Why is Self Resonant Frequency important?
SELF RESONANT FREQUENCY
Practical inductive components are not perfect inductors; they have stray resistances and capacitances associated with them. For certain components, especially those with a low inductance value, the impedance of the stray capacitance can become significant when compared to that of the inductance.
XL = 2πfL XC = ½ πfC
In most power electronic circuits the power magnetics operate well below their resonant frequency in order to avoid undesirable circuit oscillations and a production test for resonant frequency is not required.
In some circuits for resonant converters and for filters and oscillator circuits the resonant frequency is important and a check of each part in production is desirable to guarantee circuit performance.At a sufficiently high frequency, the capacitive impedance can dominate, making a measurement of the inductance impossible. Under these circumstances, any measurement instrument may report negative inductance values and measurement errors. Should these symptoms be observed, reduce the test frequency to avoid problems.
The frequency at which the inductive impedance equals the capacitive impedance (XL = XC) is known as the self resonant frequency (SRF) of the component. At this point, the phase angle of the impedance (which can be measured using the ANGL test) is zero. At test conditions where the frequency is low enough for problems with capacitive impedance to be negligible, the phase angle will be positive and close to 90 degrees. Therefore an ANGL test can be used during program development to confirm if measurement problems are due to the chosen test frequency approaching the SRF of the part under test. If the angle is significantly less than 90 degrees, consider reducing the test frequency.
See attached “Resonant Frequency determination on an AT3600 or ATi” file for details.
Note that stray fixture capacitance will add to the capacitance of the component and reduce the SRF. Performing compensation will remove the effect of stray fixture capacitance on the measurement of capacitance, but cannot remove its effect on SRF.
The Voltech AT Series testers are sophisticated high-speed production line testers that use Fourier analysis to determine amplitude and phase very accurately.