Exploration of electromagnetic compatibility and circuit protection technology

Portable electronic devices are becoming smaller and thinner, and more and more new functions or features are being integrated into the devices, making the data rate and clock frequency of portable devices higher and higher. At the same time, portable devices will inevitably 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 products.

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 the 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 in the system.

Electromagnetic compatibility design considerations In order to save energy and improve work efficiency, most electronic products currently use switching power supplies. At the same time, more and more products also contain digital circuits to provide more application functions. Switching power supply circuits and clock circuits in digital circuits 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.

Due to insufficient consideration of electromagnetic compatibility, some electronic products do not meet the requirements of electromagnetic compatibility standards. Redesign will greatly delay the time to market of the product. Therefore, changes in electromagnetic compatibility are more important. First of all, we need 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: reduce the interference source within the allowable range; sort and organize wires and cables to reduce line coupling; 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 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 a 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 pay more and more attention to the electromagnetic field effects of electrostatic discharge, such as electromagnetic interference (EMI) and electromagnetic compatibility (EMC).

Circuit design engineers generally increase protection through a certain number of transient voltage suppressors (TVS) devices. Such as solid devices (diodes), metal oxide varistors (MOVs), thyristor rectifiers, other variable voltage materials (new polymer devices), gas electron 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 getting higher and higher. For example, TVS must respond quickly to the incoming surge voltage. When the surge voltage reaches a peak of 8KV (or higher) in 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 distributor. 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 with very small differences between line capacitances, which can protect dual high-speed USB ports from transient voltage signals. It can also clamp negative transients slightly below the ground level, and clamp positive transients slightly above the 5V operating voltage range. Nowadays, circuit design engineers are increasingly using ESD suppression solutions in high-frequency circuit design. Although the trigger/clamping voltage of low-cost silicon diodes (or varistors) is very low, their high-frequency capacity and leakage current cannot meet the growing application needs. The attenuation of polymer ESD suppressors at frequencies up to 6GHz is less than 0.2dB, 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 the 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 in the design, and ensure that EMI is within the control range through layout 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 select the correct ESD protection device based on this.

Summary:

Solving the problem of circuit protection and electromagnetic compatibility is a topic that engineers are keen to solve and discuss. Electromagnetic compatibility (EMC) includes two requirements: on the one hand, it means that the electromagnetic interference generated by the equipment to the environment during normal operation 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 gradually been overcome.