How to design PCB to achieve the best EMC effect

MartinFowler

How to design PCB to achieve the best EMC effect [Copy link]

The first thing to consider in PCB EMC design is the layer setting; the number of layers of a single board is composed of the power supply, ground layers and signal layers; in the EMC design of the product, in addition to the selection of components and circuit design, good PCB design is also a very important factor.

The key to PCB EMC design is to minimize the return area as much as possible and let the return path flow in the direction we designed. The design of the layer is the basis of the PCB. How can we do a good job in the PCB layer design to optimize the EMC effect of the PCB?

1. Design ideas for PCB layers

The core of PCB stacking EMC planning and design ideas is to reasonably plan the signal return path and minimize the signal return area from the single board mirror layer as much as possible, so that the magnetic flux can be canceled or minimized.
Single board mirror
layer The mirror layer is a complete copper-clad plane layer (power layer, ground layer) adjacent to the signal layer inside the PCB. It has the following main functions:
(1) Reduce return noise: The mirror layer can provide a low-impedance path for the signal layer return, especially when there is a large current flowing in the power distribution system, the role of the mirror layer is more obvious.
(2) Reduce EMI: The existence of the mirror layer reduces the area of the closed loop formed by the signal and the return current, reducing EMI;
(3) Reduce crosstalk: It helps to control the crosstalk problem between signal lines in high-speed digital circuits. By changing the height of the signal line from the mirror layer, the crosstalk between the signal lines can be controlled. The smaller the height, the smaller the crosstalk;
(4) Impedance control to prevent signal reflection.
Selection of mirror layer
(1) Both the power supply and ground planes can be used as reference planes, and have a certain shielding effect on the internal lines;
(2) Relatively speaking, the power supply plane has a higher characteristic impedance and a larger potential difference with the reference level. At the same time, the high-frequency interference on the power supply plane is relatively large;
(3) From the perspective of shielding, the ground plane is generally grounded and used as a reference point for the reference level. Its shielding effect is far better than that of the power supply plane;
(4) When selecting a reference plane, the ground plane should be preferred, and the power supply plane should be secondary.

2. Principle of flux cancellation

According to Maxwell's equations, all electrical and magnetic interactions between discrete charged bodies or currents are transmitted through the intermediate region between them, regardless of whether the intermediate region is a vacuum or a solid substance. In PCB, magnetic flux always propagates in the transmission line. If the RF return path is parallel to its corresponding signal path, the magnetic flux on the return path and the magnetic flux on the signal path are in opposite directions. At this time, they are superimposed on each other, resulting in the effect of flux cancellation.

III. The essence of magnetic flux cancellation

The essence of magnetic flux cancellation is the control of the signal return path. The specific schematic diagram is as follows:



IV. The right-hand rule explains the effect of magnetic flux cancellation

How to use the right-hand rule to explain the effect of magnetic flux cancellation when the signal layer is adjacent to the ground layer? The explanation is as follows:
(1) When current flows through a conductor, a magnetic field is generated around the conductor, and the direction of the magnetic field is determined by the right-hand rule.
(2) When there are two conductors close to each other and parallel, as shown in the figure below, the current in one conductor flows outward and the current in the other conductor flows inward. If the currents flowing through the two conductors are the signal current and its return current, respectively, then the two currents are equal in magnitude and opposite in direction, so their magnetic fields are also equal in magnitude and opposite in direction, so they can cancel each other out.





Examples of design for five- and six-layer boards



For six-layer boards, plan 3 is preferred


Analysis:
(1) Since the signal layer is adjacent to the return reference plane, and S1, S2, and S3 are adjacent to the ground plane, there is the best magnetic flux cancellation effect. The wiring layer S2 is preferred, followed by S3 and S1.
(2) The power plane is adjacent to the GND plane, and the distance between the planes is very small, which has the best magnetic flux cancellation effect and low power plane impedance.
(3) The main power supply and its corresponding ground are arranged on layers 4 and 5. When setting the layer thickness, increase the spacing between S2-P and reduce the spacing between P-G2 (correspondingly reduce the spacing between G1-S2 layers) to reduce the impedance of the power plane and reduce the impact of the power supply on S2.


For six-layer boards, alternative solution 4


Analysis:
For situations where local or small amounts of signals have high requirements, solution 4 is more suitable than solution 3, as it can provide an excellent wiring layer S2. Worst
EMC effect, solution 2


Analysis:
In this structure, S1 and S2 are adjacent, S3 and S4 are adjacent, and S3 and S4 are not adjacent to the ground plane, resulting in poor magnetic flux cancellation.

To summarize

the specific principles of PCB layer design:
(1) There should be a complete ground plane (shielding) under the component surface and welding surface;
(2) Avoid two signal layers directly adjacent to each other as much as possible;
(3) All signal layers should be adjacent to the ground plane as much as possible;
(4) There should be an adjacent ground plane for key signal wiring layers such as high frequency, high speed, and clock.

toothache

The post is so well written! I have always been worried about EMC. After reading this article, I benefited a lot. I hope the author will post more good articles.

qwqwqw2088

Good graphic information, collection for those who are not familiar with multi-layer boards, especially six-layer board design examples