PCB For Electromagnetic Compatibility and Interference Design

1. The general concept of electromagnetic compatibility

The root cause for consideration is the presence of electromagnetic interference. Electromagnetic interference (ElectromagneTIc Interference, EMI for short) is the process in which destructive electromagnetic energy is transmitted from one electronic device to another electronic device through radiation or conduction. In general, EMI refers specifically to radio frequency signals (RF), but electromagnetic interference can occur in all frequency ranges.

Electromagnetic Compatibility (ElectromagneTIc CompaTIbility, referred to as EMC) refers to electrical and electronic systems, equipment and devices operating at the designed level or performance within the specified safety limits in a set electromagnetic environment without damage due to electromagnetic interference or unacceptable performance degradation capability. The electromagnetic environment mentioned here refers to the sum of all electromagnetic phenomena existing in a given place. This shows that electromagnetic compatibility means that electronic products should have the ability to suppress external electromagnetic interference on the one hand; 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 electronic products have changed from analog design to digital design. With the development of digital logic devices, issues related to EMI and EMC have begun to become the focus of products, and have received great attention from designers and users. The United States Communications Commission (FCC) announced the radiation standards for personal computers and similar equipment in the mid-to-late 1970s, and the European Community proposed mandatory requirements for radiation and anti-interference in its 89/336/EEC electromagnetic compatibility guidance document.

my country has also successively formulated national standards and national military standards on electromagnetic compatibility, such as "Terminology for Electromagnetic Compatibility" (GB/T4365-1995), "Terms for Electromagnetic Interference and Electromagnetic Compatibility" (GJB72-85), "Radio Interference and Anti- Specifications for Interference Measuring Equipment" (GB/T6113-1995), "Measurement Methods and Allowable Values of Radio Interference Characteristics of Electric 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 is getting higher and higher, and the ability to receive weak signals is getting stronger and stronger, the frequency band of electronic products is getting wider and wider, and the size is getting smaller and smaller, and the anti-interference ability of electronic equipment is required to be stronger and stronger. The electromagnetic waves generated by some electrical and electronic equipment are likely to form electromagnetic interference to other electrical and electronic equipment around them, causing malfunctions or affecting signal transmission. In addition, excessive electromagnetic interference will form electromagnetic pollution, endanger people's health and destroy the ecological environment.

If all kinds of electrical equipment in a system can work normally without mutual electromagnetic interference causing performance changes and equipment damage, it is said that the electrical equipment in this system is compatible with each other. However, with the diversification of equipment functions, the complexity of structure, the increase of power and frequency, and their sensitivity is getting higher and higher, it is becoming more and more difficult to obtain this state of mutual compatibility. 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 device does not produce electromagnetic emissions exceeding a certain limit, and at the same time it is required to have a certain anti-interference ability. Only by restricting and improving each device in these two aspects can the system be fully compatible.

It is generally believed that there are two ways of transmission of electromagnetic interference: one is conduction; the other is radiation. In actual engineering, interference between two devices usually involves coupling in many ways. It is precisely because the coupling of multiple channels exists at the same time, crosses repeatedly, and jointly generates interference, which makes electromagnetic interference difficult to control.

Common electromagnetic interference mainly includes the following types:

2.1. Radio Frequency Interference. Due to the proliferation of existing radio transmitters, radio frequency interference poses a great threat to electronic systems. Cellular telephones, handheld radios, radio remote control units, pagers, and other similar devices are now very common. A high generating power is not required to cause harmful interference. Typical faults occur in the range of radio frequency field strength of 1 to 10V/m. In Europe, North America and many Asian countries, it is legally mandatory for all products to prevent radio frequency interference from damaging other equipment.

2.2. Electrostatic discharge (ESD). Modern chip technology has made great progress, and components have become very dense on a small geometric size (0.18um). These high-speed, multi-million-transistor microprocessors are highly sensitive and easily damaged by external electrostatic discharges. Discharge can be caused by direct or radiative means. Direct contact discharges generally cause permanent damage to equipment. Electrostatic discharge caused by radiation may cause equipment disorder and malfunction.

2.3. Power interference. As more and more electronic devices are connected to the power backbone, there will be some potential disturbances in the system. These disturbances include power line disturbances, electrical fast transients, power surges, voltage variations, lightning transients, and power line harmonics. For high frequency switching power supplies, these disturbances become significant.

2.4. Self-compatibility. A system's digital portion or circuitry can interfere with analog equipment, creating crosstalk between wires, or a motor can cause disturbances in digital circuitry.

In addition, an electronic product that can work normally at low frequencies will encounter some problems that low frequencies do not have when the frequency is increased. Such as reflection, cross-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 meeting the functional requirements of the market, designed products must also adopt appropriate design techniques to prevent or remove the influence of EMI.

3. Design EMC considerations

For the EMI problem in high-speed (Printed Circuit Board, printed circuit board) design, there are usually two ways to solve it: one is to suppress the influence of EMI, and the other is to shield the influence of EMI. There are many different manifestations of these two methods, especially the shielding system minimizes the possibility of EMI affecting electronic products.

Radio frequency (RF) energy is generated by switching currents within a printed circuit board (PCB) as a by-product of digital components. Every logic state change in a power distribution system produces a momentary surge. In most cases, these logic state changes do not generate enough ground noise voltage to cause any functional impact, but when a component's When the edge rate (rise time and fall time) becomes quite fast, enough RF energy is generated to affect the normal operation of other electronic components.

