EMC design component selection and circuit design are critical

In the EMC design consideration of PCB, the first thing involved is the setting of layers; the number of layers of a single board consists of the number of power supply, ground layers and signal layers; in the EMC design of products, 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 minimize the return flow area and allow the return flow path to flow in the direction we designed. Layer design is the foundation of PCB. How to design PCB layer well to optimize the EMC effect of PCB?

1. PCB layer design ideas:

The core of PCB stack-up EMC planning and design ideas is to reasonably plan the signal return path, minimize the signal return area from the single board mirror layer, and cancel or minimize the magnetic flux.

1. Single board image layer

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

(1) 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.

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

(3) Reduce crosstalk: It helps to control the crosstalk problem between signal lines 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. Selection of mirror layer

(1) The 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 a large potential difference from the reference level. At the same time, 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 a reference plane, the ground plane should be preferred, followed by the power plane.

2. Principle of magnetic flux cancellation:

According to Maxwell's equations, all electrical and magnetic effects between discrete charged bodies or currents are transmitted through the intermediate region between them, regardless of whether the intermediate region is a vacuum or a solid substance. In PCB, 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. At this time, they superimpose each other. Then the effect of flux cancellation is obtained.

3. The essence of magnetic flux cancellation is the control of the signal return path. The specific schematic diagram is as follows:

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

(1) When a current flows through a wire, a magnetic field will be generated around the wire. 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 current flowing through the two wires is the signal current respectively And its return current, then these 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

1. For six-layer boards, give priority to option 3;

analyze:

(1) Since the signal layer is adjacent to the reflow reference plane, and S1, S2, and S3 are adjacent to the ground plane, which has 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, and the distance between the planes is very small, which has the best magnetic flux cancellation effect and low power plane impedance.

(3) The main power supply and its corresponding ground are laid on the 4th and 5th layers. When setting the layer thickness, 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 the power supply on S2.

2. When cost requirements are high, option 1 can be adopted;

analyze:

(1) In this structure, since the signal layer is adjacent to the reflow reference plane, and S1 and S2 are adjacent to the ground plane, it has the best magnetic flux cancellation effect;

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

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

3. For six-layer boards, alternative 4

analyze:

For local and small-volume signal requirements, Option 4 is more suitable than Option 3. It can provide excellent wiring layer S2.

4. 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, so the magnetic flux cancellation effect is poor.

6. Summary

Specific principles for PCB layer design:

(1) There is a complete ground plane (shielding) below the component surface and soldering surface;

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

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

(4) Key signal wiring layers such as high frequency, high speed and clock must have an adjacent ground plane.

I hope that the brief introduction in this article can help everyone deepen their understanding of PCB layer design and optimize the PCB design in advance, once and for all.