In a complex electromagnetic environment, each electronic and electrical product must be able to withstand a certain amount of external electromagnetic interference to work properly, and it cannot generate electromagnetic interference that cannot be tolerated by other electronic and electrical products in the electromagnetic environment. In other words, it is necessary to meet the electromagnetic sensitivity limit value requirements of the relevant standards and the electromagnetic emission limit value requirements. This is the problem that the electromagnetic compatibility of electronic and electrical products should be solved. It is also the electromagnetic compatibility of electronic and electrical products. Necessary conditions for certification. Many engineers are often helpless or unsatisfactory in how to correctly select and use electromagnetic compatibility components when designing product electromagnetic compatibility. Therefore, it is necessary to discuss this.
Electromagnetic compatibility components are the key to solving the problems of electromagnetic interference emission and electromagnetic sensitivity. Proper selection and use of these components is a prerequisite for good electromagnetic compatibility design. Therefore, we must master these components in depth, so that it is possible to design electronic and electrical products that meet the requirements of the standard and have the best performance and price ratio. Each electronic component has its own characteristics, so it requires careful consideration in design. Next, we will discuss some common electronic components and circuit design techniques used to reduce or suppress electromagnetic compatibility.
Components Group
There are two basic groups of electronic components: leaded and leadless components. Leaded wire components have a parasitic effect, 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 pins should be as short as possible. Compared with leaded components, the parasitic effects of leadless and surface mount components are smaller. Typical values are: 0.5nH of parasitic inductance and about 0.3pF of terminal capacitance.
From the standpoint of electromagnetic compatibility, surface mount components work best, followed by radial lead components, and finally axial parallel lead components.
One、Capacitor of EMC Components
In EMC design, the capacitor is one of the most widely used components, mainly used to form various low-pass filters or used as decoupling capacitors and bypass capacitors. A lot of practice shows that in the 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 easy use. If the selection or use of capacitors is improper, it may not achieve the intended purpose at all, and may even increase the degree of EMI.
In theory, the larger the capacitance, the smaller the capacitive reactance and the better the filtering effect. Some people also have this habitual awareness. However, large-capacity capacitors generally have large parasitic inductances and low self-resonant frequencies (such as typical ceramic capacitors, f0=5 MHz at 0.1 μF, f0=15 MHz at 0.01 μF, and f0=50 MHz at 0.001 μF). The decoupling effect of frequency noise is poor, and there is no decoupling effect at all. Discrete component filters will begin to lose performance when the frequency exceeds 10 MHz. The larger the physical size of the element, the lower the turning point frequency. These problems can be solved by choosing 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 1/3~1/5 of the traditional capacitance parasitic inductance, and the self-resonant frequency can reach the same capacity of the leaded capacitor 2 times (some sources say it can be up to 10 times), ideal for RF applications.
Traditionally, ceramic capacitors are generally used for RF applications. But in practice, ultra-small polyester or polystyrene film capacitors are also suitable because their size is comparable to ceramic capacitors.
The three-terminal capacitor can extend 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 band. To obtain better filtering effect in VHF or higher frequency bands, especially to protect the shield from being penetrated, a feed-through capacitor must be used.
Two、Inductors of EMS Components
Inductance is an element that can link a magnetic field and an electric field. Its inherent ability to interact with a magnetic field makes it potentially more sensitive than other elements. Similar to capacitors, clever use of inductors can also solve many EMC problems. There are two basic types of inductance: open loop and closed loop. The difference is in the internal magnetic field loop. 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 following figure.
Magnetic field in inductor
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 the open loop inductor passes through the air, which will cause radiation and cause electromagnetic interference (EMI) problems. When choosing an open-loop inductor, the winding type is better than the rod or solenoid type, because this way the magnetic field will be controlled in the magnetic core (ie the local magnetic field in the magnet).
Open loop inductance
For closed-loop inductors, the magnetic field is completely controlled at the core, so this type of inductor is more ideal in circuit design. Of course, they are also more expensive. One advantage of the spiral loop closed-loop inductor is that it not only controls the magnetic ring at the core, but also eliminates all external incident 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 types of inductors are used in EMC applications: ferrite beads and ferrite clips. Iron and ferrite can be used as an inductive 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, the ferrite core inductor is more suitable for EMC applications.
Three、The Choice of Filter instructure
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 connection between the capacitor and the inductor is different. 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, but poorly in a low-impedance circuit.
