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I bought 100uH inductor for my boost converter, one toroidal (DTPU100A3) and one bobbin (DSZ-9/100/2,2-V), both form faryster. In the beginning, I thought both will give me same results, but the toroidal needs frequency below 40kHz to achive expected results. At the same time, it generate high-frequency noise and heats up. The second one gives same results or even better at frequency above 100kHz, without heating and noise.

I also though the toroidoal will not heat beacuse of its size but it looks like I'm wrong.

So where is the difference?

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  • \$\begingroup\$ What’s the saturation current for each? What’s the core material? \$\endgroup\$
    – winny
    Apr 1, 2018 at 10:44
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    \$\begingroup\$ Note that bobbins inherently have an air gap. \$\endgroup\$ Apr 1, 2018 at 10:53
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    \$\begingroup\$ I also though the toroidoal will not heat beacuse of its size but it look like I'm wrong. Suppose there's a very small coil with extremely low (almost zero) losses, will it heat up? My point: the size isn't the only parameter which matters. So where is the difference? Compare the datasheets of each model and then you'll know. Another parameter not in the datasheet might be cost. \$\endgroup\$ Apr 1, 2018 at 11:15
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    \$\begingroup\$ I am beginning with inductance elements so I really do not know much about it. There’s your problem! You need to learn about these things. Look up saturation current, ferrite and iron powder. Once you understand Vt=NAB=LI and L=Al*N^2, feel free to continue. \$\endgroup\$
    – winny
    Apr 1, 2018 at 11:44
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    \$\begingroup\$ Colorful toroidal core → iron powder. This paint coating is a measure against rust. Iron powder cores are great for low frequencies and useless for anything over 20kHz. \$\endgroup\$
    – Janka
    Apr 1, 2018 at 14:49

2 Answers 2

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An inductor is not characterized simply by its inductance (how many "henry"). Even if it were a linear inductor (i.e. the kind of inductor they teach you when learning the basics, that is a simple coil with no magnetic core) you should take into account the resistance of the wire and its physical construction to estimate its temperature rise.

When you put a (ferro-/ferri-)magnetic core into a coil, that "simple" inductor turns into a wild non-linear beast, bringing you the pleasures of an host of other issues

I dare say an inductor with a magnetic core is one of the nastiest component (among the common parts) to fully understand for an engineer.

Since you are a beginner, I won't enter into the gory details, but I'll just mention the fact that the core material will greatly influence the behavior of the inductor.

When the current in the coil varies, the magnetic domains inside the core will try to follow the magnetic field variations and will change orientation. Depending on the specific material, this will create some amount of heat (imagine a room packed full of surfers with their surf-board on their shoulder trying to change direction all at once).

Different materials have different response to magnetic field changes, so their performance is affected by the current frequency in a different way.

Bottom line: you can't compare two cored inductors knowing just their inductance. You should be sure at least that their core is made of the same material.

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I believe that ready-made inductors available from distributors today, come in several broad categories:

  1. EMI suppression chokes. These are deliberately lossy, have a low Q, you should have a hard time finding a switching frequency where the choke does not overheat.

  2. SMPS flyback accumulator chokes. These have a pretty good Q and should have a defined saturation current.

  3. various RF chokes and cores for tuned circuits for different bands. Ferrites / ferromagnetic materials are generally not much good for resonant applications into the higher hundreds of MHz.

Toroidal cores look like a no-brainer for an SMPS in that they typically result in pretty low EMI radiation: owing to the lack of an airgap and the regular shape, they don't leak very much. But the lack of an airgap has a downside, which is comparably lower saturation current if memory serves. Which makes them less suitable for power applications... In addition to what JonRB has posted in his comment.

Your typical SMPS accumulation inductor is a pot core of sorts, with the winding encapsulated inside. From outside it's an opaque ferrite brick with two surface-mount leads protruding outside.

BTW the open solenoid shape is likely pretty bad in terms of EMI radiation.

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