understanding ferrite beads in EMI suppression

Discussion in 'General Electronics Chat' started by DaveH, Jul 21, 2009.

  1. DaveH

    Thread Starter Active Member

    Jan 1, 2009
    I was thinking about this subject because I'm using an IC where the manufacturer says you need to place ferrite beads in series with the chip's power supply rails.

    I understand the physics of inductance and how an inductor in series with a load can reduce the rate of change of current through the load and therefore the effects of parasitic inductance - correct me if I'm wrong, but my understanding is dI/dt will be reduced and whatever parasitic L there is lying around nearby around will have smaller amounts of noisy V induced in it.

    I then started thinking the following: well if you're reducing the high rate of change current transients in the power rails that means the load (in this case a digital IC) will never draw the full peaks of current that it would otherwise - therefore surely that will screw up it's internal operation completely - of course that doesn't happen as using ferrites like this is extremely common - therefore that means that as the thousands or milliions of transistors in the IC switch on and off, they don't actually need their full saturation current?

    There is another post about this on the forum, where a link was provided to a document where it said (paraphrasing):

    "an inductor in series with a switch presents the switch with an inductive load at turn on so that it switches on with zero current".
  2. studiot

    AAC Fanatic!

    Nov 9, 2007
    To answer you last part first,

    The presence of switching noise it not a problem for digital circuits because of the significant voltage changes at switching of the digital and error correction circuitry.
    However it does present a problem to sensitive analog circuitry - this is the EMC dimension.

    I am guessing that this is a switched mode power supply IC.

    The ferrite bead (and probably a small ceramic capacitor ) limits the common mode switching spikes which appear on both the supply and ground lines.