Filter classification (based on function)
Filter classification (based on structure)
Filter selection
Four、Magnetic beads of EMC components
The magnetic bead is composed of an oxygen magnet, the inductance is composed of a magnetic core and a coil, the magnetic bead converts the AC signal into heat energy, and the inductance 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 it can be seen from the model 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 an energy storage element, and the magnetic bead is an energy conversion (consumption) device. Inductance is mostly used in power supply filter circuits, focusing on suppressing conductive interference; magnetic beads are mostly used in signal circuits, mainly used in EMI. Magnetic beads are used to absorb UHF signals. Like some RF circuits, PLLs, oscillating circuits, and UHF memory circuits (DDR, SDRAM, RAMBUS, etc.), magnetic beads need to be added to the power input part, and the inductance is a kind of storage Energy components, used in LC oscillation circuits, low-frequency filter circuits, etc., its application frequency range rarely exceeds 50MHz.
Five、Diodes for EMC components
Diodes are the simplest semiconductor devices. Due to their unique characteristics, certain diodes help to solve and prevent some problems related to EMC.
Six、Selection of analog and logic active devices
The key to 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 tuning devices and basic band devices. The tuning device functions as a band-pass element, and its frequency characteristics include: center frequency, bandwidth, selectivity, and out-of-band spurious response; the basic tie device functions as a low-pass element, and its frequency characteristics include: cut-off frequency, pass-band characteristics, and out-of-band suppression Features and spurious responses. In addition, there are input impedance characteristics and balanced unbalance characteristics of the input terminal.
The sensitivity characteristics of analog devices depend on sensitivity and bandwidth, and sensitivity is based on the inherent noise of the device.
The sensitivity characteristics of logic devices depend on the DC noise tolerance and noise immunity.
There are two types of electromagnetic emission sources for active devices: Conducted interference is transmitted through power lines, grounding lines and interconnecting lines, and increases with increasing frequency; Radiated interference is radiated through the device itself or through interconnecting lines, and varies with the square of the frequency And increase. Transient ground current is the initial source of conducted and radiated interference. Reducing transient ground current must reduce ground impedance and use decoupling capacitors.
The shorter the toggle time of the logic device, the wider the spectrum occupied. For this reason, the rise/fall time of the signal should be increased as much as possible while ensuring the realization of the function.
Digital circuits are one of the most common sources of broadband interference, and their electromagnetic emissions can be divided into differential and common modes.
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 For common mode radiation, either grid ground or ground plane can be used, or common mode chokes can also be used. At the same time, it is also very important to choose “clean ground” as the grounding point.
Surface Mount Technology (SMT) is a new type of electronic assembly technology developed in the late 1970s, which includes Surface Mount Devices (SMD), Surface Mount Components (SMC), Surface Mount Printed Circuit Boards (SMB), and Surface Mount Equipment, Online Testing etc.
The most used electronic machine SMT is the computer, followed by communications, military, and consumer electronics.
In the 1990s, SMT developed a new type of circuit board that can be used to make multi-chip component MCM. At present, the input/output ports of chip integrated circuits have increased to hundreds, and the center-to-center spacing of the pins has been reduced to 0.3 mm. At present, surface mounting technology is intertwining and penetrating with micro-assembly technology. Due to the ultra-miniaturization of SMD/SMC, the size of the soldering area of the substrate is reduced to less than 1 square inch, and no matter the electromagnetic emission or electromagnetic sensitivity problems can be solved well.
Seven、Selection of electromagnetic shielding materials
Materials with high electrical conductivity and magnetic permeability can be used as shielding materials. Commonly used are steel plate, aluminum plate, aluminum foil, copper plate, copper foil, etc. It can also be shielded by spraying nickel paint or copper paint on the plastic case.
In addition to the shielding effectiveness of the shielding chassis, which is related to the conductivity, magnetic permeability, and thickness of the selected shielding material, it also largely depends on the structure of the chassis, that is, its conductive continuity. There are gaps in any practical shielding chassis. These gaps are caused by the temporary overlap between the shielding plates. Due to the conductive discontinuity of the gap, electromagnetic leakage will occur at the gap. Therefore, for permanent laps, welding can be used to eliminate gaps. If riveting or screw connection is used, the spacing must be sufficiently small. For non-permanent lap joints, shielding materials such as electromagnetic sealing gaskets are very effective means.
1. Electromagnetic sealing gasket
The 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 the electromagnetic sealing gasket, you need to be familiar with the following characteristic parameters:
The transfer impedance assumes that a current I flows on the joint surface of the gasket and the shield plates on both sides, and the voltage between the shield plates on both sides is V, then the transfer impedance is defined as Zr=V/I. The lower the transfer impedance, the smaller the electromagnetic leakage between the shielding plates on both sides, and the higher the shielding effectiveness of the gap after the gasket is added.
