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I am going to try using a QX5252 to drive 300ma led. The specs imply a 30uH inductor in the 'boost' circuit would do the trick, which I would wind on a small ferrite ring I have, the kind used to suppress rfi by running ribbon cables through it.

But those rings are not 'meant' for hi Q inductors. Are they maybe specially made to be lossy for rfi suppression, and hence not a good choice for a switch mode power application?

A previous question on SE advised about ferrite properties here, and, later, here.

I tried to tag this with 'Q' or quality factor, but it wasn't there.

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There are various formulations of "ferrite" with different properties. Some are deliberately intended to absorb energy, like ferrite tiles. Something intended to be common mode filter may be made out of more lossy material, even if most of its AC blocking properties come from the series inductance it adds to a line.

Without a spec for the ferrite material you have, there is no way to know without actually measuring it. Unfortunately, measuring power loss in inductors is not as simple as other measurements where you simply put a meter to the device. One way to measure loss is to put a high frequency sine wave accross the inductor and measure the phase angle between the voltage and current. For a ideal inductor, these will be 90° apart. For a ideal resistor, they would be 0° apart. A lossy inductor will be somewhere in between. From the phase angle you can calculate the power dissipated by the inductor. This will vary with frequency and go up more steeply near the end of the core material's useful frequency range.

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