Common mode choke inductor’s design factors

author: MagTop
02/11/2022
Toroidal Common mode Choke inductor
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In some motherboards, we can see the common mode inductor, but in most motherboards, we will find that the element is omitted, and even some of the position is not reserved. Is this motherboard qualified?

Undeniably, the common mode inductor has a good inhibitory effect on the common mode interference of the high-speed interface of the motherboard, which can effectively avoid the electromagnetic radiation formed by EMI through the cable to affect the normal work of other peripherals and our health. However, it is also necessary to point out that the EMI prevention design of board cards is a huge and systematic project, and the design of common mode inductors is only a small part of it. High - speed interface with common - mode inductor design of the board, not necessarily the overall anti-EMI design is excellent. 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 make the mistake of missing the forest for the wood.
Only by understanding the overall EMI prevention design of the board can we evaluate the merits and demerits of the board. So, what does a good board design generally do in terms of EMI performance?

1. The main board Layout design For good motherboard wiring design, most clock wiring will use shielding measures or close to ground to reduce EMI. For multi-layer PCB design, the principle of open-loop is adopted in the adjacent PCB wiring layers, so that the wires from one layer to another will avoid the formation of loops in the design. If the wire forms a closed loop, it plays the role of an antenna, which will enhance the EMI radiation intensity.
The unequal length of signal lines will also cause the imbalanced impedance of the two lines and form common-mode interference. Therefore, signal lines will be treated as snake lines in the design of board cards to make their impedance as consistent as possible and reduce common-mode interference. At the same time, the serpentine wire also minimizes the curved swing when it is routed to reduce the area of the annular area and thus the radiation intensity.

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

Of course, these are just a few of the principles of EMI resistant PCB design. Layout design of the main board is a very complex and profound knowledge, and even many DIYer have such a consensus: Layout design is excellent or not, has a very significant impact on the overall performance of the main board.