Electronic Design - How to do PCB design to make EMC effect better?

In the EMC design consideration of PCB, the first thing involved is the setting of layers; 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 we designed. The layer design is the basis of the PCB. How to do a good job in the PCB design to make the PCB's EMC effect better?

1. The design idea of PCB layer:

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.

1. Single board mirroring layer

The mirror image layer is a complete copper-clad plane layer (power layer, ground layer) inside the PCB adjacent to the signal layer. The main functions are as follows:

(1) Reduce the 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.

(2) Reduce EMI: The existence of the mirror layer reduces the area of the closed loop formed by the signal and reflow, and reduces EMI;

(3) Reduce crosstalk: It is helpful to 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;

(4) Impedance control to prevent signal reflection.

2. The choice of mirroring layer

(1) Power and ground planes can be used as reference planes, and have a certain shielding effect on 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 a reference level reference point, 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.

3. 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.

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

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

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

(2) 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 currents flowing through the two wires are signal currents respectively Compared with its return current, 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.

5. Six-layer board design example

For six-layer boards, option 3 is given priority;

analyze:

(1) Since the signal layer is adjacent to the reflux 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, the distance between the planes is very small, there is the best magnetic flux cancellation effect and low power plane impedance.

(3) The main power supply and its corresponding ground are placed on the 4th and 5th layers. When the layer thickness is set, increase the distance between S2-P and reduce the distance between P-G2 (correspondingly reduce the distance between G1-S2 layers). Spacing) to reduce the impedance of the power plane and reduce the impact of power on S2.

2. Option 1 can be adopted when the cost requirements are high;

analyze:

(1) With this structure, since the signal layer is adjacent to the reflux reference plane, S1 and S2 are close to the ground plane, which has the best magnetic flux cancellation effect;

(2) Since the power plane passes through S3 and S2 to the GND plane, the magnetic flux cancellation effect is poor and the power plane impedance is high;

(3) The wiring layers S1 and S2 are preferred, followed by S3 and S4.

3. For six-layer boards, option 4

analyze:

For occasions with high local and small signal requirements, Option 4 is more suitable than Option 3. It can provide an excellent wiring layer S2.

4. The worst EMC effect, option 2

analyze:

In this structure, S1 and S2 are adjacent, S3 and S4 are adjacent, and S3 and S4 are not adjacent to the ground plane, and the magnetic flux cancellation effect is poor.

6. Summary

Specific principles of PCB layer design:

(1) PCB component surface and soldering surface are complete ground plane (shield);

(2) Try to avoid two signal layers directly adjacent to each other;

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

(4) The wiring layer of key signals such as high frequency, high speed, clock, etc. must have an adjacent ground plane.