In the context of a complex electromagnetic environment, each electronic and electrical equipment must not only be able to resist a certain amount of external electromagnetic interference to maintain normal operation, but also must ensure that it does not emit electromagnetic interference beyond the ability of other electronic and electrical equipment in the environment. This means that the equipment must meet the requirements of the established standard on the maximum tolerance value of electromagnetic sensitivity and the maximum allowable value of electromagnetic radiation, which is the core problem of electromagnetic compatibility design of electronic and electrical products to solve, and also the basic premise of the product through electromagnetic compatibility certification. However, in the face of EMC design, many engineers often feel confused or ineffective in how to accurately select and apply EMC components, so it is particularly important to explore this topic in depth.

Electromagnetic compatibility components are the key to solve the problems of electromagnetic interference emission and electromagnetic sensitivity. The correct selection and use of these components is the premise of electromagnetic compatibility design. Therefore, we must deeply grasp these components, so that it is possible to design electronic and electrical products that meet the standard requirements and have the best performance and price ratio. Each electronic component has its own characteristics, therefore, requires careful consideration in the 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 groups of electronic components: pin-free and pin-free. Leaded elements have parasitic effects, especially at high frequencies. This pin forms a small inductance, about 1nH/mm/ pin. The end of the pin can also produce a small capacitive effect, about 4pF. Therefore, the length of the pin should be as short as possible. Pin-free and surface-mounted components have less parasitic effects than those with pins. Typical values are 0.5nH parasitic inductance and about 0.3pF terminal capacitance.

From an electromagnetic compatibility point of view, surface mount components work best, followed by radial pin components, and finally, axial parallel pin components.

Capacitance of EMC components

Capacitors are one of the most widely used components in EMC design, mainly used to form various low-pass filters or as decoupling capacitors and bypass capacitors. A lot of practice shows that: in EMC design, proper selection and use of capacitors 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 selected or used improperly, it may not achieve the intended purpose at all, and even increase the level of EMI.

In theory, the larger the capacity of the capacitor, the smaller the reactance, and the better the filtering effect. Some people also have this habit of knowing. However, the capacitors with large capacity are generally large parasitic inductors, and the self-resonant frequency is low (such as typical ceramic capacitance, f0 of 0.1μF = 5MHz, f0 of 0.01μF = 15MHz, f0 of 0.001μF = 50MHz), and the decoupling effect on high-frequency noise is poor, or even does not play a decoupling role at all. Filters for discrete components will begin to lose performance at frequencies above 10 MHz. The larger the physical size of the component, the lower the turning point frequency. 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 traditional capacitor parasitic inductance, and the self-resonant frequency can reach 2 times of the same capacity of the lead capacitor (there are also data that can reach 10 times), which is the ideal choice for RF applications.

Traditionally, chip capacitors have been chosen for RF applications. In practice, however, ultra-small polyester or polystyrene film capacitors are also suitable because their size is comparable to that of porcelain chip capacitors.

The three-terminal capacitor can extend the capacitor frequency range of small porcelain chips from less than 50 MHz to more than 200 MHz, which is very useful for suppressing noise in the VHF band. In order to obtain better filtering effect at VHF or higher frequency bands, especially to protect the shield from penetration, it is necessary to use the feed capacitance.

2. Inductance of EMC components

An inductor 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. Like capacitors, smart use of inductors can solve many EMC problems. Here are the two basic types of inductors: open loop and closed loop. What makes them different is the magnetic field loop inside. In the open-loop design, the magnetic field is closed by air; In the closed-loop design, the magnetic field completes the magnetic circuit through the magnetic core, as shown in the figure below.

Magnetic field in inductor

An advantage of inductors over capacitors is that they have no parasitic reactance, so there is no difference between the surface mount type and the lead type.

The magnetic field of an open-loop inductor passes through the air, which will cause radiation and introduce electromagnetic interference (EMI) problems. When choosing an open-loop inductor, a winding type is preferable to a rod or solenoid type because the magnetic field will then be controlled within the core (i.e. the local magnetic field within the magnet).

