Ferrite vs. iron powder toroid for buck converters?

I'm wondering about the difference between small (13 mm outer diameter) ferrite and the yellow white iron powder toroids. Will the ferrite toroids saturate at 5 A current?

I'm planning on using the cores for buck converters (mostly 3 A at probably below 200 kHz).

These are the ones I am looking at:

Ferrite: https://www.ebay.com/itm/Metal-Core-Power-Inductor-Ferrite-Rings-Toroid-Cord-25x10x15mm/310980203521 (also available in 13 mm outer diameter)

Most of the buck converters seem to use the yellow white iron powder toroids, like this: https://www.ebay.com/itm/5Pcs-Toroid-Core-Inductors-Wire-Wind-Wound-mah-100uH-6A-Coil-DIY/221981982278.

From searching on the Internet, the yellow white toroids seem to have a permeability of 75, and the ferrite has a permeability of 2300 or so. Is this important for saturation?

I have some toroids and an LCR meter, and the ferrite toroid needs only a few turns of wire to get a 1 mH inductor, vs. many more turns for the iron powder core. Will this matter if the peak current through the inductor is limited?

I'm guessing the ferrite toroids are great at low currents (0-100 mA) and low frequencies (<100 kHz, as I can get more inductance with fewer turns). But, are they also good for higher currents (like 5-6 A peak)?

(PS: Also another reason I ask, is that at my place, the ferrite cores are half the price of the iron powder cores.)

• I was trying to figure this out not long ago. I ended up going with the iron powder power inductor cores(yellow and white), but I've learned they have a functional limit of ~1Mhz due to pole switching losses or some such, so I've been wondering what kind of inductor is used in higher frequency power converters.
– K H
Jan 11, 2019 at 6:42
• If you have a lot of time for R&D and measurement equipment to make your own power inductors, that's fine. But if you want working converters, I would recommend to buy fully-characterized inductors from professionals, Coilcraft, TDK, Murata, KEMET, etc.etc., and get components that are recommended by IC manufacturers in their BOMs. Jan 11, 2019 at 8:21
• @Ale..chenski That doesn't help me learn anything. Besides, I'm not building something to pass regulations. Also, the price for the cores here is about 6 cents each, probably a hundred times cheaper than getting them with datasheets. Jan 11, 2019 at 8:43
• @Indraneel For discontinuous operation, should you need to plan it, you can work out the details from $\frac{A_c\cdot\, l_m}{\mu_r}=\frac{\mu_0\:V_\text{peak}\:I_\text{max}\:t_\text{on}}{B_\text{max}^{\,2}}$. ($t_\text{on}=\frac{D}{f}$, $D$ is the duty cycle and $f$ is the frequency.) For continuous operation, it's $\frac{A_c\cdot \,l_m}{\mu_r}=\frac{\mu_0\:L\:I_\text{max}^{2}}{2\:B_\text{max}^{\,2}}$. $A_c$ is the cross-section of the core, $l_m$ is the magnetic path length, and $\mu_r$ and $B_{max}$ depend on the core material. The core material should support $f$, too.
– jonk
Jan 11, 2019 at 16:54
• @jonk So if $I_{max}$ has to increase, then either $A_c$ or $l_m$ has to increase. But $\mu_r$ is very high for ferrites. Is this why iron powder is used? Or gapped ferrite core? $B_{max}$ is 0.4 tesla for ferrites, right? Jan 11, 2019 at 17:28

There is a color standard for painted toroids, and yellow means it has hysteresis to prevent saturation and is meant for filter inductors. But a side effect is that it has very low permeability. Black ferrite is usually a good choice for transformers. Blue is an expensive Permalloy that is more efficient than ferrite. Green is low frequency filters made with silicon steel tape wrapped to form a toroid.

This chart is generic as it is not including fine details such as permeability, and does not state if iron, steel, ferrite or permalloy, which is a nickel-iron alloy.

PC power supplies can put out over 1,000 watts and they use E cores as they are easy to wind by machine, and can have a cross section large enough to handle as much as 10 ampere/turns, and a tiny 10 mil air gap helps a lot. Large toroids need expensive winding machine heads so toroids are better used at low voltages were the number of windings is low, such as car stereo power supplies.

NOTE: Sometimes practical reasons determine what material and shape of transformer are used, which is not always the best choice. Cost and size compete with efficiency. The opinions of engineering and marketing and sales are not the same, and who wins determines what is used. "Just good enough" wins most of the time.

Hysteresis is a gap in which a iron or ferrite core needs a more intense magnetic field to magnetize, retaining a bit of the field after the current has been removed. It takes a stronger current of reverse polarity to reverse the cores magnetic field. In general a LCR meter which works with little drive currents will show a core with hysteresis built into its material has a much lower inductance than a core of the same turns of wire and the same cross section, but is made of ferrite or Permalloy.

To cover all the variations of cores made by many manufactures you would need a book full of charts specific to each core material. For any given core of any shape you need the manufactures datasheet or chart for that core to get an idea of permeability and any hysteresis factors and peak current values vs. pulse width. To quote Ali..chenski's comment:

Ferromagnetic materials are characterized by more than one parameter, by shape (and corner values) of its hysteresis loop, saturation field, losses at various frequencies, etc. You can't map this multidimensional parametric space onto a single color-coded lineup, every material has variety of properties, and there are hundreds of specific material compositions designed for different uses. Without datasheets a core can be anything.

• And what about the unpainted black ferrites? That's the cheapest one at my place. One can also see them inside CFL lamps. I already know they work at low currents very well with a MC34063, and also as jewel thief. But how about 3A buck converter with LM2596? Jan 11, 2019 at 6:46
• For a given design based on a PWM IC the manufacture often specifies core material or a part number that you can search with. There a many toroid manufactures all over the world.
– user105652
Jan 11, 2019 at 6:55
• Well, the LM2596 datasheet says ferrite E core or ferrite bobbin or powdered iron toroid. So, is this because the peak current is already too high for ferrite cores without an air gap? Jan 11, 2019 at 7:04
– user105652
Jan 11, 2019 at 7:28
• Yes, I have seen that chart before. It is for iron powder cores. There are other similar ones with the yellow/white color code (Mu=75). Jan 11, 2019 at 7:53

Powdered iron is cheap and more forgiving when it comes to saturation due to the more gradual BH curves. There is a downside when for buck and most other DC/DC convertors.

The inductor ripple current will cause more core losses in the powdered iron than in most ferrites. It is quite normal to have AC ripple currents at about 33% of the max DC load current. So on an orthodox hard-switched peak current mode switching regime, which is most easy to buy chips for, are specified to do you will get lower efficiency on powdered iron.

When I run powdered iron I set up for very low ripple currents to make core losses very low.

• Powdered iron cores have to be carefully thermally managed to prevent temperature-related aging (which leads to higher losses, higher temperatures and eventual destructive thermal runaway). Ferrite and MPP material do not have this problem. Jan 11, 2019 at 14:50
• @ Adam Lawrence .Very Very good comment.Do not buy powdered Iron off strange men .In a previous life I found this out the hard way . Jan 12, 2019 at 5:12
• Even cores from reputable manufacturers (like Micrometals) need to be watched. Jan 14, 2019 at 12:32