Using common mode choke in differential configuration for differential mode EMI filtering

Question

Why is it not popular using common mode chokes in differential mode configuration for differential mode noise filtering?

Mostly when using fly-wire type chokes with filters, where the chokes are simply kept inside the enclosure with its wires connected to X and Y capacitors placed on the PCB (due to its size which makes it costly to be placed on board), can't this kind of arrangement help avoid placing one common mode choke and two separate single winding chokes and go with one common mode choke for common mode filtering and another common mode choke connected in differential mode configuration as shown in attached schematics?

Could saturation be a problem for low current design? If yes, I dont understand why not if two separate chokes are used. I am assuming the core is properly sized to not go to saturation during normal operation.

Answer 1

Differential signals are often used where EMC is a concern, as interference is usually common mode and a differential signal on a close pair will radiate less. The EMC enemy is more often common mode noise or emissions. Your suggestion would be more useful in situations where you need multiple signal paths and want to do as much as possible to ensure they're all identical.

Answer 2

The common PI filter uses shunt caps to ground for CM suppression and a differential Cap for differential noise suppression. The % of suppression depends on the match tolerances of L1a/L1b and C1a/C1b and the impedance ratios, for an imbalance CM noise creates a differential noise from the difference.

Magnetics rarely operate well (linear) over 2 to 3 freq. decades due to both self-winding capacitances at the high-end or insufficient impedance or mutual coupling at the low end. So multiple chokes may be used with different inductance and cascading impedance increases for EMI reduction. Yet Chokes may be useable over 3.5 f decades and cores are used with low hysteresis.

Since passive LC low pass filters often rely on Specs for DC current or 50 Ohm T lines, DC lines or ACs they tend to be lower inductance and primary rating is current.

Common mode chokes may crossover in the range of DM chokes which are designed for tight balancing and assists the CMRR of a CM impedance from a CM source noise to a pair of CM shunt attenuation capacitors. The tolerance error of the C pair also affects the CMRR of the LC filter pair and the load R dampens the resonant gain.

So in short, CM chokes tend to be better matched and have higher inductance for each leg to achieve a minimum impedance at some test frequency range. While DM chokes are lower impedance to match the load and thus be well dampened at the breakpoint frequency.