Due to the inherent switching characteristics of switching power supplies, they tend to generate significant electromagnetic compatibility (EMC) interference. As engineers working in power supply design, electromagnetic compatibility, or PCB layout, understanding the root causes of EMC issues—especially layout-related ones—is crucial. One of the most important aspects is learning how to prevent the spread of electromagnetic noise, commonly referred to as "dirty spots." This article will focus on key considerations for designing power PCBs.
### 29 Fundamental Relationships Between Layout and PCB Design
#### Basic Principles:
1. Any wire has impedance; current always follows the path of least resistance. The strength of electromagnetic radiation depends on the current, frequency, and loop area. Common-mode interference and rapid voltage changes (large dv/dt) are linked to the mutual capacitance of the ground. Strategies for mitigating EMI and improving immunity share many similarities.
2. The layout should be divided into distinct sections based on power supply, analog circuits, high-speed digital components, and other functional blocks.
3. Minimize the loop areas associated with large di/dt currents, reduce the length of high dv/dt signal lines (or their loop areas, keeping widths moderate to avoid increasing distributed capacitance), and strive for straighter traces while reducing their hidden areas to minimize radiation.
4. Inductive crosstalk arises primarily from large di/dt loops (acting like loop antennas). The intensity of induction is directly proportional to mutual inductance. Thus, minimizing this mutual inductance is critical (achieved by reducing loop areas and increasing distances). Capacitive crosstalk, on the other hand, is mainly caused by large dv/dt signals. The induced intensity is proportional to mutual capacitance. To reduce this, decrease the effective coupling area, increase distances, and lower mutual capacitance (as distance increases, mutual capacitance decreases faster). This step is particularly vital.
5. Aim to use the loop cancellation principle when routing traces, further shrinking the loop area for large di/dt currents. This approach is illustrated in Figure 1 (similar to twisted pairs), which enhances interference resistance and extends transmission distances:
*Figure 1. Loop Cancellation (Freewheeling of Boost Circuit)*
In addition to the above points, here are some supplementary tips to consider:
- Always aim for symmetry in your layout to balance currents and reduce imbalances that could lead to EMI.
- Use ground planes effectively to shield sensitive circuits and provide a low-impedance return path for signals.
- Keep high-speed signals away from the edges of the board to minimize external interference.
- Ensure proper decoupling capacitors are placed close to IC power pins to filter out noise.
- Avoid sharp corners in traces, as they can create hotspots for EMI.
- Use shielding materials where necessary to protect critical components.
By adhering to these guidelines, engineers can significantly enhance the performance of their power PCB designs, ensuring compliance with EMC standards and reducing potential interference issues. Proper attention to detail during the layout phase can make a substantial difference in the final product's reliability and efficiency.
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