Electromagnetic Compatibility (EMC) refers to the ability of a device or system to operate in its electromagnetic environment without causing unacceptable electromagnetic interference to any device in its environment. Therefore, EMC includes two aspects: on the one hand, it means that the electromagnetic interference generated by the equipment in the normal operation of the equipment cannot exceed a certain limit; on the other hand, it means that the equipment has certain electromagnetic interference in the environment. Degree of immunity, ie, electromagnetic susceptibility.
In fact, most engineers understand that electromagnetic compatibility is generally speaking: the equipment or system can work in its electromagnetic environment, and does not constitute any unbearable electromagnetic harassment to the environment. The EMC test includes two major aspects: testing the intensity of the electromagnetic disturbance sent to the outside world in order to confirm compliance with the limits specified in the relevant standards; and conducting a sensitivity test in an electromagnetic environment with specified electromagnetic disturbance intensity so that Verify compliance with the noise immunity requirements of the relevant standards. For engineering and technical personnel engaged in the design of SCM application systems, it is very necessary to master certain EMC testing techniques.
This article mainly introduces three methods to deal with the failure of electromagnetic compatibility testing, and the specific follow-up system Xiaobian together to find out.
(1) Test environment
In order to ensure the accuracy and reliability of the test results, the electromagnetic compatibility measurement has high requirements for the test environment. The measurement site has open outdoor space, shielded room or anechoic chamber.
(2) Test Equipment
Electromagnetic compatibility measurement equipment is divided into two categories: one is electromagnetic interference measurement equipment, the equipment connected to the appropriate sensor, you can measure electromagnetic interference; the other is in the electromagnetic sensitivity measurement, equipment simulation of different interference sources, through appropriate A coupling/decoupling network, sensor, or antenna that is applied to various types of devices under test as a measure of sensitivity or interference.
(3) Measurement method
There are many kinds of measurement methods for electromagnetic compatibility tests depending on the standard, but they can be grouped into four categories; conducted emission test, radiated emission test, conducted sensitivity (immunity) test, and radiation sensitivity (immunity) test.
(4) Test Diagnosis Procedure
(5) Test Preparation
1 Test site conditions: The EMC test laboratory is a radio-wave semi-dark room and shielded room. The former is used for radiation emission and radiation sensitivity tests, and the latter is used for conducted emission and conduction sensitivity tests.
2 Ambient level requirements: The level of electromagnetic environment conducted and radiated is preferably well below the limits specified by the standard. Generally, the ambient level is at least 6 dB below the limit.
3 test table.
4 isolation of measuring equipment and tested equipment.
5 sensitivity criteria: generally provided by the measured side, and the actual monitoring and discrimination, to measure and observe the way to determine the degree of performance degradation.
6 Placement of the device under test: To ensure the repeatability of the test, there are usually specific provisions for the placement of the device under test.
(6) Test types
Conducted emission test, radiation emission test, conducted immunity test, and radiation immunity test.
(7) Common measuring instruments
Electromagnetic Interference (EMI) and Electromagnetic Sensitivity (EMS) tests require the use of many electronic instruments such as spectrum analyzers, electromagnetic field interference meters, signal sources, function amplifiers, and oscilloscopes. Because EMC test frequency is wide (20Hz ~ 40GHz), amplitude is large (μV level to kW level), mode is many (FM, AM, etc.), posture is many (flat, oblique, etc.), so the correct use of electronic instruments is very important. The appropriate instrument for measuring electromagnetic interference is a spectrum analyzer. A spectrum analyzer is an instrument that displays the voltage amplitude as a function of frequency, and the waveform it displays is called the spectrum. The spectrum analyzer overcomes the shortcomings of the oscilloscope in measuring electromagnetic interference, can accurately measure the interference intensity at each frequency, and can directly show each spectral component of the signal by the spectrum analyzer.
