Common or Normal-mode noise is a noise current that has leaked via a stray capacitance that passes through a ground and returns to the power supply line. These types of noise are conducted emissions; however, noise currents are flowing in power supply lines, so noise is radiated.
To suppress this type of noise, EMI suppression filters are installed on all lines on which noise is conducted. In the example illustrations shown above, the following two suppression methods are applied.
1. Noise is suppressed by installing an inductor to the signal line and ground line, respectively.
2. A metallic casing is connected to the signal line using a capacitor. Thus, noise is returned to the noise source in the following order:
signal/GND lines ➝ capacitor ➝ metallic casing ➝ stray capacitance ➝ noise source.
Common-mode choke coils are used to suppress common-mode noise. This type of coil is produced by winding the signal or supply wires on a ferrite core. Since magnetic flux flows inside the ferrite core, common mode choke coils work as an inductor against common mode current.
The common-mode refers to signals or noise that flow in the same direction in a pair of lines. As an example, consider a configuration where two copper wires are wound around a ring-shaped ferrite core.
In this structure, two patterns may exist: one in which the currents in the wires flow in the same direction (Common Mode), and another where they flow in opposite directions (Differential Mode).
You may also refer to the following Power Management video for additional overview information about power.