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What is a ferrite bead? How is it different from an inductor? How is it rated? What are its important parameters? What allows me to chose between an inductor or ferrite bead to filter out frequencies that are greater than 8 kHz in a signal with a current in milliamps? (The signals here I'm referring are input and output signals of a DC/DC Converter.)

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  • \$\begingroup\$ A ferrite bead is a component to build an inductor... \$\endgroup\$
    – PlasmaHH
    Commented May 24, 2015 at 10:05
  • \$\begingroup\$ Single design questions are required. \$\endgroup\$ Commented May 24, 2015 at 11:37

3 Answers 3

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Here's a fairly generic picture of a ferrite bead: -

enter image description here

  • At low frequencies (<10 MHz) its impedance is dominated by its reactance with resistive losses being low. It's basically operating like a piece of ferromagnetic material i.e. it is an inductor
  • As frequency rises, inductive reactance drops away due to eddy current losses in the core material. This also gives rise to extra resistive losses.
  • At high frequencies "losses" dominate the impedance curve thus making a series-connected ferrite bead useful as an attenuator of unwelcome high frequency RF interference.
  • At very high frequencies parasitic capacitance makes the bead less useful.

Ferrite beads tend to be useful from 10 MHz onwards. At 8 kHz they are generally useless.

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  • \$\begingroup\$ What is the irony that a real capacitor behaves like an inductor at high frequencies and a real inductor behaves like a capacitor at high frequencies, how come? \$\endgroup\$
    – quantum231
    Commented Jul 6, 2023 at 17:32
  • \$\begingroup\$ @quantum231 which answer did you read that in? Keywords like irony and behave only appear in your comment. \$\endgroup\$
    – Andy aka
    Commented Jul 6, 2023 at 17:35
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Ferrite beads have uses in EMI filtering/minimisation, so that's generally in the 10s of MHz & beyond, and in radio. Commonly spec'd as "Ohms @ X freq", e.g. "60ohms @ 100MHz". Often paired with decoupling caps. In their simplest form they're literally a lump of ferrite circling a wire - hence "bead", though there's more complicated physical arrangement these days now too.

To filter the frequencies you're talking about, you'd almost certainly be looking for an "inductor" along with carefully chosen caps & resistors.

Yes, this is a fairly vague answer to a very general question :)

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Very good resource to understand ferrite bead is here. https://resources.altium.com/p/how-do-ferrite-beads-work-and-how-do-you-choose-right-one

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