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Selecting inductor for buck converter.

Discussion in 'Electronic Design' started by acct94528928, Mar 15, 2008.

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  1. acct94528928

    acct94528928 Guest

    I'm trying to find a suitable inductance for the TI TPS54283
    "2A Dual non-synchronous converter with integrated high-side mosfet".
    Calculating the inductance value in [H] and the current [A] is no problem.
    The DCR [ohm] should aswell be kept low. So far so good, but then there seems
    to be few other parameters to need to be determined:

    * Allowable DCR range
    * Q @ Frequency
    * Self resonant frequency
    * Shielding
    * Tolerance
    * Type:
    Drum core
    Ferrite core
    High power, High current
    Metal composite core
    Powdered iron core

    How does one find suitable values for these characteristics ..?

    Links:
    http://focus.ti.com/lit/ds/symlink/tps54283.pdf
    http://search.digikey.com/scripts/DkSearch/dksus.dll?Cat=196627
     
  2. MooseFET

    MooseFET Guest

    Apply ohms law and hot much temperature rise you can stand.
    You don't usually think in turns of Q for such inductors. Many makers
    will spec a suggested AC current level. That is a more direct
    indication than Q. You need the Q to be good all the way from the
    switching frequency up to about its tenth harmonic. Think of the Q as
    a resistor in parallel with the inductance and consider the power into
    that resistance.
    Way above the operating frequency of the switcher.
    This is needed mostly of EMI reasons.
    Assume about 25%
     
  3. mook Johnson

    mook Johnson Guest


    You must calculate the current waveform that will be going through the
    inductor. You want both the average DC level and the p-p current ripple.
    Find an inductor that can handle the DC+peak current expected on the output
    + ~20% for margin.

    Look at the frequency range the current is rated for and compare to you
    switching frequency. You want these to overlap so the switching losses will
    have been measured and guaranteed by the manufacturer. Self resonant
    frequency should be at least 5 times the switching frequency preferably
    higher.

    Shielding is very nice so the PSU noise is less likely to radiate into
    adjacent circuits.

    For tolerance, make sure the supply can handle the max and min inductance.

    Type is typically linked to the degree of shielding availale. Drun core
    with shields, torroids and ferrite and composite cores are common.
     
  4. MooseFET

    MooseFET Guest

    Also check the saturation effect at the peak. You want to not be too
    far into saturation at the peak of the waveform. It is not uncommon
    for the peak current rating of an inductor to be where significant
    saturation is happening. This will increase the rate of rise in the
    current. Stay short of this point if you can.

    I say 10 times. A lot depends on the exected duty cycle. Do an FFT
    on the expected waveform.
     
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