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Consider the following schematic, where there is a 10V offset voltage source with 1V ripple, two selfs mounted in common mode choke and a 100 Ohm resistor. Everything here is "ideal" (no parasitic resistance or capacitance etc.).

common mode

Since the current that enters into one winding is equal to the current that enters into the other winding, I would expect that this pair of windings has no effect regarding the AC analysis of the voltage between the two terminals of the resistor. But look at the AC analysis of this voltage in the image below: obviously, the ripple is strongly filtered. How can it be?

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    \$\begingroup\$ You just have the wrong coupling in the common mode chocke. Either flip horizontally one coil or change k to -1. \$\endgroup\$
    – carloc
    Sep 17 '17 at 8:26
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You're wiring the common mode filter wrong (look at the dots!). In this way, you're creating a differential mode filter. The current of V1, in fact, enters both on L2 and on L1 from the dot side. This way, the magnetic flux, instead of cancelling each other, is summed.

In a common mode filter, instead, the differential current enters the first winding the one dot, and exits the other winding from the dot.

enter image description here

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  • \$\begingroup\$ I thought the dots means: if a current enters at one dots, then the induced current in the other side exits at the second dot. If this is true, how can the main induction cancel in your schematic? the induction of the first self on the second self would rather add with the self induction of the second self no? \$\endgroup\$
    – MikeTeX
    Sep 17 '17 at 8:55
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    \$\begingroup\$ The dot means: if the current enters from the dot, the magnetic flux will have a sign, let's say positive. If in both windings the current enters from the dot, you're creating two magnetic fields, with the same polarity, thus they will sum up. If the current enters from one dot, and exits from the other, you're creating two fluxs with opposite polarities. If the currents are equal, and if the two windings have the same properties, the fluxes will cancel each other. In my scheamatics, the main current enters from one dot, and exits from the other. \$\endgroup\$
    – next-hack
    Sep 17 '17 at 9:11
  • \$\begingroup\$ Nice. It is exactly the contrary to what I thought. +1 and accepted. \$\endgroup\$
    – MikeTeX
    Sep 17 '17 at 9:20
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    \$\begingroup\$ When differential currents crosse the CM choke, the differential-mode fluxes cancel in the core and what is left are leakage inductances. These inductances combined with the capacitors filter out differential mode. \$\endgroup\$ Sep 29 '17 at 20:33

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