Analysis on Electromagnetic Compatibility and Circuit Protection Technology

　　Portable devices face many potential electromagnetic interference (EMI)/radio frequency interference (RFI) sources, such as switching loads, power supply voltage fluctuations, short circuits, lightning, switching power supplies, RF amplifiers and power amplifiers, and high-frequency noise of clock signals. Therefore, the technical level of circuit design and electromagnetic compatibility (EMC) design will play a very critical role in the quality and technical performance indicators of the product.

　　There are usually two types of electromagnetic interference, namely conducted interference and radiated interference. Conducted interference refers to the coupling (interference) of a signal on an electrical network to another electrical network through a conductive medium. Radiated interference refers to the coupling (interference) of an interference source to another electrical network through space. Therefore, the study of EMC issues is actually the study of the relationship between interference sources, coupling paths, and sensitive devices. Electromagnetic compatibility design is to optimize the design of electromagnetic interference generated in electronic products to make them products that meet electromagnetic compatibility standards.

　　As long as there is an electric field or magnetic field in an electronic circuit, electromagnetic interference will be generated. In high-speed PCB and system design, high-frequency signal lines, integrated circuit pins, various connectors, etc. may become radiation interference sources with antenna characteristics, which can emit electromagnetic waves and affect the normal operation of other systems or other subsystems within the system.

　　In order to save energy and improve work efficiency, most electronic products are currently powered by switching power supplies. At the same time, more and more products also contain digital circuits to provide more application functions. The switching power supply circuit and the clock circuit in the digital circuit are the main sources of electromagnetic interference in electronic products at present, and they are the main content of electromagnetic compatibility design. In the design, the analog signal part, the high-speed digital circuit part and the noise source DC-DC power supply need to be reasonably separated to minimize the signal coupling between them. In terms of device layout, the principle of keeping related devices as close as possible can be followed to obtain better anti-noise effect. In addition, the design of power lines and ground lines in printed circuit boards is an important means to overcome electromagnetic interference.

　　Some electronic products do not meet the requirements of electromagnetic compatibility standards due to insufficient consideration of electromagnetic compatibility. Redesign will greatly delay the time to market of the product. Therefore, changes in electromagnetic compatibility are more important. First of all, it is necessary to diagnose the product according to the actual situation, find out the interference source and the ways and means of mutual interference, and make targeted rectifications based on the analysis results. For example: weaken the interference source within the allowable range; sort out wires and cables to reduce coupling between wires; improve the grounding system; select highly conductive materials and ferromagnetic materials to achieve electromagnetic shielding, etc. When these measures cannot effectively improve the electromagnetic compatibility performance of the product, changing the wiring structure of the circuit board is the fundamental way to solve the problem.

　　ESD protection of circuits Electrostatic discharge (ESD) is knowledge that engineers engaged in hardware design and production must master. Many developers often encounter such a situation: the product developed in the laboratory has passed the test completely, but after the customer uses it for a period of time, abnormal phenomena will occur, and the failure rate is not very high. In general, these problems are mostly caused by surge shock, ESD shock and other reasons. In the assembly and manufacturing process of electronic products, more than 25% of the damage to semiconductor chips is attributed to ESD. With the widespread application of microelectronics technology and the increasingly complex electromagnetic environment, people are paying more and more attention to the electromagnetic field effects of electrostatic discharge, such as electromagnetic interference (EMI) and electromagnetic compatibility (EMC).

　　Circuit designers generally add protection through a number of transient voltage suppressor (TVS) devices. Such as solid devices (diodes), metal oxide varistors (MOVs), thyristor rectifiers, other variable voltage materials (new polymer devices), gas tubes and simple spark gaps. With the emergence of a new generation of high-speed circuits, the operating frequency of devices has risen from a few kHz to GHz, and the requirements for high-capacity passive devices for ESD protection are also increasing. For example, TVS must respond quickly to the incoming surge voltage. When the surge voltage reaches 8KV (or higher) peak at 0.7ns, the trigger or adjustment voltage of the TVS device (in parallel with the input line) must be low enough to act as an effective voltage divider. ON Semiconductor's NUC2401 is a common-mode filter with integrated low-capacitance ESD protection that can provide the necessary bandwidth, appropriate common-mode attenuation and sensitive internal circuit ESD protection for high-speed USB 2.0 signals, maintaining signal integrity. Vishay's VBUS054B-HS3 is a single-chip ESD solution that protects dual high-speed USB ports from transient voltage signals with very small differences between line capacitances. It can also clamp negative transients slightly below the ground level, while clamping positive transients slightly above the 5V operating voltage range. Today, circuit designers are increasingly using ESD suppression solutions in high-frequency circuit designs. Although low-cost silicon diodes (or varistors) have very low trigger/clamping voltages, their high-frequency capacitance and leakage current cannot meet the growing application needs. Polymer ESD suppressors have an attenuation of less than 0.2dB at frequencies up to 6GHz, and the impact on the circuit is almost negligible.

　　Electromagnetic compatibility and circuit protection are unavoidable issues for the design of all electronic products. In addition to being familiar with relevant standards for electromagnetic compatibility, circuit design engineers must also comprehensively consider the performance of the device itself, parasitic parameters, product performance, cost, and each functional module in the system design, and ensure that EMI is within the control range through layout and wiring optimization, adding decoupling capacitors, magnetic beads, magnetic rings, shielding, PCB resonance suppression and other measures. When formulating a circuit protection design plan, the most important thing is to first master the corresponding technical solutions and design methods, and then choose the correct ESD protection device based on this.

　　Summarize:

　　Electromagnetic compatibility (EMC) includes two requirements: on the one hand, the electromagnetic interference generated by the equipment in the normal operation process cannot exceed a certain limit; on the other hand, it means that the equipment has a certain degree of immunity to the electromagnetic interference in the environment, that is, electromagnetic sensitivity. With the enthusiastic research of engineers, the problem has been gradually overcome.