How to design electromagnetic compatibility and electromagnetic interference for PCB circuits
1.General concepts of electromagnetic compatibility
The root cause to consider is the presence of electromagnetic interference. Electromagnetic interference (EMI) is the process by which destructive electromagnetic energy is transmitted from one electronic device to another through radiation or conduction. Generally speaking, EMI refers to radio frequency signals (RF), but electromagnetic interference can occur in all frequency ranges.
(Electromagnetic Compatibility, EMC for short) refers to the operation of electrical and electronic systems, equipment, and devices at the designed level or performance within the specified safety limits in a set electromagnetic environment without being damaged or impossible due to electromagnetic interference Ability to receive performance degradation. The electromagnetic environment referred to here refers to the sum of all electromagnetic phenomena existing in a given place. This indicates that electromagnetic compatibility means that the electronic product should have the ability to suppress external electromagnetic interference; on the other hand, the electromagnetic interference generated by the electronic product should be below the limit and must not affect the normal operation of other electronic equipment in the same electromagnetic environment.
Today's electronics have shifted from analog to digital design. With the development of digital logic devices, issues related to EMI and EMC have begun to become the focus of the product, and have received great attention from designers and users. The United States Communications Commission (FCC) published radiation standards for personal computers and similar devices in the mid-to-late 1970s, and the European Commission introduced mandatory requirements for radiation and immunity in its 89/336 / EEC electromagnetic compatibility guidance document. China has also formulated national standards and national military standards related to electromagnetic compatibility, such as "electromagnetic compatibility terms" (GB / T4365-1995), "electromagnetic interference and electromagnetic compatibility terms" (GJB72-85), "radio interference and anti- Specification for interference measurement equipment "(GB / T6113-1995)," Measurement methods and allowable values of radio interference characteristics of power tools, household appliances and similar appliances "(GB4343-84). These electromagnetic compatibility specifications have greatly promoted electronic design technology and improved the reliability and applicability of electronic products.
2. The importance of EMC in design
As the sensitivity of electronic equipment becomes higher and higher, and the ability to accept weak signals becomes stronger, the frequency band of electronic products becomes wider and wider, and the size becomes smaller and smaller, and the anti-interference ability of electronic equipment is required to become stronger and stronger. The electromagnetic waves generated by some electrical and electronic equipment when they work can easily cause electromagnetic interference to other electrical and electronic equipment around them, causing malfunctions or affecting signal transmission. In addition, excessive electromagnetic interference will cause electromagnetic pollution, endanger people's physical health, and damage the ecological environment.
If various electrical equipment in a system can work normally without mutual electromagnetic interference causing performance changes and equipment damage, people will call the electrical equipment in this system compatible with each other. However, with the diversification of equipment functions, the complexity of the structure, the increase in power and the increase in frequency, at the same time their sensitivity is getting higher and higher, and this mutually compatible state is becoming more and more difficult to obtain. In order for the system to achieve electromagnetic compatibility, it must be based on the electromagnetic environment of the system. It is required that each electrical equipment does not generate electromagnetic emissions exceeding a certain limit, and at the same time it must have a certain ability to resist interference. Only by making constraints and improvements in these two aspects for each device can we ensure that the system is fully compatible.
Generally speaking, there are two ways to transmit electromagnetic interference: one is the conduction method; the other is the radiation method. In actual engineering, interference between two devices usually involves the coupling of many ways. It is precisely because the coupling of multiple pathways exists at the same time, crosses repeatedly, and jointly produces interference, which makes electromagnetic interference difficult to control.
Common electromagnetic interferences are as follows:
(1) Radio frequency interference. Due to the proliferation of existing radio transmitters, radio frequency interference poses a great threat to electronic systems. Cell phones, handheld radios, radio remote control units, pagers and other similar devices are now very common. No harmful power is required to cause harmful interference. Typical faults occur in the RF field strength range of 1 to 10V / m. In Europe, North America, and many Asian countries, avoiding RF interference to damage other equipment has become a legal requirement for all products.