3.1. Causes of electromagnetic interference on PCB

Improper practice can often cause EMI on the PCB that exceeds specifications. Combined with the characteristics of high-frequency signals, the EMI related to PCB level mainly includes the following aspects:

a. Improper use of encapsulation measures. For example, devices that should be packaged in metal are packaged in plastic.

b. Poor PCB design, low quality finish, poor grounding of cables and connectors.

c. Inappropriate or even wrong PCB layout.

Including improper setting of clock and periodic signal routing; improper layering arrangement of PCB and signal wiring layer; improper selection of components with high frequency RF energy distribution; insufficient consideration of common mode and differential mode filtering; ground loops cause RF and ground bounce; insufficient bypassing and decoupling, etc.

To achieve system-level EMI suppression, some appropriate methods are usually required: this mainly includes shielding, gaskets, grounding, filtering, decoupling, proper wiring, circuit impedance control, etc.

3.2. EMC shielding design

Today's electronics industry has paid more and more attention to the needs of SE/EMC (Shielding Effectiveness, SE, isolation room shielding effect), and with the use of more electronic components, electromagnetic compatibility has also received more attention. Electromagnetic shielding is to use the principle of metal isolation to control the electrical method of electromagnetic interference induction and radiation transmission from one area to another. It 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 comply with the EMC specification simply and effectively. When the frequency is below 10MHz, the electromagnetic wave is mostly in the form of conduction, while the electromagnetic wave with higher frequency is mostly in the form of radiation. New materials such as single-layer solid shielding material, multi-layer solid shielding material, double shielding or more than double shielding can be used for EMI shielding during design. For low-frequency electromagnetic interference, a thick shielding layer is required. It is most suitable to use materials with high magnetic permeability or magnetic materials, such as nickel-copper alloys, to obtain the maximum electromagnetic absorption loss, and for high-frequency electromagnetic waves, metal shielding can be used Material.

In actual EMI shielding, the effectiveness of electromagnetic shielding largely depends on the physical structure of the chassis, that is, the continuity of conduction. Seams and openings on the chassis are sources of electromagnetic wave leakage. Moreover, the cable passing through the chassis is also the main reason for the decrease in 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 properly designing the size of the opening and the distance from the radiation source to the opening. Usually, the way to solve the electromagnetic leakage of the chassis gap is to use an electromagnetic sealing gasket at the gap. Electromagnetic sealing gasket is a kind of conductive elastic material, which can maintain the electrical continuity at the gap.

Common electromagnetic sealing gaskets are: conductive rubber (conductive particles are added to the rubber, so that the composite material has both the elasticity of rubber and the conductivity of metal.), double conductive rubber (it is not mixed in all parts of the rubber) Conductive particles are added, so that the benefits obtained are that the elasticity of the rubber is maintained to the greatest extent and the conductivity is guaranteed), metal braided mesh sleeves (metal braided mesh sleeves with rubber as the core), spiral pipe gaskets (made of stainless steel, beryllium copper or tin-plated beryllium copper rolled coil), etc. In addition, when the ventilation rate is relatively high, a cut-off waveguide ventilation plate must be used. This plate is equivalent to a high-pass filter, which does not attenuate and pass through electromagnetic waves higher than a certain frequency, but passes through electromagnetic waves lower than this frequency. A large attenuation is carried out, and the reasonable application of this characteristic of the cut-off waveguide can well shield the EMI interference.

3.3 Reasonable PCB design for electromagnetic compatibility

With the large-scale improvement of system design complexity and integration, electronic system designers are engaged in circuit design above 100MHZ, and the operating frequency of the bus has reached or exceeded 50MHZ, and some even exceeded 100MHZ. When the system works at 50MHz, there will be transmission line effects and signal integrity issues; and when the system clock reaches 120MHz, PCBs designed based on traditional methods will not work unless high-speed circuit design knowledge is used. Therefore, high-speed circuit design technology has become a design method that electronic system designers must adopt. 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 operating above this frequency has already accounted for a certain amount (for example, 1/3) of the entire electronic system, 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, and it is the rapidly changing rising and falling edges of the signal (or signal jumps) that cause unexpected results of signal transmission. To achieve a high-frequency PCB design that meets EMC standards, the following technologies are usually required: including bypass and decoupling, grounding control, transmission line control, and wiring terminal matching.

Decoupling refers to the removal of RF energy from high-frequency devices entering the power distribution network when the devices switch, while bypassing refers to diverting 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 resonate. At resonance, the LCR circuit will have very low impedance and effective RF bypass. When the frequency is higher than the self-resonance of the capacitor, the capacitor gradually becomes an inductive impedance, and the effect of bypassing or decoupling decreases. Therefore, the bypassing and decoupling effects of capacitors are affected by the length of the leads, the wiring between the capacitor and the device, and the dielectric filler. An ideal decoupling capacitor can also supply all the current needed by the logic device to switch states. In fact, the impedance between the power and ground planes determines how much current the capacitor can supply.

When selecting bypass and decoupling capacitors, the self-resonant frequency of the required capacitor can be calculated from the logic family and the clock speed used, and the value of the capacitor should be selected based on the frequency and the capacitive reactance in the circuit. When selecting the package size, try to choose a capacitor with lower lead inductance, which is usually expressed as an SMT (Surface Mount Technology) capacitor instead of a through-hole capacitor (such as a DIP package capacitor). In addition, in product design, parallel decoupling capacitors are often used to provide a larger operating frequency band and reduce grounding imbalance. In a parallel capacitor system, when the frequency is higher than the self-resonant frequency, the large capacitor exhibits inductive impedance and increases with frequency; while the small capacitor exhibits capacitive impedance and decreases with frequency, and at this time the entire capacitor circuit The impedance is less than that of a capacitor alone.