Design Theory Explanation of Common Mode Inductor

On some motherboards, we can see the common mode inductor, but on most motherboards, we will find that this component is omitted, and some even have no reserved position. Is this motherboard qualified?

It is undeniable that the common-mode inductor has a very 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 other peripherals and our health. But at the same time, it should be pointed out that the anti-EMI design of the board is a very large and systematic project, and the design using common mode inductors is only a small part of it. A board with a common-mode inductor design at the high-speed interface may not necessarily have a good overall anti-EMI design. Therefore, from the common mode filter circuit, we can only see one aspect of the board design, which is easily overlooked by everyone, making the mistake of not seeing the forest for the trees.

Only by understanding the overall anti-EMI design of the board, can we evaluate the pros and cons of the board. So, what kind of work does yxiu's board design generally do in terms of anti-EMI performance?

Motherboard Layout (wiring) design

For yxiu's motherboard wiring design, most clock traces will use shielding measures or be close to the ground wire to reduce EMI. For multi-layer PCB design, the open-loop principle will be adopted in the adjacent PCB wiring layer, and the wires will go from one layer to another layer, so that the wires will not form a loop in the design. If the trace forms a closed loop, it will act as an antenna and enhance the EMI radiation intensity.

The unequal length of the signal lines will also cause the impedance imbalance of the two lines to form common-mode interference. Therefore, in the design of the board, the signal lines will be processed in a serpentine line to make the impedance as consistent as possible and reduce the common-mode interference. . At the same time, the serpentine line will also minimize the bending swing when routing, so as to reduce the area of ​​the ring area, thereby reducing the radiation intensity.

In high-speed PCB design, the length of the trace is generally not an integer multiple of 1/4 of the clock signal wavelength, otherwise resonance will occur and serious EMI radiation will occur. At the same time, the wiring should ensure that the return path is the smallest and smooth. For the design of the decoupling capacitor, its setting should be close to the power pin, and the area surrounded by the power trace and the ground wire of the capacitor should be as small as possible, so as to reduce the ripple and noise of the power supply and reduce EMI radiation .

Of course, the above are only a small part of the principles in PCB anti-EMI design. The layout design of the motherboard is a very complicated and profound knowledge, and even many DIYers have the consensus that whether the layout design is good or not has a very significant impact on the overall performance of the motherboard.

Separation of motherboard wiring

If you want to completely isolate the electromagnetic interference between the motherboard circuits, it is absolutely impossible, because we have no way to isolate the electromagnetic interference one by one."Bag"up, so other methods are used to reduce the level of interference. The metal wire in the main board PCB is the culprit of transmitting the interference current. It transmits and emits the electromagnetic interference signal like an antenna, so in the appropriate place"cut off"These"antenna"It is a useful method of preventing EMI."antenna"If it is broken, then surround it with a circle of insulators, and its interference to the outside world will naturally be greatly reduced. If the filter capacitor is used at the disconnection, the leakage of electromagnetic radiation can be further reduced. This design can significantly increase the stability of high-frequency operation and prevent EMI radiation. Many large motherboard manufacturers have used this method in their designs.

Legend:"disconnect"It is designed to prevent electromagnetic interference from being transmitted through these interfaces to form electromagnetic radiation. The bright lines on the circuit board are clearly visible in the figure. Especially after adopting this design for the USB interface part, the possibility of EMI current radiation to the outside can be greatly improved to a great extent.

Motherboard interface design

I don't know if you have noticed that the motherboard will come with a thin iron shield with an opening. In fact, this is also used to prevent EMI. Although the EMI shielding performance of the chassis is good, electromagnetic waves will still leak out from the openings on the surface of the chassis, such as the openings of the PS/2 interface, USB interface, and parallel and serial ports. The size of the hole determines the degree of leakage of electromagnetic interference. The smaller the aperture of the opening, the greater the attenuation of electromagnetic interference radiation. For a square hole, L is the length of its diagonal.

After using the baffle, the raised metal contacts on the baffle will be grounded well with the input and output parts of the main board through the chassis, which not only attenuates EMI, but also reduces the size of the square hole and further reduces the L value, so that Shield EMI radiation more effectively.

The above three points are just a few main anti-EMI designs in the mainboard design except for the circuit design. It can be seen that the anti-EMI design of the mainboard is an overall concept. If the overall design is unqualified, it will bring large electromagnetic waves. Radiation, and these are not what a small common-mode inductance can make up for.