Open-loop inductance

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

There are two main types of inductor core materials: iron and ferrite. Ferrite 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 inductor types are used in EMC applications: ferrite beads and ferrite clips. Iron and ferrite can be used as inductance core skeleton. 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.

Third, the selection of filter structure

The filter in EMC design usually refers to the low-pass filter composed of L and C. One of the main differences between filters of different structures is that the capacitors and inductors are connected differently. The effectiveness of the filter is not only related to its structure, but also to the impedance of the connected network. For example, a filter with a single capacitor works well in a high-impedance circuit, while the effect is poor in a low-impedance circuit.

Filter classification (based on function)

Filter classification (based on structure)

Filter selection

4. Magnetic beads for EMC components

The magnetic bead is composed of an oxygen magnet, the inductor is composed of a magnetic core and a coil, the magnetic bead converts the AC signal into heat energy, the inductor stores the AC and slowly releases it.

How magnetic beads work

Magnetic bead selection

The circuit symbol of the magnetic bead is the inductor, but the model can be seen that the magnetic bead is used in the circuit function, the magnetic bead and the inductor are the same principle, but the frequency characteristics are different.

The inductor is the energy storage element, and the magnetic bead is the energy conversion (consumption) device. The inductor is mainly used in the power filter circuit, focusing on restraining the conductive interference; Magnetic beads are mostly used in signal loops, mainly for EMI. Magnetic beads are used to absorb ultra-high frequency signals, such as some RF circuits, PLL, oscillating circuits, including ultra-high frequency memory circuits (DDR, SDRAM, RAMBUS, etc.) need to add magnetic beads in the power input part, and the inductor is a kind of energy storage element, used in LC oscillating circuits, low frequency filter circuits, etc., its application frequency range rarely exceeds 50MHz.

Diodes for EMC components

Diodes are the simplest semiconductor devices. Due to their unique characteristics, certain diodes help solve and prevent some of the problems associated with EMC.

6. Selection of analog and logic active devices

The key of electromagnetic interference emission and electromagnetic sensitivity is the selection of analog and logic active devices. Attention must be paid to the inherent sensitivity and electromagnetic emission characteristics of active devices.

Active devices can be divided into tuned devices and basic frequency band devices. The tuning device acts as a bandpass element, and its frequency characteristics include center frequency, bandwidth, selectivity and out-of-band spury-response. The basic tie device acts as a low-pass element, and its frequency characteristics include cutoff frequency, passband characteristic, out-of-band suppression characteristic and spurned response. In addition, there are input impedance characteristics and input balance and unbalance characteristics.

The sensitivity characteristics of the simulator component depend on the sensitivity and bandwidth, and the sensitivity is based on the inherent noise of the device.

The sensitivity characteristics of logic devices depend on DC noise tolerance and noise immunity.

Active devices have two types of electromagnetic emission sources: conducted interference is transmitted through power lines, ground wires and interconnects, and increases with frequency; Radiation interference radiates through the device itself or through the interconnect and increases with the square of the frequency. Transient ground current is the initial source of conduction interference and radiation interference. To reduce transient ground current, it is necessary to reduce ground impedance and use decoupling capacitance.

The shorter the turnover time of the logic device, the wider the spectrum. Therefore, the rise/fall time of the signal should be increased as much as possible under the premise of ensuring the realization of the function.　

Digital circuit is one of the most common broadband interference sources, and its electromagnetic emission can be divided into two forms: differential mode and common mode.　

In order to reduce the emission, the frequency and signal level should be reduced as much as possible; In order to control the differential mode radiation, the signal lines, power lines and their return lines on the printed circuit board must be close together to reduce the loop area; In order to control common-mode radiation, a grid ground wire or ground plane can be used, and a common-mode choke can also be used. At the same time, it is also very important to choose "clean" as the ground point.　

Surface mount technology (SMT) is a new electronic mounting technology developed in the late 1970s, including surface mount device (SMD), surface mount component (SMC), surface mount printed circuit board (SMB) and surface mount equipment, online testing and so on.

