Common mode interference and differential mode interference of switching power supply
There are two types of interference on power lines: common mode interference and differential mode interference. Common mode interference exists between any power source and ground or between wires and ground. Common mode interference is sometimes referred to as longitudinal mode interference, asymmetric interference, or ground interference. This is the interference between the current carrying conductor and the earth. Differential mode interference exists between power supply phase lines and neutral lines, as well as between phase lines. Differential mode interference, also known as normal mode interference, transverse mode interference, or symmetric interference. This is interference between current carrying conductors. Common mode interference indicates that the interference is coupled into the circuit by radiation or crosstalk, while differential mode interference indicates that the interference originates from the same power supply circuit. Usually, these two types of interference coexist, and due to the imbalance of line impedance, the two types of interference can also transform into each other during transmission, making the situation very complex. After long-distance transmission, the attenuation of differential mode components is greater than that of common mode due to the difference in line to line impedance and line to ground impedance. For the same reason, common mode interference also radiates to adjacent spaces during line transmission, while differential mode interference does not. Therefore, common mode interference is more likely to cause electromagnetic interference than differential mode interference. Different interference methods require different interference suppression techniques to be effective. A simple method to determine interference is to use a current probe. The current probe first individually wraps around each wire to obtain the induction value of a single wire, and then wraps around two wires (one of which is the ground wire) to detect their induction situation. If the induction value increases, the interference current in the circuit is common mode; On the contrary, it is differential mode.
