Why should the lead length between the common mode inductor and the connector be controlled within 5cm?

The lead length between the common mode inductor and the connector should be controlled within 5cm. The core reason is to reduce interference "escape" caused by parasitic parameters.

An excessively long lead will introduce significant parasitic inductance (approximately 1nH/mm) and distributed capacitance. At high frequencies (such as above 100MHz), these parasitic parameters can turn the lead into an "antenna", causing common mode interference to radiate out through the lead without being suppressed by the inductor. At the same time, long leads can disrupt the impedance matching of the filter (such as the source impedance/load impedance mismatch between the common mode inductor and the connector), leading to interference reflection and reducing filtering efficiency.

5cm is an empirical threshold, at which the parasitic inductance is approximately 50nH, providing a low radiation gain for interference above 100MHz, ensuring effective suppression of interference.​