EMC/EMI solutions and design experience in designing magnetic protection circuits

With the development of electrical and electronic technology, household appliances are becoming more and more popular and electronic, radio and television, post and telecommunications, computers and their networks are becoming more and more developed, and the electromagnetic environment is becoming more and more complex and deteriorating. We are gradually paying attention to the working environment of the equipment and the impact of the electromagnetic environment on electronic equipment. The electromagnetic interference ( EMI ) and electromagnetic compatibility ( EMC ) of electrical and electronic products are receiving more and more attention from engineers and manufacturers.

It is important to find the object to be protected for electrostatic protection (ESD)

When designing electromagnetic protection circuits, engineers must clearly know what to protect in the system. It is important to find the object to be protected. How to find out which are the core and which are easily interfered among 10,000 devices? When the circuit to be protected is found, it is necessary to start electrostatic analysis. What kind of static electricity makes it ineffective? What is the reason? After analyzing various reasons, it is necessary to take electrostatic protection measures and select corresponding devices.

Conductive ESD protection: To protect the electrostatic current in the circuit, some protection devices are mainly used to form a protection circuit in front of the sensitive device to guide or dissipate the current. Such protection devices include: ceramic capacitors, varistors, TVS tubes, etc.

Radiative ESD protection: For the impact of static electricity on sensitive circuits, the protection method is mainly to minimize the generation and energy of the field, increase the protection capability through structural improvement, and protect sensitive circuits. The protection of the field is usually difficult, and a method called equipotential body has been explored in the practice of improvement. Through effective connection, the shell forms a body with the same potential, which suppresses discharge. It has been proven that this method is effective and easy to implement.

General methods for protecting against static electricity (the first three are for direct discharge, and the last two are for coupling of associated fields)

Reduce the accumulation of static electricity;

Insulate the product to prevent static electricity;

Provide branch circuits to divert static current to sensitive circuits;

Shielding the circuit in the discharge area;

Reduce loop area to protect circuits from magnetic fields generated by electrostatic discharge.

Starting electromagnetic compatibility design from understanding electromagnetic induction

For a good electronic product, in addition to the functions of the product itself, the technical level of circuit design (ECD) and electromagnetic compatibility design (EMCD) plays a very critical role in the quality and technical performance indicators of the product. When many people engage in electronic circuit design, they start by understanding electronic components, but they have no idea where to start when engaging in electromagnetic compatibility design. In fact, engaging in electromagnetic compatibility design starts with electromagnetic field theory, that is, from the understanding of electromagnetic induction.

Just imagine that when multiple electronic devices are working in the same space, an electromagnetic field of a certain intensity will be generated around them. Under the influence of the field or human factors, various interferences will interfere with the equipment through conduction, radiation and other means, making the system unstable or even freezing - the culprit is electromagnetic interference.

Electromagnetic interference is common in electronic products. It not only affects each other between devices, but also exists between components and between systems. The two main ways are conducted interference and radiated interference, and conducted interference is further divided into common mode interference and differential mode interference. The causes of interference are complex, and the core is electrostatic discharge interference. How to ensure the stable operation of the system without being affected by the outside world? The following is the "three-step" rule compiled by the Electronic Components Technology Network for you, which provides the necessary guide to completely solve electrostatic discharge interference and improve design efficiency. Three major countermeasures to reduce radiated interference

1. One is shielding, and the other is to reduce the area of ​​each current loop (magnetic field interference) and the area and length of the charged conductor (electric field interference).

2. When the length of the current carrier is exactly equal to an integer multiple of one-quarter wavelength of the interference signal, the interference signal will resonate in the circuit. At this time, the radiation interference is the strongest. This situation should be avoided as much as possible.

3. Magnetic field radiation interference is mainly caused by the magnetic flux generated by the high-frequency current loop flowing into the receiving loop. Therefore, the area of ​​the high-frequency current loop and the area of ​​the receiving loop should be minimized.