When solving electromagnetic interference problems, the most important issue is to determine the source of the interference. Only after the source of the interference is accurately located can the solution to the interference be proposed. Determining the source of interference based on the frequency of the signal is the easiest method, because in all the characteristics of the signal, the frequency characteristics are the most stable, and the circuit designers often have very clear signal frequencies in various parts of the circuit. Therefore, as long as the frequency of the interference signal is known, it can be inferred which part of the interference is generated. For electromagnetic interference signals, because the amplitude is often much smaller than the normal working signal, it is very simple to do this measurement with a spectrum analyzer. Because the spectrum analyzer's IF bandwidth is narrow, it is possible to filter out signals that are different from the frequency of the interference signal, accurately measure the frequency of the interference signal, and determine the circuit that generates the interference signal.
(1) Solution to Conducted Problems
1 Reduce EMI current by connecting a high impedance in series.
2 Short-circuit EMI currents to ground or to other loop conductors by paralleling a low impedance.
3 Cut off the EMI current through the galvanic isolation device.
4 By its own action to suppress EMI currents.
(2) Electromagnetic Compatibility Capacitive Solutions
A common phenomenon is that one side of the filter capacitor is not viewed as directly connected to a separate impedance, but as a connection to the transmission line. Typically, the transmission line becomes "long" when the length of an input-output line reaches or exceeds 1/4 wavelength. Actually, this change can be approximated by the following formula: l ≥ 55/f
In the formula: l unit is m, f is unit MHz. This formula takes into account the average propagation speed, which is 0.75 times that of free space theory.
a. Dielectric materials and tolerances: Most of the capacitors used in EMI filtering are nonpolar capacitors.
b, differential mode (line to line) filter capacitive capacitors
c, common mode (line to ground / chassis) filter capacitor
Common mode (CM) decoupling typically uses small capacitors (10 to 100nF). A small capacitor can short circuit an unwanted high frequency current to the chassis before it enters the sensitive circuit or when it is far from the noise circuit. In order to get a good high-frequency attenuation circuit, reducing or eliminating parasitic inductance is the key. It is therefore necessary to use ultra-short wires, and it is particularly desirable to use leadless components.
(3) Inductive and Tandem Loss EMC Solutions
In terms of capacitance, if Zs and Z1 are not purely resistive, their actual values ​​should be used when calculating the frequency. When a capacitor is connected in series with a power supply or signal circuit, it must satisfy:
1 The operating current flowing through should not cause the overheating of the inductor or excessive overshoot.
2 The current flowing through cannot cause magnetic saturation of the inductor, especially for high permeability materials.
There are several solutions:
Magnetic core material
Ferrite and ferrite-loaded cables;
Inductance, differential mode and common mode;
Grounding choke
Combined inductor and capacitor components.
(4) Solution to the radiation problem
In many cases, radiated EMI problems may occur during the conduction phase and be eliminated. There are also solutions that can suppress the interference device in the radiating transmission path and work like a field shield. According to the shielding theory, the effectiveness of such shielding depends mainly on the frequency of the electromagnetic interference source, the distance from the shielding device, and the characteristics of the electromagnetic interference field—electric, magnetic, or plane waves.
1 conductor strip. Use copper or aluminum tape to quickly and easily create a direct shield and low resistance connection or bus. They are convenient for temporary solutions and relatively permanent solutions. The thickness is between 0.035 and 0.1mm, and the backside is provided with a conductive adhesive for mounting. If a copper conductive tape is used, it passes a resistance of about 20 mΩ/cm2. Applications: Electrical shields; location of leaking points in the event of a fault; as an emergency solution, plastic connectors are metalized and shielded from ordinary flat cables.
2 mesh shield and zipper jacket. Tin coated steel mesh belt: It is mainly used as an easy-to-install bandage type shield cover installed on an already-assembled power tariff sheath. In order to reduce the magnetic field radiation or sensitivity of electricity charges, steel mesh belts are an effective solution.
