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).
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.
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.
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.
<Advantages of common mode choke coils>
Common mode choke coils have two advantages.
(1) Even when the frequencies of signals and noise overlap, their different conduction modes enable suppression of only noise.
(2) Performance does not decrease even with a large differential mode current, as the core does not become saturated.
The most important feature of common mode choke coils is their ability to distinguish between noise and signals even of the same frequency. Recently, more and more electronic devices are starting to use high-speed differential transmission as their method for transmitting signals. USB, SATA, and HDMI are typical examples of high-speed differential transmission. In high-speed differential transmission lines, extremely high frequency signals are transmitted, and filters like ferrite beads that separate noise from signals by differences in frequency cannot distinguish between the two. Emphasizing the impact of the signal means that it cannot suppress noise very well, but focusing on noise suppression attenuates part of the signal, therefore influencing signal integrity. Common mode choke coils, on the other hand, separate signals from noise using transmission modes, so that high-speed signals that flow through the coils are not affected if they are differential modes. In high-speed differential transmission lines, signals are generally only differential modes. The problematic noise is mostly common mode noise, so common mode choke coils can be used to effectively suppress common mode noise without affecting high-speed signals.
Cables are connected to business power lines and secondary AC adapters that receive inputs of power. These cables become antennae, and the noise that is released becomes a problem. When using inductor-type filters that act on differential modes, such as ferrite beads or normal mode choke coils, the core becomes magnetically saturated by large currents that flow through, causing a dramatic reduction in their performance as inductors. Common mode choke coils are very useful in these types of applications. In common mode choke coils, no magnetic saturation occurs, as flux generated by differential mode currents cancels each other out and disappears. Common mode choke coils are therefore used to suppress noise in power lines through which large currents flow.
<Examples of common mode choke coils >
Figure 5 shows some examples of common mode choke coils.
AC power lines are subject to high voltage, so careful attention is given to safety when constructing coils for these lines. Conversely, as high-speed signal lines require smaller-sized coils, chip-type coils are used in these applications. Other coils available on the market are a winding wire-type that winds a wire around a ferrite core and a film-type that uses film coils. The winding wire-type features high performance, while the film-type features smaller size. Figure 6 shows an example of the structure of a winding wire chip-type common mode choke coil. Two lines are wound around the coil together, so that the outgoing line and the returning line lie next to each other, and the magnetic coupling between the lines increases to raise selectivity between common modes and differential modes.
<Precautions for using common mode choke coils>
The discussion up to now has stated that common mode choke coils do not affect differential modes, but this is only true for ideal common mode choke coils. In reality, some of the flux produced by the opposing coils leaks is not cancelled out, resulting in a small amount of inductance. This differential mode inductance is very small, but its influence must be considered in applications that use extremely high-frequency signals. Figure 7 shows an example of an actual impedance curve for a chip-type common mode choke coil. We know that the differential mode impedance becomes high at around 1 GHz. Chip-type common mode choke coils have recently been developed that suppress differential mode impedance even more. Appropriate chip-type common mode choke coils should be chosen for Display Ports, USB3.0, and other devices that use extremely high-frequency signals.
Have a look at the guide we have made available for selecting chip-type common mode choke coils for use in high-speed differential lines.
Selection Guide for Common Mode Choke Coils for High-Speed Differential Transmission Lines
Person in charge: Murata Manufacturing Co., Ltd. Yasuhiro Mitsuya