(6) Self Resonance Frequency of Inductor | Tips for COIL users | SAGAMI ELEC CO., LTD.
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(6) Self Resonance Frequency of Inductor | Tips for COIL users(Part-1)

(6) Self Resonance Frequency of Inductor

The sixth topic is about "self resonance frequency of inductor". Actual components are different from ideal ones in many aspects. Some aspects show unexpected characteristics.

What is self resonance frequency?

Normal frequency characteristic of impedance of inductor (Z=R+jX) is measured like a blue line in Graph-1(plotted only jX). (Graph is according to our CER1277B, 100uH) For your reference, red line shows the frequency characteristics of 100uH at ideal conditions.

In Graph-1, the frequency which shows reversed impedance polarity is called Self Resonance Frequency = SRF.

Impedance(X) characteristic
Graph-1 Impedance(X) characteristic

Surce of SRF

In actual world, if electrodes have width (size), capacitance (capacitor) occurs. Such capacitance is called as the parasitic capacitance, the distributed capacitance, the floating capacitance, and the stray capacitance.

Equivalent circuits
Figure-1 Equivalent circuits

Therefore, in general the capacitance (capacitor) Cp is added to the equivalent circuit of inductor in parallel as Figure-1.

This capacitance Cp and own inductance Ls of inductor resonate, and the frequency response is as Graph-1.

Generally, the resonance phenomenon doesn't happen only (with?) the inductance. However, it happens by the resonance itself without connecting capacitor in parallel, so it is called self resonance frequency (SRF).

The SRF is inevitable, but special coils exist, which decrease Cp and move the SRF to higher value by devising the coil structure.

Impedance characteristic and Equivalent circuits
Graph-2 Impedance characteristic and Equivalent circuits

Actual SRF of inductor

Measurement value for the SRF and stray capacitance (Cp) of our power inductor (CER8065B) are described in the Table-1.

Table-1 C2012CB and 7E06LB specifications
Inductance
Ls(µH)
Self
Resonance
Frequency(MHz)
Self
Resonance
Frequency(MHz)
10 26.0 3.7
100 5.2 9.4
1000 1.2 17.6

In general, the value of stray capacitance does not increase in proportion to the inductance. However, although it doesn't increase so much as the inductance, when the inductance value increases, the stray capacitance will also increase and the self resonance will decrease.

The differences of impedance characteristic for each inductance (position of SRF) are shown in Graph-3.

Impedance characteristics with different inductance
Graph-3 Impedance characteristics with different inductance

Precautions for use related to SRF

  1. When the inductors are mounted on a printed wiring board, the stray capacitance increases due to wiring. Therefore, the SRF moves to lower area compared with the case of single inductor.
  2. The stray capacitance of inductor is relatively small. So, the self resonance frequency value may significantly change due to increased stray capacitance (capacitance between printed patterns) by mounting.
  3. Usually, if the frequency is less than or equal to 1/10 of the self resonance frequency, such effect should not be a big problem.
  4. Impedance value increases around the SRF. So if you make better use of that, you may expect to obtain the effect more than inductance value. However it requires a care, for the SRF may vary widely because it is not created intentionally.

Author

Yasuo Hoshino
born in1954, is a prominent coil specialist at SAGAMI ELEC CO., LTD.
He joined Sagami Musen Works, a former company of SAGAMI ELEC, in 1976 and has been working in the engineering department for over 40 years.
During his career at Sagami, he has served as a technical manager and a member in board.
He supported the firm as an technical advisor and retired at the end of March 2024.His technical advice helped many engineers.
Photography is his life's work. His favorite animals are cats and cricket.

Notes
  • Some of the products listed in this document are no longer in production.
  • As some time has passed since the article was written, the information provided may still contain outdated content.

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