The magnetic field of Inductor

Hello. I have an Inductor like below to use in a buck converter. Does somebody know where is the direction of magnetic field in Inductor like that.

Almost all of the magnetic field is confined to the ferrite core (around which the wire is wrapped). The direction depends on which way the current is flowing. Look up "right hand rule".

Sorry but I couldn't understand yet. Could you tell which one of the below figure? thanks. |500x148

Sorry but I couldn't understand yet.

So to simplify the answer there is NO external magnetic field from a toroidal coil, so none of those pictures mean anything.

Why is it important to you?

Because Noise is so important for me, and I want to know how to replace in my PCB. thanks.

leoncorleone: Because Noise is so important for me, and I want to know how to replace in my PCB. thanks.

It won't make any difference what way round you place it.

(b) is correct. Curl the fingers of your right hand and stick the thumb out: the right thumb points in the direction of (conventional) current, the fingers indicate the magnetic field direction around the coductor.

For a toroid consider the direction of current through the hole, its then simple to see.

[ BTW you probably never need to know the direction for most electronics applications! ]

As mentioned most of the magnetic field is contained in a toroid coil.

When noise is critical designers use fully shielded coils as far away from sensitive circuits as possible.

When noise is really critical, don't use a switcher.

For RF use any metal shield will do, it doesn't have to be magnetic (mu-metal) since the eddy currents cancel the high frequency field very efficiently. Often RF transformers sit in an Al can to isolate their field from the next one along - Al is used to keep a high quality factor (low damping).

I knew a cranky old CATV RF engineer who would always orient his coils at 90 degree angles to minimize crosstalk.

I don't think it mattered, but can't hurt, unless your boss is Steve Jobs.

I knew a cranky old CATV RF engineer ....

Sure that is fine on open coils but this is a toroid so an RF engineer wouldn't be playing with those.

I suggest you read Ruthroff: “Some RF transformers.”