The hardness of the hardness pad should be moderate. If the hardness is too low, it is easy to cause poor contact and the shielding efficiency is low. If the hardness is too high, it requires greater pressure, which causes difficulty in structural design.
The compression permanent deformation gasket only has a shielding effect when a certain deformation occurs under the action of external force. When the external force is removed, the liner will not fully return to its original shape, that is, permanent deformation occurs. Of course, the smaller the compression set of the pad, the better.
Gasket thickness The thickness of the gasket should meet the requirements of the unevenness of the contact surface, and use its elasticity to fill the gap to achieve the purpose of conductive continuity.
Commonly used electromagnetic sealing gaskets have the following types:
The elastic mesh sleeve woven with metal wire for the metal wire gasket is pure metal contact, and the contact resistance is low; however, the metal wire will show greater inductance at high frequencies, which reduces the shielding effectiveness. So it only applies to the frequency range below l GHz.
The rubber core braided mesh sleeve sets the metal wire braided mesh sleeve on the foam rubber core or the silicone rubber core, which has good elasticity and conductivity.
The conductive rubber gasket is filled with metal particles or metal wires in silicone rubber to form a conductive elastic substance. Since the capacitive reactance between the conductive particles in the conductive rubber decreases at high frequencies, the filler metal particles have higher shielding effectiveness at high frequencies. If the metal wires are filled in the same direction, pure metal contact can also be achieved, but because the metal wires show greater inductance at high frequencies, which reduces the shielding effectiveness, so only suitable for low frequencies when filling the metal wires.
Beryllium copper finger reeds can be made into various finger reeds by utilizing the good conductivity and elasticity of copper. Due to the pure metal contact, the DC resistance is low, and the inductance is small, so it has a high shielding efficiency at low and high frequencies.
The spiral tube gasket is a spiral tube made of tin or copper or stainless steel. It has good elasticity and conductivity, and it is currently the highest shielding gasket.
2. Conductive compound
Conductive compounds include various conductive adhesives and various conductive fillers. Epoxy conductive adhesive can be used for conductive bonding between metals, between metal and non-metal, and between various hard surfaces. Can replace solder to complete the connection of microwave device leads; can repair printed circuit board, can be used for conductive ceramic bonding, antenna element bonding, glass defrosting bonding, conductive / thermal conduction bonding, microwave waveguide component bonding, etc. Silicone grease conductive adhesive is used to fix elastic conductive rubber on metal surface and can be used in aerospace, aviation, military and other electronic equipment. The conductive filler is a highly conductive paste-like material that is used in gaps where shielding pads cannot be added and remains elastic after curing.
3. Cut-off waveguide ventilation plate
The vents and other openings of the shielded chassis are the main sources of electromagnetic radiation. It is difficult to achieve satisfactory shielding effectiveness by using small holes or adding wire mesh. The theory proves that when the cross-sectional size of the metal tube meets certain conditions, it can transmit electromagnetic waves in a certain frequency range, called a waveguide. The waveguide has a cut-off frequency. When the frequency is lower than the cut-off frequency, the electromagnetic wave is cut off and cannot be transmitted. According to this principle, it can be designed as a cut-off waveguide. The cut-off waveguide ventilation plate is composed of many cut-off waveguides arranged in sequence. In order to improve the ventilation efficiency, the cross-section of each cut-off waveguide is designed as a hexagon, so it is also called a honeycomb-shaped 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 shielding effectiveness. The use of high conductivity materials or coated ventilation plates can achieve high shielding effectiveness.
4. Conductive glass and conductive diaphragm
The display screen or display window must meet not only the visual requirements but also the requirements for preventing electromagnetic radiation. To this end, conductive glass can be used to achieve shielding. The conductive glass can be composed of two pieces of optical glass with a wire mesh in between. The greater the density of the wire mesh, the higher the shielding effectiveness, but the worse the light transmission becomes. The conductive glass can also be composed of a metal film plated on the surface of optical glass or organic glass. In addition, a metal film can be plated on the transparent polyester film to make a flexible transparent conductive film. The light transmittance of this kind of membrane can reach 70% (80%, and the membrane is very thin, only 0.13mm, can be directly attached to the surface of conventional glass or plexiglass, especially suitable for high transparency and medium shielding efficiency Instrument dial, LCD display, panel indicator hole, color display and other parts.
