Starting electromagnetic compatibility design from understanding electromagnetic induction

For a good electronic product, in addition to the product's own functions, the technical level of circuit design (ECD) and electromagnetic compatibility design (EMCD) plays a very critical role in the product's quality and technical performance indicators. 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 a "three-step" rule, which provides the necessary guide to completely solve electrostatic discharge interference and improve design efficiency.

Three countermeasures to reduce radiation 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, using 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 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 (Power Trace); general beads can be used to suppress noise signals of a specific frequency; CMF is used to suppress noise radiation problems in 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 by pass to Ground at the I/O end to direct the noise to the ground.

3. Use shielding to enclose electromagnetic waves in a shielding cover. 4. Expand the PCB ground area as much as possible.

5. 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 at the beginning or end of the differential 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.