The most common application of SMT in electronic machines is computers, followed by communications, military, and consumer electronics.

In the 1990s SMT developed a new type of circuit board that could be used to make multi-chip component MCMS. At present, the input/output ports of chip integrated circuits have been increased to hundreds, and the center spacing of pins has been reduced to 0.3 mm. At present, surface mounting technology and micro-assembly technology are interweaving and permeating. Due to the ultra-miniaturization of SMD/SMC, the size of the substrate welding area is reduced to less than I square inches, and both electromagnetic emission and electromagnetic sensitivity problems can be well solved.

Seven, electromagnetic shielding material selection

Materials with high electrical and magnetic conductivity properties can be used as shielding materials. Commonly used steel plate, aluminum plate, aluminum foil, copper plate, copper foil and so on. Shielding can also be achieved by spraying nickel paint or copper paint on the plastic chassis.

In addition to the conductivity, permeability and thickness of the selected shielding material, the shielding efficiency of the shielding enclosure depends to a large extent on the structure of the enclosure, that is, its conductive continuity. There are gaps in any practical shielding case, and these gaps are caused by the temporary lap between the shielding plates. Electromagnetic leakage occurs at the gap due to the conductive discontinuity of the gap. Therefore, for permanent lap joints, the gap can be eliminated by welding. If riveted or screwed, the spacing must be small enough. For non-permanent lap joints, the use of electromagnetic sealing gaskets and other shielding materials is a very effective means.

1. Electromagnetic sealing gasket

Electromagnetic sealing gasket is a material with good elasticity and high conductivity. Filling this material in the gap can maintain conductive continuity, which is a good way to solve the electromagnetic leakage of the gap. When selecting electromagnetic sealing gaskets, you need to be familiar with the following characteristics:

The transfer impedance is defined as Zr = V/I if the current I flows through the joint surface of the liner and the two shielding plates, and the voltage between the two shielding plates is V. The lower the transfer impedance, the smaller the electromagnetic leakage between the two shielding plates, and the higher the shielding efficiency of the gap after padding.　　

Hardness liner hardness should be moderate, hardness is too low, easy to cause poor contact, low shielding efficiency; The hardness is too high and requires greater pressure, which makes the structural design difficult.　

The compression permanent deformation liner has a shielding effect only when a certain deformation occurs under the action of external forces. When the external force is removed, the liner will not completely return to its original shape, that is, permanent deformation has occurred. Of course, the smaller the compressive permanent deformation of the liner, the better.　

The thickness of the liner should be able to meet the requirements of the roughness of the contact surface, and the gap should be filled with its elasticity to achieve the purpose of conductive continuity.

Commonly used electromagnetic sealing gaskets have the following types:

The elastic mesh sleeve woven with metal wire is pure metal contact, low contact resistance; However, the wire will present a large inductive reactance at high frequency, which reduces the shielding efficiency. Therefore, it is only applicable to the frequency range below l GHZ.

Rubber core woven mesh cover the wire woven mesh cover on the foam rubber core or silicone rubber core, with good elasticity and electrical conductivity.　

The conductive rubber liner is filled with metal particles or wires inside the silicone rubber to form a conductive elastic substance. Because the bulk reactance between the conductive particles in the conductive rubber is reduced at high frequencies, the shielding efficiency of the filled metal particles is higher at high frequencies. If the wire is filled with the same direction, pure metal contact can also be achieved, but because the wire presents a large inductive reactance at high frequencies, the shielding efficiency is reduced, so it is only suitable for low frequencies when filling the wire.　

Beryllium copper finger reeds can be made into various finger reeds by using the good electrical conductivity and elasticity of copper. Due to the pure metal contact, the DC resistance is low, and the inductive reactance is small, so the low frequency and high frequency have high shielding efficiency.　

Spiral tube gasket A tin-plated spiral tube made of copper or stainless steel, with good elasticity and electrical conductivity, and is currently the most effective shielding gasket.

