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Discussion of EMC and ESD problems in high speed PCB design

The portable electronic device is compact and slim size, more and more new features or new features have been integrated into the device, making the data rate and the clock frequency is more and more portable devices. At the same time, portable devices will face many potential electromagnetic interference (EMI) / radio frequency interference (RFI) source of risk, such as high frequency noise, load switch power supply voltage fluctuation, short circuit, lightning, switching power supply, RF amplifier and power amplifier and clock signal etc.. Therefore, the technical level of circuit design and electromagnetic compatibility (EMC) design will play a crucial role in the quality and technical performance of the product.
There are usually two kinds of electromagnetic interference, that is, conduction interference and radiation interference. Conducted interference is the coupling of electrical signals across an electrical network to another electrical network via a conductive medium. Radiated interference is the interference of a source by coupling its signal through space (interference) to another electrical network. Therefore, the research on the EMC problem is actually the study of the relationship between the three sources of interference, coupling and sensitive devices. EMC design is aimed at the electromagnetic interference generated in electronic products to optimize the design, so as to meet the electromagnetic compatibility standards of products.
In electronic circuits, as long as there is an electric field or a magnetic field, electromagnetic interference will occur. In the high-speed PCB design, high frequency signal line, integrated circuit pin, various types of connectors are likely to become radiation sources of interference with the antenna characteristics, can emit electromagnetic waves and affect the normal work of other systems or other subsystems within the system.
EMC design considerations
In order to save energy and improve work efficiency, most of today's electronic products are powered by switching power supply. At the same time, more and more products also contain digital circuits, in order to provide more application functions. The clock circuit of switching power supply circuit and digital circuit is the most important electromagnetic interference source in the electronic products at present. They are the main contents of EMC design.
In the design, the analog signal part, the high speed digital circuit part and the noise source DC-DC power supply three parts should be reasonably separated, so that the signal coupling between each other can be minimized. In the aspect of device layout, the principle of the closest connected devices can be observed, so that better noise immunity can be achieved. In addition, the design of power line and ground wire in printed circuit board is an important means to overcome electromagnetic interference.
Some electronic products due to the lack of consideration of electromagnetic compatibility, the products do not meet the standard of electromagnetic compatibility requirements, time to market, re design greatly delayed products so the change of electromagnetic compatibility is important. First of all, according to the actual situation of the product diagnosis, find the interference source and mutual interference ways and methods, according to the analysis results, targeted rectification. Such as: the interference source within the allowable range of weakening; classification arrangement of wires and cables to reduce line to line coupling; to improve the earth system; choose high conductivity materials and ferromagnetic materials, electromagnetic shielding. When these measures can not effectively improve the electromagnetic compatibility of products, changing the wiring structure of the circuit board is the fundamental solution to the problem.
ESD protection of circuit
Electrostatic discharge (ESD) is the knowledge that must be mastered by engineers involved in hardware design and production. Many developers often encounter this situation: the products developed in the laboratory, the test passed completely, but customers use a period of time, there will be abnormal phenomenon, the failure rate is not very high. Under normal circumstances, these problems are mostly caused by surge impact, ESD shock and so on. In the assembly and manufacturing of electronic products, more than 25% of the damage to semiconductor chips is attributed to ESD. With the wide application of microelectronic technology and the increasingly complicated electromagnetic environment, more and more attention has been paid to the electromagnetic field effects such as electromagnetic interference (EMI) and electromagnetic compatibility (EMC) in electrostatic discharge.
Circuit design engineers generally increase protection by means of a certain number of Transient Voltage Suppressors (TVS) devices. Such as solid devices (diodes), metal oxide varistors (MOV), 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 several kHz to GHz, and the requirements for high capacity passive devices for ESD protection are also getting higher and higher. For example, TVS must respond quickly to the arrival of the surge voltage, when the surge voltage reaches 8KV at 0.7ns (or higher) peak, TVS devices trigger or adjust the voltage (in parallel with the input line) must be low enough to serve as an effective voltage divider. A Morimi NUC2401 is a semiconductor with common mode filter integrated low capacitance ESD protection function, can provide high speed USB 2 signal necessary bandwidth, appropriate mode attenuation and sensitive ESD internal circuit protection, maintain the integrity of the signal. Vishay VBUS054B-HS3 is a single chip ESD solution with very little difference between line capacitors. It protects double high-speed USB ports in case of transient voltage signals. It can also clamp the negative transients slightly below the ground level and clamp the positive transients at slightly higher than the operating voltage range of 5V.
Now, circuit designers are increasingly adopting ESD suppression schemes in high frequency circuit design. Although the trigger / clamp voltage of a low-cost silicon diode (or varistor) is very low, its high frequency capacity and leakage current can not meet the growing demand. The decay of polymer ESD suppressors at frequencies up to 6GHz