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Here is an excerpt of an inductor datasheet from RS Components for Shielded metal composite power inductors:

excerpt from datasheet showing one test frequency for each inductor model

All the tables have a test frequency. Does this mean that the inductance value, DC resistance, maximum DC current and saturation current, are all dependent on frequency of an input and thus not fixed?

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    \$\begingroup\$ Testing costs money so many if not most do not test the span to provide curves. I wish they did, but alas. \$\endgroup\$
    – DKNguyen
    Sep 18 at 0:28
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    \$\begingroup\$ quantum231 - Hi, Please remember the site rule which requires that when a post includes content (e.g. text, image, photo etc.) copied or adapted from elsewhere, that content must be correctly referenced. As a minimum, for online material the source webpage or PDF etc. should be linked (see that rule regarding references for books / articles etc.). In order to help you, I found what I believe to be the source PDF link & added it. For the future, please remember it's your responsibility to do that :) Thanks. \$\endgroup\$
    – SamGibson ♦
    Sep 18 at 1:23

3 Answers 3

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Lots of vendors provide more than just one frequency. Murata for example provides SPICE models and has an online tool for viewing inductor properties:

Simsurfing 1217AS-H-220M entry

https://ds.murata.co.jp/simsurfing/powerinductor.html

As you can see, inductance does vary with frequency, especially as you approach the resonant frequency of the device. For a lot of things though the exact value is less critical, so just knowing that you're going to run it below the resonant frequency is good enough, which is why you see that in a lot of datasheets. But for other applications where the exact values matter (e.g. RF filters), you may want to check that the part you're ordering will be ok over the range of interest.

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  • \$\begingroup\$ ok, lets clear this, an inductor's inductice reactance dominates before we reach resonance. This means that the impedance increases until we reach resonance. Then, the parasitic capacitive reactance dominates. This causes the reactance to start to decrease. However, this is the frequency impedance curve on the left. How come the inductance itself is changing with frequency as shown on the right? Inductance depends on things like number of coils in wire and area of the coil and core material right? \$\endgroup\$
    – quantum231
    Sep 18 at 5:28
  • \$\begingroup\$ @quantum231 What a circuit sees is only impedance, not "inductance". This graph is provided for convenience for users, so they know where it stops being a good inductor. If you connected this inductor to a circuit which expected a perfect inductor, and injected a signal at some frequency, the graph on the right shows that your circuit would behave as if an inductor of whatever value was connected. Note though that this graph is only approximating the impedance response; the phase response will likely be something completely different. \$\endgroup\$
    – Graham
    Sep 18 at 13:14
  • \$\begingroup\$ @quantum231 to add to Graham's point, at low frequency the material properties vary slightly with frequency. As the device approaches resonance, current reflects back and forth through the device due to its parasitic capacitance creating the appearance of a different inductance (at least to an external circuit). \$\endgroup\$
    – user1850479
    Sep 18 at 14:00
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    \$\begingroup\$ @quantum231: At any particular frequency, any network consisting of ideal resistors, capacitors, and inductors will behave as an ideal inductor or capacitor in series or in parallel with an ideal resistor (all four combinations are possible). At increasing frequencies, the device's behavior would be consistent with an increasingly high-valued inductor in parallel with decreasing resistance. \$\endgroup\$
    – supercat
    Sep 18 at 17:24
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Yes, inductance can change with frequency, depending on the core material and construction. Many core materials will exhibit changes in permeability with frequency.

AC resistance (dissipative resistance, not inductance) will also change with frequency due to skin and proximity effects.

Of course core losses will increase with frequency and flux density too.

It would be useful if the manufacturers would show a sweep of inductance vs. frequency, but inductors don't tend to be well specified. Even giving the test frequency is a plus.

One possibility for better data is to wind your own inductor, companies like Micrometals supply very good data on their materials and cores. For example, here's a curve of permeability vs. frequency for -52 iron powder (which shows why inductance would vary with frequency for this core):

enter image description here

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  • \$\begingroup\$ We need to know a lot about a component before we stick it into a product and sell it. So I do not understand why insufficient information would be provided in the datasheet. Inductance itself changing with frequency is confusing for me? Inductance depends on things like number of coils in wire and area of the coil and core material right? \$\endgroup\$
    – quantum231
    Sep 18 at 5:30
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    \$\begingroup\$ @quantum231 Sometimes you need to know a lot, other times the details aren't critical. You can always acquire a sample and test it if the details matter. \$\endgroup\$
    – John Doty
    Sep 18 at 13:18
  • \$\begingroup\$ @quantum231 I edited my answer to give more information. \$\endgroup\$
    – John D
    Sep 18 at 15:38
  • \$\begingroup\$ When we intend to use inductors on PCB, can we really create that tiny inductor component ourself? \$\endgroup\$
    – quantum231
    Sep 19 at 15:38
  • \$\begingroup\$ @quantum231 It depends on your requirements. I've wound many inductors that were mounted on PCBs, but you can probably get the smallest size solution with an off-the-shelf part. \$\endgroup\$
    – John D
    Sep 19 at 15:56
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Yes it can change with frequency but it’s unlikely to matter more than single digit percents in the sub MHz range.

More significant is the initial tolerance (usually at least 10%) and the 20% droop you can get over its rated current.

(Saturation is usually defined as the current when inductance has dropped 20%. Some core materials hold steady then fall off a cliff, others are a straight line sloping down meaning you’d lose ~10% of inductance at half the saturation current. Good data sheets will have this curve).

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  • \$\begingroup\$ Inductors behave like capacitors above saturation current limit i.e DC resistance dominates? \$\endgroup\$
    – quantum231
    Sep 18 at 5:31
  • \$\begingroup\$ Yes. Inductor is a coil of wire so as it loses its magnetic properties it becomes more like a regular piece of wire - just an R. Though all indicators will retain some L and the actual curve varies widely. \$\endgroup\$
    – asdf30
    Sep 18 at 11:57
  • \$\begingroup\$ By the way, since inductors will usually contain some sort of core material which increases the inductance value, why would someone use aircore inductors at all? Air core means that its inductance is going to be quite poor. \$\endgroup\$
    – quantum231
    Sep 19 at 15:40

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