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LM5146-Q1-EVM12V: Clarification on filters for the design

Paul Popoviciu
Prodigy 20 points
Part Number: LM5146-Q1-EVM12V
Other Parts Discussed in Thread: LM5146

The schematic for the LM5146-Q1-EVM12V has an EMI filter that contains a ferrite bead, a transformer, and a pi filter, but is this not overkill? How does each filter compare to each other in terms of price and noise reduction? The schematic is on page 21 of this pdf: https://www.ti.com/lit/ug/snvu591/snvu591.pdf. If I were to also make a board using the LM5146 at 255kHz, would a pi filter be enough for noise reduction?

  • +1
    TI__Mastermind 22299 points

    Paul,

    The common mode choke and ferrite bead are useful for high-frequency EMI in the FM band, 30MHz-108MHz. 

    Pi filter will not help much at these frequencies.

    The pi filter is for noise reduction at the fundamental frequency(255kHz).

    If you're only worried about noise at switching frequency, then a pi filter is sufficient.

    Hope this helps,

    -Orlando

  • 0 in reply to Orlando Murray
    Prodigy 20 points

     Thanks for your help. If I also wanted to filter out input noise from my power supply to the DC converter, would you recommend the ferrite bead or the common mode choke? The noise I am filtering is 200mV p-p at approx 20 MHz. 

  • +1 in reply to Paul Popoviciu
    TI__Mastermind 22299 points

    Hi Paul, yes then the ferrite bead stage will help with 20MHz noise.

    Note that for 20MHz frequencies, the smaller capacitances like 100nF and 10nF are more effective for filtering.

    Be sure to check the ceramic capacitor datasheets; At frequencies above the self-resonant frequency (SRF) the parasitic inductance starts to dominate and the impedance starts to grow larger instead of reducing. 

    You want to minimize the capacitors characteristic impedance at your target frequency of 20MHz for the ferrite bead "pi filter" network to be effect.

    Hope this helps,

    -Orlando