Technical column
CASES
EMC design problems and solutions for electronic equipment
Release time:
2014-09-30 00:00
Source:
1. EMC overview
Electromagnetic compatibility, the English name is "Electromagnetic Compatibility" , referred to as EMC . A clear definition is given in my country's national military standard. " Equipment (subsystem, system) will not cause or suffer impermissible degradation due to electromagnetic emissions from other equipment in the same electromagnetic environment; nor will it cause other equipment (subsystem, system) in the same electromagnetic environment, cause or suffer impermissible degradation from exposure to its electromagnetic emissions". It can be seen that electromagnetic compatibility refers to the equipment's own quality characteristics and special ability to withstand environmental influences that exist in various electromagnetic environments without mutual interference. Because it directly affects the accuracy, reliability, compatibility, safety and environmental adaptability of electronic systems, it has become increasingly prominent in the design of contemporary electronic and electrical products. The concept of EMC originated from the military industry, and now it has involved all industries in the modern electronics industry. As far as electronic equipment is concerned, there are mandatory standards on EMC in both CE certification in Europe and FDA certification in the United States . This article starts from the basic concepts, combined with practical engineering experience, and puts forward some practical and effective methods for the difficult problems in EMC design and testing . The improvement of the EMC performance of the equipment plays the role of attracting new ideas.
2. Discussion on EMC design problems and their solutions
At present, most domestic equipment manufacturers in China do not have good EMC system design capabilities and EMC design management and control capabilities. Therefore, in actual engineering, they will encounter many EMC problems, among which the more prominent ones are electrostatic immunity, radiation interference, and conduction. interference etc. This article mainly summarizes these three daily difficult problems and their solutions as follows.
2.1 Electrostatic Discharge (ESD) Problems and Solutions
Static electricity is pervasive throughout the life cycle of electronic equipment, and it is hard to prevent. Electronic equipment crashes due to static electricity, abnormal display, and even device damage are the enemies that endanger the reliability of equipment. Especially the electrostatic problem of portable electronic equipment is more prominent. The harm mechanism of static electricity to electronic products is complex and difficult to prevent. For ±4kV contact discharge of well-grounded and accessible conductive parts , the insulating shell of the equipment can generally resist, but for ±8kV air discharge and exposed, poorly grounded metal parts or electroplated parts, surface insulating metals, etc., often There will be many problems.
Solutions to ESD can be classified into 4 categories:
(1) Surface insulation method, the design concept of this method is to increase the electrical gap, creep point distance, etc., so that the external electrostatic field cannot directly contact or indirect air discharge to sensitive electronic equipment, such as plastic shell products without gaps Design or even if there is a gap, there is an electrical distance between the gap and the internal circuit greater than 8mm or a thin reinforced insulating material that can withstand ±8kV . This method is especially suitable for solving the problem of contact discharge, but it cannot solve the problem of electrostatic field capacitive interference caused by electrostatic field coupling.
(2) Electrostatic shielding method, through the shielding method of the Faraday cage, static electricity cannot enter the interior of the metal shell device. However, since the equipment needs to dissipate heat and has an external interface, it becomes a problem of hole and seam design of the equipment. According to the typical rise time of electrostatic discharge is 0.7~1ns , and the main energy of radiated electromagnetic waves is concentrated in the harmonic below the 9th order, therefore, the highest frequency point for generally suppressing electrostatic coupling is 14GHz , and according to the commonly used electromagnetic field and antenna theory, for The energy that can pass through holes less than 0.05λ is less than 4% . Therefore, the opening size of the general metal heat dissipation hole is preferably less than 1.2mm ; in fact, the most critical thing is the interface design of the external cable (see the last two ) and Gap design (plus conductive pad).