Eight、Selection of ElectroMagnetic Interference Filter
Practice shows that even for a well-designed product with 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 electromagnetic interference filter can be used to filter out the useless interference. Therefore, proper selection and proper use of filters are very important to suppress conducted interference. From the perspective of frequency selection, electromagnetic interference filters are 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 on various signal lines to filter out high-frequency interference components. It can be divided into three types: circuit board filter, feed-through filter and connector filter. The circuit board filter is suitable for installation on the circuit board, which has the advantages of low cost and convenient installation; the feed-through filter is suitable for installation on the shielding shell, especially suitable for use when a single wire or cable passes through the shield; filtering The connector is suitable for use when multiple wires or cables pass through the shield. The filter connector is the same in shape and size as the ordinary connector, and the two are completely interchangeable. But each pin or hole of the filter connector has a low-pass filter, and its circuit can be a single capacitor, or it can be L-type or π-type.
When selecting a signal line filter, the type of filter should be selected according to the occasion used, and the circuit and performance index of the filter should be selected according to the filtering requirements. In order to ensure that the signal frequency passes through the filter smoothly, the filter cut-off frequency should be higher than the signal frequency Upper limit. In addition, the operating voltage, current and temperature range of the filter should be selected correctly. When using a signal line filter, the most important thing is to ensure that the filter has a good ground. The ground wire should be as short as possible. The filter housing should have good electrical contact with the shield. Welding methods or radio frequency electromagnetic sealing gaskets can be used.
The newly developed filter array board is to make the filter into micro-shaped devices and arrange them 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 ferrimagnetic material with cubic lattice structure. Its manufacturing process and mechanical properties are similar to ceramics, and its color is gray-black. For ferrite for electromagnetic interference suppression, the most important performance parameters are permeability μ and saturation magnetic flux density Bs. The permeability μ can be expressed as a complex number, the real part constitutes the inductance, and the imaginary part represents the loss, which increases as the frequency increases. Therefore, its equivalent circuit is a series circuit composed of inductance L and resistance R, both of which are functions of frequency. For example, a ferrite with a magnetic permeability of 850 has an impedance of less than 10Ω at 10MHz, and an impedance greater than 100Ω after more than 100MHz, which greatly attenuates high-frequency interference. In this way, a low-pass filter is formed. At low frequencies, R is small, and L plays a major role, and electromagnetic interference is reflected and suppressed; at high frequencies, R increases and electromagnetic interference is absorbed and converted into thermal energy.
Ferrite suppression components are widely used in printed circuit boards, power lines and data lines. For example, adding a ferrite suppression element to the entrance of the power line of the printed board can filter out high-frequency interference. Ferrite magnetic rings or beads are designed to suppress high-frequency interference and spike interference on signal lines and power lines. It also has the ability to absorb electrostatic discharge pulse interference.
Different ferrite suppression components have different optimal suppression frequency ranges. Generally, the higher the permeability, the lower the frequency of suppression. In addition, the larger the volume of the ferrite, the better the suppression effect. When the volume is constant, the long and thin shape is better than the short and thick suppression effect, and the smaller the inner diameter, the better the suppression effect. But in the case of DC or AC bias current, there is also the problem of ferrite saturation. The larger the cross-section of the suppression element, the more difficult it is to saturate and the greater the tolerable bias current.
Ferrite suppression components should be installed close to interference sources. For the input/output circuit, it should be as close as possible to the entrance and exit of the shielding shell.
It should also be noted that during installation, ferrite components are easily broken, and reliable fixing measures should be taken.
3. Power line filter
The power cord is the main way of electromagnetic interference into and out of equipment. To prevent these two situations from occurring, you must install a power line filter on the power interface of the device. It only allows the power frequency to pass through, but the electromagnetic interference higher than the power frequency is greatly attenuated.
The interference on the power line appears in two forms. The interference in the live and neutral circuit is differential mode interference, and the interference in the live, neutral and ground circuit is common mode interference. Although the power line filter can suppress the differential mode interference and the common mode interference, but the effect is different, the insertion loss of the two should be given separately. In addition to the special instructions that allow ungrounded filters, all power filters must be grounded, because the common-mode bypass capacitor in the filter only works when grounded.
When using a power filter, it should be installed as close as possible to the power inlet, and the input/output of the filter should be shielded and isolated to avoid direct electromagnetic coupling from the input to the output of the filter. In addition, the ground point of the filter should 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.
Nine、Conclusion
Electromagnetic compatibility components are the key to solving the problems of electromagnetic interference emission and electromagnetic sensitivity. Proper selection and use of these components is a prerequisite for good electromagnetic compatibility design. Therefore, we must master these components in depth, so that it is possible to design electronic and electrical products that meet the requirements of the standard and have the best performance and price ratio.