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Inductor selection always confuses me. There is a spec for inductors as self-resonant frequency. For example, on digikey, as SR frequency of inductors increase, the price may increase or decrease. But why? See this graph below, behavior of self resonant frequency response of the inductor:

self resonant frequency response

Assume we have a circuit board with SMPS has switching range in between 1-2 MHz and a few other analog blocks work in up to 4 MHz. In this case, is it smart to use an inductor with SR frequency below 1 MHz or over 4 MHz? And why?

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In this case, is it smart to use an inductor with SR frequency below 1 MHz or over 4 MHz?

Above the self resonant frequency the impedance of the "inductor" is capacitive i.e. it stops being like an inductor and exhibits the net impedance of a capacitor: -

enter image description here

A better picture would be the magnitude of the impedance: -

enter image description here

The inductor (and parasitic capacitance) form a parallel tuned circuit.

It's the same with capacitors - internal inductances will generally mean that at some arbitrary high frequency a capacitor will behave like a series resonant LC: -

enter image description here

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  • \$\begingroup\$ Ok that solves my problem. Could you suggest me a solution? I need an inductor with current rating at least 12 A, value must be between 33 uH and 100 uH, and my SMPS/Led Driver switching frequency is 2MHz. I couldn't find an inductor covers my requirements(digikey.com/product-search/en/inductors-coils-chokes/…) I used two of them on the link but they were useless so like you say their self resonant must be low. If I use two of this(digikey.com/product-detail/en/wurth-electronics-inc/74435588200/…) parallel for 41 uH, is it okey? \$\endgroup\$ – Alper91 Apr 26 '16 at 8:25
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    \$\begingroup\$ SRF wants to be at least 4 times higher than max switching frequency in my opinion and sometimes I go as far as 10x. You might be OK with the wurth inductor but it is a little close to my self-imposed limit. \$\endgroup\$ – Andy aka Apr 26 '16 at 8:28
  • \$\begingroup\$ @Tom please place this comment under the question and delete from under my answer. \$\endgroup\$ – Andy aka Apr 26 '16 at 9:30

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