Zippered shield jacket: Use when there is clear evidence that electricity charges are the main cause of EMI coupling.
3EMI gaskets. Application: When the following conditions exist and true SE is required, EMI gaskets are the most commonly used method to solve radiation problems, sensitive problems, ESD, electromagnetic pulse, and TEMPEST problems.
Chassis leaks have been identified as the main radiation path.
The mating surface is not smooth, flat or hard enough and does not provide good connection contact itself.
4 EMI shielding of windows and vents: Suitable for shielding of apertures.
The approximate model of the plane wave is: SE≈104(-20-lgl)-20lgf
In the formula, SE unit is dB; l is the size of the grid or mesh, the unit is mm; f unit is MHz. Of course, as the frequency decreases, the upper limit of the shielding effectiveness SE of the mesh is limited to the metal itself. In the near field, shielding H field, the shielding power SHE is not affected by the frequency, can be approximated by the following formula: SEH ≈ 10lg (πr/l)
Among them, r is the distance from the source to the shield, l is the size of the mesh, both units are mm.
5 Conductive Coatings: Used to establish an EMI shield in the plastic housing of the system, to transmit shielding effectiveness SE of existing common or deteriorated conductive surfaces, to prevent ESD or build-up of static electricity, and to increase the contact area of ​​the bonding surface or gasket.
6 Conductive Foil: Aluminum is a good conductor with no absorption loss below 10 MHz, but it has a good reflection loss for any frequency of the electric field. Application occasions, please refer to the relevant information.
7 Conductive cloth: Can be used in stereo shielding applications where the frequency range from 100kHz to GHz requires attenuation of 30 to 30dB.
3.1 Power Line Filter
The power line filter is installed between the power line and the electronic equipment and is used to simulate the parasitic electromagnetic interference in the power transmission, which plays an important role in improving the reliability of the equipment. The filter allows some frequencies to pass through and falsifies components of other frequencies. According to the characteristics of the interference source, frequency range, voltage and impedance and other parameters and load characteristics, the appropriate choice of filter.
3.2 Signal Blocking Transformers
The isolation transformer used in pulse type (digital or thyristor gate drive) or analog isolation transformer and AC power supply has the same principle as the isolation transformer used in AC power supply, but the transmission frequency band is completely different, and some of the performance requirements of the transformer for useful signal processing (eg distortion, 3dB bandwidth, loss, symmetry, impedance, pulse delay, etc.) are very strict. This type of transformer is a broadband device. The ratio of the highest frequency to the lowest frequency, fMAX/fMIN, reaches several tens of times. By isolating the common-mode ground loop at the sending end or the receiving end, the isolation transformer is to make common-mode noise without changing the differential mode signal. Since the common-mode voltage is applied to both the primary and secondary sides of the transformer, the isolator must have a high breakdown voltage: typically 1.5 kV, and in some cases up to 10 kV.
The main advantages of a signal transformer are its simplicity, durability, long-term and linearity, and its affordable price. As the frequency increases, its electromagnetic compatibility performance decreases.
Application:
When loop isolation is required, its frequency range is from DC to tens of MHz.
When analog small signals (≤10mV) are transmitted under low noise and low distortion conditions, there may be several V to several kV of common-mode voltage on the signal lines.
In the thyristor application circuit, the trigger drive circuit is isolated from the common-mode voltage;
As an on-site solution, it can be used to cut off a ground loop and build a balanced connection or unbalanced connection transmission line.
3.3 Power Isolation Transformers, Power Regulators, and Uninterruptible Power Supplies
(1) Power isolation transformer
A common isolation transformer can cut off the ground loop of the main power line in the low frequency range. When the frequency rises, the electrical isolation decreases due to the presence of the primary-side registered capacitance C1-2. In order to reduce the influence of parasitic capacitances, primary and secondary windings can be used in series, spiral, and discrete, which can reduce the parasitic capacitance to 1/3 to 10/10.
