How to properly select and use electromagnetic compatibility components
In a complex electromagnetic environment, each electronic and electrical product must not be able to withstand certain external electromagnetic interference, nor can it generate electromagnetic interference that cannot be tolerated by other electronic or 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 meet the electromagnetic emission limit value requirements. This is the problem that the electromagnetic compatibility of electronic and electrical products should be solved, and the electromagnetic compatibility of electronic and electrical products. The necessary conditions for certification. Many engineers are often at a loss as to how to properly select and use electromagnetic compatibility components when designing electromagnetic compatibility products. Therefore, it is necessary to discuss this.
Electromagnetic compatibility components are the key to solving the problem of electromagnetic interference emission and electromagnetic sensitivity. The correct selection and use of these components is the prerequisite for 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-price ratio. Each type of electronic component has its own characteristics, so it is required to be carefully considered in the design. Next we will discuss some common electronic components and circuit design techniques used to reduce or suppress electromagnetic compatibility.
There are two basic types of electronic components: leaded and unpinned. There are parasitic effects on pin-line components, especially at high frequencies. This pin forms a small inductor of approximately 1nH/mm/pin. The end of the pin also produces a small capacitive effect of approximately 4pF. Therefore, the length of the pins should be as short as possible. The lead-free and surface-mounted components have less parasitic effects than the leaded components. Typical values are: 0.5nH parasitic inductance and about 0.3pF termination capacitance.
From the point of view of electromagnetic compatibility, surface mount components work best, followed by radial pin components, and finally axially parallel pin components.
First, the capacitance of the EMC component
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 lot of practice shows that in EMC design, proper selection and use of capacitors can not only solve many EMI problems, but also fully demonstrate the advantages of good effect, low price and convenient use. If the capacitor is chosen or used improperly, it may not achieve the intended purpose at all, and even increase the degree of EMI.
In theory, the larger the capacity of the capacitor, the smaller the capacitive reactance and the better the filtering effect. Some people also have this habit of knowing. However, a large-capacity capacitor generally has a large parasitic inductance and a low self-resonant frequency (such as a typical ceramic capacitor, 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 the frequency noise is poor, and even the decoupling effect is not achieved at all. Discrete component filters 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 of a particular structure.
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 parasitic inductance of the conventional capacitor. The self-resonant frequency can reach the same capacity of the leaded capacitor. 2 times (also known as up to 10 times), ideal for RF applications.
Traditionally, RF applications have chosen ceramic capacitors. However, in practice, ultra-small polyester or polystyrene film capacitors are also suitable because they are comparable in size to ceramic capacitors.
Three-terminal capacitors can extend the small ceramic capacitor frequency range from below 50 MHz to above 200 MHz, which is useful for suppressing noise in the VHF band. To achieve better filtering in the VHF or higher frequency band, especially if the protective shield is not penetrated, a feedthrough capacitor must be used.
Second, the inductance of the EMC component
An inductor is a component that can connect a magnetic field to an electric field. Its inherent ability to interact with a magnetic field makes it potentially more sensitive than other components. Similar to capacitors, smart use of inductors can solve many EMC problems. Below are two basic types of inductors: open loop and closed loop. They differ in the internal magnetic field loop. In an open-loop design, the magnetic field is closed by air; in a closed-loop design, the magnetic field completes the magnetic path through the core, as shown in the following figure.
Magnetic field in the inductor
One advantage of an inductor over a capacitor is that it has no parasitic inductance, so there is no difference in surface mount type and lead type.
The magnetic field of the open-loop inductor passes through the air, which causes radiation and introduces electromagnetic interference (EMI) problems. When selecting an open-loop inductor, the shaft is better than a rod or solenoid because the magnetic field will be controlled to the core (ie the local magnetic field within the magnet).
Open loop inductor
For closed-loop inductors, the magnetic field is completely controlled at the core, so this type of inductance is more desirable in circuit design, and of course they are also more expensive. One advantage of a spiral-loop closed-loop inductor is that it not only controls the magnetic ring to the core, but also eliminates all external incidental field radiation.
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 used in EMC applications: ferrite beads and ferrite clips. Iron and ferrite can be used as the inductor core skeleton. 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.
Third, the choice of filter structure
A filter in an EMC design generally refers to a low pass filter composed of L, C. One of the main differences between filters of different structures is that the capacitance and inductance are connected differently. The effectiveness of the filter is not only related to its structure, but also to the impedance of the connected network. Filters such as single capacitors work well in high impedance circuits and poorly in low impedance circuits.
