To put it simply, some manufacturers don't list their inductors' self resonance frequency.

I'm a bit baffled as to why they don't do this, and I have some concerns using inductors like this one on high frequency switching applications such as DC to DC converters, as they typically use frequencies in the order of hundreds of kHz, some even a few Mhz.

The only spec they list for that particular inductor is "Frequency - Test" of 1kHz. What does this mean? Will there be any problems if I use this inductor on a DC to DC converter switching at a frequency of, let's say, 100kHz?

I suspect the reason for this omission is the type of inductor, toroidal in this case. Do toroidal inductors have such a low parasitic capacitance that its effect is not noticeable even at very high frequencies, and manufacturers don't even bother to list it? Or is it the opposite, and manufacturers are expecting designers to only use them at very low frequencies?

And whatever the case may be, how does the parasitic capacitance of toroidal inductors compare to other types of inductors?

Also, maybe I shouldn't be using these type of inductors for stuff like SMPSs? If so, what kind of inductors should I be using? What if I need high power, and thus high current handling capabilities?

Any help would be immensely appreciated.

  • \$\begingroup\$ doesn't any resonant frequency depend on both the rated inductance and some associated capacitance? there can be a self-resonant frequency that is a property only of the coil if there is some kinda self-capacitance of the coil. how would that self-capacitance be related to the physical properties of the coil? such as the dimensions of the annulus of the inductor and the number of turns (so this would be related to the cross-section of the wire). \$\endgroup\$ – robert bristow-johnson Feb 18 '16 at 3:33
  • 3
    \$\begingroup\$ Yes, the self-resonance frequency depends on the inductance of an inductor and the parasitic capacitance of that same inductor. \$\endgroup\$ – Chi Feb 18 '16 at 3:57
  • \$\begingroup\$ Sumida America Components Inc. CDRH127/LDNP-681MC is similar to type used on many of our 100kHz switching power supplies past several years. Or Sumida America Components Inc. RCH110NP-681K has SRF of 2.1MHz, rated 680µH 650mA 1.1 Ohm Max DCR. \$\endgroup\$ – MarkU Feb 18 '16 at 4:23

The reason that inductor doesn't have a self resonant frequency specified is because it's not intended for DC to DC converter applications, where the SRF is important. It's too lossy to be a good inductor for that purpose, it's intended purely as a supply filter inductor. In that application, the loss will likely kill the Q before you get to resonance.

Generally, if a data sheet doesn't have a specification for the component at the frequency you want to use it, don't use that supplier. Either (a) (as above) you're not looking at the right type of component or (b) the supplier is a bit flaky and trying to fob you off with rubbish. Don't let the fact that you may have to dig deep on a supplier's site to find the data put you off though, ceramic capacitor voltage coefficients are always very hard to find.

  • \$\begingroup\$ Well, it's interesting that while the manufacturer of that particular inductor doesn't give the SFR, it says it's intended use is DC to DC converters. Funny. \$\endgroup\$ – Chi Feb 18 '16 at 16:32
  • \$\begingroup\$ In the energy storage inductor, or the filter inductors? \$\endgroup\$ – Neil_UK Feb 18 '16 at 17:46
  • \$\begingroup\$ It only says "DC/DC converter, EMI filter applications". A little ambiguous IMO. \$\endgroup\$ – Chi Feb 22 '16 at 0:21

I wouldn't use an inductor that doesn't specify its SRF unless it was a sub 100 kHz application and only then after exploring all other options.

A word of extra caution though. It's likely that the switching transistor will have drain to source capacitance that is tens to hundreds of pico farads and this is usually the main resonance problem.

  • \$\begingroup\$ Good point about the drain source capacitance! \$\endgroup\$ – Chi Feb 18 '16 at 14:22

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