Common EMI Suppression Methods

Currently, common methods for suppressing EMI include shielding, spread spectrum method, use of filters, etc., as well as prevention and control through integrated grounding, wiring, overlapping and other aspects.

Electromagnetic shielding methods are mostly used to shield electromagnetic noise above 300MHz. In addition, the use of shielding composite materials is also a common method. For example, mobile phones often use vacuum plating to cover the inside of the plastic shell with a layer of shielding material such as nickel to isolate the emission of electromagnetic waves.

The spread spectrum method is used to spread the clock signal to reduce the peak signal waveform amplitude to reduce the peak level of the signal. Currently, some BIOS have provided built-in spread spectrum functions that can be set by users. Yu Xiaoqi pointed out that the use of spread spectrum method requires a balance between signal distortion and EMI reduction, generally 1% to 1.5%. If it exceeds 3%, the signal will usually be too distorted and unfeasible. The use of filters or filter circuits is most commonly used by general design engineers because of their low cost and SMD (surface mount) process processing requirements. The use opportunities and modes of filters are determined according to different prevention and control needs. For example, high-current beads can be used on the power circuit path (PowerTrace); general beads can be used to suppress noise signals of a specific frequency; CMF is used to suppress noise radiation problems of differential mode lines such as USB, 1394, and LVDS.

However, there are many solutions to EMI suppression, and they must be selected according to the time and place. As long as they are effective, they are good prevention methods. There is no specific method that is particularly superior.

The solutions to the thorny problem of radiated and conducted EMI are summarized as follows:

1. Add an LC filter circuit to the interference source.

2. Add DeCap bypass to Ground at the I/O end to direct the noise to the ground.

3. Use shielding to enclose the electromagnetic waves in a shielding cover.

4. Try to expand the PCB ground area as much as possible.

5. Try to use as few flat cables or physical wires as possible inside the product.

6. The physical wires inside the product should be made into twisted wires as much as possible to suppress noise radiation, and DeCap should be added to the I/O end of the flat cable.

7. Add a common mode filter (Common Mode Filter) at the beginning or end of the differential mode signal line.

8. Follow certain analog and digital wiring principles.

In addition, the formation of EMI can be divided into two categories: common mode radiation and differential mode radiation. Common mode radiation includes common mode interference of common ground impedance and common mode interference of electromagnetic field on wires. The former is the common mode interference caused by the noise source and the victim circuit sharing the same ground resistance. The solution can be to cut the ground to avoid the common ground interference problem; the latter is the interference caused by the electromagnetic field formed by high electromagnetic energy on the wiring between devices. The interference problem of field on the line can be dealt with by the response method of shielding and isolation.

As for differential mode radiation, the most common is differential mode interference between wires. The interference path is that the interference noise in a certain wire infects other wires and feeds into the victim circuit. It is a kind of near-field interference. This type of interference problem can be handled by widening the distance between wires.

Four solutions to improve EMC of mobile phone sensitivity

Electronic products are now developing towards high performance, which has led to very complex changes in electronic devices. For example, the IC operating voltage of mobile phones is getting lower and lower, and the energy is getting smaller and smaller. Smart phones, including PCs, and wireless circuits will have related interference. The third is that the speed of interface communication is getting higher and higher. The fourth is that there are more and more electronic controls in automotive applications. All these have caused our electronic version to become more and more complex, which has made our methods more and more important. We believe that this change itself will generate more and more demands after continuous changes.

Electronics product market, safety regulations and testing trends

In fact, IEC divides electromagnetic compatibility standards into three categories: basic standards, general standards and product standards. Basic standards are divided into emission standards and immunity standards. General standards divide the environment into two categories: A and B. Category A belongs to industrial areas, and category B is for civilian use. Therefore, we should be clear about which indicator you really want to achieve. For a certain product, if there is neither a product EMC standard nor an applicable product-class EMC standard, the general EMC standard should be adopted. A product standard is adopted first, and then the product-class standard is approached for assessment. If we do not have an EMC standard for a certain product and do not have an appropriate product-class standard, we will use the general standard.