Filter classification (based on function)
Filter classification (based on structure)
Fourth, the magnetic beads of the EMC components
The magnetic beads are composed of an oxygen magnet. The inductance is composed of a core and a coil. The magnetic beads convert the AC signal into heat energy, and the inductor stores the AC and slowly releases it.
Magnetic bead working principle
Magnetic bead selection
The circuit symbol of the magnetic bead is the inductance. However, it 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 an energy storage component and the magnetic beads are energy conversion (consumption) devices. Inductors are mostly used in power supply filter loops, focusing on suppressing conducted 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, PLLs, oscillator circuits, and ultra-high frequency memory circuits (DDR, SDRAM, RAMBUS, etc.), which need to be added to the power input part, and the inductor is a kind of storage. The energy components are used in LC oscillation circuits, low-frequency filter circuits, etc., and their application frequency ranges rarely exceed 50 MHz.
Five, the diode of the EMC component
Diodes are the simplest semiconductor devices. Due to their unique characteristics, certain diodes help solve and prevent some of the problems associated with EMC.
Sixth, the selection of analog and logic active devices
The key to electromagnetic interference emissions and electromagnetic sensitivity is the choice 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 bandpass component, and its frequency characteristics include: center frequency, bandwidth, selectivity, and out-of-band spurious response; the basic tie device functions as a low-pass component, and its frequency characteristics include: cutoff frequency, passband characteristics, and out-of-band rejection. Features and spurious response. In addition, there are input impedance characteristics and balance imbalance characteristics at the input.
The sensitivity characteristics of analog devices depend on sensitivity and bandwidth, while sensitivity is based on the inherent noise of the device.
The sensitivity characteristics of logic devices depend on DC noise margin and noise immunity.
Active devices have two sources of electromagnetic emissions: conducted interference is transmitted through power lines, ground lines, and interconnects, and increases with increasing frequency; radiated interference is radiated through the device itself or through interconnects, and squared with frequency And increase. Transient ground current is the initial source of conducted and radiated interference. Reducing transient ground currents must reduce ground impedance and use decoupling capacitors.
The shorter the flip time of the logic device, the wider the spectrum. To this end, the rise/fall time of the signal should be increased as much as possible while ensuring the implementation of the function.
Digital circuits are one of the most common sources of broadband interference, and their electromagnetic emissions can be divided into 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; For common mode radiation, a grid ground or ground plane can be used, or a common mode choke can be used. At the same time, it is also important to choose “clean” as the grounding point.
Surface Mount Technology (SMT) is a new electronic assembly technology developed in the late 1970s that includes surface mount devices (SMD), surface mount components (SMC), surface mount printed circuit boards (SMB), and surface mount devices, online. Test, etc.
The most common application of electronic whole machine SMT is computer, followed by communication, military and consumer electronics.
In the 1990s, SMT developed a new type of circuit substrate that can be used to make multi-chip components MCM. At present, the number of input/output ports of the chip integrated circuit has been increased to hundreds, and the center pitch of the pins has been reduced to 0.3 mm. Surface mount technology is currently intertwined and infiltrated with micro-assembly technology. Due to the ultra-small size of SMD/SMC, the size of the substrate pad is reduced to less than 1 square inch, which can be solved well regardless of electromagnetic emission or electromagnetic sensitivity.
Seven, the selection of electromagnetic shielding materials
Materials with higher electrical conductivity and magnetic permeability can be used as shielding materials. Commonly used are steel plates, aluminum plates, aluminum foil, copper plates, copper foils, and the like. It is also possible to apply shielding by spraying nickel or copper paint on a plastic case.
In addition to the conductivity, permeability and thickness of the selected shielding material, the shielding effectiveness of the shielded chassis is also largely dependent on the structure of the chassis, ie its conductive continuity. There are gaps in any practical shielded enclosure that are caused by temporary overlap between the shields. Electromagnetic leakage occurs at the gap due to the conduction discontinuity of the gap. Therefore, for permanent lap joints, a weld can be used to eliminate the gap. If riveting or screwing is used, the spacing must be small enough. For non-permanent lap joints, shielding materials such as electromagnetic sealing gaskets are very effective.
