Electromagnetic compatibility design and circuit ESD protection of electronic equipment need to consider what issues
The size of portable electronic devices is becoming smaller and smaller, and more and more new functions or new features are continuously integrated into the device, making the data rate and clock frequency of portable devices higher and higher. At the same time, portable devices are bound to face many potential sources of electromagnetic interference (EMI) / radio frequency interference (RFI), such as switching loads, supply voltage fluctuations, short circuits, lightning, switching power supplies, RF amplifiers and power amplifiers, and clocks. High frequency noise of the signal, etc. Therefore, the technical level of circuit design and electromagnetic compatibility (EMC) design will play a key role in product quality and technical performance indicators.
Electromagnetic interference usually has two situations, namely conducted and radiated interference. Conducted interference refers to the coupling (interference) of signals on one electrical network to another electrical network through a conductive medium. Radiated interference refers to an interference source that couples (interferes) its signal to another electrical network through space. Therefore, the research on EMC problem is actually the study of the relationship between interference source, coupling path and sensitive equipment. Electromagnetic compatibility design is optimized for electromagnetic interference generated in electronic products, making it a product that meets electromagnetic compatibility standards.
As long as an electric field or a magnetic field exists in the electronic circuit, electromagnetic interference occurs. In high-speed PCB and system design, high-frequency signal lines, integrated circuit pins, various types of connectors, etc. may become radiation interference sources with antenna characteristics, can emit electromagnetic waves and affect other systems or other subsystems in the system. normal work.
Electromagnetic compatibility design considerations
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. Switching power supply circuits and clock circuits in digital circuits are the most important sources of electromagnetic interference in electronic products. 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 mutual signal coupling. In terms of device layout, follow the principle that the interconnected devices are as close as possible, so that better noise immunity can be obtained. In addition, the design of power and ground lines in printed circuit boards is an important means of overcoming electromagnetic interference.
Due to insufficient consideration of electromagnetic compatibility, some electronic products have not met the requirements of electromagnetic compatibility standards. Redesign will greatly delay the time-to-market of products. 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 ways and ways of interference sources and mutual interference, and carry out targeted rectification based on the analysis results. Such as: weakening the allowable range of interference sources; sorting and arranging wires and cables to reduce the coupling between lines; improving the grounding system; selecting high-conductivity materials and ferromagnetic materials to achieve electromagnetic shielding. When these measures are not effective in improving the electromagnetic compatibility of the product, changing the wiring structure of the board is the fundamental solution to the problem.
ESD protection of the circuit
Electrostatic discharge (ESD) is a must-have for engineers engaged in hardware design and production. Many developers often encounter situations where the products developed in the lab pass the test completely, but after a period of time, the customer will be anomalous and the failure rate is not very high. In general, most of these problems are caused by surges, ESD strikes, and the like. In the assembly and manufacturing process of electronic products, more than 25% of semiconductor chip damage is attributed to ESD. With the widespread use of microelectronics technology and the increasing complexity of the 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 design engineers typically add protection through a number of transient voltage suppressor (TVS) devices. Such as solid devices (diodes), metal oxide varistors (MOVs), thyristors, other variable voltage materials (new polymer devices), gas tubes and simple spark gaps. With the advent 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 increasing. For example, TVS must respond quickly to incoming surge voltages. When the surge voltage reaches 8KV (or higher) peak at 0.7ns, the trigger or regulation voltage of the TVS device (parallel to the input line) must be low enough to be effective. Voltage distributor. ON Semiconductor's NUC2401 is a common-mode filter with integrated low-capacitance ESD protection that provides the necessary bandwidth for high-speed USB 2.0 signals, proper common-mode attenuation, and sensitive internal circuit ESD protection to maintain signal integrity. . Vishay's VBUS054B-HS3 is a single-chip ESD solution with very small differences between line capacitances to protect dual high-speed USB ports from transient voltage signals. It is also possible to clamp a negative transient that is slightly below ground level while clamping the positive transient at a voltage range slightly above 5V.
Today, circuit design engineers are increasingly adopting ESD suppression schemes in high-frequency circuit design. Although low-cost silicon diodes (or varistors) have very low trigger/clamp voltages, their high frequency capacity and leakage current cannot meet the growing demand for applications. The polymer ESD suppressor has an attenuation of less than 0.2 dB at frequencies up to 6 GHz, 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 familiarity with EMC standards, circuit design engineers must also consider the performance of the device itself, parasitic parameters, product performance, cost, and each functional module in the system design. Through layout and routing optimization, add decoupling capacitors and magnetic beads. , magnetic ring, shielding, PCB resonance suppression and other measures to ensure that EMI is within the control range. When developing a circuit protection design, the most important thing is to first grasp the technical solutions and design methods, and select the correct ESD protection device accordingly.