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Ferrite beads in SMPS design with Lm2678?

Tony Philip
Intellectual 260 points
Other Parts Discussed in Thread: LM2678

Hi,

I am quite new to using ferrite beads for noise cancellation in SMPS . I wanted to know if its possible to attenuate the output noise of the LM2678 output after the L-C filter further by using ferrite beads?

Could you also please specify how to find the appropriate bead size and specification, I heard that beads have different composition according to their use.

  • 0
    Intellectual 260 points
    Also does Chokes Help decrease the Nosie too?
  • 0 in reply to Tony Philip
    TI__Genius 10095 points
    Ferrite beads are usually specified as having a certain impedance at a given frequency and a rated current.

    For buck converters they are sometimes placed before the input capacitors and after the output capacitors in the schematic.

    The noise that is typically trying to be attenuated is the rising edge of the switching current. This current can rise in 10ns and thus creates frequencies in the 100MHz range. The combination of a ferrite bead rated for the load current followed by an additional capacitor can create a divider to attenuate some of this very high frequency noise. The capacitor must have a lower impedance than the ferrite bead at the frequency of interest so usually a small case size ceramic coupled with vias to the ground plane or a three terminal capacitor is used.

    Chokes or inductors tend to have more inductance and thus are better at attenuating frequencies below 100Mhz than a ferrite bead. However the additional windings causes parasitic capacitance, which decreases their high frequency (> ~50Mhz) performance. Check the self resonant frequency of a choke to see at what frequency the parasitic capacitance makes the choke performance degrade.

    regards,
    Marc
  • 0 in reply to Marc Davis-Marsh
    Intellectual 260 points

    The Lm2678 has a switching frequency of 2.5MHz.. Does this mean that I can atteneate the noise on it?

    Also if the load current is 5A , what should be the IRMS specification of the CM Choke?

  • 0 in reply to Tony Philip
    TI__Genius 10095 points

    The Irms of the choke should ~ 6A or greater.  The RMS is 5A + [Inductor Current Ripple / SQRT(3)].  The inductor current ripple is designed to be approximately 1/3rd of the full DC load. so  Irms = 5A + [5/3/ SQRT(3) ~6A.  This assumes the output capacitor doesn't shunt any ripple current, which is a worse case assumption. 

    The LM2678 has an internal switching frequency of 260kHz.  Usually to attenuate noise in this frequency range you use the LC of the Buck.  The Beads are usually for the higher frequency switching noise. 

    Regards,

    Marc