[Basic course series on passive components (LC)] Basics of LC ~Part 8 What is a common mode choke coil?

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Basic Analog Discrete Würth Elektronik

Inductors and capacitors are essential components in electronic circuits. I will explain the basic role of its operation.

The content of this time is Part 8 "What is a common mode choke coil?"

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Common mode noise and differential mode noise

Conduction noise includes common mode noise and differential mode noise (normal mode).

The direction of the current is different as shown in Fig.1 and Fig.2.

Figure 1 Current direction of common mode noise

Figure 2 Current direction of differential mode noise

common mode noise	It refers to the case where part of the return current returns through the ground or housing, etc., and is called common mode because the direction of the noise current is the same.
differential mode noise	When a noise source flows through a line, the return current flows through the paired line. It is called differential mode because the forward and return directions are opposite.

What is a common mode choke coil?

A common mode choke coil has the structure shown in Fig. 3 and consists of two windings on one iron core. If the number of turns is the same and the winding direction is reversed, the operation will differ depending on the direction of the noise current.

Fig. 3 Common mode choke coil structure

Common mode choke coils behave differently depending on the operating mode, as shown in Fig. 4. For common mode current, the magnetic flux is added and it works as an inductor, and the high impedance eliminates noise.

Conversely, since the magnetic flux is canceled for the differential mode current, it does not function as an inductor and does not affect the differential mode signal.

Fig. 4 Direction of current and magnetic flux depending on operation mode

Impedance characteristics of common mode choke coil

Fig. 5 shows an example of impedance characteristics of a common mode choke coil.

Fig. 5 Example of impedance characteristics of common mode choke coil

From the 7448228002 datasheet

The common mode choke coil operates to cancel the magnetic flux in the differential mode, so the impedance is theoretically zero, but as shown in the red impedance characteristics in Fig. 5, the impedance is actually completely reduced due to the influence of the leakage magnetic flux. cannot cancel each other and have an impedance.

This is very small at low frequencies and can be neglected, but at higher frequencies it becomes larger and this characteristic must be taken into account depending on the frequency of the signal used.