Electromagnetic sealing gasket
The electromagnetic sealing gasket is a material with good elasticity and high electrical conductivity. Filling this material in the gap can maintain electrical continuity and is a good way to solve the electromagnetic leakage of the gap. When choosing an electromagnetic sealing gasket, you need to be familiar with the following characteristic parameters:
The current I flows through the junction surface of the transfer impedance pad and the shield plates on both sides, and the voltage between the shield plates on both sides is V, and the transfer impedance is defined as Zr=V/I. The lower the transfer impedance, the smaller the electromagnetic leakage between the shield plates on both sides, and the higher the shielding effectiveness of the gap after padding.
The hardness of the hardness pad should be moderate, the hardness is too low, the contact is poor, the shielding effectiveness is low, the hardness is too high, and the pressure is required, which makes the structural design difficult.
The compression permanent deformation pad only has a shielding effect when a certain deformation occurs under the action of an external force. When the external force is removed, the pad does not completely return to its original shape, ie permanent deformation occurs. Of course, the smaller the compression permanent deformation of the liner, the better.
The thickness of the liner thickness liner should meet the requirements of the unevenness of the contact surface. With its elasticity, the gap is filled to achieve the purpose of electrical continuity.
Commonly used electromagnetic sealing gaskets are of the following types:
The wire mesh gasket is woven with a wire mesh and is made of pure metal. The contact resistance is low. However, the wire exhibits a large inductance at high frequencies, which reduces the shielding effectiveness. Therefore, it is only applicable to the frequency range below l GHz.
The rubber core woven mesh sleeve has a wire mesh woven mesh sleeve on the foam rubber core or the silicone rubber core, and has good elasticity and electrical conductivity.
The conductive rubber gasket is filled with metal particles or wires in the silicone rubber to constitute an electrically conductive elastic substance. Since the capacitive reactance between the conductive particles in the conductive rubber is lowered at a high frequency, the filling metal particles have a high shielding effect at a high frequency. If the wire is filled in the same direction, it can also be made of pure metal contact. However, since the wire exhibits a large inductive reactance at a high frequency, the shielding performance is lowered, so that the filling of the wire is only suitable for the low frequency.
The beryllium copper finger reed can be made into various finger springs by utilizing the good electrical conductivity and elasticity of the copper. Due to the pure metal contact, the DC resistance is low and the inductance is small, so the shielding performance is high at both low frequency and high frequency.
The spiral tube gasket is made of tinned copper or stainless steel spiral tube, which has good elasticity and electrical conductivity and is the most effective shielding liner.
2. Conductive compound
The conductive compound includes various conductive pastes, various conductive fillers, and the like. Epoxy conductive adhesive can be used for conductive bonding between metals, between metal and non-metal, and between various hard surfaces. It can replace the solder to complete the lead connection of the microwave device; it can repair the printed circuit board, which can be used for conductive ceramic bonding, antenna component bonding, glass defrosting bonding, conductive/thermal bonding, microwave waveguide component bonding, etc. Silicone grease is used to bond elastic conductive rubber to metal surfaces and can be used in aerospace, aerospace, military and other electronic equipment. The conductive filler is a highly conductive paste-like material used in the gap where the shield gasket cannot be added, and remains elastic after curing.
3. Cut-off waveguide vent
The vents and other openings in the shielded enclosure are the primary source of electromagnetic radiation. It is difficult to achieve satisfactory shielding effectiveness by using a small hole or a wire mesh. The theory proves that when the cross-section of the metal tube meets certain conditions, electromagnetic waves of a certain frequency range can be transmitted, which is called a 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 ventilating plate is composed of a plurality of cut-off waveguides. 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 a honeycomb ventilating plate. When the shielding performance is very high, two two-way waveguide louvers can be used to form a double-layer ventilating plate. The conductivity of the ventilating sheet material is an important factor in the shielding effectiveness, and a high-conductivity material or a plated ventilating plate can achieve high shielding effectiveness.
4. Conductive glass and conductive diaphragm
The display or display window must meet the visual requirements as well as the requirements for protection against electromagnetic radiation. For this purpose, conductive glass can be used for shielding. The conductive glass can be composed of two optical glass intermediate clamp metal meshes. The higher the density of the metal mesh, the higher the shielding performance, but the poorer the light transmittance becomes. The conductive glass can also be composed of a metal film coated on the surface of an optical glass or an organic glass. Further, a transparent transparent conductive film can also be formed by plating a metal film on the transparent polyester film sheet. This diaphragm has a light transmission of 70% (80%, and the diaphragm is very thin, only 0.13mm, can be directly attached to the surface of conventional glass or plexiglass, especially suitable for high transparency and moderate shielding effectiveness. Instrument dial, LCD display, panel indicator light, color display and other parts.
