Following our previous discussion of chip-type three-terminal capacitors, in this lesson we will discuss common mode choke coils.
<Common mode choke coils separate noise from signals using conduction modes>
In our previous discussions of chip ferrite beads and chip-type three-terminal capacitors, we explained how they utilize differences in frequency, as noise frequencies are relatively higher than signal frequencies. As such, they function as low-pass filters that selectively suppress only noise. Common mode choke coils are also a type of noise filter, but rather than using differences in frequency, they separate noise from signals by differences in conduction mode. We must therefore first learn the distinction between common modes and differential modes.
<Common modes and differential modes>
Normally, in the electrical circuit of a circuit board, the current flowing out from a certain part reaches another circuit through the load, and returns to the origin via a different route on the circuit board. (In many cases, the return route is the ground plane of the circuit board.) This type of flow is called differential mode (or normal mode).
Figure 1. Conduction route of a differential mode
Another conduction route also exists, though not as a clear-cut wire. A tiny amount of stray capacitance is generated between the wires on the circuit board and the reference ground surface, creating a conduction route where the capacitance flows commonly through all wires on the circuit board and returns in the opposite direction along the reference ground surface. This route is called common mode.
Figure 2. Common mode conduction route
Although the stray capacitance between the wires and the reference ground surface is quite small, impedance drops as the signal frequency rises even with the tiny amount of stray capacitance, so that the common mode current flows more easily. Normally, the common mode current is not actively sent through the electrical circuit, but if the ground of a power supply circuit or driver IC vibrates, the entire circuit it drives will vibrate, resulting in common mode noise. If a cable is externally connected to the circuit, common mode current will also flow through the cable. As it will have an electrical potential with respect to the ground, the current will be released as noise radio waves.
<Common mode choke coils are noise filters that act only on common mode currents>
Common mode choke coils are noise filters that discriminate between signals and noise from the above mentioned common modes and differential modes, or conduction modes. Simply put, they are filters that act only on common modes.
Figure 3 shows a principle diagram for common mode choke coils.
Figure 3. Working principles of common mode choke coils
Common mode choke coils are made up of two conducting wires wrapped around a single core (a ferrite core when used in high-frequency applications). They therefore have four terminals. The wires are wrapped around the core in opposite directions. When common mode currents flow through coils with this type of structure, flux is generated by the electromagnetic induction phenomenon that occurs in each coil. However, as the direction of the generated flux is the same, both fluxes become stronger to increase their action as inductors. Conversely, differential mode currents flowing through the coil generate flux in opposing directions that cancel each other out. As a result, it no longer acts as an inductor against the differential mode current. Common mode choke coils are therefore filters that only act as inductors for common modes, and not against differential modes.