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:
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?
Best Answer
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: -
A better picture would be the magnitude of the impedance: -
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: -