Q&A 2

Two inductors or one common mode choke?

From Suja, electronics engineer, India

Question: Can I put two inductors (same part number from the same manufacture) in both power and ground lines for filtering?


For differential noise, I wouldn’t choose the first topology, because you need an L-C (single stage filter) or L-C-L-C (two stage filter). You shouldn’t need another inductor in the minus line, but the only reason you want to do it is you want to make a common mode action as well. I don’t think it is a good idea. Because the inductors are not the same (even they come from the same production line patch), you won’t be able to compensate the tolerance between the two. The common mode noise will have different impedance on both lines, so you end up turning a common mode noise to differential mode again. 

I would choose the second topology. You can even select a common mode choke which has slightly larger leakage inductance. Bifilar wound common mode choke often has almost zero leakage inductance, but sectional wound common mode choke has sometimes 2-5% leakage inductance. The leakage inductance, together with the capacitor acts like L-C filter for differential mode noise.