Differential mode

Differential noise is typically generated by the fast switch transients.
In an AC/DC switch mode power supply the primary source for the differential noise is typically the PFC switch components.
Capacitors must be placed correctly to reduce the loop for the switch currents and the switching must be done as “soft” as possible.
The input switch currents can be calculated or measured and the input filter must be designed accordingly.
The X capacitors in the input filter must together with a Differential mode choke – or the leakage inductance of the Common mode choke secure sufficient attenuation to make the unit comply with the noise regulations.
Luckily some manufacturers specify the leakage inductance of their Common mode chokes, so that this extra inductance can be used in the differential mode filter calculations.