Hi Vishnuk,
1.How can I select a toroid core ?. I found some codes T30-10 etc.
There are many ways to wind a 2.2µH inductor. Using a toroidal core is just one of them. You can also wind the coil in air, or you can use a solenoidal ferrite core, etc.
If you have decided you want to ujse a toroidal core, then it's time to go on and choose one. For this you first need to know some basic things in addition to the required inductance:
- The current the coil has to carry continuously;
- The peak current at which there shall not be too much saturation, or else, how much inductance reduction is acceptable at what current;
- The operating frequency, and the losses you are willing to accept;
- Size, weight, cost restrictions;
So, selecting the optimal core is really a little art, and experience helps a lot. But to get started, you can use the core data given by manufacturers. The aim is to choose a core that allows getting the required inductance with a reasonable number of turns, that does not saturate when the max current is flowing, that has enough winding space to accomodate the required wire thickness with that number of turns, that has low enough loss, and that isn't too large, too heavy, nor too expensive.
2. Is the wire gauge a factor?. If I selected one core, How to determine which gauge wire need to use?
As a first approximation, for relatively low frequency coils where skin effect isn't too important, a cross sectional area of 1mm² for every 3A of average current is a good starting point. In very small coils you can often get away with significantly less cross section, while in high frequency coils you might need thicker wire. To validate and correct the design, once you know the number of turns and thus the total wire length, you can calculate the AC resistance of that wire, considering skin effect, for the operating frequency, and then calculate how much power loss that resistance will cause, and decide whether or not it's acceptable. But many times your current will be a mix of AC and DC, and for the DC part there is no skin effect. So, for applications where the AC component is small, you can simply use the DC resistance.
3. Atlast how can I know whether the 2.2uH inductance reached?
Core manufacturers tell you how many turns on which core produce how much inductance. To confirm that all is well, you can measure the inductance. Methods are an inductance meter, of course, or a dipmeter, or you can make a test circuit that basically measures the current rise per time for a given applied voltage, using an oscilloscope.
I have iron powder core (black coloured).
Caution with that. Those black cores are usually ferrite, not powedered iron. Ferrite comes in a large range of permeability, roughly from 10 to 10000! And you need to have considerable experience to have any chance at telling them apart by sight, so you have to measure them!
Powdered iron toroids typically come in colors. For example, a red core with one black side is material 2, a yellow one with one black side is material 6, and a yellow one with one white side is material 26. These are numbers used by the American Micrometals company, probbaly the best known. Other manufacturers might use different numbers, but often use the same color code. Powdered iron cores come in permeabilities from 1 (which means it's just plastic, without any iron!) to about 100. And there is no logical relationship between the material's number and its permeability! You have to look up the data.
My application for this inductor is in a boost circuit, which switches in 0.55 MHz. The current through inductor I need is 6A.
For that frequency it's better to use ferrite rather than powdered iron. But with ferrite you will need a core that has an air gap, or is otherwise open, such as a solenoid or a bobbin core. A ferrite toroid (ungapped) would have to be too large to store enough energy.
You can easily use an air-wound coil for this, without any core at all, but it will radiate a huge amount of interference.
A low permeability powdered iron toroid, such as material 2, would probably work well too, and be smaller and radiate less noise than an air coil. But not a "high permeability" powdered iron core like material 26. Too lossy.
My choice would probably be a ferrite bobbin core.
Well, the resonance freq matters here?
For the boost converter to work properly, you need to keep the stray and inter-turn capacitance low enough so that the resonant frequency of that coil is very much higher than 550kHz. But that's easy, because you will need just a few turns on a core, and still not very many without a core!
Is there any way to get the spec of the Core, which I have now (from old inductor) ? I mean TO-37, TO-30 etc.
Yes, but it's not too easy. You need to make a test winding (10 turns should be OK) and measure the resulting inductance. That allows you to calculate the AL value of that core. And then you measure its physical dimensions: Inner diameter, outer diameter, height. Thhen you go to the manufacturers' catalogs, you look for toroids having that size, and then you try to match your measured AL value to the published AL values of all the cores of that size, made from different types of ferrite. That should tell you at least roughly what core you have.
There are two problems with this. One is that the AL value of ferrite cores usually has a big tolerance, so you may not know well which of two or three candidate materials you have there. The other is that there are many ferrite materials, from several different manufacturers, that have roughly the same permability, and thus give roughly the same AL value for a given core size, but which differ in other characteristics, such as loss factor, saturation flux density, Curie temperature, temperature coefficient, and so on. To make sure you really have identified a ferrite core, you need to measure all of those too, and that's rather hard!
For your converter, I suggest that you simply wind an air coil to test it. Place the coil at some distance from the rest of the circuit, to keep it from interfering the small-signal parts of it. The longish wires will just add to the inductance. And when you can, look for some nice little ferrite bobbin core to wind the definitive coil on.
There are many programs and online calculators that can help you choose cores, calculate turn numbers, calculate air coils, etc. I sometime suse one called "mini ring core calculator". It's not perfect, but quite usable and practical.
You might also want to look up this page, to learn about basic electromagnetics:
https://ludens.cl/Electron/Magnet.html