People may feel strange about the word electromagnetic interference (EMI), but the impact of EMI is something that almost everyone has experienced. For example, when watching TV, some people nearby use electric drills, hair dryers and other electrical appliances, which will make the TV screen appear snowy, and the sound of the sound in the sounder... This kind of phenomenon has long been commonplace and used, but the harm of electromagnetic interference But far more than that. In fact, electromagnetic interference has caused civil aviation system failure, poor communication, computer operation errors, malfunction of self-controlled equipment, and even endangering personal safety. Therefore, it has become a top priority to strengthen the popularity of electromagnetic capacitance (EMC) knowledge and improve EMI suppression technology.
Electromagnetic compatibility refers to a state in which electronic circuits and systems do not affect each other and are electromagnetically compatible with each other. For the study of EMC technology, foreign countries began in the 1930s. Some countries and international organizations such as the Federal Communications Commission (FCC), the German Institute of Electrical and Electronics Engineers (VDE), and the International Special Committee on Radio Interference (CISPR) have successively A number of guiding documents and procedures have been developed, and a relatively complete system has been formed and strictly adhered to. The US computer industry is fully implementing the FCC regulations. China's electromagnetic compatibility work started relatively late, and relevant standards have been introduced since the 1980s. The application is difficult due to lack of experience and technology. How to catch up with the international advanced level as soon as possible, so that China's electronic products can meet the increasingly urgent domestic demand and occupy a place in the international market, has become a major issue for everyone. This article is willing to talk about electromagnetic interference suppression technology, and to learn from each other.
The definition of electromagnetic interference refers to the disturbance caused by external noise and unwanted electromagnetic waves in the reception. The degree of electromagnetic interference of a system or a line within a system can be expressed as follows:
N=G&TImes;C/I
G: noise source strength;
C: the coupling factor that noise is transmitted to the disturbed place through some means;
I: Sensitivity of the interfered circuit.
The three elements G, C, and I constitute the three elements of electromagnetic interference. Electromagnetic interference suppression technology is the various measures taken around these three elements. It is summarized into three parts: one, suppressing electromagnetic interference sources; second, cutting off the electromagnetic interference coupling path; and third, reducing the sensitivity of electromagnetic sensitive devices. The following three aspects are introduced separately.
First, suppress interference sources
In order to smash the source of interference, it is first necessary to determine where the source of interference is. The closer to the source of the interference, the better the suppression effect. In general, the sudden change of current and voltage, that is, where di/dt or du/dt is large, is the source of interference; specifically, relay opening and closing, capacitor charging, motor operation, and integrated circuit switching work may become sources of interference. In addition, the mains power supply is not an ideal 50Hz sine wave, but full of various frequency noise, is a source of interference that cannot be ignored. The suppression method can use a low-noise circuit, a transient suppression circuit, a rotating device suppression circuit, a voltage stabilization circuit, etc.; the selection of the device uses electronic components with low noise, high frequency characteristics, and high stability as much as possible. It is important to note that improper device selection in the suppression circuit can create new sources of interference.
Second, cut off electromagnetic interference coupling
The electromagnetic interference coupling pathway is mainly conducted and radiated. Noise is most commonly coupled to the circuit via wires. The main measure to suppress conducted interference is the series filter. The filter is divided into low pass (LPF), high pass (HPE), band pass (BPF), and band stop (BEF). Different types of filters are selected according to the difference between signal and noise frequency. If the noise frequency is much higher than the signal frequency, an LC low-pass filter is often used. This filter has a simple structure and a good filtering effect. However, for military or TEMPEST technology and for demanding civilian products, a feedthrough filter must be used.
Feed-thru filters (Feed-thruFilters) are also called pass-through filters. The circuit structure is C-type, T-type and LC-type. It is characterized by excellent high frequency characteristics and can work above 1 GHz. This is determined by its "coaxial" nature, which increases the self-resonant frequency due to its parasitic inductance. The feedthrough filter is small in size, light in weight, and allows for a large current, and can be widely used in various occasions.
Noise transmitted through the power supply line can be filtered out with a power filter. The power supply filter that only complies with the VDE0871 standard has an insertion loss of 20-100 dB in the range of 30K-30MHZ. The power filter can be connected not only to the input of the grid but also to the output of the noise source circuit to suppress the noise output, and it can be used for both AC and DC. The power filter port is divided into high-impedance and low-impedance terminals. The correct connection should be selected according to the input and load impedance. The principle of connection is to match the impedance mismatch, that is, the high resistance input is connected to the filter end, and the low resistance load is connected to the high resistance end of the filter; vice versa.
For the wiring design of the transmission line and the printed circuit board, it should be noted that the incoming and outgoing lines, the signal lines and the power lines are separated as much as possible. For the key lines, devices such as lossy line filters, three-terminal capacitors, and magnetic rings can be used for interference suppression. For the interface end, there are D-type, round and square connector products with filtering in foreign countries. This type of connector is equipped with a capacitor or an inductor on the common connector to form a filter circuit, which is characterized by not occupying. PCB space, without adding volume, is extremely important for high-density design of modern components. Recently, there are also manufacturers in China, the quality is not lower than the foreign level, and can replace imports.
For radiated interference, the main measure is to use shielding technology and layering technology. Shielding technology is a science. Choosing the right shielding material and shielding it at the right place is critical to the shielding effect. Especially the design of the shielded room. There are a wide variety of shielding materials to choose from, including various metal plates, finger-shaped beryllium copper alloy reeds, copper mesh, braided copper tape, conductive rubber, conductive adhesive, conductive glass, and the like. Should be selected as needed. The design of the shielded room should fully consider the shielding and overlapping of doors and windows, vents, and inlet and outlet. In addition to electrostatic shielding, magnetic shielding as well as grounding and grounding techniques must be considered.
Third, reduce the sensitivity of electromagnetic sensitive devices
The sensitivity of electromagnetic sensitive devices is a double-edged sword; on the one hand, people want high sensitivity of the receiving device to improve the ability to receive signals; on the other hand, the sensitivity is more likely to be affected by noise. Therefore, according to the specific situation, the use of derating design, avoiding design, network passivation, functional passivation and other methods is the solution to the problem.
In summary, there are many methods for suppressing electromagnetic interference, and one or more comprehensive applications can be selected. But no matter what method you choose, you should start to consider the system's electromagnetic compatibility from the beginning of the design, instead of remedying it. According to reports, if the EMC design is carried out at the beginning of the product development, approximately 90% of the conducted and radiated interference can be controlled. According to reliability, safety, quality requirements, environmental control, efficiency / cost trade-off, choose the appropriate electromagnetic interference suppression technology, which is the research content of electromagnetic compatibility.
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