Common mode choke. This detail can be seen on boards from switching power supplies, next to the network connection socket. Choke consists of two identical coils, which are wound with wire of the same diameter and have the same number of turns. The coils are located on a core made primarily of ferrite. Why is this part needed in power supplies?
This choke is needed to suppress the common mode signal (interference) and pass the differential (useful) signal without significant restrictions. In switching power supplies, common mode noise appears during their operation. To prevent this interference from entering the network or vice versa, a choke is used for this.
If a differential signal is applied to the input of the inductor, then the magnetic fluxes of the two windings compensate each other and the signal will pass through the inductor almost without resistance. But if a common-mode signal is applied to the input of the inductor, then the magnetic fluxes of the two coils are summed up, the inductance of the inductor will be large and resistance will be provided to the signal .
Now I’ll show you what this looks like in practice. I unsoldered one common mode choke from the board, which has four contacts. Two contacts, two points – the inductor input, the other two contacts – the inductor output. I connected an 8 kOhm resistor to the input and output. The choke input is connected to the network
I apply two signals to the choke input. This is 2 a sinusoid with a frequency of 1 MHz and an amplitude of 7 Volts (full swing). I connected the input of an oscilloscope to the output of the inductor to observe the signal.
First, I applied a differential signal to the inductor input. These are two sinusoids out of phase by 180 degrees. You see that the inductor passed this useful signal.
And now I apply two signals to the choke input, the phases of which coincide. And what happened? The choke resisted these two signals and there will be almost no these signals at the choke output. The choke did not pass the common-mode signal.
But you need to know that a common mode choke works well only from a certain frequency. My choke works well at a frequency of 1 MHz and above, but at a frequency of 100 kHz my choke does NOT work.
