Electrical – Common mode choke vs individual inductors for offline flyback smps

common-mode-chokeemcinductor

I am working on Viper22a based offline switcher and I was going through similar app notes by ON semi and ST. I found two different implementations of noise filtering on incoming mains.

This one uses individual inductors (each of value 1 mH):

Inductor

And this one uses a common mode choke (2.2 mH):

Choke

Is there any benefits of using a common mode choke over inductors?

Best Answer

Two isolated chokes only work in differential mode, while the Cm choke raises the CM impedance which is essential due to the imbalanced differentical impedance of both a switched load to ground and transient line impulses.

But CM impedance creates a high Z series path , which with shunt caps creates also a differential LPF . This is for attenutating large CM noise internal AND external but with the two shunt caps to ground creates a high a Z attenuation ratio , resulting in differential reduction of noise.

So the two coils MUST be tightly coupled and large in value (mH) for low frequency noise 50/60Hz line. For even better line filters low value CM (10uH) choke is cascaded in series for higher switching noise attenuation back to conducted line noise required for CE and FCC qualification.