PCB Technical - PCB multilayer: magnetic flux elimination method to effectively control EMC

In the EMC design of PCB, the first concern is the layer setting; The layers of the board are composed of power supply, ground layer and signal layer. 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. The layer design is the basis of PCB, how to do a good job of PCB layer design to make the EMC effect of PCB optimal?

Design idea of PCB layer: The core of EMC planning and design idea of PCB layer is to reasonably plan signal backflow path, minimize the backflow area of signal from the board mirror layer, and make the magnetic flux cancel or minimize.

1. Single board mirror layer The mirror layer is a complete copper-coated plane layer (power supply layer, grounding layer) adjacent to the signal layer inside the PCB. The main functions are as follows:

(1) Reduce backflow noise: the mirror layer can provide a low impedance path for the signal layer backflow, especially when there is a large current flow in the power distribution system, the role of the mirror layer is more obvious.

(2) EMI reduction: the existence of the mirror layer reduces the area of the closed loop formed by the signal and reflux and reduces 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.

2. Selection of mirror layer

(1) Both the power supply and the ground plane can be used as the reference plane, and have a certain shielding effect on the internal wiring;

(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) When selecting the reference plane, the ground plane should be preferred, and the power plane should be selected second.

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, regardless of whether the intermediate region is 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.

Specific principles of PCB layer design:

(1) There is a complete ground plane (shield) below the component surface and welding surface;

(2) Try to avoid direct adjacent of two signal layers;

(3) All signal layers are adjacent to the ground plane as far as possible;

(4) Wiring layer of high frequency, high speed, clock and other key signals should have an adjacent ground plane.