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Filtering the DC power supplies

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Advanced Member level 3
Nov 3, 2018
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Hi, I have looked at some FPGA development boards schematic where they also use components like FERRITE-96, see attachment. Is that common mode chock or any other devices ? In case of common mode chock, what are the key parameters to select such components ?

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"FERRITE-96" can´t be a common mode choke, because common mode chokes have at least 4 connections.

It rather is a "ferrite bead" ... and "96" could mean "N96" core material.


What is the selection criteria ferrite bead. Like in FPGA development boards we have several busk converters. Do we need such ferrite bead at the output or just at the input to the dc-dc converter ?

You can review the Evalboard bill of material published by Xilinx. It reveals L19, L64 etc. as Laird 28F0121-1SR-10.


The problematic thing with this and similar ferrite beads is the saturation characteristic, see below.


The part is suited to mask EMI issues if you operate the DC/DC converter with minimal load during EMI tests, but doesn't fix it. Thus I doubt that the selection is well considered.

As for your other question, consider that the input current of a buck switch is discontinuous (square wave) while output current is continuous (triangle). Respectively input needs additional filtering.
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N96 refers to a nominal impedance of 96 ohms at 100 MHz (and Idc = 0).
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The second image shows some sort of LC filter module, judging from its symbol it contains both a common mode choke and a single ended filter. Whether such a thing is suitable in end-user application is debatable.

Hi, yes they used common mode chock with the external power supply 12 V which is U16 in the picture in first post then the filtered 12 V is supplied to several dc-dc buck converters to generate low voltages for kintex development board, there they used ferrite beads to power the dc-dc converter but not used at the outputs where they have inductors and capacitors.

The use of common mode chock and ferrite beads is to reduce EMI, right ? but how do we select these devices.

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