why common mode choke can reduce common mode noise?

Hi,

what additional explanation do you expect?

High impedance --> less noise current --> less noise voltage.

Klaus

More detailed:
Let′s view only one path: Maybe it′s inductance is 1mH
--> for commom mode (noise) both path′s inductance is added to 1mH + 1mH = 2mH
--> for differential mode (usual energy flow) both path′s inductance is subtracted to 1mH - 1mH = 0mH

For common mode the vector in both paths go from left to right.
For differential mode: The upper path vector goes from left to right, but the lower path vector goes from right to left.

Klaus

Hi, I don't understand: Why accumulated magnetic flux equals to high impedance but not low impedance?

based on your example:
--> for commom mode (noise) both path′s inductance are in parallel, the total inductance should be 1mH/2=0.5mH. Why total inductance is 2mH.

Hi,

Take a pencil and a sheet of paper.
Draw the common mode circuit.
Then draw the current/voltage vectors for common mode case.

This is true for independent inductances only. But you have two windings on one core. They are not independent. Thus they add/subtract.

Klaus