(2) Electrostatic discharge (ESD). Modern chip technology has made great progress, and components have become very dense in very small geometries (0.18um). These high-speed, millions of transistor microprocessors have high sensitivity and are easily damaged by external electrostatic discharge. The discharge can be caused directly or by radiation. Direct contact discharges generally cause permanent damage to the equipment. Electrostatic discharge caused by radiation may cause equipment disturbances and abnormal operation.
(3) Power interference. As more and more electronic devices are connected to the power backbone network, there will be some potential interference in the system. These disturbances include power line disturbances, electrical fast transients, surges, voltage changes, lightning transients, and power line harmonics. For high-frequency switching power supplies, these interferences become significant.
(4) Self-compatibility. Digital parts or circuits of a system may interfere with analog equipment, create crosstalk between wires, or a motor may cause disturbances in digital circuits.
In addition, an electronic product that can work normally at low frequencies will encounter some problems that are not found at low frequencies when the frequency increases. Such as reflection, string winding, ground bounce, high frequency noise, etc.
An electronic product that does not comply with EMC specifications is not a qualified electronic design. In addition to designing products to meet market functional requirements, appropriate design techniques must be employed to prevent or eliminate the effects of EMI.
3. EMC considerations for design
For high-speed (Printed Circuit Board, printed circuit board) design EMI problems, there are usually two methods to solve: one is to suppress the effects of EMI, and the other is to shield the effects of EMI. These two methods have many different manifestations, especially the shielding system minimizes the possibility of EMI affecting electronic products.
Radio frequency (RF) energy is generated by switching currents in printed circuit boards (PCBs), which are byproducts of digital components. In a power distribution system, each logic state change will generate an instantaneous surge. In most cases, these logic state changes will not generate enough ground noise voltage to cause any functional impact. When the edge rate (rise time and fall time) becomes relatively fast, enough RF energy is generated to affect the normal operation of other electronic components.
3.1 Causes of electromagnetic interference on PCB
Improper practices often cause out-of-specification EMI on the PCB. Combined with the characteristics of high-frequency signals, the main aspects related to PCB-level EMI include the following aspects:
(1) Improper use of packaging measures. For example, devices that should be packaged in metal are packaged in plastic.
(2) The PCB design is poor, the completion quality is not high, and the grounding of the cable and connector is poor.
(3) Improper or even wrong PCB layout.
Including improper setting of clock and periodic signal traces; improper hierarchical arrangement of PCBs and signal wiring layers; improper selection of components with high-frequency RF energy distribution; insufficient consideration of common mode and differential mode filtering; ground loops causing RF And ground bounce; insufficient bypass and decoupling, etc.
To achieve system-level EMI suppression, some appropriate methods are usually needed: this mainly includes shielding, padding, grounding, filtering, decoupling, proper wiring, circuit impedance control, and so on.
3.2 Electromagnetic compatibility shielding design
Today's electronics industry is paying more and more attention to the need for SE / EMC (Shielding Effectiveness, SE), and with the use of more electronic components, electromagnetic compatibility has become more concerned. Electromagnetic shielding is based on the principle of metal isolation to control electromagnetic interference from one area to another and induce and radiate electricity. Usually includes two types: one is electrostatic shielding, which is mainly used to prevent the influence of electrostatic field and constant magnetic field; the other is electromagnetic shielding, which is mainly used to prevent the influence of alternating electric field, alternating magnetic field and alternating electromagnetic field.
EMI shielding can make the product simple and effective in compliance with EMC standards. When the frequency is below 10MHz, most of the electromagnetic waves are conducted, while the higher frequency electromagnetic waves are mostly radiated. New designs such as single layer solid shielding material, multilayer solid shielding material, double shielding or more than double shielding can be used for EMI shielding. For low-frequency electromagnetic interference, a thick shielding layer is required. The most suitable is to use a material with high permeability or magnetic material, such as nickel-copper alloy, to obtain the maximum electromagnetic absorption loss. For high-frequency electromagnetic waves, metal shielding material.
