I'm working on a DC-DC converter to power a 12V-Sensor on a 5V supply.
I've chosen the attached circuit.
I though have some problems when choosing the correct Inductor. I think, a 220uH -> 240uH would work fine (according to Texas Instruments (Formula 5)).
I chose this Inductor, which has an inductance of 240 uH.
Now that it came, I saw, that it is coiled counterrotating so that the magnetic field will cancel itself out. Will this part still do its job, and if not, what Inductor would fulfill my needs?
I appreciate your help
The magnetic field outside the toroid is going to cancel out, not inside. This just means the inductor
will spray less magnetic energy outwards (toroids are already very well behaved in this regard though).
The key parameters for an inductor for power conversion are saturation limit, and losses at the switching
This is why selecting them is complex.
The prime requirement for an inductor in this context is energy storage efficiency, you want a small package
able to handle the maximum current without saturating, typically meaning a gapped inductor which can
store more energy (the airgap holds most of the energy in fact), and low magnetic losses.
The toroid you have selected is ungapped so will need to be larger than is necessary, though it will
produce less EMI as a result.
Without knowing the specific core material properties there's no way to know if its appropriate for
your switching frequency (which is what?).
You'll notice the majority of power inductors in DC-DC converters are little potted inductors with
a gap at the rim, such as Global Sources - Expired Product
This design has the benefit of small size and limited EMI (the magnetic field is directed upwards
a short distance from the ferrite body, not directed at other parts of the pcb).
There are main complexities in inductor selection, its a big set of compromizes. Try it out and
be prepared to experiment.
Gapped inductors are used for various reasons, as well as increasing the saturation current (and
thus allowing higher power handling), the gap stabilizes the value of inductance against changes
in the core's properties, since the magnetic properties of air are very stable(!)
For transformer use there is no need to store magnetic energy (the energy is supposed to transfer
between windings), so gaps are usually avoided as they reduce performance