1
\$\begingroup\$

I am a little bit confused by the types of inductors you can buy. After looking through numerous datasheets, it seems to me that most inductors have a high Q-Factor only at high frequencies (around 100 MHz), and the Q-Factor for frequencies below 1MHz is not even plotted in the datasheet.

I, on the other hand, am looking for an inductor with a high Q-Factor for frequencies around 200kHz. Is there some special type that is designed for rather low frequencies? Or do I just have to accept that for some physical reason Q-factors are always low in the sub-MHz range? I can not really find that attribute anywhere?

Thanks, Peter

\$\endgroup\$
2
  • \$\begingroup\$ Q is difficult at low frequency because wire resistance R is independent of frequency and reduces Q. For some applications you can compensate with active electronics : useful search term "Q multiplier", often seen in amateur radio. \$\endgroup\$
    – user16324
    Commented Jun 10, 2020 at 10:21
  • \$\begingroup\$ As a point-of-reference, an inductor built to resonate near 1 MHz (along with a high-quality capacitor) might be able to achieve a Q near one thousand. That takes supreme effort, expensive and difficult-to-obtain Litz wire (more like Litz rope). The result is quite large, and nearby conductors should be avoided. A real pain. \$\endgroup\$
    – glen_geek
    Commented Jun 10, 2020 at 13:08

2 Answers 2

Reset to default
2
\$\begingroup\$

do I just have to accept that for some physical reason Q-factors are always low in the sub-MHz range?

The value of Q factor is inductive reactance divided by series resistance and, given that the inductive reactance magnitude is \$\omega L\$ (or \$2\pi F L\$), at lower frequencies Q factor is necessarily smaller. If you need to understand the Q factor of a candidate inductor calculate \$2\pi F L\$ and divide it by series resistance of the coil.

But make sure that the operating frequency of the coil is below the coil's self resonant frequency (SRF). If you don't ensure this, you cannot regard the coil as having a "normal" Q factor.

\$\endgroup\$
2
  • \$\begingroup\$ Ah, okay, that makes sense. So do I understand you correctly, that, as long as I stay well below the SRF, I can use a given coil with any frequency I want? In that case for maximum Q factor, I would be looking for a coil that a) matches the inductance, b) has the lowest available DC resistance while c) tolerating the maximum currents of my application. Or do I have to take something else into account? \$\endgroup\$
    – PeterO
    Commented Jun 10, 2020 at 10:12
  • \$\begingroup\$ Yes, that's pretty much it. \$\endgroup\$
    – Andy aka
    Commented Jun 10, 2020 at 10:22
1
\$\begingroup\$

For maximum Q on low frequency projects, I typically will wind custom toroid's with specific core permeabilities optimized for the Q/frequency/turns/inductance/wire gauge and physical constraints of the inductor itself.

Using Q curves from a manufacturer like MICROMETALS.com will help visualize these variables and guide you to the solutions you seek.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.