In actual EMI shielding, the effectiveness of electromagnetic shielding depends largely on the physical structure of the chassis, that is, the continuity of conduction. The seams and openings on the chassis are sources of electromagnetic wave leakage. In addition, the cable passing through the chassis is also the main cause of the shielding effectiveness. The electromagnetic leakage of the opening on the chassis is related to the shape of the opening, the characteristics of the radiation source, and the distance from the radiation source to the opening. The shielding effectiveness can be improved by appropriately designing the opening size and the distance from the radiation source to the opening. Generally, the way to solve the electromagnetic leakage of the slot of the case is to use an electromagnetic sealing gasket in the slot. Electromagnetic sealing gasket is a conductive elastic material, which can maintain the continuity of conduction in the gap. Common electromagnetic sealing gaskets are: conductive rubber (doped with conductive particles in rubber, so that this composite material has both rubber elasticity and metal conductivity.), Double conductive rubber (it is not mixed in all parts of rubber Incorporating conductive particles, the benefits obtained in this way are both to maintain the elasticity of the rubber to the greatest extent and to ensure electrical conductivity), metal braided mesh sleeves (rubber-based metal braided mesh sleeves), spiral tube liners (using stainless steel, Spiral tube made of beryllium copper or tin-plated beryllium copper). In addition, when the ventilation requirements are relatively high, a cut-off waveguide ventilation board must be used. This board is equivalent to a high-pass filter and does not attenuate electromagnetic waves higher than a certain frequency, but for electromagnetic waves lower than this frequency, For a large attenuation, reasonable application of this characteristic up to the waveguide can well shield EMI interference.
3.3 Reasonable PCB Design for EMC
With the large-scale increase in the complexity and integration of system design, electronic system designers are engaged in circuit design above 100MHZ, and the operating frequency of the bus has also reached or exceeded 50MHZ, and some even exceeded 100MHZ. When the system is operating at 50MHz, transmission line effects and signal integrity issues will occur; and when the system clock reaches 120MHz, PCBs based on traditional methods will not work unless knowledge of high-speed circuit design is used. Therefore, high-speed circuit design technology has become a design method that electronic system designers must adopt. The controllability of the design process can only be achieved by using the design techniques of high-speed circuit designers.
It is generally believed that if the frequency of a digital logic circuit reaches or exceeds 45MHZ ~ 50MHZ, and the circuit working above this frequency has already occupied a certain amount of the entire electronic system (such as 1/3), it is called a high-speed circuit. In fact, the harmonic frequency of the signal edge is higher than the frequency of the signal itself, which is an unexpected result of signal transmission caused by the rapidly changing rising and falling edges (or signal transitions) of the signal. To achieve EMC-compliant high-frequency PCB design, the following technologies are usually required: including bypass and decoupling, ground control, transmission line control, and wiring termination matching.
(1) Bypass and decoupling
Decoupling refers to removing RF energy that enters the distribution network from high-frequency devices during device switching, while bypassing is the transfer of unwanted common-mode RF energy from components or cables.
All capacitors are composed of LCR circuits, where L is the inductance, which is related to the length of the wire, R is the resistance in the wire, and C is the capacitance. At a certain frequency, the LC series combination will produce resonance. In the resonant state, the LCR circuit will have very small impedance and effective RF bypass. When the frequency is higher than the self-resonance of the capacitor, the capacitor becomes inductive impedance, and the effect of bypassing or decoupling decreases. Therefore, the effect of capacitors for bypassing and decoupling is affected by the length of the leads, the traces between the capacitor and the device, and the dielectric filler. The ideal decoupling capacitor can also provide all the current required for logic device state switching. In fact, the impedance between the power supply and the ground layer determines how much current the capacitor can provide.
When the bypass and decoupling capacitors are selected, the self-resonant frequency of the required capacitor can be calculated by the logic series and the clock speed used, and the capacitance value is selected according to the frequency and the capacitive reactance in the circuit. When choosing the package scale, try to choose a capacitor with a lower lead inductance as much as possible. This usually manifests itself as an SMT (Surface Mount Technology) capacitor instead of a through-hole capacitor (such as a DIP packaged capacitor). In addition, in product design, parallel decoupling capacitors are often used to provide a larger operating frequency band and reduce ground imbalance. In a parallel capacitor system, when the frequency is higher than the self-resonant frequency, a large capacitor exhibits an inductive impedance and increases with increasing frequency; a small capacitor exhibits a capacitive impedance and decreases with increasing frequency. The impedance is smaller than that of a single capacitor.