The 20H principle refers to the distance between the power layer and the stratum within 20H, which is also to suppress the edge radiation effect. Electromagnetic interference radiates outward from the edge of the plate. Shrink the power layer so that the electric field can only be transmitted within the range of the ground. Effectively enhance EMC. If the voltage is reduced by 20H, 70% of the electric field can be confined to the earth edge. A 100H contraction can limit 98% of the electric field.
The 20H rule is used to ensure that the edge of the power plane is at least 20 times smaller than the edge of the 0V plane than the interval between two planes. This rule is often required as a side firing technique to reduce the edge radiation effect from the 0V/ power planar structure.
However, the 20H rule only provides significant effects under certain conditions. These specific conditions include:
1. The rise/fall time of current fluctuation in the power bus is less than 1ns.
2. The power supply plane shall be on the inner layer of PCB, and the upper and lower layers adjacent to it shall be 0V plane. The two 0V planes extend outward at least 20 times the distance between them and the power supply plane.
3. The power bus structure will not generate resonance at any frequency of interest.
4. The total derivative of PCB shall be at least 8 layers or more.
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