At another company I worked for we found an application with a ferrite on the output of a regulator, presumably to further low-pass noise from coming through into a sensitive area. This ferrite material's inductive vs. resistive curves were such that there was a significant frequency area where it was NOT acting as a lossy inductor, but a rather high Q inductor. It ended up resonating with the circuit capacitance! In this case simply selecting a different ferrite material fixed the problem, but it reminds us that ferrites are not always low-Q lossy inductors :) Brad Henson Raytheon Raymond Anderson <Raymond.Anderson To: si-list@xxxxxxxxxxxxx @Sun.COM> cc: Sent by: Subject: [SI-LIST] Re: Ferrite Bricks on power Supplies si-list-bounce@fr eelists.org 03/01/2004 10:42 AM Please respond to Raymond.Anderson 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. ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List technical documents are available at: http://www.si-list.org List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List technical documents are available at: http://www.si-list.org List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu