"introduction
In fact, most engineers understand that the electromagnetic compatibility is generally: the device or system works normally in its electromagnetic environment, and does not constitute an unbearable electromagnetic harassment in the environment. The EMC test includes two major aspects: testing the electromagnetic harassment intensity to the outside world to confirm whether it meets the restrictions required by the relevant standards; the sensitivity test is performed under the electromagnetic environment conditions specified in the electromagnetic harassment strength. Confirm that it meets the refraction requirements for the relevant standards. For engineering technicians engaged in microcontroller application system, it is necessary to master certain EMC testing techniques.
1 single-chip system EMC test
(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, and the measurement venue has outdoor open space, shielding chamber or electric wave dark room.
(2) Test equipment
Electromagnetic compatible measuring equipment is divided into two categories: one type is an electromagnetic interference measuring device. The device can be measured by electromagnetic interference; the other is the electromagnetic sensitivity measurement, the equipment simulates different interference sources, by appropriate Coupling / decoupling network, sensors or antenna, is applied to various types of measured devices for sensitivity or interference measurement.
(3) Measurement method
Electromagnetic compatibility testing is based on standards, there are many measurement methods, but it can be divided into 4 categories; conductive launch test, radiation launch test, transmission sensitivity (immunity) test and radiation sensitivity (immunity) test.
(4) Test preparation
1 Test site conditions: EMC test laboratory is a half dark room and shield chamber. The former is used for radiation emission and radiation sensitivity testing, which is used to conduct emission and conductivity sensitivity testing.
2 Environmental level requirements: The electromagnetic environment level of the conduction and radiation is preferably much lower than the limit value of the standard, which generally enables at least less than the limit value of 6db.
3 Test table.
4 Measuring the equipment and the isolation of the device under test.
5 Sensitivity discrimination criteria: It is generally provided by the test, and triggered and discriminating, the degree of performance reduction is determined by measurement and observation.
6 Placement of the test device: To ensure the repetition of the experiment, the placement of the measured device usually has specific provisions.
(5) Test type
Conduct emission test, radiation transmission test, conduction immunity test, radiation immunity test.
(6) Common measuring instrument
Electromagnetic Interference (EMI) and electromagnetic sensitivity (EMS) testing requires many electronic instruments, such as spectrum analyzers, electromagnetic field interference measuring instruments, signal sources, functional amplifiers, oscilloscopes, and the like. Since the EMC test frequency is wide (20 Hz ~ 40GHz), the amplitude is large (μV to KW), the mode is much large (fm, AM, etc.), the gesture is much (flat, oblique, etc.), so properly use the electronic instrument very important. Suitable instruments for measuring electromagnetic interference are spectrum analyzers. The spectrum analyzer is an instrument that disseminated the voltage amplitude with the regular changes in the frequency, and the waveform it shows is called a spectrum. The spectrum analyzer overcomes the shortcomings of the oscilloscope in the measurement of electromagnetic interference, and can accurately measure the interference strength on each frequency, and the spectrum analyzer can directly display the respective spectral components of the signal.
When solving electromagnetic interference issues, the most important problem is to determine the source of interference. Only when the interference source is accurately positioned, the measures to solve the interference can be proposed. Determining the frequency of the signal is the easiest way to determine the frequency of the signal, since the frequency characteristics are most stable in all the features of the signal, and the circuit designer tends to be very clear about the signal frequency of each portion in the circuit. Therefore, as long as the frequency of the interference signal is known, it is possible to speculate which site is generated. For electromagnetic interference signals, this measurement is very simple to do this measurement with a spectrum analyzer due to its amplitude is often less than the normal working signal. Since the intermediate frequency bandwidth of the spectrum analyzer is narrower, the signal filtering with the frequency of the interference signal can be filtered off, and the interference signal frequency is accurately measured, thereby judgeting a circuit generating an interference signal. Be
2 Electromagnetic compatible troubleshooting technology
(1) Solution of the conductive type problem
1 Reduce EMI current by a high impedance in series.
2 Short the EMI current to the ground or introduce another loop conductor by parallel a low impedance.
3 cut off the EMI current through the current isolation device.
4 The EMI current is suppressed by its own effect.
(2) Electromagnetic compatibility capacitive solution
A common phenomenon is to be connected to a separate impedance from a separate impedance without regarding a separated impedance. Typically, the transmission line changes "long" when a length of an input and output line reaches or exceeds 1/4 wavelength. It can actually use the following formula to approximate this change: L ≥ 55 / f
Where: the L unit is M, F unit is MHz. This formula takes into account the average propagation speed, which is 0.75 times the freedom of space theory.
a. Dielectric material and tolerance: most of the capacitance used in electromagnetic interference filtering is a non-polar capacitor
b. Differential mode (line to line) filter capacitive capacitor
c. Common mode (line to floor / casing) filter capacitor
A common mode (CM) decoupling usually uses small capacitance (10 to 100 nF). Small capacitors can short-circuited to the casing before or when they enter the sensitive circuit, or when it is farther away from the noise circuit. In order to obtain a good high frequency attenuation circuit, it is critical to reduce or eliminate parasitic inductance. Therefore, it is necessary to use ultrashine wires, especially the use of non-lead components.
