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):


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


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


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.

  • \$\begingroup\$ Thanks a lot Tony. I will go for CMC then because it appears to be a better choice \$\endgroup\$ Oct 6 '16 at 20:49
  • \$\begingroup\$ They come in thousand of sizes and shapes... look at any old PC PSU. X1 rated caps are also special plastic hiV rated \$\endgroup\$ Oct 6 '16 at 20:53
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    \$\begingroup\$ uhm I believe two single coils work both differential mode (Zdm=2ZL) and common mode (Zcm=ZL/2). The main point is that a twin coil can instead give high Zcm while keeping Zdm low. Whether this is good or not depends on what you are after. Somewaht higher Zdm (first design posted) may help keeping current harmonics low acting as a passive PFC. \$\endgroup\$
    – carloc
    Oct 7 '16 at 6:53

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