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Common Mode Chokes as AC-Block

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dknguyen

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Hello. I've been looking at common mode chokes and there was something I was wondering. Their frequency response is much much better than a regular inductor. So could you use a common mode choke as a DC-pass on a single-line by running the line through both channels of the choke?

Or is there a reason I've never seen this.

The neutral in my circuit is moving up and down with a virtual ground to the BLDC caused by the switching of my motor driver. I already have chokes that supress the common mode noise at the amplifier inputs (which is basically the only path that common mode noise between either power rail and neutral can get in. But there is still basically just a direct copper connection from this floating neutral right into the power supply neutral so common mode signals can travel straight through.

I can't seem to find a 3-line choke that will do. TO demonstrate the problem is my Example schematic (with two chokes for no reason just to make things more clear). THere is a straight path along neutral for common mode currents to travel even though most of the other paths have been blocked. Of course, there is the common mode path between +5V and -5V, but that has been blocked by the front-end amplifier chokes.

The two remaining schematics try to show how the chokes on the amplifiers block most common mode noise that can get through to the power supply, except for the one path through the neutral which I want to block by using the choke on a single line.
 

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Do the images work now?

EDIT: Okay, there is still a very obvious path for the CMN to get into the converter in my original proposed circuit, but maybe with the choke at neutral it can turn it into differential mode noise that can be absorbed by the capacitors?...nope. DOesn't look like it.

I just can't seem to find an SMD 200-350mA 10khz-20MHz 3-line choke.

MORE EDIT: It seems no matter what I do I can't block all paths for common mode noise going into a 3-terminal supply. So I came up with this circuit instead which basically makes it as if I'm working with a two-terminal power supply. But are there disadvantages to using two capacitors to form the neutral rather than using the common connection present on the supply?
 

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I got my answer- you cannot use a common mode choke in place of an inductor because a common mode choke's regular operation cancels out magnetic fields and causes it not to saturate to differential signals. If you use it as a regular inductor it's current rating is much much much less which is why inductors of the same rating as a CM choke are much larger.

And since I can't find a suitable 3-phae choke, I guess I have to stick the CM choke at the input of the converter. BUt is there a reason it seems to be recommended to put the CM choke at a DC converter's input to stop common mode currents form flowing? I never seem to see it on the output of the converter.
 
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