PCB最佳EMC效果如何設計？

在 EMC 設計中 PCB，首先關心的是圖層設置；電路板的層數由電源層、地層和信號層組成。 In EMC design of products, besides the selection of components and circuit design, good PCB design is also a very important factor.

The key to the EMC design of PCB is to minimize the backflow area and make the backflow path flow in the direction we designed. 層數設計是PCB的基礎，如何做好PCB層數設計，使PCB的EMC效果達到最佳？

一、PCB層設計思路

The core of PCB laminated EMC planning and design is to reasonably plan signal backflow path to minimize the backflow area of signal from the board mirror layer, so as to eliminate or minimize magnetic flux.

Single board mirroring layer

鏡面層是PCB內部與信號層相鄰的一層完整的鍍銅平面層（電源層、接地層）。主要功能如下：

(1)降低迴流噪聲：鏡層可以為信號層回流提供低阻抗路徑，尤其是在配電系統中有大電流流動時，鏡層的作用更加明顯。

(2)降低EMI：鏡層的存在減少了信號和回流形成的閉環面積，降低了EMI；

(3) reduce crosstalk: help to control the crosstalk problem between signal lines in high-speed digital circuit, change the height of the signal line from the mirror layer, you can control the crosstalk between signal lines, the smaller the height, the smaller the crosstalk;

(4) Impedance control to prevent signal reflection.

Selection of mirror layer

(1) 電源和地平面都可以作為參考平面，對內部走線有一定的屏蔽作用；

(2) Relatively speaking, the power plane has a high characteristic impedance, and there is a large potential difference with the reference level, and the high-frequency interference on the power plane is relatively large;

(3) From the perspective of shielding, the ground plane is generally grounded and used as the reference point of the reference level, and its shielding effect is far better than that of the power plane;

(4) 選擇參考平面時，應優先選擇地平面，其次選擇電源平面

Two, magnetic flux cancellation principle

According to Maxwell’s equations, all electrical and magnetic action between separate charged bodies or currents is transmitted through the intermediate region between them, whether it is a vacuum or solid matter. In a PCB, the flux is always propagated in the transmission line. If the rf backflow path is parallel to the corresponding signal path, the flux on the backflow path is in the opposite direction to that on the signal path, then they are superimposed on each other, and the effect of flux cancellation is obtained.

磁通抵消的性質

The essence of flux cancellation is the control of signal backflow path, as shown in the following diagram:

The right hand rule explains the magnetic flux cancellation effect

How to use the right hand rule to explain the magnetic flux cancellation effect when the signal layer is adjacent to the stratum is explained as follows:

(1) 當電流流過導線時，導線周圍會產生磁場，磁場的方向由右手定則確定。

(2)當有兩根靠得很近並平行的導線時，如下圖所示，其中一根導電體流出，另一根導電體流過，如果有電流流過導線是電流及其返回的電流信號，那麼兩個相反方向的電流相等，所以它們的磁場相等，但方向相反，所以它們相互抵消。

Five, six board design examples

對於六層，首選方案3

分析：

(1) 由於信號層與回流參考面相鄰，S1、S2、S3與接地面相鄰，因此達到了最佳的磁通抵消效果。因此，S2 是首選路由層，其次是 S3 和 S1。

(2)電源平面與GND平面相鄰，平面之間的距離很小，具有最好的磁通抵消效果和低電源平面阻抗。

(3) 主電源及其對應的地佈位於第4層和第5層。設置層厚時，應增大S2-P之間的間距，減小P-G2之間的間距（層間距）。 G1-S2應相應減小），以減小電源平面的阻抗和電源對S2的影響。

對於六層，選項 4

分析：

Scheme 4 is more suitable than Scheme 3 for local, small number of signal requirements, which can provide an excellent wiring layer S2.

Worst EMC effect, plan 2