Electronic Design - PCB laminated design and EMC suppression

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

The key to PCB EMC design is to reduce the reflow area as much as possible so that the reflow path flows in the direction of our design. The layer design is the basis of the PCB. How to do a good job in the PCB layer design to make the PCB's EMC effect optimal?

PCB layer design ideas:

The core of PCB stack EMC planning and design thinking is to reasonably plan the signal return path, and minimize the signal return area from the mirror layer of the single board, so that the magnetic flux can be canceled or minimized.

Veneer mirror layer

The mirror layer is a complete copper-clad plane layer (power layer, ground layer) inside the PCB adjacent to the signal layer. It has the following functions:

Reduce return noise: The mirror layer can provide a low-impedance path for the signal layer to return, especially when there is a large current flowing in the power distribution system, the role of the mirror layer is more obvious.

Reduce EMI: The presence of the mirror layer reduces the area of the closed loop formed by the signal and reflow, and reduces EMI;

Reduce crosstalk: help control the crosstalk problem between signal traces 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;

Impedance control: prevent signal reflection.

Mirror layer selection

Both power and ground planes can be used as reference planes, and have a certain shielding effect on internal wiring;

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;

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

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

Principle of magnetic flux cancellation:

According to Maxwell's equation, all electrical and magnetic interactions between discrete charged bodies or currents are transmitted through the intermediate area between them, regardless of whether the intermediate area is a vacuum or a physical substance. In the PCB, the 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, and they are superimposed on each other. The effect of flux cancellation is obtained.

The essence of magnetic flux cancellation is the control of the signal return path

When a current flows through the wire, a magnetic field is generated around the wire, and the direction of the magnetic field is determined by the right-hand rule.

When there are two parallel wires close to each other, as shown in the figure below, the current of one conductor flows out, and the current of the other conductor flows in. If the current flowing through the two wires is the signal current and its Reflux current, 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.

The specific principles of : a complete ground plane (shield) below the component surface and soldering surface; try to avoid two signal layers directly adjacent; all signal layers should be adjacent to the ground plane as much as possible; key signals such as high frequency, high speed, clock, etc. The wiring layer must have an adjacent ground plane.

PCB layer design