What Is a Common Mode Inductor?

Common mode choke, also called common mode choke, is commonly used in computer switching power supplies to filter common mode electromagnetic interference signals. In the board design, the common mode inductor also plays the role of EMI filtering, which is used to suppress the electromagnetic waves generated by high-speed signal lines from radiating to the outside.

Mixed on the motherboard inside the computer

Why can common mode inductors prevent EMI? To figure this out, we need to start with the structure of the common-mode inductor.

Filter circuit for common mode inductor, La and Lb are common mode

For an ideal inductance model, when the coil is wound, all magnetic flux is concentrated in the center of the coil. But usually, the toroidal coil will not be wound for a full circle, or the winding is not tight, which will cause leakage of magnetic flux. The common mode inductor has two windings with a considerable gap between them, which will cause magnetic flux leakage and form

Advantages of the iron core type:

High initial magnetic permeability (this is the basic requirement for common mode inductance), high saturation magnetic induction strength, temperature is more stable than ferrite (it can be understood as small temperature rise), frequency characteristics are more flexible, because the magnetic permeability is high, it can be small Make a large amount of sensitivity, and the frequency of adaptation is relatively wide;

Overall advantages:

Because the initial magnetic permeability is 5-20 times that of ferrite, the suppression effect on conducted interference is far greater than that of ferrite;

The high saturation magnetic induction intensity of nanocrystals is better than that of ferrite, so it is not easy to saturate under large current;

The temperature rise is lower than that of the UF series. Someone's actual test: it is nearly 10 degrees lower at room temperature (personal test values are for reference only);

The flexibility of the structure makes it adaptable, and can be adapted to different needs by changing the processing technology (see the magnetic ring inductor used on energy-saving lamps, the use is quite flexible);

The distributed capacitance will be smaller because the winding area is wider and the volume is relatively small;

The number of turns used in the circle is smaller, the distribution parameters are smaller, and the efficiency is dominant.

Overall disadvantage:

The diameter of the magnetic ring is small, the machine is difficult to thread, and it needs to be manually wound. It takes time and effort, high processing cost and low efficiency. This is especially important at a time of increasing cost pressure.

Compared with UF, the pressure resistance is not great: it can be seen that many magnetic ring common modes are separated by cable ties, which is not very reliable. Some of them are separated by a certain distance. How about reliability? If the inductance is relatively large, the wires will be crowded together, and there is a little doubt about safety.

Inconvenient installation and high failure rate.

application:

Because of the cost factor, most of the magnetic rings are used on high-power power supplies. Someone described it as "low-power magnetic rings are too high-end", which makes sense.

Of course, because of its small size and small power supply that requires volume, a magnetic ring is also a good choice.

Compared with comprehensive performance, it is better than UF series. If the cost pressure is not high, you can consider using a magnetic ring. For an actual test conduction, the margin of the magnetic ring is much lower. And the amount of sensitivity is still smaller than UF.

Let's talk about the common mode of the UF / UU series

Material: Basically it is ferrite. Of course, this ferrite is also different. Generally there are MXO-manganese-zinc and NXO-nickel-zinc. The main advantages of nickel-zinc are: the initial magnetic permeability is low (less than 1000u), but it can work at a relatively high frequency (greater than 100MHZ) and maintain the magnetic permeability unchanged. Very strong and great.

NXO has a higher resistivity than MXO. The similar damping effect of ferrites on high-frequency clutter is used to release high-frequency clutter as thermal energy, which explains the temperature problem of common-mode inductance.

Overall advantages:

The most important point is that the cost is low (someone uses this for 0.9 yuan), which can be wrapped with a machine and is efficient. UU9.8 or UU10.5 is commonly used;

With a skeleton, the winding process should be better controlled and a higher inductance can be made;

Better withstand voltage and reliability? For magnetic ring common mode;

Good plugin, good installation. There are no problems with the four pins and the holes are correct. It is basically used on power supplies with small currents, because the wire diameter cannot be very thick, so the current cannot be too large;

Overall disadvantage:

Space factor: The package position is large, maybe because it is relatively strong, not as small and exquisite as the magnetic ring;

The fever is relatively serious, which is also based on my actual measurement: 90V input can be as fast as 90 degrees at room temperature under full load;

application:

Generally used in strict cost control or low power applications ^[1]

On some motherboards, we can see the common-mode inductor, but on most motherboards, we will find that this component is omitted, and even some locations are not reserved. Are such motherboards qualified?

It is undeniable that the common mode inductor has a good suppression effect on the common mode interference of the high-speed interface of the motherboard, and can effectively prevent EMI from forming electromagnetic radiation through the cable to affect the normal operation of the remaining peripherals and our health. But it also needs to be pointed out that the anti-EMI design of the board is a rather large and systematic project, and the design using common mode inductor is only a small part of it. The high-speed interface has a common-mode inductor design board, which may not be excellent in the overall anti-EMI design. Therefore, from the common-mode filter circuit, we can only see one aspect of the board design, which is easy to be ignored by everyone, and the mistake of seeing no wood is seen.

Only by knowing the overall anti-EMI design of the board can we evaluate the advantages and disadvantages of the board. So, what does a good board design generally do in terms of anti-EMI performance?

1. Motherboard Layout Design

For excellent motherboard wiring design, most of the clock traces are shielded or close to ground to reduce EMI. For multi-layer PCB design, the open-loop principle will be adopted in the adjacent PCB routing layers, and the wires from one layer to the other will be designed to avoid the wires from forming a loop. If the trace forms a closed loop, it acts as an antenna and will increase EMI

Missing common mode inductance = low EMI resistance? This statement is obviously quite one-sided.

It is true that because the national EMI related regulations are not perfect, some manufacturers have drilled this space in order to save material, and they have saved the compression cost (including the omission of common mode inductance) in the overall anti-EMI performance. The consequence is that the motherboard's anti-EMI performance is extremely low; but for those motherboards that are excellent in overall design and do not shrink in materials, even if there is no common-mode inductor, their overall anti-EMI performance can still meet relevant requirements. Such products are still qualified. Therefore, it is not appropriate to judge whether the motherboard is good or bad based on whether there is a common mode inductor.

1 Overview

The design of the power filter can usually be considered from two aspects: common mode and differential mode. The most important part of the common mode filter is the common mode

(1) The wires wound on the core of the coil should be insulated from each other to ensure that no short circuit occurs between the turns of the coil under the transient overvoltage;

(2) Do not saturate the magnetic core when a large instantaneous current flows through the coil;

(3) The magnetic core in the coil should be insulated from the coil to prevent breakdown between the two under transient overvoltage;

(4) The coil should be wound as single layer as possible. This will reduce the parasitic capacitance of the coil and enhance the coil's ability to withstand transient overvoltages.

Usually, pay attention to selecting the frequency band to be filtered at the same time. The larger the common-mode impedance, the better. Therefore, you need to look at the device information when selecting the common-mode inductor, which is mainly selected based on the impedance-frequency curve. In addition, please pay attention to the influence of differential mode impedance on the signal when choosing, mainly pay attention to the differential mode impedance, especially pay attention to the high-speed port ^[1] .