(3) The filter grounding absorption method is divided into situations such as isolation of the internal circuit board from the metal case, single-point grounding or multi-point grounding. Its purpose is mainly to prevent static electricity from entering the internal circuit board through the interface cable. The isolation method is a structural design that can ensure that static electricity is difficult to enter the interior along the cable, through the cable metal connector, so that the static electricity can only flow in the shell design, also known as the energy release method, generally through reasonable PCB wiring and structural design, so that high The energetic electrostatic charge is discharged in the form of an arc discharge. However, it is necessary to prevent the influence of the large transient current magnetic field on the surface; single-point grounding and multi-point grounding are achieved by adding ESD absorption or discharge devices (such as Zener diodes, varistors, etc.) at the interface, such as the power supply at the port Or add a differential-mode or common-mode ESD absorbing device at the signal end, so that static current flows directly to the casing to protect the internal circuit. Generally, the ground area needs to reach a certain size.
(4) The method of combining insulation, discharge and ground conduction (the most commonly used in general systems). The first is the input and output ports of the equipment, and the second is the keyboard, panel and other gaps of the equipment. There is nothing special about these parts themselves, but if there are conductive PCB boards or components that are not well grounded near the gaps of these parts, the ±8kV air discharge will break down the air, which may directly cause abnormal operation of the circuit, or due to Spark discharges (arcing) can cause severe radio frequency interference and cause system shutdowns. These problems can generally be avoided through good structural and system design.
2.2 Conducted interference problems and solutions
Conducted interference means that the interference source reaches the sensitive equipment in the way of conduction coupling. There are typically three coupling paths between an interfering source and a sensitive device. They are the near-field induction between the public power supply, the public ground loop, and the signal line. In contemporary electronic equipment, the more significant conduction interference comes from the power supply. At present, switching power supply is widely used, but the radiation and conduction interference caused by its " switching frequency " cannot be ignored. Of course, if the power supply design is exquisite, these problems can also be solved well, such as adjusting the switching waveform generation circuit, considering the design of the transformer, improving the common-mode and differential-mode filters, and so on. In addition to system design considerations, the best way to solve the conduction interference and anti-interference of the power port is to add a professional power filter; similarly, the best way to solve the conduction interference of the signal and control ports is to add a professional signal filter . The filter can not only suppress electromagnetic disturbance, but also protect the internal circuit from external electromagnetic interference. But it is wrong to rely on the filter alone, because even the best filter has its maximum filtering ability, and only the joint design of the disturbance source and the propagation path is the best design.
2.3 Radiation interference problems and their solutions
2.3.1 Radiation Interference
Radiation disturbance and radiation susceptibility is another difficult problem of EMC problem. Not only because of the wide frequency band of radiation-related tests, but also because of the multiple ways that cause radiation interference problems, such as too strong noise sources, gaps in the path, wires, shielding effectiveness and leakage, etc. Sensitive sources have high sensitivity and poor immunity. It is difficult to implement the design and design of the product in a consistent manner. These factors determine that it is difficult to completely solve this problem through shielding, filtering, grounding, management and other methods. Therefore, solving the problem of radiation interference is one of the key factors to measure the quality of product EMC design.
2.3.2 Solutions to radiation interference
Generally, the suppression of interference is considered to start from the three elements of interference — interference source, interference path, and sensitive equipment — and radiation interference is no exception. The first is to consider reducing the radiation intensity of the interference source, and then the way to cut off the interference coupling.
(1) Board-level radiation interference suppression. The control of radiated interference starts from the wiring of the circuit board. In order to control the radiation of the circuit board, the ring area formed by the signal path must be reduced, which is an extremely difficult task. Clock currents are often the worst source of radiation in a system. All the energy of the clock current is concentrated in a narrow band dominated by the fundamental frequency [3] . When wiring, the area of the ring carrying the system clock current should be minimized, and all clocks should be connected to the ground loop as short as possible. For example, the ground layer can play this role when using multi-layer board wiring. For those clock signals that are led out of the board, LC filtering should be performed on the board (in the selection of parameters, care should be taken not to cause serious distortion or low amplitude of the signal), so that the clock signal carries as little high-order harmonic components as possible. . In addition, using a case-grounded crystal can also reduce radiated interference from the clock.
