I need to design some \$3.7 mH\$ coils for two low-pass filters and a \$5\mu H\$ coil for a boost converter. But am having some trouble finding suitable ferrite cores, since there will be a significant amount of current running thru all of them. So I thought I'd ask here for some guidelines about calculating and choosing the right ferrite cores for my applications.

First, I am aware of the equation: $$B=\mu\_0\times\mu\times\frac{N\times I}{l\_e}$$ where \$B\$ is magnetic field strength, \$N\$ is number of turnes, \$I\$ is current thru the coil and \$l\_e\$ is effective length of the core.

However I was suggested by an employee at one of the company that produces ferrite cores, that whilst this is a valid equation to use, for high flux densities I am better of using one of these equations: $$U\_{rms-sine}:=B\_{max}\times A\_e\times N\times f\times\pi\times\frac{2}{\sqrt{2}}$$ $$U\_{rms-square}:=B\_{max}\times A\_e\times N\times f\times\pi\times 2$$

Where \$B\_{max}\$ is typically used around 300mT, \$U\$ is the applied voltage and \$f\$ is the frequncy of the applied voltage. And I should calculate for \$A\_e\$, for effective core size. 

From this on I've lost contact with him, so am kinda lost. While I trust him, about these equations, I am also unsure (shouldn't they depend on current?) on how to approach this problem, and solve it.

Here are some numbers with what I am working.

Low-pass filter:

* current thru the coil maximum at around 7.5A
* voltages across the coil will be from 12V to maximum around 27V (square wave will be applied to the filter)

Boost converter:

* input voltage of maximum 15V (square wave)
* input current of maximum 15A

These numbers already include some margin.

Also if I will get some large values for the core size can I connect more of them together? 

How, parallel or serial? By first equation I should connect them in series (lesser turns=lesser B), but by second/third I should connect them in parallel (more turns = smaller Ae), so which is it?

**EDIT:**

Low-pass filters are in a H-bridge, for class D amplifier. And boost converter is what will be powering it.

**EDIT2:**

Frequency of the square wave applied to the low-pass filters will be around 20kHz. And frequency of the square wave produced by boost converter will be somewhere between 100kHz and 500kHz.