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6 questions and answers about EMC of PCB design.

6 questions and answers about EMC of PCB design.

1. Why do EMC design for products?
Answer: meet the functional requirements of the product, reduce the debugging time, make the product meet the requirements of EMC standards, so that the product will not produce electromagnetic interference to other equipment in the system.

2. How many aspects can EMC design be carried out?
Answer: Circuit Design (including device selection), software design, PCB design, shielding structure, signal line / power line filtering, circuit grounding design.

3. In the field of electromagnetic compatibility, why is it always described in decibels (DB)?
Answer: because the range of amplitude and frequency to be described is very wide, it is easier to use logarithmic coordinates to represent graphs, while DB is represented by logarithm.

4. I don't know much about EMC, but now the speed of data transmission in circuit design is getting faster and faster. When I made PCB, I also encountered some PCB EMC problems, but I thought it was too potential. I want to learn in this area, not to follow the trend. I really feel that EMC is becoming more and more important in circuit design in the future. As I said before, I don't know much about it, and I don't know how to start. I want to ask what basic knowledge is needed and what basic courses should be learned to do well in EMC Cheng. How to learn is a better way. I know that it is not easy to learn any subject well. I have never thought of getting him through in a short time. I just hope to give some suggestions and try to avoid detours.

Answer: About EMC, we need to first understand the EMC standards, such as  GB9254 and simple testing principles. In addition, we need to understand the use of EMI electronic components, such as capacitors, beads, differential mode inductors, common mode inductors, etc. at the PCB level, we need to understand the PCB layout, stack structure, the impact of high-speed wiring on EMC and some rules. Another point is to master some analysis and solutions for EMC problems. These are the basic knowledge that a hardware personnel must master in the future!

5. I am a novice in PCB design. I would like to ask you what kind of knowledge I should master if I want to do a good job in PCB layout? In addition, where can I find the knowledge about safety regulations in PCB design? I hope you can give me your advice!

Answer: for PCB design, you should master the following:
a. Familiar with PCB design software, such as powerpcb / candence, etc;
b. Understand and be familiar with the specific structure of the designed product, as well as the schematic circuit knowledge, including digital and analog knowledge;
c. Master PCB process flow, process and maintainable processing requirements;
d. To master the knowledge of EMI, EMI, and Si;
e. If the related work involves RF, it is necessary to master RF knowledge;
f. For PCB design of safety knowledge mainly depends on gb4943 or UL, general insulation distance requirements can be obtained by looking up the table!

6. Basic principles of EMC design

Answer: electronic circuit design criteria electronic circuit designers often only consider the function of the product, but not the function and electromagnetic compatibility. Therefore, when the product completes its function, it also produces a lot of functional harassment and other disturbances. Moreover, the sensitivity requirements cannot be met. The EMC design of electronic circuit should be considered from the following aspects:
Selection of electronic components in most cases, the degree to which the basic components of the circuit meet the electromagnetic characteristics will determine the degree to which the functional units and the final equipment meet the electromagnetic compatibility. The main criteria for selecting suitable electromagnetic components include out of band characteristics and circuit assembly technology. Because electromagnetic compatibility (EMC) is often determined by the response characteristics of components far away from the fundamental frequency. In many cases, circuit assembly determines the out of band response (such as lead length) and the degree of coupling between different circuit components. The specific rules are as follows:
(1) at high frequency, compared with the lead type capacitor, the core capacitor or support capacitor with small lead inductance should be used to filter.
(2) when lead-in capacitor must be used, the influence of lead inductance on filtering efficiency should be considered.
(3) aluminum electrolytic capacitor may have transient dielectric breakdown for several microseconds, so solid capacitor should be used in the circuit with large ripple or transient voltage.
(4) use parasitic inductance and resistor with small capacitance. Sheet resistor can be used in UHF band.
(5) large inductance has large parasitic capacitance. In order to improve the insertion loss of low frequency part, single section filter should not be used, but multi section filter composed of several small inductors should be used.
(6) when using the core inductor, we should pay attention to the saturation characteristic, especially the high-level pulse will reduce the inductance of the core inductor and the insertion loss in the filter circuit.
(7) use shielded relay as far as possible and make the shielding case grounded.
(8) select the input transformer with effective shielding and isolation.
(9) the power transformer used for sensitive circuit should have electrostatic shielding, and the shielding shell and transformer shell should be grounded.
(10) shielded wires must be used for interconnection signal lines inside the equipment to prevent disturbance coupling between them. (11) in order to connect each shielding body with its own pin, the plug socket with enough pins shall be selected.


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