In order to decide what type of core I need, I need to calculate the Area Product Ap which is given as:

$$A_p \simeq \frac{LI^2}{JK_uB_{max}} $$

Where I is the current, L is the inductance, J is the current density, Ku is window utilization and Bmax is the max flux.

Now, given that it falls to me to decide what values of Bmax and J to pick from, are there certain rules of thumb that could help?

Thanks!

## Best Answer

Over the years, I've found this to be the most reliable method of designing hand-built inductors using ferrites.

Here is an example produced by Ferroxcube for their E38 planar core sets: -

Highlighted is 3F3 material (because I use it) and it tells you \$A_L\$ and effective permeability (\$\mu_e\$). It also gives you choices should you decide to implement an air gap (quite often needed with inductors). Below is the BH curve that will tell you what flux density you will get for a given H field: -

As you should be able to see, operating with a lower H field gives more linear results. If the core starts to saturate then the inductance starts to drop and you will get non-linear distortions in your signals (not normally regarded as a good thing).

Of course, your H field may be too big so you have the option of using a bigger core (repeating the steps above) or adding a gap.

Adding a gap means: -

If gapping doesn't get you where you need to be choose a bigger core and repeat the above.