# Design steps for a choke inductor

I want to filter out high frequency noise on my power line. At first, I was considering to use an LC filter on the line; but then, I learned about choke filters are used exactly for this purpose. However, I can't find anything specifically about choke inductors on Google since 'choke' word alone is common in the language and if I added the word 'inductor', I end up with inductor tutorials containing the word 'choke'.

The noise is superimposed on the mains voltage for some background reason. The voltage on the resistive load without any filtering is as expressed below.

$$V_\text{LOAD} = 120\sqrt{2}\sin(2\pi60t) + 5\sin(2\pi50000t) \quad\text{Volts}.$$

I want to design this choke filter myself by using a ferrite E-core which already exists in my inventory. I roughly know the basics for the transformer design. At the nominal working frequency, the primary side inductive reactance must be small enough to keep the magnetizing current small. I understand that my choke must behave as a voltage transformer at 50kHz by inducing a reverse voltage same in magnitude. I also understand that it should have negligible effect on the 60Hz line voltage.

But how? What are the critical design parameters that will make the choke allow the 60Hz signal while blocking the 50kHz one? I think the number of turns will be same, thus the winding self inductances will be the same as well. The winding inductances must be on of the critical parameters. What are the other parameters of design? Can you please shortly summarize me the design steps?

Also, I browsed some commercial products. They specify a saturation current limit. Why does a choke inductor saturate? I mean, the current flows through the windings in reverse direction which will create reversely flowing fluxes in the core and they will cancel out each other making the net flux zero technically. What causes the core saturation in choke inductors?

impedance = $j\omega (L - M)$ where L is the inductance of a winding and M is the mutual inductance between windings. M is usually very close in value to L therefore the net inductance for differential signals is lower than for common mode signals. For common-mode currents, the impedance is $j\omega (L + M)$ i.e. significantly more than for differential currents.