(3) Impiration, series loss electromagnetic compatibility solution
For the capacitance, if Zs and Z1 are not a pure resistance, use their actual values when calculating frequencies. When the capacitor is connected in series in the power supply or signal circuit, it must be satisfied:
1 The working current flowing should not cause an inductance overheating or too much, and there is no difference;
2 flowing current cannot cause inductance magnetic saturation, especially for highly magnetine materials.
Solutions have the following:
Magnetic core material;
Ferrite and the loading of ferrite;
Inductance, differential mode and common mode;
Ground choke;
Combined inductor capacitive elements.
(4) Solution of radiation type problem
In many cases, radiation electromagnetic interference problems may be generated and excluded in the conduction phase, and some solutions are to suppress the interference device on the radiation transmission channel, as working like a field shield. According to the mask theory, the effect of this shield mainly depends on the frequency of the electromagnetic interference source, the distance between the shielding device and the characteristics of the electromagnetic interference field - electric field, magnetic field or plane wave.
1 conductor belt. Use copper or aluminum strips to create a direct shield and low-resistance connectivity or bus. They are very convenient for temporary solutions and relatively permanent solutions. The thickness is between 0.035 to 0.1 mm, and the back is a conductive binder to mount. If a copper conductive strip is used, it passes from 20 mΩ / cm2 by electric resistance. Applications: Electrical shield; disclosure point positioning in the event of a fault; as an emergency solution, the plastic connector becomes a metal, shields the normal flat cable.
2 Net shield strips and zipper jackets. Tin's steel mesh belt: Mainly used to install a well-equipped electric toll jacket as an easy-to-mounted bandage type shield. In order to reduce the magnetic field radiation or sensitive problem of electricity bills, steel mesh is an effective solution.
Zipper shielding jacket: When there is obvious indication that the electricity bill is mainly used for the cause of EMI coupling.
3EMI seal. Applications: When the conditions are present, and the true SE is required, the EMI seal is the most commonly used method of solving radiation problems, sensitive problems, ESD, electromagnetic pulses, and Tempest issues.
The chassis leaks have been confirmed as the main radiation path.
The meshing surface is not smooth enough, flat or not hard enough, itself cannot provide a good connection contact.
4 EMI shielding of windows and ventilation: Suitable for masking of the aperture.
The probably the plan of the plane is: SE≈104 (-20-LGL) -20LGF
In the formula, the SE is db; L is the size of the mesh or mesh, the unit is mm; F unit is MHz. Of course, as the frequency decreases, the upper limit of the shielding efficiency SE of the mesh is limited to the metal itself. In the nearland, the shield of the H field, its shielded power She is not affected by the frequency, can be approximately drawn by the following formula: SEH ≈10LG (πr / L)
Wherein R is the distance between the source to the shield, L is a mesh size, both of which are mm.
5 Conductive Coatings: Apply an EMI shield in the system's plastic housing to transmit existing ordinary or deteriorated conductive surfaces, prevent ESD or electrostatic accumulation phenomenon, increase the contact area of the binding surface or sealing spacer.
6 Conductive foil: Aluminum is a good conductor, there is no absorption loss at 10 MHz, but it has better reflection loss for any frequency of the electric field. Please refer to the relevant information.
7 Conducting cloth: Can be applied to any 100 kHz to GHz-level frequency range requires a three-dimensional shielding occasion of 30 to 30 dB attenuation. Be
3 Application of new material for electromagnetic compatibility
3.1 Power Cord Filter
The power cord filter is mounted between the power cord and the electronic device, which is used to develop parasitic electromagnetic interference in electrical energy transmission, and it is important to improve the reliability of the equipment. The filter allows some frequencies to be combined, and ingredients for other frequencies are proposed. The filter is appropriately selected depending on the characteristics of the interference source, frequency range, voltage and impedance requirements.
3.2 Signal Block Transformer
Pulse type (digital or thyristor door driver) or analog isolation transformer is the same as the principle of isolation transformers used in AC power supplies, but the transmission frequency band is completely different, and useful signal processing on some performance requirements of the transformer (For example, distortion, 3DB bandwidth, loss, symmetry, impedance, pulse delay, etc.) are very strict. This transformer is a broadband device, and the maximum frequency and the lowest frequency ratio fmax / fmin reaches several tens of times. By cutting the common mode loop in the transmitting end or the receiving end, the isolation transformer is simultaneously propelled while the difference mode signal is not changed. Since the common mode voltage is added to the primary side of the transformer, this isolator must have a higher breakdown voltage: typical value is 1.5kV, and some occasions are up to 10kV.
