[SOLVED] Inductor quality value for switching regulators

[Zak28]

Most ferrites have very high electrical resistivity and that means that the eddy losses are very low.

What you are seeing is called hysteresis loss; it is dependent on the area of the hysteresis loop. Most of the ferrites have very low hysteresis loss.

If you want a low loss ferrite for 1200 kHz, you have to search because at high frequency the µ drops off; this has to do with the domain rotations and µ becomes complex with real and imaginary parts.

If you want to use high frequency (>1MHz) you need to reduce the level of magnetization and live with greater dissipation.

I randomly looked up the TDK site https://product.tdk.com/info/en/catalog/datasheets/ferrite_mn-zn_material_characteristics_en.pdf and it does specify all the important parameters quite explicitly.

Note that variations can be large (20-30% difference with published values are not uncommon) and you need to design conservatively.

Most datasheets do not mention anything about the material the windings are on. That document isn't of much use for inductors which doesn't have its material specified.

 

[c_mitra]

Easy peasy is obviously referring to AC resistance caused by skin and proximity effect of the winding.

I guessed it but it is wrong to call it "AC resistance" because of several reasons.

1. You cannot define an AC resistivity because it depends on the geometry of the conductor. A foil or a cylinder with same cross-sectional area will have different "resistance"

2. The skin depth is actually dependent on the DC resistivity- this suggests that these two quantities cannot be independent parameters.

3. The terminology is wrong (IMHO); the wikipedia says: (Screenshot image)

4. it is rarely called AC resistance because it is also responsible for the inductance of the conductor.

 

[Easy peasy]

it is rarely called AC resistance because it is also responsible for the inductance of the conductor.

I can't really agree with that, AC resistance is affected greatly by proximity effect in a coil of wire.

for a single straight wire, AC resistance can be defined pretty exactly, as can it's self inductance,

AC resistance is called just that by the majority of power electronics engineers working in the field ...

 

[c_mitra]

for a single straight wire, AC resistance can be defined pretty exactly, as can it's self inductance,

Right, in a field free space. But I do not know if the single straight wire of circular cross section is parallel to a metal object; or is surrounded not by vacuum (what will be the impedance).

Real part of the (complex) impedance is conventionally called the resistance but it has poor physical significance (because of the strong dependence on the geometry and other environment).

Unfortunately I do not have most of my old books.

Many power transformers use rectangular wires (some use thick foils) for winding; it is impossible (if my memory is correct) to derive any analytical formula for (the impedance) these cross sections. But the effect (the skin effect) is still there.

 

[FvM]

Please remember that the thread topic is about inductor quality values, or more generally speaking, about it's frequency dependent complex impedance. The contributions about "AC resistance" are clearly referring to the inductor impedance (in this case it's real part), not general properties of the conductor material. While skin effect describes the frequency dependent behavior of a straight wire, proximity effect handles the interaction with other parts of the winding.

There's a complete chapter in Snelling, Soft Ferrites about properties of the winding, see https://www.edaboard.com/showthread.php?386026-magnet-wire-parasitic-properties

Proximity effect in foil windings is overseeable, you can assume an almost uniform current distribution in many cases.