2. Conducting compound

Conductive compounds include various conductive adhesives and various conductive fillers. Epoxy conductive adhesive can be used for conductive bonding between metals, between metals and non-metals, and between various hard surfaces. Can replace solder, complete microwave device lead connection; It can be used for conductive ceramic bonding, antenna component bonding, glass defrosting bonding, conductive/thermal bonding, microwave waveguide component bonding, etc. Silicone grease conductive adhesive is used to bond the elastic conductive rubber to the metal surface, and can be used in aerospace, aviation, military and other electronic equipment. Conductive caulk is a highly conductive past-like material used in crevices where shielding padding is not possible and remains elastic after curing.　

3. Cut-off waveguide vent plate

The vents and other openings in the shielded enclosure are the main sources of electromagnetic radiation. It is difficult to achieve satisfactory shielding efficiency by opening small holes or adding wire mesh. Theory proves that when the cross-section size of the metal tube meets certain conditions, electromagnetic waves in a certain frequency range can be transmitted, which is called the waveguide. The waveguide has a cutoff frequency, when the frequency is lower than the cutoff frequency, the electromagnetic wave is cut off and cannot be transmitted. According to this principle, a cut-off waveguide can be designed. The cut-off waveguide ventilation plate is composed of many cut-off waveguides arranged in turn. In order to improve the ventilation efficiency, the cross-section of each cut-off waveguide is designed to be hexagonal, so it is also called honeycomb ventilation plate. When the shielding efficiency is very high, two cut-off waveguide ventilation plates can be used to form a double-layer ventilation plate. The conductivity of the ventilation plate material is an important factor in the shielding efficiency, and the ventilation plate with high conductivity material or coating can obtain high shielding efficiency.　

4. Conductive glass and conductive diaphragm

The display screen or display window should meet both the visual requirements and the requirements of anti-electromagnetic radiation, so conductive glass can be used to achieve shielding. Conductive glass can be composed of two optical glass sandwiched metal mesh, the higher the density of the metal mesh, the higher the shielding efficiency, but the worse the light transmission becomes. Conductive glass can also be composed of a metal film plated on the surface of optical glass or plexiglass. In addition, the transparent polyester film can also be plated with metal film to make a flexible transparent conductive film. The light transmission of this diaphragm can reach 70% (80%), and the diaphragm is very thin, only 0.13mm, can be directly affixed to the surface of conventional glass or plexiglass, especially suitable for instrument dial, liquid crystal display, panel indicator hole, color display and other parts requiring high transparency and medium shielding efficiency.

Eight, electromagnetic interference filter selection

Practice has shown that even for a well-designed product with the correct shielding and grounding measures, there will still be conducted interference emission or conducted interference into the product. Filtering is an effective method to compress the interference spectrum. When the interference spectrum is different from the frequency band of the useful signal, the useless interference can be filtered out by electromagnetic interference filter. Therefore, it is very important to select and use the filter properly to suppress the conducted interference. From the point of view of frequency selection, electromagnetic interference filters belong to low-pass filters, which are divided into signal line filters and power line filters.

1. Signal line filter

The signal line filter is a low-pass filter used in various signal lines to filter out high-frequency interference components. It can be divided into circuit board filter, feed-through filter and connector filter. The circuit board filter is suitable for installation on the circuit board, and has the advantages of low cost and convenient installation. The feed-through filter is suitable for installation on the shield housing, especially for use when a single wire or cable passes through the shield; The filter connector is suitable for use when multiple wires or cables pass through the shield. The filter connector is identical in shape and size to the ordinary connector, and the two are completely interchangeable. But there is a low-pass filter on each pin or hole of the filter connector, and its circuit can be a single capacitor, or it can be L-type or π-type.

