What is a chip common mode choke coil? What are the characteristics?

The common mode choke coil is distinguished according to the interference and the transmission mode of the signal.

The previously used chip ferrite beads and chip three-terminal capacitors use the principle that the interference frequency is slightly higher than the signal frequency, and are used as a low-pass filter to selectively cancel the interference. Although the common mode choke coil is also an interference filter, the principle is to distinguish between interference and signal, not frequency, depending on the conduction mode. Therefore, it is necessary to understand the two modes of conduction, common mode and differential mode.

Common mode and differential mode

Usually, the current flowing from somewhere in the electrical circuit on the substrate will flow through the load to other circuits and flow back through other routes on the substrate. (Many times the return route is the ground plane of the substrate) This is the differential mode flow.

Figure 1 Conduction route of differential mode

On the other hand, although there is no clear wiring, there are other conductive lines. Since a weak parasitic capacitance is generated between each wiring on the substrate and the ground as a reference, a conduction path is formed which flows out from the ground and is folded back from the opposite direction to the ground. This is called common mode.

Figure 2 Common mode conduction line

Although the parasitic capacitance between the ground and the ground is very weak, when the signal frequency increases, the weak parasitic capacitance decreases, and the common mode current easily passes. Generally, the common mode current does not actively pass through the electronic circuit, but when the ground of the power supply circuit or the driver IC is loose, the circuit driven by the whole is unstable, thereby forming common mode interference. In this circuit, if there is a cable connected to the outside, a common mode current will pass through the cable, and since the potential is relatively unstable, an interference wave is formed.

Common mode choke coil is a squelch filter that only acts on common mode current

The common mode choke coil is a squelch filter that distinguishes noise and signals by the above-described common mode and differential mode conduction modes. In a word, it is a filter that acts only on the common mode.

Figure 3 is a schematic diagram of a common mode choke coil.

Figure 3 The working principle of the common mode choke coil

The common mode choke coil is wound on two cores (a ferrite core at high frequency). There are 4 joints. The two winding directions are opposite each other. When a common mode current passes through such a coil, each coil generates a magnetic flux due to an electromagnetic induction phenomenon, but since the generated magnetic flux directions are the same, thereby enhancing each other, the inductance performance is improved. When the differential mode current is passed through the coil, the magnetic fluxes cancel each other due to the opposite direction of the generated magnetic flux. The effect of the inductance on the differential mode current disappears. In this way, a filter that has no effect on the differential mode in the common mode choke coil and only acts on the common mode is formed.

The common mode choke coil has two advantages.

1 Even if the frequency of the signal and noise coincide, the noise can be removed as long as the conduction is different.

2 Even if the current is passed through the differential mode, the core will not be saturated and the performance will be degraded.

The biggest feature of common mode choke coils is the ability to separate noise and signals without the aid of frequency. Recently, the use of high-speed differential transmission as a signal transmission method for electronic devices has been increasing. USB, SATA, and HDMI are representative of high-speed differential transmission. The signal frequency in the high-speed differential transmission line is so high that it is not possible to properly separate the noise and the signal using a frequency like a ferrite bead. If you pay attention to the impact on the signal, you can't clear the noise; if you pay attention to clear the noise, some of the signal will also attenuate, affecting the quality of the signal. The common mode choke coil separates signals and interference according to the transmission method, so even if it passes through a high speed signal, it will have no effect as long as it is a differential mode. The signal in the high-speed differential transmission line has only the principle differential mode. The main interference here is the common mode. Therefore, by using the common mode choke coil, the common mode interference can be effectively eliminated without affecting the high speed signal.

Figure 4 Comparison of interference cancellation in high-speed differential transmission lines

This cable can sometimes become an interference transmitting antenna due to the input of the power supply's industrial power cord or the secondary side cable of the AC adapter, causing problems. If a ferrite bead or a differential mode inductive filter such as a normal mode choke coil is used here, the magnetic current is saturated by the large current here, and the inductance performance is drastically lowered. At this time, the common mode choke coil functions. In the common mode choke coil, the magnetic fluxes generated by the differential mode current cancel each other out, and magnetic saturation does not occur. Therefore, the common mode choke coil is also widely used in the noise countermeasure of the large current power line.

Common mode choke coil example

Figure 5 is an example of a common mode choke coil.

Figure 5 Example of a common mode choke coil

When used in an AC power cord, due to high voltage, sufficient safety considerations have been taken. For high-speed signal lines, the sheet structure is mainly considered in order to meet the requirements of miniaturization. In addition, a coil type that is wound on a ferrite core and a filter type that applies a filter coil are also being commercialized. The coil type is characterized by high performance, while the filter type is characterized by small size. Fig. 6 is a view showing an example of the structure of a wound-line type chip common mode choke coil. The two wires are wound at the same time to make the outgoing line and the return line adjacent, and the magnetic relationship between the lines and the lines is combined with each other, and the selectivity of the common mode and the differential mode is increased.

Figure 6 Example of the structure of a coiled-type chip common mode choke coil (below)

Note on common mode choke coil

The description so far is for the common mode choke coil does not affect the differential mode, but in fact this is the ideal case of the common mode choke coil. In fact, a part of the magnetic flux generated between the coils remains and becomes a part of the resistor. Although this differential mode resistance is very weak, it is necessary to consider this effect when the signal frequency is high. Fig. 7 is a graph showing the actual chip-type common mode choke coil resistance curve. It can be seen from the figure that the differential mode resistance rises at about 1 GHz. Recently, chip common mode choke coils with low-voltage differential mode resistors are being gradually commercialized. When dealing with signals with high frequencies such as Display Port and USB3.0, the selection of corresponding chip-mode common-mode choke coils is very important. of.

For a chip-type common mode choke coil selection guide with high-speed differential lines, please refer to https://

Figure 7 Common mode choke coil resistance characteristics