What is the optimal distance between the common mode inductor and the X capacitor? Why? ​

The optimal distance between the common mode inductor and the X capacitor is usually recommended to be within 3-5cm. This is because the common mode inductor primarily suppresses common mode interference, while the X capacitor mainly filters differential mode interference. Both components need to work together to form an EMI filter. If the distance is too long, the parasitic inductance between the leads will increase, leading to a disruption of the impedance matching of the high-frequency (such as above 100MHz) filtering network. Interference signals may "bypass" the filter through parasitic parameters, reducing the overall suppression effect. Close arrangement can minimize parasitic inductance and capacitance, ensuring effective attenuation of wide-frequency interference by the filter.