The main advantage of the signal transformer is its simple, durable, lasting and linear, and price is moderate. When the frequency increases, its electromagnetic compatibility performance is lowered.
Application occasion:
When loop isolation is required, its frequency ranges from DC to tens of MHz;
When the simulated small signal (≤10mv) is transmitted under low noise and low distortion conditions, a common mode voltage of several V to a few kV may exist on the signal line;
In the thyristor application circuit, the trigger drive circuit is isolated from the common mode voltage;
As a live solution, it can be used to cut off a ground loop and build a balanced connection or a non-balanced connection transmission line.
3.3 Power Isolation Transformer, Power Supply Regulator and Uninterruptible Power Supply
(1) Power isolation transformer
Ordinary isolation transformers can cut off the ground loop of the main power line at the low frequency range. When the frequency is increased, electrical isolation is lowered due to the presence of a side room storage capacitor C1-2. In order to reduce the influence of parasitic capacitance, a falling system, a spiral, discrete one and secondary windings can be used, which can reduce the parasitic capacitance as the original 1/3 ~ / 10.
(2) Faraday shield transformer
A layer of aluminum foil or copper foil is covered between one and secondary coils, and it is not in contact with the coil to avoid short circuit. Faraday shielding or electrostatic shield ground. The application range is as follows:
Applying to the inlet power or power distribution box, as a simply 1: 1 isolation transformer, isolation 50/60 Hz ground road;
Restalling a neutral AC power source in the same system, maintaining electrical isolation with the total power distribution point;
Application to the ground fault detector in the transition frequent trigger system is prevented when there is a large-ground leakage current in the system;
It can be used in conjunction with the power cord filter, and the attenuation characteristics of the power cord filter start only at several tens or hundreds of kHz.
3.4 Transient Suppressor
Transzs and solid state transformers are elements having a nonlinear V-I characteristic curve that can be used as a regulator element. The voltage value equal to or greater than the breakdown voltage VBR is clamped when the voltage passes the device. The device has a fast response speed, but there is a limit on the energy value of the process.
3.5 Connection, grounding continuity and reducing RF impedance devices
1 Grounding woven layer or metal bandwidth and the flat wire is smaller inductance than the same cross-sectional circular wire. As a preferred selection, a flat metal strip can be used; a flat knitted layer with a flat terminal; a circular, a plurality of stranded jumpers.
2 Printed circuit board (PCB) ground pad. In order to establish a more direct low-exctricted electromagnetic interference current receiver, a ground pad is required. Typically there is a spring clip in the middle of the resin gasket to provide strong reliable pressure on the OV copper plate while the PCB on one side. Since the spring is made of copper tin material, the electrical contact performance is good, the contact resistance is the magnitude of MΩ.
3 Metal electricity trough and metal weaving layer thereof. The mechanical cable bracket, the common wire, and the metal braided layer is to transmit a partially grounded EMI circulation between several interconnected devices. It can be regarded as a common mode short circuit channel between different chassis or ground, but in addition to DC or AC50 / 60Hz, this method cannot be applied to a longer distance; available for computer room, factory workshop or other There are many large venues of non-shielded electricity, impossible or difficult to convert them to shielded electricity bills or load pipe.
4 The ground impedance is reduced, and the high metal floor ground liner is high. In order to reduce the input of conduction transient interference and the effects of the surrounding environmental radio frequency field on the system, it can be improved by setting an indoor reference grounding plate or grounding network. In this way, it can be easily improved in the frequency of up to several hundred MHz, and the ground potential deviation between different devices in the same room can also be reduced.
Alternatively, in the room, it is recommended to mount the elevated metal bottom plate (RMF), using the brick rib as a ground reference grid; convert the plastic shock absorbent into a conductive shock absorbing pad, you can establish a good Long-lasting electrical connection.
5 Temporary ground floor. This after each solution is initially used by IBM's installation planning engineers, that is, installs a copper plate or plated steel sheet. For those who do not "actually", there is a large capacitance (300 ~ 1000pF) between the temporary grounding plate and the building special structure, which provides the Fala-shielding layer of the electromagnetic interference filter, transient protector, and isolation transformer. There is a more absorption device. At high frequency, this virtual or yellow-green grounding wire is more effective.
4 conclusion
In addition to passing the standard in actual EMC test applicationsIn addition to the identification test of the qualification laboratory, there are two feasible methods to be recognized by the industry: TCF and Self Ceritification. Anti-interference ability test is a very practical test item. The best way to achieve electromagnetic compatibility is that all numbers and analog circuits are treated as circuits for high-frequency signal response, with high frequency design methods to process electricity shielding, PCB wiring and common mode filtering. It is also important to use the entire plane and the power supply surface, which is also true for analog circuits, so that it is conducive to limit high-frequency common mold ring. Most transient interference are high frequencies and produce strong radiation energy. "
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