Due to the inherent switching characteristics of switching power supplies, they tend to generate significant electromagnetic compatibility interference. As a power supply engineer, an electromagnetic compatibility specialist, or a PCB layout designer, it's crucial to comprehend the root causes of electromagnetic compatibility issues, particularly when it comes to layout design. Engineers must learn how to prevent the spread of electromagnetic noise. This article focuses primarily on key aspects of power PCB design.
Here are 29 essential guidelines regarding layout and PCB design:
1. Basic principles include: all traces have impedance; current always follows the path of least resistance; radiation intensity depends on current, frequency, and loop area; common-mode interference and large dv/dt signals relate to ground mutual capacitance. The strategies for mitigating EMI and improving immunity are similar.
2. The layout should be organized into distinct zones based on power supply, analog circuits, high-speed digital components, and various functional blocks.
3. Minimize the loop area of large di/dt paths, shorten the length of large dv/dt signal lines (or areas), ensure that the width isn't excessively broad to avoid increasing distributed capacitance. Generally, make traces as wide as possible but eliminate unnecessary sections. Route signals in straight lines to minimize radiation.
4. Inductive crosstalk arises mainly from large di/dt loops (loop antennas). The induced intensity is directly proportional to mutual inductance. Thus, reducing the mutual inductance of these signals is critical (primarily achieved by reducing loop areas and increasing distances). Capacitive crosstalk is predominantly caused by large dv/dt signals. The induced intensity correlates with mutual capacitance. All efforts to reduce mutual capacitance between these signals are vital (methods include decreasing coupling effective areas, increasing distances, and reducing mutual capacitance with greater spacing).
5. Aim to apply the loop cancellation principle when routing, further shrinking the area of large di/dt loops, as illustrated in Figure 1 (akin to twisted pairs). Using the loop cancellation principle improves anti-interference capability and increases transmission distance.
[Figure 1 shows an example of loop cancellation (freewheeling of the boost circuit) aligned at the center.]
By following these principles, engineers can significantly enhance the electromagnetic compatibility of their designs while ensuring efficient and reliable performance. Additional considerations may involve selecting appropriate shielding materials and optimizing grounding techniques to further mitigate potential interference issues.
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