[SI-LIST] Re: Ferrite Bricks on power Supplies
- From: "Tom Dagostino" <tom@xxxxxxxxxxxxx>
- To: <cchalmers@xxxxxxxxxxx>
- Date: Tue, 2 Mar 2004 08:49:06 -0800
Chris
It all depends on why they are there in the first place. If they are there
to keep the switching noise of the regulator from getting into the system,
they likely are required. If they are there to keep noise from the system
from disturbing the regulator's feedback loop, they are required. It is
hard to say. Have you contacted the power supply vendor for details?
I doubt a power supply company, in a very price competitive market, would
spend the money to put extra expensive components that take up space and add
weight for no particular reason.
Tom Dagostino
Teraspeed Consulting Group LLC
Device Modeling Division
13610 SW Harness Lane
Beaverton, OR 97008
503-430-1065
tom@xxxxxxxxxxxxx <mailto:tom@xxxxxxxxxxxxx>
Teraspeed Consulting Group LLC
2926 SE Yamhill St.
Portland, OR 97214
http://www.teraspeed.com
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Chris Chalmers
Sent: Tuesday, March 02, 2004 1:09 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Ferrite Bricks on power Supplies
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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