Selecting inductor for buck converter.

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%

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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.

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.