What is Self Resonant Frequency?

What is Self Resonant Frequency (SRF) in Inductors?
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Raghav Kapur - everything RF

Sep 1, 2017

The self resonant frequency of an inductor is the frequency at which the parasitic capacitance of the inductor resonates with the ideal inductance of the inductor resulting in an extremely high impedance. At this frequency the device looks like an open circuit. 

The parasitics of an inductor vary based on the type of inductor - wire wound inductor, multi-layer inductor, conductive film inductor etc.

For example, the equivalent circuit of a wire-wound inductor can be seen below. As can be seen in the circuit, there are two parasitic capacitance's that are in parallel to the inductor - One is the lead capacitance and the other is the coil capacitance. The coil capacitance is a result of the individual turns of the coil being close to one another. The lead capacitance is usually very small and is usually ignored.

As frequencies rise the impedance of the parasitic capacitance drops until it’s magnitude equals that of the ideal inductance. The point where this happens is called the self resonant frequency. 
The inductance only acts like an inductor up to its Self Resonant Frequency (SRF). At the SRF the impedance becomes very high and the inductor can be used as a choke to attenuate signals near at the SRF.
 Inductance and Impedance for a 100 nH Wirewound Inductor (Source: Coilcraft)