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Discussion on Power and Ground in Electromagnetic Compatibility of PCB

The improving of electronic products is closely related with the progress of electronic technology. With high-speed development of electronic technology, electronic products have developed towards miniature and density that brings about much interference to PCB electromagnetic compatibility (EMC) design in which power and ground are the most essential part. Therefore, confronted with the development of electronic products and interference of electromagnetic design, optimization should be implemented on EMC design based on the certainty of EMC interference.

Analysis on Interference of Power and Ground in Electromagnetic Compatibility

Power circuit is the media connecting electronic circuit and power grid while noise is the main reason for the interfering electromagnetic compatibility design. With the development of PCB design, voltage in electromagnetic compatibility design is also the main element leading to the instability of circuit. The interference is mainly indicated as the following aspects. First, the application of electronic components in electronic products brings about convenience to the utilization of electronic products and requires higher commands for the internal design of electronic products. Optimization is required if electronic product technology upgrading speed fails to be compatible with electromagnetic compatibility design. At this time, once the logic chips of electronic products such as DPS chip and CPU suffer from interference, the performance of electronic products will also decrease. Electromagnetic interference in PCB electromagnetic compatibility design is caused by the resistance generated by power lines and ground lines. As a result, confronted with the situation of bad electromagnetic compatibility, compatibility design of ground lines and power lines should be analyzed and optimized so that electromagnetic performance will be improved. Meanwhile, for high-speed circuits that have high current speed, they have special PCB design and fast changing current should be aligned with electromagnetic compatibility design. Moreover, when multiple circuits simultaneously apply the same power line, great interference and burden will also take place to circuits. Circuit signals are influenced as well with some limitation, too. The mutual application between circuits will lead to the generation of public impedance interference. Meanwhile, public impedance interference has more obvious effect than single line interference.

Processing Strategies of Electromagnetic Compatibility Design

• Electromagnetic compatibility design and processing of power line

As an essential part of PCB electromagnetic compatibility design, electromagnetic design and processing of power line plays a fundamental role in stabilizing PCB circuits, covering the following aspects:

1). Set and adjust width of power line according to the intensity of the current passing through PCB and the scientific setting of power line width is capable of greatly reducing the current resistance in the process of loop operation.

2). Pay much attention to routing direction of power line and ground line. Generally speaking, the routing direction of power line and bottom line should be compatible with the flow direction of current. Nevertheless, in terms of PCB electromagnetic compatibility design, the routing direction of power line and bottom line should be compatible with the flow direction of data because the noise problem will be solved in this process.

3). Set length of pins reasonably. Mounting components application is a significant step to increase the suitability of pins. When applying mounting components, it's necessary to decrease the loop area supplied by capacitance and mounting components are capable of reducing the bad influence of distributed capacitance of components. During the procedure of electromagnetic compatibility design, the influence of component distributed capacitance is a key element leading to the generation of noise. The reason why the balance of component distributed inductance just lies in the shrinking of pin length.

• Electromagnetic compatibility design and processing of ground line

EMC design and processing of ground line is mainly to decrease the interference of ground loop and eliminate the bad influence of noise on PCB electromagnetic compatibility, which can be implemented from the following aspects:

1). The formation of loop current is the key cause of ground loop interference. However, to practically reduce the formation of loop current, the first job is to design ground line in terms of its electromagnetic compatibility. Specifically, the application of isolator and common mode choke is right the essential measure to decrease loop current. When loop current is being formed, public impedance is the major element producing effect. In order to avoid the conflict between loop current and loop ground line design, a layer of thick ground lines are required to be paved adjacent to ground loop to stop the formation of loop current that causes noise interference. Moreover, the accuracy of extreme position should be ensured. For ground line plane in a multi-layer PCB, specific setting has to be carried out. Meanwhile, in the process of PCB EMC design, adjusting the assembling of shifter is actually an important measure to adjust noise interference, which means that adjustment on shifter is capable of reducing noise when noise interference goes beyond certain limit.

2). The resistance of public part is the main element leading to EMC design interference. Nevertheless, for the smooth implementation of EMC design of ground line, the electromagnetic compatibility design of public part is the most important job and either thickening ground line or coating processing is capable of avoiding the resistance of public part. Therefore, the change of ground mode is capable of processing and optimizing parallel single point. Meanwhile, in the process of series and parallel design, the generation of single-point ground can also eliminate public resistance as much as possible.

3). Digital ground and analog ground should be independent from each other. On the one hand, digital ground and analog ground should be independent from each other; on the other hand, digital ground should be designed independently and analog ground has to be ensured to fail to interfere with digital ground. In the process of parallel and series mutual ground, single-point grounding is the commonest mode that fails to reduce interference as much as possible to stop from interference led by circuit with low frequency. Therefore, circuit with high frequency should be connected with series and parallel circuit.

• Hazardous substance detection

Hazardous substance detection for electronic products mainly consists of application of detection method, determination of detection projects and recycling of discarded exported electronic products.

a. Sample quantity and selection of method for hazardous substance detection for electronic products.

b. Determination of detection items. Similar with commodities in the market, raw material for electronic products has different quality and types. The raw material should be determined according to the specific environmental protection project by electronic product suppliers and manufacturers, which also benefits the improvement of detected result. Detection should be implemented from the following aspects:
1). Ensure type, quantity and indexes of electronic products reach corresponding standard coupled with features of manufacturing procedure craft.
2). Detect from all positions and angles. Legal and authoritative detection has to be implemented so that the detected result is both complete and accurate.
3). Full understand physical and chemical features to reduce the influence of detected environment on electronic products to the minimum and reduce measurement error. Electronic products with different properties are required to correspond to different detection grades so that detected data can be more accurate and scientific.

c. The recycling and destruction of discarded electronic products.

After detection, discarded electronic products are required to be recycled in time that are incompatible with standard and do harm to people's health. If necessary, discarded electronic products have to be destroyed to avoid bad influence.

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