(2) Faraday shielded transformer
A layer of aluminum foil or copper foil is wrapped between the primary and secondary coils so that they do not come in contact with the coils to prevent short circuits. The Faraday shield or electrostatic shield is grounded. The scope of application is as follows:
Applied to the room power supply or power distribution box as a simple 1:1 isolation transformer to isolate the 50/60 Hz ground loop;
Regenerate neutral AC power to ground in a certain part of the same system to maintain electrical isolation from the main power distribution point;
When applied to the system, when there is a large ground leakage current, it prevents the transition from triggering the ground fault detector in the system frequently.
Can be used in conjunction with power line filters, and the attenuation characteristics of power line filters only start at tens or hundreds of kHz.
3.4 Transient suppressor
Varistors and solid state varistors (transzorbs) are components with a non-linear VI characteristic that can be used as a regulated component. When the voltage passes through the device, it is clamped at a voltage equal to or greater than the breakdown voltage VBR. The device has a fast response, but there is a limit to the amount of energy that can be processed.
3.5 Lap, Ground Continuity, and Reduction of RF Impedance Devices
1 A grounded braid or metal band has a smaller inductance than a flat wire with a similar cross-section. As a preferred choice of reference, it is possible to use: flat metal strips; flat braided layers with flat ground terminals; round, multi-stranded jumpers.
2 Printed circuit board (PCB) grounding pads. To create a more direct, low-impedance EMI current receiver, use a grounding gasket. Usually there is a spring clip in the middle of the resin-type gasket to provide strong reliable pressure on one side of the OV copper board and one side of the PCB mounting chassis. Since the spring is made of copper-tin material, the electrical contact performance is good and the contact resistance is on the order of mΩ.
3 metal electricity wire trough and its common metal braid. The role of metal cable trays, common conductors, and metal braids is to transmit a partially grounded EMI loop between several interconnected devices. Think of it as a common-mode short circuit between different chassis or ground, but in fact, this method can't be applied to longer distances except for DC or AC 50/60Hz; it can be used in computer room, factory floor or other There are many large venues that are not shielded from electricity, and it is impossible or difficult to replace them with shielded electricity bills or pipes.
4 The ground impedance is reduced, and the raised metal floor is grounded. In order to reduce the impact of transients on conduction and the RF field in the surrounding environment on the system, it can be improved by setting up an indoor reference ground plane or grounding network. In this way, it is easy to achieve a 20 dB improvement in frequency up to several hundred MHz, and it is also possible to reduce the ground potential difference between different devices in the same room.
Another technique: In the room, it is recommended to install a raised metal base plate (RMF) and use the ribs of the floor tiles as a ground reference grid; replacing the plastic shock-absorbing gasket with a conductive shock-absorbing gasket can be established very well. Lasting electrical connection.
5 temporary ground plate. This post-solution was originally used by IBM's installation planning engineers to install a copper or plated steel plate. For those applications where there is no “real worldâ€, due to the large capacitance (300 to 1000 pF) between the temporary grounding plate and the building structure, this provides EMI filters, transient protectors, and Faraday shields for isolation transformers. There are more absorption devices. At high frequencies, this virtual ground is more effective than long, green, or yellow-green ground wires.
In addition to passing the qualification test of the standard qualification laboratory in actual EMC test applications, there are two possible methods that are also recognized by the industry: TCF and Self CeriTIficaTIon (self-certification). The anti-jamming capability test is a very practical test item. The best way to achieve electromagnetic compatibility is to consider all digital and analog circuits as circuits that respond to high-frequency signals, and use high-frequency design methods to handle the tariff mask, PCB layout, and common-mode filtering. The use of a monolithic ground plane and a power plane is also important, as is the case for analog circuits, which helps to limit high frequency common mode loops. Most of the transients are high frequencies and produce very strong radiant energy.
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