Hi

Question about common-mode choke selection.

I have two conductors, each carrying an AC current of 0A to .01A, at a frequency of 40 kHz, in phase with each other.

I assume that if i pass each signal through the two conductors of a common-mode choke, in opposite directions, then the current between the two signals will cancel out, and only the difference current will appear as magnetic flux signal. Correct?

How can i select the correct inductance rating Henries, if my frequency of interest is AC current of 0A to .01A, at a frequency of 40 kHz?

Thx

 

Bottom of this page has ap notes that might help -

https://www.coilcraft.com/cmc/index.cfm


Regards, Dana.

 

Thx, Dana. I have read that article.

I think i mistakenly made a duplicate post. Here's info from my other post:

"The value of inductance required of the choke is simply thel load in Ohms divided by the radian frequency at and above
which the signal is to be attenuated. For example, attenu-tion at and above 4000 Hz into a 50-ohm load would require
a 1.99 mH (50/(2π x 4000)) inductor."

www.coilcraft.com/pdfs/doc191_CMFiltDesign.pdf

So it depends on my load . Generally, as desired pass frequency goes up, Henries goes down. For my case I think I want about 0.1 mH.

 

More info from coilcraft. This isn't a product endorsement, just technical info:

for 40 kHz and such low current: they recommend
LPD3015-334MR https://www.coilcraft.com/lpd3015_cm.cfm This solution was found using our Common Mode Choke Finder tool: https://www.coilcraft.com/apps/finder/cmffinder.cfm

Their measurements start at 100 kHz at the low end, the graph indicates the CM attenuation will still be good at 40 kHz.

The dotted line represents differential mode. This will not be attenuated as shown by the flat line close to zero.

The dashed line represents common mode (noise). This will be attenuated.

-3dB attenuation means the common mode noise (power) will be cut in half.

Any more CM attenuation is better. Typically, at least 3dB attenuation (-3dB) helps cut down the noise.

for -3dB into a 50 ohm load, you want at least 0.1 mH = 100 uH.

The LPD3015-334 has higher inductance, so it’s coilcraft's suggestion.