(2) Radiation interference suppression for board-to-board connections. In addition to radiation from circuit boards, inter-board interconnection cables can also cause serious interference. There are several types of inter-board interconnection cables: coaxial, three-core, twisted pair and ribbon cables, and their ability to reduce radiation is weakened in turn. Usually the best way to handle the clock signal is to use coaxial cable, because coaxial cable has the greatest mutual inductance between the core wire and the shield of all types of cables. However, because of the high connection efficiency of ribbon cables (flat cables), ribbon cables are widely used in many electronic devices. At this time, the configuration of the ground wire in the cable is very important. The best wiring method between the wires is: ground - signal - ground - signal - ground, phase-to-phase configuration, so that the ring area of each signal can be as small as possible, so that Reduce radiation intensity. Ribbon cables can also be shielded, such as shielded cables covered with a thin layer of aluminum foil, but such shielded cables tend to be difficult to terminate well (i.e. have a 360 degree ring connection), so the effectiveness is significantly reduced. In order to reduce the radiation interference caused by the inter-board connection cables, the length of the cables should be kept as short as possible, so that the radiation efficiency of the cables will be significantly reduced. A large number of hard connections (direct socket or SMT welding) can be used to reduce the interference caused by the inter-board connection . Radiation is reduced.
(3) System-level radiation interference suppression. Shielding is the most important means. Many medical electronic devices use metal casings, which can play a certain role in shielding and reduce the radiation of the equipment to the outside world. System-level radiation interference suppression not only needs to manage and control the cable radiation interference level of each unit (including design realization, production process realization, engineering installation realization, etc.), but also needs to control the ground loop effect of each unit, signal impedance, etc. The fit, shielding and filtering overlapping processes should be managed in a unified manner. The so-called product design level is guaranteed by scientific management, and management is not considered in this article.
The energy efficiency of electromagnetic shielding is divided into two aspects, that is, the reflection loss and absorption loss of the electromagnetic wave by the shielding body. At the same time, the effectiveness of electromagnetic shielding mainly depends on two points: the integrity of the shielding material and the shielding body. From the perspective of EMC testing, as long as conditions permit, iron (steel) is generally given priority as a shielding material. In addition, in order to improve its reflection loss, materials such as galvanized or tin can be plated on the surface. For some portable devices, due to the requirement of light weight, plastic shells are generally used. In this case, the method of spraying shielding paint on the inner wall of the plastic shell is usually used to achieve the purpose of shielding. Generally, the surface resistance after spraying is required to be less than 0.5 Ohm , which is significantly improved compared with the situation where no shielding material is sprayed.
In addition, the shielding effectiveness of various electrical discontinuities on the shielding body is the decisive factor determining the overall shielding effectiveness, so special attention should be paid to those factors that cause the greatest leakage of electromagnetic energy during shielding design, such as ventilation holes of equipment, cable entrances and exits, equipment external cables, seams of the case, etc. In the shielding design, the size of the direct gap and the indirect overlapping gap should be much smaller than the wavelength λ of the electromagnetic wave to be suppressed . General military equipment requires the gap to be no greater than 0.01λ , and civilian equipment is not greater than 0.05λ . The general experience is that it must be less than 0.25λ . The design of the gap must be considered in conjunction with the filtering and lapping design of the cable to prevent one from losing the other.
It is also worth mentioning that there is a certain relationship between the conducted interference and the radiated interference of the equipment, which is prominently manifested in the AC/DC switching power supply. When a large ringing appears on the switching waveform, in addition to causing a large conduction interference, it will also bring a large radiation interference. However, unlike the radiation interference caused by the clock of a digital circuit, it does not have a prominent single peak in the high-frequency spectrum, but presents a hillside-shaped curve. When appropriate filter circuit measures are taken to weaken the ringing, the radiation interference it brings is also eliminated.
3. Conclusion
With people's understanding of the EMC work of modern electronic equipment, the importance of this work has been recognized by more and more enterprises. Therefore, strengthening the experience exchange and technical discussion among engineering technicians and scholars in related fields is bound to promote the development of EMC work in modern electronic equipment in China. This article briefly discusses some understandings and solutions to some difficult problems in the EMC design process, for your reference only.