inductor in DC-DC converter circuits

[metebalci]

Should the LH in this circuit (IAB01 is a DC-DC converter, the circuit is from its datasheet) have particular properties (core type, shielded etc.) ?

[T3sl4co1l]

No. EMI filtering parts are generally very noncritical.

Tim

[Jim from Chicago]

 article which may interest you: https://www.allaboutcircuits.com/technical-articles/choosing-and-using-ferrite-beads/

[metebalci]

Thanks. What does the single solid line over L mean ? I guess no line is air and double is iron but what is single ?

[Jwillis]

I guess no line is air and double is iron but what is single ?

FeSi (Ferrosilicon) Powder Core

[T3sl4co1l]

I doubt it's very specific.  Anything from a single solid or dotted line to two or three, is usually used to imply a core of some sort, with solid generally meaning laminated iron and dashed meaning powder or ferrite; but no one is particularly careful about the meaning anymore.

Tim

[metebalci]

I guess no line is air and double is iron but what is single ?

FeSi (Ferrosilicon) Powder Core

I almost never see FeSi in catalogs, but I guess it is what is meant by various alloys ?

[metebalci]

I doubt it's very specific.  Anything from a single solid or dotted line to two or three, is usually used to imply a core of some sort, with solid generally meaning laminated iron and dashed meaning powder or ferrite; but no one is particularly careful about the meaning anymore.

Tim

Is there a set of basic rules on how to select an inductor core (not for this circuit, I mean in general) ? I try to find but the properties seem to be overlapping (other than air), is that why you say the particular meaning is not indicated in the diagrams ?

Two examples I have: - and first time I am using an inductor as it can be understood -

- this circuit above, for EMI filtering when using the DC-DC converter, I think the value was 6.8mH, what should I primarily look for ?

- another one, for example I have lets say 18mH, this is a few kHz oscillator like circuit, I may want low DCR for this one, so what should I primarily look for ?

[Jwillis]

The different alloys used in inductors have different properties obviously . But many times the choices in materials is a balance between properties and cost. FeSi cores have high DC bias and High AC losses but are also cheaper than other alloys or iron core. This makes a big difference when going into high production when you can save a few cents per unit. So in the case of your DC/DC converter it makes sense to use the least expensive component if going in high production.

If your interested in the EMI filter used for your converter you'll want to read up on LC filters. Specifically PI filters. LC filters can also significantly reduce ripple coming from a rectifier as well.
 
But for the hobbiest or someone than is learning its probably best not to over think it and just go with the more common FerroZinc. Most cored inductors will work fine for DC.
Aside from the Inductance value and current capability you want to keep your inductors foot print as small as possible for your application.
 

[T3sl4co1l]

Probably 6.8uH, not mH; mH would be quite a lot for power supply filtering.  The impedance Zo = sqrt(L/C), for L the inductor and C the series equivalent of the two capacitors around the inductor (the inductor-capacitor loop sees both C's acting in series), should generally be much less than the DC load.  This is so that, on a step change for the load current (say, zero up to nominal-max or back down), the load voltage peaks by about ΔI * Zo.  It can be worse, for periodic current changes, if the Q of the LC circuit is above 1.  Generally one or both capacitors should have ESR ~= Zo to achieve this.  If strong high frequency attenuation is desired, a smaller C can be used in parallel with the large + ESR ("bulk") capacitor.

Electrolytics generally have modest ESR values, so make a good option here.

Since the filter is making a low Q anyway, a low Q inductor may well be preferable.  These are usually powdered iron type, if anything.  But you'll still need ESR of something (usually the capacitor(s), as above) to get enough total losses.  So it's not very important what type of inductor is used here.

Nor does shielding matter, because the AC voltage is at best fractional volts.

Core types matter more when the inductor is used directly with switching waveforms (the central inductor in a buck/boost/forward converter, or the transformer of a flyback), or for resonant (power or signal) or filtering purposes where a high Q factor is required (high selectivity / narrow bandwidth while getting low insertion loss).  In these cases, special grade powdered iron / composite, or ferrite, are used.  Or at high frequencies (>10MHz?), air core inductors are increasingly practical.

"Powder" cores include mostly iron alloys, in a fine-grained form, pressed into shapes and bonded with resin (epoxy or whatever).  The exact composition varies.  Many are pure iron ("carbonyl iron"), prepared in a granular (layered, onion-like) form by chemical process; others are ground into filings and dust, hence you might get more-or-less the same "electrical steel" (silicon alloy) used in transformer laminations but reduced to dust, hence "FeSi powder".  Still others incorporate Ni, Mo, Al and more, for higher permeability (permalloy) or lower losses (MPP, Sendust, etc.), usually at higher cost.

Ferrites are chemically "ferrite", i.e. compounds of something with Fe2O4(2-) ("ferrate(III)", in the same way "sulfite" is SO3(2-) and "sulfate" is SO4(2-), etc.).  The most common grade of which uses a mixture of Mn and Zn as the "something", hence MnZn ferrite.  These have high Tc, Bsat, mu, and somewhat low frequency range (10s kHz to a few MHz).  NiZn ferrites are the other important ("soft") ferrite, with lower Tc, Bsat, mu and higher frequency range (10s, even 100s of MHz).  There are also "hard" (= remains magnetized) ferrites using strontium and such, used for speaker magnets and etc.; and more esoteric ones like YIG (yttrium iron garnet, a different crystal structure -- the others are in the spinel family -- much lower mu but useful properties in the microwave range, used for circulators (I think?) and tunable oscillators).

None of this is very important for electronic purposes; simply get the inductor with Idc and Isat at least what are required for the circuit, and a suitable Q factor, preferably Q measured at a frequency near where you're going to be using it.  And then the rest is size / aspect ratio, cost, and availability.

Tim

[metebalci]

Probably 6.8uH, not mH; mH would be quite a lot for power supply filtering.

Yes 6.8uH, my mistake. Thanks Tim for the detailed explanation, very informative.

[planet12]

One parameter of the input filtering inductor I don't see mentioned in this thread is "DC saturation current".

The saturation current is important, as once the core saturates, the inductance will drop dramatically, impairing filtering performance.

You want to pick an inductor with a saturation current rating that's a good 20% higher than the highest input current you expect to see (the input current will depend on your load, the conversion ratio, and the efficiency of the converter, you'll need to work it out).