buck reg.)
>
>Best Regards
>
>Chris
>
>
>Tom Dagostino wrote:
>
> >Power supplies need to have low impedance.  But in the frequency range
>where
> >the ferrite bead has high impedance the power supply distribution on the
> >circuit board controls the impedance seen by the load, not the output
> >impedance of the power supply itself.
> >
> >
> >
> >
> >
>
>Tom is absolutely correct. "Most" garden variety VRM's and switching power
>supplies have a 'heartbeat' frequency ranging from a few 100kHz up to
>perhaps
>10MHz in the high performance units. The output impedance  of these units is
>very low at DC and stays quite low up to some corner frequency which is
>usually in the 100's of kHz to a couple MHz range. (this is usually a
>function of the switching regulators loop dynamics) Above that corner
>frequency the output impedance begins to climb.
>
>
>By the time you get to 100 MHz the power supply output impedance is quite
>high and the components controlling the power distribution system (PDS)
>impedance at that frequency are the decoupling capacitors and perhaps the
>power plane capacitance.  Inclusion of the ferrite component in the
>output of
>the power supply or VRM doesn't materially effect the PDS impedance at low
>frequencies because as Tom pointed out, the ferrite component impedance only
>becomes Hi-Z at higher frequencies. One thing that you might want to
>consider
>though is that the inclusion of extra inductance in the power supply path
>between the power supply and the bulk decoupling capacitors may effect the
>slew rate capabilities of the power supply under transient load conditions.
>
>
>
>
>-Ray Anderson
>Sun Microsystems inc.
>
>
>
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