PCB design to maximize EMC effects

1. Design idea of PCB layer

The core of PCB laminated EMC planning and design idea is to plan the signal backflow path reasonably, so as to reduce the signal backflow area from the mirror layer of the veneer as far as possible, so as to cancel or minimize the magnetic flux.

Mirror veneer layer

Mirror layer is a complete copper-coated plane layer (power layer, connection layer) adjacent to the signal layer inside PCB.

It has the following functions:

(1) Reduction of backflow noise: the mirror layer can provide a low impedance path for the backflow of the signal layer, 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 mirror layer reduces the area of the closed loop formed by the signal and the backflow, and reduces EMI;

(3) reduce the crosstalk: it is helpful to control the crosstalk between the signal lines in high-speed digital circuits. By changing the height of the signal lines 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 mirrored layers

(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 a large potential difference with the reference level, while the high-frequency interference on the power plane is relatively large;

(3) From the perspective of shielding, the ground plane is generally grounded, and as the reference point of the reference level, 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 secondary

2, the principle of magnetic 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, whether it is a vacuum or a solid substance.

In PCB, the flux always propagates in the transmission line. If the RF backflow path is parallel to the corresponding signal path, then the flux on the backflow path and the flux on the signal path are in the opposite direction. At this time, they are superimposed on each other, and the effect of flux cancellation is obtained.

3, the nature of flux cancellation

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

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

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

(2) when there are two close to each other and parallel to the wire, as shown in the figure below, one of the conductors of electricity to drain out, the other a conductor of electricity to flow, if the electric current flows through the wire are current and its return current signal, then the two opposite direction of current is equal, so their magnetic field are equal, but the direction is opposite,

So they cancel each other out.

5, six layer plate design examples

For six-ply plates, Scheme 3 is preferred

Analysis:

(1) Since the signal layer is adjacent to the backflow reference plane, and S1, S2 and S3 are adjacent to the ground plane, the best magnetic flux cancelling effect is obtained. 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 small, and it has the best magnetic flux offset effect and low power plane impedance.

(3) The main power supply and its corresponding ground distribution are in the fourth and fifth layers. When the layer thickness is set, increase the spacing between S2-P and reduce the spacing between P-G2 (accordingly reduce the spacing between G1-S2 layers), so as to reduce the impedance of the power plane and the influence of the power supply on S2.

For six layers, Scheme 4

Analysis:

Scheme 4 is more suitable than Scheme 3 for situations where local and small signals are required. It can provide excellent wiring layer S2.

Worst EMC effect, scheme 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, so the magnetic flux offset effect is poor.

conclusion

Specific principles of PCB layer design:

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

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

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

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