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Measurements >> Other Measurements >> Inductor behavior at low frequencies (10KHz-1MHz)
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Message started by forghani on Feb 11^th, 2004, 3:28pm

Title: Inductor behavior at low frequencies (10KHz-1MHz)
Post by forghani on Feb 11^th, 2004, 3:28pm

Hi,

I am measuring the power inductors, for dc-dc converter chips, at 10mA using a 4192A. The inductors I measured were discrete 0.15uH, 1uH, and 10uH. The measuured inductace goes systematicly down by about 5% from 1Khz to upto a frequeuncy, say 2MHz, and then it starts to rise after that. I can understand the rise of the inductance after a certain frequency because of the stray capacitor that is in parallel with the inductor (SRF effect); however, I can not understand why it is going down at lower frequencies. Is it just a measurement instrument error or there is some parasitics effect I am not aware of?
The inductors use ferrite core.

Title: Re: Inductor behavior at low frequencies (10KHz-1M
Post by Ken Kundert on Feb 11^th, 2004, 5:37pm

Forghani,
I have seen very similar behavior in capacitors as a result of dielectric absorption (see Modeling dielectric absorption in capacitors at www.designers-guide.com/Modeling). Ferromagnitism is the dual of dielectric absorbtion and so has a similar effect. Effectively what is happening is that at low frequencies the magnetic domains have more time to line up with the magnetic field imposed by the winding, which supplements the field and hence the inductance. The time constants of the domains are widely distributed and so you see this gradual increase in the inductance as frequencies decrease.

Turns out, the same is true for skin effect (see Modeling skin effect in inductors also at www.designers-guide.com/Modeling). With skin effect, the impedance exhibited by an inductor is roughly
Z = jwL + sqrt(jw)/H
and if you define
L_eff = Z/(jw)
then
L_eff = L + 1/(sqrt(jw)H)
which increases slowly as w --> 0.

Which effect is to blame depends on the frequency range and type of inductor. I suspect in your case it is ferromagnitism that is the culprit.

When measuring the inductance, be sure to also measure the real part of the inductor's impedance. It is key to being able to extract and build an accurate model.

-Ken

Title: Re: Inductor behavior at low frequencies (10KHz-1M
Post by forghani on Feb 12^th, 2004, 12:41pm

Thank you for the reply.
I tested an air core inductor, and I can see the reduction in the inductance with almost the same rate. I found out that the difference in accuracy of my test instrument from low frequency to the higher frequency is about the same amount of reduction in the inductor value. However, I can not explain if it is a measurement insturment resolution error, why I do not see a random error but I see a systematic error?