# 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?

## 1 Answer

You have drawn a differential choke not a common-mode choke - a CM choke has dots at the same end and doesn't easily saturate because current flowing sets up cancelling flux thus nearly entirely preventing saturation. The differential choke in your circuit very easily saturates because the fluxes from the two coils are additive.

The CM choke blocks only common mode currents i.e. currents of the same direction trying to enter and pass through the choke - this is a good example of transformer action. For differential currents (as per the signal you want to pass) the windings are in effect anti-phase and have a low impedance: -

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.

I get the feeling that you are expecting a CM choke to deal with high frequency differential currents differently to low frequency currents. This is theoretically not the case but, in reality, because leakage inductances exist (not all the flux is shared between the two coils), there is a tendency for higher frequencies to receive more attenuation.

For more info read this on page 6

• Sorry, I didn't mention because of lack of knowledge. The noise is added to the mains power at the source. I think it is called "differential noise" in this case. The noise is not added from ambient. Actually, I'm trying to filter out output of a cheap UPS. It gives too much ripple at the output. I'm trying to design a chock which will allow the 60Hz signal while blocking the 50kHz ripples. I edited my question accordingly. Sep 15, 2014 at 12:27
• You should just use normal individual chokes to filter out high frequency stuff - using a differential choke is not going to do this effectively. Sep 15, 2014 at 12:33