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How to correctly select and use EMC components

In a complex electromagnetic environment, in addition to being able to withstand certain external electromagnetic interference for normal operation, each electronic and electrical product cannot generate electromagnetic interference that other electronic and electrical products in the electromagnetic environment cannot withstand. In other words, it is necessary to meet both the electromagnetic sensitivity limit requirements specified in relevant standards and the electromagnetic emission limit requirements. This is a problem that should be solved for the electromagnetic compatibility of electronic and electrical products, and it is also a necessary condition for electronic and electrical products to pass the electromagnetic compatibility certification. Many engineers are often helpless or ineffective in selecting and using EMC components correctly when designing products for electromagnetic compatibility. Therefore, it is necessary to explore this issue.


Electromagnetic compatibility components are the key to solving electromagnetic interference emissions and electromagnetic sensitivity issues, and the correct selection and use of these components is a prerequisite for good electromagnetic compatibility design. Therefore, we must thoroughly understand these components, so that it is possible to design electronic and electrical products that meet the requirements of standards and have the best cost performance ratio. Each electronic component has its own characteristics, so it requires careful consideration during design. Next, we will discuss some common electronic component and circuit design techniques used to reduce or suppress electromagnetic compatibility.

Component group

There are two basic sets of electronic components: pin based components and non pin based components. Pinned wire components have parasitic effects, especially at high frequencies. This pin forms a small inductance, approximately 1nH/mm/pin. The end of the pin can also produce a small capacitive effect, approximately 4 pF. Therefore, the length of the pin should be as short as possible. "Compared to components with pins, components without pins and with surface mounting have a smaller parasitic effect.". Typical values are: a parasitic inductance of 0.5 nH and a terminal capacitance of about 0.3 pF.

From an electromagnetic compatibility perspective, surface mount components have the best effect, followed by radial pin components, and finally axial parallel pin components.


 

1、 Capacitance of EMC components

In EMC design, capacitors are one of the most widely used components, mainly used to form various low-pass filters or as decoupling capacitors and bypass capacitors. A large number of practices have shown that the proper selection and use of capacitors in EMC design can not only solve many EMI problems, but also fully reflect the advantages of good results, low price, and convenient use. If the capacitor is improperly selected or used, it may not achieve the expected purpose at all, and may even exacerbate the level of EMI.



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Theoretically, the larger the capacitance, the smaller the capacitive reactance, and the better the filtering effect. Some people also have this habit of understanding. However, capacitors with large capacity generally have large parasitic inductance and low self resonant frequency (such as a typical ceramic capacitor, 0.1 μ F0 of F=5 MHz, 0.01 μ F0 of F=15 MHz, 0.001 μ F (f0=50 MHz) has a poor decoupling effect on high-frequency noise, even having no decoupling effect at all. Discrete element filters will begin to lose performance when the frequency exceeds 10 MHz. The larger the physical size of the component, the lower the frequency of the turning point. These problems can be solved by selecting capacitors with special structures.

The parasitic inductance of a chip capacitor is almost zero, and the total inductance can also be reduced to the inductance of the component itself, usually only 1/3 to 1/5 of the parasitic inductance of a traditional capacitor. The self resonant frequency can be up to twice that of a lead capacitor of the same capacity (it is also reported to be up to 10 times), making it an ideal choice for radio frequency applications.

Traditionally, ceramic capacitors have been selected for RF applications. However, in practice, ultra small polyester or polystyrene film capacitors are also suitable because their size is comparable to that of ceramic chip capacitors.

Three-terminal capacitors can expand the frequency range of small ceramic capacitors from below 50 MHz to above 200 MHz, which is very useful for suppressing noise in the VHF frequency band. To achieve better filtering effects in VHF or higher frequency bands, especially to protect the shield from penetration, feedthrough capacitors must be used.


 

2、 Inductance of EMC components

Inductance is a component that can connect magnetic and electric fields, and its inherent ability to interact with magnetic fields makes it potentially more sensitive than other components. Similar to capacitors, using inductors intelligently can solve many EMC problems. The following are two basic types of inductors: open loop and closed loop. Their difference lies in the internal magnetic field loop. In an open loop design, the magnetic field is closed through air; In closed loop design, the magnetic field completes the magnetic circuit through the magnetic core.



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Magnetic field in inductance

An advantage of an inductor over a capacitor is that it has no parasitic inductance, so there is no difference between its surface mount type and lead type.

The magnetic field of an open loop inductor passes through the air, causing radiation and electromagnetic interference (EMI) problems. When selecting an open loop inductor, the shaft wound type is better than the rod or solenoid type because this will control the magnetic field within the magnetic core (i.e., the local magnetic field within the magnet).

Open loop inductance

For closed-loop inductors, the magnetic field is completely controlled at the magnetic core, so this type of inductor is more ideal in circuit design, although they are also more expensive. One advantage of a spiral loop closed loop inductor is that it not only controls the magnetic ring at the magnetic core, but also eliminates all external incidental field radiation by itself.


There are two main types of magnetic core materials for inductors: iron and ferrite. Ferromagnetic core inductors are used in low-frequency applications (tens of KHz), while ferrite core inductors are used in high-frequency applications (to MHz). Therefore, ferrite core inductors are more suitable for EMC applications.

Two special types of inductors are specifically used in EMC applications: ferrite magnetic beads and ferrite magnetic clips. Iron and ferrite can be used as the framework of an inductive magnetic core. Iron core inductors are often used in low-frequency applications (tens of KHz), while ferrite core inductors are often used in high-frequency applications (MHz). Therefore, ferrite core sensors are more suitable for EMC applications.




keywords: EMC
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