Common Mode Choke Query

[Techman_7]

Is the impedance of CM choke for differential noise will be 1.5 times as that of CM noise.

For example, CM inductance is of 47uH then the differential mode inductance will be 47uH + (47uH/2) = 47 + 23.5 = 70.5uH

This is because for DM noise the CM inductance should be splitted between two equal halves.

Is this analysis correct?

 

[FvM]

No. The ratio of CM to DM inductance depends on choke design and coupling factor. It varies between Ldm = 4 Lcm (separated chokes) to Ldm < 0.01 Lcm (perfect coupling on high ur core).

 

[Techman_7]

But if we consider differential mode current, the two windings of CM choke will be in series for DM current.
Isn't it ?

 

[FvM]

It depends on coupling factor. For perfectly coupled windings, DM inductance cancels out. With moderate coupling (separated windings on a toroidal core, see photo), Ldm is typically about 0.1* Lcm.

 

[Easy peasy]

if you wire a CM choke as series aiding ( instead of series cancelling - i.e. normal mode ) then you will get 4 x the L of a single wdg, as L prop to N^2.