This article teaches you how to properly select electromagnetic compatibility (EMC) components

In a complex electromagnetic compatibility environment, in addition to being able to resist certain external electromagnetic interference, each electronic and electrical product cannot generate affordable electromagnetic interference on other electronic and electrical products in the electromagnetic environment. 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 designing electromagnetic compatibility of products, so it is necessary to discuss.

For the related theories of electromagnetic compatibility, the Electronic Components Technology Network conducted in-depth discussions through online EMC semimonthly talks, EMC podiums, and offline EMC seminars. Now let's visually and systematically review the meaning of electromagnetic compatibility, the three elements of electromagnetic interference, and the principle of suppressing electromagnetic interference through some pictures. Then, according to the design principles and structural characteristics of different components of EMC, it mainly explains the structural characteristics of different components. The selection and application skills in EMC design are instructive.

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

Component group

There are two basic sets of electronic components: pin based and non pin based components. Pin line components have parasitic effects, especially at high frequencies. The pin forms a small inductance, approximately 1 nH/mm/pin. Pin ends can also produce a small capacitance effect of about 4 pFs. Therefore, the length of the pin should be as short as possible. Compared to components with pins, pin free, surface mount components have less parasitic effects. Typical values are: 0.5 nH parasitic inductance and about 0. 3 pF terminal capacitance.

From the perspective of electromagnetic compatibility, 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 decoupling capacitors and bypass capacitors. A large number of practices have shown that the correct selection and use of capacitors in EMC's design can not only solve many EMI problems, but also fully reflect the advantages of good effect, low price, and convenient use. If the capacitor is improperly selected or used, it may not achieve its intended purpose at all, or even aggravate the level of EMI.

Theoretically, the larger the capacity of a capacitor, the smaller the capacitive reactance, and the better the filtering effect. Some people also have this habit. However, large capacity capacitors generally have large parasitic inductance and low self resonant frequency (e.g., 0.1 for typical ceramic capacitors μ F0 of F=5MHz, 0.01 μ F0 of F=15 MHz, 0.001 μ F (f0=50MHz), the decoupling effect of high-frequency noise is very poor, or even not at all. If the filter frequency of the separating element exceeds 10 MHz, it will begin to lose performance. 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 the 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 the traditional capacitor. The self resonant frequency can reach 2 times of the lead capacitance of the same capacity (some data say up to 10 times), making it an ideal choice for RF applications.

Traditionally, ceramic capacitors are commonly used in RF applications. In practice, however, ultra small polyester or polystyrene film capacitors are also suitable because they are comparable in size to ceramic 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. here VHF or higher frequency bands can achieve better filtering effects, especially in order to protect the shield from penetration, it is necessary to use feedthrough capacitors.

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, intelligent use of inductors 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 a closed-loop design, the magnetic field completes the magnetic circuit through the magnetic core.

Magnetic field in inductance

Compared to capacitors, inductors have the advantage of no parasitic inductance, so there is no difference between the surface mount type and the lead type.

The magnetic field of an open loop inductor passing through the air can cause radiation and electromagnetic interference (EMI) problems. When selecting an open loop inductor, winding the shaft is better than a rod or solenoid because the magnetic field will be controlled in the magnetic core (i.e., the local magnetic field in the magnet).

Open loop inductance

For closed-loop inductors, the magnetic field is completely controlled within the magnetic core, so this type of inductor is more ideal and, of course, more expensive in circuit design. An advantage of a spiral loop closed loop inductor is that it not only controls the magnetic ring in the magnetic core, but also eliminates all external radiation from the attached field.

There are two main types of magnetic core materials: iron and ferrite. Ferromagnetic core inductors are used in low-frequency applications (dozens of types) at kHz, while ferrite core inductors are used in high-frequency applications (at 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 skeleton of an inductive magnetic core. Iron core inductors are commonly used in low-frequency applications (tens of kHz), and ferrite core inductors are commonly used in high-frequency applications (MHz). Therefore, ferrite core sensors are more suitable for EMC applications.

3、 Selection of filter structure

The filter in EMC design usually refers to L, C composed of low-pass filters. One of the main differences between filters with different structures is the different connection methods of capacitors and inductors. The effectiveness of a filter is not only related to its structure, but also to the impedance of the connected network. For example, a single capacitor filter at high resistance

The effect is good in anti impedance circuits, but poor in low impedance circuits.

4、 Magnetic Beads for EMC Components

The magnetic bead consists of an oxygen magnet, and the inductance consists of a magnetic core and a coil. Magnetic beads convert AC signals into thermal energy, and inductance stores AC and slowly releases it.

The circuit symbol for magnetic beads is inductance, but it can be seen from the model that magnetic beads are used for circuit functions. The principle of magnetic beads and inductances is the same, but the frequency characteristics are different.

Inductors are energy storage elements, and magnetic beads are energy conversion (consumption) devices. Inductors are mainly used in power filter circuits, with the emphasis on preventing conductive interference; Magnetic beads are mainly used in signal circuits, mainly in EMI. Magnetic beads are used to absorb ultra-high frequency signals, such as some RF circuits, PLLs, oscillation circuits, including ultra-high frequency storage circuits (DDR, SDRAM, RAMBUS, etc.), which require the addition of magnetic beads to the power input section. Inductance is an energy storage element used in the power input section, such as LC oscillation circuits, intermediate and low frequency filter circuits, and its application frequency range rarely exceeds 50 MHz.

5、 EMC element diode

Diodes are the simplest semiconductor devices. Due to its unique characteristics, some diodes help solve and prevent, prevent and prevent. EMC related issues.

9、 Conclusion

Electromagnetic compatibility components are the key to solving electromagnetic interference emissions and electromagnetic sensitivity issues. The correct selection and use of these components is a prerequisite for EMC design. Therefore, we must thoroughly understand these components in order to design electronic and electrical products that meet the requirements of standards and have the best cost performance.