Eight, the choice of electromagnetic interference filter
Practice has shown that even for a product that is well designed and has the correct shielding and grounding measures, there will still be conducted interference emissions or conducted interference into the product. Filtering is an effective method of compressing the interference spectrum. When the interference spectrum is different from the frequency band of the wanted signal, the EMI filter can be used to filter out unwanted interference. Therefore, proper selection and proper use of the filter is important to suppress conducted interference. From the perspective of frequency selection, the electromagnetic interference filter is a low-pass filter, which is divided into a signal line filter and a power line filter.
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, feedthrough filter and connector filter. The circuit board filter is suitable for mounting on a circuit board, and has the advantages of low cost and convenient installation; the feedthrough filter is suitable for being mounted on the shielding case, and is particularly suitable for use when a single wire or cable passes through the shielding body; The 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 normal connector, and the two are completely interchangeable. However, each pin or hole of the filter connector has a low-pass filter whose circuit can be a single capacitor or an L-type or a π-type.
When selecting the signal line filter, the type of the filter should be selected according to the occasion used. 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 cutoff frequency of the filter should be higher than the signal frequency. The upper limit. In addition, the operating voltage, current, temperature range, etc. of the filter should be correctly selected. When using a signal line filter, the most important thing is to ensure that the filter has good grounding and the grounding wire should be as short as possible. The filter housing should have good electrical contact with the shield, either by soldering or by radio frequency electromagnetic sealing gasket.
The newly developed filter array board is made up of micro-shaped devices and arranged in an array, which can be quickly mounted on the bottom plate or partition of the electronic product to achieve sealing or isolation.
2. Ferrite electromagnetic interference suppression element
Ferrite is a ferrimagnetic material of a cubic lattice structure. Its manufacturing process and mechanical properties are similar to ceramics, and its color is grayish black. For ferrites for suppressing electromagnetic interference, the most important performance parameters are magnetic permeability μ and saturation magnetic flux density Bs. The magnetic 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 consisting of an inductor L and a resistor R, both L and R being a function of frequency. For example, a ferrite having a magnetic permeability of 850 has an impedance of less than 10 Ω at 10 MHz, and an impedance of more than 100 Ω after exceeding 100 MHz, so that high-frequency interference is greatly attenuated. Thus, a low pass filter is constructed. At low frequencies, R is small, L plays a major role, electromagnetic interference is reflected and suppressed; at high frequencies, R increases, and electromagnetic interference is absorbed and converted into heat.
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 inlet end of the power line of the printed board, high frequency interference can be filtered out. Ferrite magnetic rings or magnetic beads are designed to suppress high-frequency interference and spike interference on signal lines and power lines. They also have the ability to absorb electrostatic discharge pulse interference.
Different ferrite suppression elements have different optimal suppression frequency ranges. Generally, the higher the magnetic 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 one, and the smaller the inner diameter, the better the suppression effect. However, in the case of DC or AC bias current, there is also a problem of ferrite saturation. The larger the cross-section of the suppressing element, the less likely it is to saturate and the greater the bias current that can be withstood.
The ferrite suppression element should be installed close to the source of the interference. For input/output circuits, it should be as close as possible to the inlet and outlet of the shield case.
It should also be noted during installation that the ferrite components are easily broken and reliable fixing measures should be taken.
3. Power line filter
The power line is the main way for electromagnetic interference to incoming and outgoing devices. To prevent these two conditions from occurring, a power line filter must be installed at the power connector of the device. It only allows the power supply frequency to pass, while the electromagnetic interference above the power supply frequency is greatly attenuated.
Interference on the power line occurs in two forms. The interference in the live and neutral loops is differential mode interference, and the interference in the live, neutral, and ground loops is common mode interference. Although the power line filter has a suppression effect on differential mode interference and common mode interference, the effect is different, and the insertion loss of both should be given separately. All line filters must be grounded except that the filter is allowed to be ungrounded, since the common mode bypass capacitor in the filter only works when grounded.
When using the line filter, install it as close as possible to the power inlet and shield the input/output of the filter from electromagnetic interference from the input directly 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. Power line filter specifications include: maximum leakage current, withstand voltage, rated operating frequency, rated operating voltage, rated operating current and temperature range.
Electromagnetic compatibility components are the key to solving the problem of electromagnetic interference emission and electromagnetic sensitivity. The correct selection and use of these components is the prerequisite for 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-price ratio.