[SI-LIST] Re: Ferrite Bricks on power Supplies
- From: Raymond Anderson <Raymond.Anderson@xxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Mon, 01 Mar 2004 10:42:57 -0800
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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