However, no matter how careful the layout and schematic design is, sometimes the conducted EMI cannot be reduced to the required level. This is because noise depends not only on circuit parasitics, but also on the strength of the current. In addition, the switch opening and closing action will generate discontinuous currents, and these discontinuous currents will generate voltage ripple on the input capacitor, thereby increasing EMI.
Therefore, it is necessary to adopt some other methods to improve the performance of conducted EMI. This article mainly discusses introducing an input filter to remove noise, or adding a shield to lock the noise.
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Figure 1 Schematic diagram of EMI filter
Figure 1 is a simplified EMI filter including common mode (CM) and differential mode (DM) filters. Generally, DM filters are mainly used to filter noise (DM noise) less than 30MHz, and CM filters are mainly used to filter noise (CM noise) from 30MHz to 100MHz. In fact, these two filters have a certain suppression effect on the EMI noise of the entire frequency band.
Figure 2 shows the input lead noise without a filter, including both positive and negative noise. The peak and average levels of these noises are noted. Among them, the system under test mainly uses the chip LMR14050SSQDDARQ1 to output 5V / 5A, and supplies power to the subsequent chip TPS65263QRHBRQ1, while outputting 1.5V / 3A, 3.3V / 2A, and 1.8V / 2A. Both chips operate at a switching frequency of 2.2MHz. In addition, the conducted EMI standard shown in the figure is CISPR25 Class 5 (C5). For more information on this system, please refer to the application note 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 mid-band DM noise (2MHz to 30MHz) by nearly 35dBμV / m. In addition, high-frequency noise (30MHz to 100MHz) is also reduced, but it still exceeds the limit. This is mainly because the DM filter has a limited ability to remove high-frequency CM noise.
Figure 3 Noise characteristics under the C5 standard (with DM filter)
Figure 4 shows the noise characteristics after adding the CM and DM filters. Compared with Figure 3, the increase of the CM filter reduces the CM noise by nearly 20dBμV / m. And EMI performance also passed the CISPR25 C5 standard.
Figure 4 C5 standard noise characteristics (with CM and DM filters)
Figure 5 shows the noise characteristics with CM and DM filters in different layouts, where the filters are the same as Figure 4. However, compared with Figure 4, the noise of the entire frequency band has increased by about 10dBμV / m, and the high-frequency noise even exceeds the average value of the CISPR25 C5 standard.
Figure 5 C5 noise characteristics (with CM and DM filters, different layouts)
The difference in noise results between Figure 4 and Figure 5 is mainly due to the differences in PCB layout, as shown in Figure 6. In the wiring of Figure 5 (right side of Figure 6), a large area of copper (GND) surrounds the DM filter and forms some parasitic capacitance with the Vin trace. 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, all copper plating around the filter needs to be removed, as shown in the wiring on the left side of Figure 6.
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Figure 6 Different PCB layouts
In addition to adding filters, another effective way to optimize EMI performance is to add shields. This is because a metal shield connected to GND prevents noise from radiating outward. Figure 7 recommends a method of placing the shield. This shield covers exactly all the components on the board.
Figure 8 shows the EMI results after adding filters and shields. As shown in the figure, almost the entire band of noise is eliminated by the shield, and the EMI performance is very good. This is mainly because the long input lead, which is equivalent to an antenna, can couple a lot of radiated noise, and the shield can just isolate them. In this design, IF noise is also coupled to the input leads in this way.
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Figure 7 PCB 3D model with shield
Figure 8 Noise characteristics under C5 standard (with CM, DM filter and shield)
Figure 9 also shows the noise characteristics with filters and shields. 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 outside. Despite this shield, some radiated noise can still bypass the EMI filter and be coupled to the power lines 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 (same layout) are almost the same. This is because with the addition of EMI filters, the high-frequency radiated noise that can be coupled to the input lines is almost gone.
Figure 9 Noise characteristics under C5 standard (with CM, DM filter and shielded metal box)
In summary, adding EMI filters or shielding can effectively improve EMI performance. However, at the same time, the layout of the filter and the placement of the shield need to be carefully considered.
