toroid core length

[kinarfi]

I hope you have seen this application note by Jim Williams: https://www.electro-tech-online.com/custompdfs/2012/02/an35f.pdf

He gives a lengthy and practical discussion of inductor selection. While he doesn't discuss Q specifically, he states emphatically the need for good inductors. Based on that need, he decided not to try to make the inductors in house and to use only inductors from outside vendors that are designed for switching regulators. He gives some recommended sources. I have used CoilCraft.

While it might be fun to experiment with making an inductor, for your power supply, I would suggest following William's advice.

John

Yes, I've read that, then I went and weighed my inductors,, most of which came out of power supply units for servers that were being replaced where my son works, seem they were of the HP variety.
I was lucky enough to have been invited to his seminars in Salt Lake City, Utah, very informative and interesting, I think he said he's the one who wrote Spice, which I use extensively. The seminar was sponsored by Nu Horizons Electronics and Würth Elektronik who sampled me some of their inductors mentioned earlier.
My latest diversion has been to modify what's working to a SEPIC design, (It worked, but then I had to go watch the game ), using a toroid inductor with 4 separate winding, they were paralleled via their connection to their circuit board. Looks like about 4 to 6 turns of #16 awg wire for each winding and about 9 uh per winding, (from memory). I series two of the windings for each part of the SEPIC transformer (~20uh) for a 1 to 1 ratio, My next diversion will be to make it a 1 to 3 ratio, up the frequency of the circuit by changing the capacitor. The FET I'm using is an IRF2805 which is rated at 75 volts, but if I were to use a lower rated FET with a lower Rdon and protect it with a series high voltage switching diode, would I lose the benefit of the lower Rdon due to the voltage drop of the diode. My high voltage FETS have an Rdon close to a 1/2 ohm and the low voltage FETs have an Rdon of less the 10 milliohms or would it even work, I'll find out.
Kinarfi

 

[kinarfi]

The FET I'm using is an IRF2805 which is rated at 75 volts, but if I were to use a lower rated FET with a lower Rdon and protect it with a series high voltage switching diode, would I lose the benefit of the lower Rdon due to the voltage drop of the diode. My high voltage FETS have an Rdon close to a 1/2 ohm and the low voltage FETs have an Rdon of less the 10 milliohms or would it even work, I'll find out.
Kinarfi

What was I thinking, that was dumb, putting a diode in line with a low voltage FET just drops the fed back voltage .7 volt, if it is put in so as to block the voltage, it blocks every thing.
The 1 to 3 ratio didn't work either, the inductors I have are probably just to far of specs for what I wanted to do. My other thought is why would I want to use SEPIC instead of fly back, flyback is working good. Hope I'm using the correct terms.
Kinarfi

 

[colin55]

The way it works is this:
The most efficient inductor is when you fill the centre of the toroid or "C" cores with wire.
I have used the word "efficient" because I want to compare the two values you ask in the original posting. If the magnetic path is increased, the inductance will be reduced.
In other words, keep the inductor as compact as possible.

 

[kinarfi]

The way it works is this:
The most efficient inductor is when you fill the centre of the toroid or "C" cores with wire.
I have used the word "efficient" because I want to compare the two values you ask in the original posting. If the magnetic path is increased, the inductance will be reduced.
In other words, keep the inductor as compact as possible.

With what I have learned from this thread, my question about length referred to a toroid's length from end to end, as in the length a pencil would penetrate, not he magnetic path, so an increase in 'length as I was referring to would increase the cross sectional area where as an increase inner and outer diameter would be needed to increase the magnetic path. I deduce that the construction of the most efficient toroid cores would be if the core was round as opposed to square because round would use the least wire for the same cross section, but the ones I have are not, so there must be a good reason for this due to manufacturing concerns. I wound 2 inductors that had interesting results, will post more on that after I get some more info.
This has been a good thread for me.

Thanks
Kinarfi

 

[colin55]

If it is just a length, it is called a rod and not a toriod and nothing we have said applies.

 

[ronsimpson]

What Kifarfi calls length is width. Cross-sectional area is how much core there is to saturate. When Kifarfi said he is using a core that is twice as long it actually has twice the cross-sectional area.

 

[kinarfi]

What Kifarfi calls length is width. Cross-sectional area is how much core there is to saturate. When Kifarfi said he is using a core that is twice as long it actually has twice the cross-sectional area.

Thanks Ron, I knew there had to be a proper name for it, so I went looking on the internet, most use inner and outer diameter, TDK refers to the other dimension as thickness, Digikey refers to it as mmL, BiteMark refers to it as heigt, http://www.toroidcore.com refers to it as width, Wurth Electonic calls it C and in A, B, C, Triad magnetics uses H, I think I like width the best, no more length.
Kinarfi

 

[ronsimpson]

OK The bottom line is: The flux passes through the core, no matter what shape. There are several formulas that tell us how much power the core can store and what inductance ets. The area of the core is very important. This is the area if the core is cut in half. Cros section of one leg of the core. In the case of a EE core it is of the center leg. If the area if different if different places it is the smallest place.

The magnetic path length is the length the flux takes to make a loop.