When choosing a signal line filter, the type of filter should be selected according to the occasion of use, and the circuit and performance indicators of the filter should be selected according to the filter requirements. In order to ensure that the signal frequency passes smoothly through the filter, the cut-off frequency of the filter should be higher than the upper limit of the signal frequency. In addition, the working voltage, current and temperature range of the filter should be correctly selected. When using a signal line filter, the most important thing is to ensure that the filter is well grounded, and the grounding line should be as short as possible. The filter housing should have good electrical contact with the shield, which can be welded or used as a radio-frequency electromagnetic sealing liner.

The newly developed filter array board is to make the filter into a micro-shaped device and arrange it into an array, which can be quickly installed on the bottom plate or partition of electronic products to achieve sealing or isolation.

2. Ferrite electromagnetic interference suppression element

Ferrite is a ferromagnetic material with a cubic lattice structure. Its manufacturing process and mechanical properties are similar to ceramics, and its color is gray and black. For ferrites used to suppress electromagnetic interference, the most important performance parameters are permeability μ and saturation magnetic flux density Bs. The permeability μ can be expressed as a complex number, with the real part constituting the inductance and the imaginary part representing the loss, which increases with frequency. Therefore, its equivalent circuit is a series circuit composed of inductors L and resistors R, both of which are functions of frequency. For example, a ferrite with a permeability of 850 has an impedance of less than 10Ω at 10MHz, and an impedance greater than 100Ω after exceeding l00MHz, which greatly attenuates high-frequency interference. In this way, a low-pass filter is formed. At low frequency, R is very small, L plays a major role, and electromagnetic interference is suppressed by reflection; At high frequencies, R increases, and electromagnetic interference is absorbed and converted into heat energy.

Ferrite suppression components are widely used in printed circuit boards, power lines and data lines. For example, by adding a ferrite suppression element to the power line inlet of the printed board, high-frequency interference can be filtered out. Ferrite magnetic ring or magnetic bead is specially used to suppress high-frequency interference and peak interference on signal lines and power lines, and it also has the ability to absorb electrostatic discharge pulse interference.

Different ferrite suppression elements have different optimal suppression frequency ranges. Usually the higher the permeability, the lower the frequency of inhibition. In addition, the larger the volume of ferrite, the better the inhibition effect. When the volume is fixed, the inhibition effect of long and thin shape is better than that of short and thick, and the smaller the inner diameter, the better the inhibition effect. However, in the case of DC or AC bias, there is still the problem of ferrite saturation, the larger the cross section of the suppression element, the less saturated, the greater the tolerable bias.　

Ferrite suppression elements should be installed close to the source of interference. For the input/output circuit, it should be as close as possible to the inlet and outlet of the shield shell.　

When installing, it should also be noted that the ferrite component is fragile, and reliable fixing measures should be taken.　　

3. Power line filter

Power line is the main way of electromagnetic interference into and out of equipment. To prevent these two situations, a power line filter must be installed in the power interface of the device. It only allows the power frequency to pass through, and the electromagnetic interference above the power frequency is greatly attenuated.

The interference on the power line comes in two forms, the interference in the live line and neutral line circuit is differential mode interference, and the interference in the live line, neutral line and ground line circuit is common mode interference. Although the power line filter can inhibit both differential mode interference and common mode interference, the effect is not the same, and the insertion loss of the two should be given separately. All power supply filters must be grounded, except those that are specifically stated to allow ungrounded filters, because the common-mode bypass capacitor in the filter only functions when grounded.

When the power filter is used, it should be installed as close as possible to the power supply entrance, and the input/output end of the filter should be shielded and isolated to avoid electromagnetic interference from the input end directly coupled to the output end of the filter. In addition, the ground point of the filter should also be as close as possible to the ground point of the device. The technical specifications of the power line filter include: maximum leakage current, withstand voltage, rated operating frequency, rated operating voltage, rated operating current and temperature range.

Ix. Concluding remarks

Electromagnetic compatibility components are the key to solve the problems of electromagnetic interference emission and electromagnetic sensitivity. The correct selection and use of these components is the premise of electromagnetic compatibility design. Therefore, we must deeply grasp these components, so that it is possible to design electronic and electrical products that meet the standard requirements and have the best performance and price ratio.