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This article introduces to you what are the skills to effectively reduce conducted radiation interference in EMC, the content is very detailed, interested friends can refer to, hope to be helpful to you.

The problem of electromagnetic interference (EMI) in design has always been a headache, especially in the automotive field. In order to reduce the electromagnetic interference as much as possible, designers usually reduce the noise source by reducing the loop area of high di / dt and switch conversion rate when designing the schematic diagram and drawing the layout. However, sometimes, no matter how carefully the layout and schematics are designed, it is still not possible to reduce the conduction EMI to the desired level. This is because the noise depends not only on the parasitic parameters of the circuit, but also on the current intensity. In addition, the opening and closing of the switch will produce discontinuous currents, which will produce voltage ripples on the input capacitor, thus increasing the EMI. Therefore, it is necessary to use some other methods to improve the performance of conducted EMI. The following mainly discusses the introduction of an input filter to filter out the noise, or the addition of a shield to lock the noise.

Fig. 1 schematic diagram of EMI filter figure 1 is a simplified EMI filter, including a common-mode (CM) filter and a differential-mode (DM) filter. Generally speaking, DM filter is mainly used to filter noise less than 30MHz (DM noise), while CM filter is mainly used to filter noise from 30MHz to 100MHz (CM noise). But in fact, these two filters can suppress the EMI noise of the whole frequency band to a certain extent. Figure 2 shows an input lead noise without a filter, including positive and negative noise, and marks the peak and average levels of these noises. Among them, the tested system mainly uses the chip LMR14050SSQDDARQ1 to output 5V/5A, and supplies power to the subsequent chip TPS65263QRHBRQ1, while outputting 1.5V LMR14050SSQDDARQ1, 3A, 3.3V and 2A and 1.8V/2A. Both chips operate at the switching frequency of 2.2MHz. In addition, the conductive EMI standard shown in the figure is CISPR25 Class 5 (C5). For more information about the system, please refer to the Application Notes SNVA810.

Figure 2 noise characteristics under the C5 standard (no filter) figure 3 shows the EMI results after adding a DM filter. As can be seen from the figure, the DM filter attenuates the intermediate band DM noise (2MHz to 30MHz) of nearly 35dB μ V / m. In addition, the high-frequency noise (30MHz to 100MHz) also decreased, but still exceeded the limit level. This is mainly because the ability of DM filter to filter CM noise in high frequency band is limited. Figure 3 noise characteristics under the C5 standard (with DM filter) figure 4 shows the noise characteristics with the addition of CM and DM filters. Compared with figure 3, the increase of CM filter reduces the CM noise of nearly 20dB μ V / m. And the EMI performance also passed the CISPR25 C5 standard. Figure 4 noise characteristics under the C5 standard (with CM and DM filters) figure 5 shows the noise characteristics of filters with CM and DM under different layouts, where the filter is the same as figure 4. However, compared with figure 4, the noise in the entire frequency band increases by about 10dB μ V / m, and the high frequency noise even exceeds the average of the CISPR25 C5 standard. Figure 5 noise characteristics under the C5 standard (with CM and DM filters, different layouts) the difference in noise results between figures 4 and 5 is mainly due to differences in PCB wiring, as shown in figure 6. In the wiring of figure 5 (on the right side of figure 6), a large area of copper clad (GND) surrounds the DM filter and forms some parasitic capacitance with the Vin wiring. These parasitic capacitors provide an effective low impedance path for high frequency signal bypass filters. Therefore, in order to maximize the performance of the filter, it is necessary to remove all copper cladding around the filter, such as the wiring on the left side of figure 6. Fig. 6 different PCB cabling in addition to adding filters, another effective way to optimize EMI performance is to add shields. This is because the metal shield connected to the GND can prevent the noise from radiating outward. Figure 7 recommends a method of placing the shield. The shield happens to cover all the components on the board. Figure 8 shows the EMI results after adding a filter and a shield. As shown in the figure, the noise of the whole frequency band is almost eliminated by the shield, and the performance of the EMI is very good. This is mainly because the long input leads equivalent to antennas are coupled with a large amount of radiated noise, and the shield happens to isolate them. In this design, the if noise will also be coupled to the input lead in this way. Figure 7 PCB 3D model with shield figure 8 noise characteristics under the C5 standard (with CM,DM filter and shield) figure 9 also shows the noise characteristics with filter and shield. Unlike figure 8, the shield in figure 9 is a metal box that wraps the entire circuit board and only the input leads are exposed. Despite this shield, some radiated noise can still bypass the EMI filter and be coupled to the power cord on the PCB, which will result in worse noise characteristics than figure 8. Interestingly, the noise characteristics of the high frequency bands in figure 4, figure 8 and figure 9 (the same layout and wiring) are almost the same. This is because the radiated noise of high frequency band which can be coupled to the input line is almost non-existent after the addition of EMI filter. Figure 9 noise characteristics under the C5 standard (with CM,DM filter and shielded metal box) generally speaking, the addition of EMI filter or shield can effectively improve EMI performance. But at the same time, the layout and wiring of the filter and the placement of the shield need to be carefully considered. On EMC effectively reduce conducted radiation interference skills is shared here, I hope that the above content can be of some help to you, can learn more knowledge. If you think the article is good, you can share it for more people to see.

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