How to do the best EMC design of PCB?
In the EMC design considerations of PCBs, the first step is the setting of the layers; the number of layers of the 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 the EMC design of PCB is to minimize the reflow area and let the reflow path flow in the direction we designed. The layer design is the basis of the PCB. How to do a good PCB layer design to optimize the EMC effect of the PCB?
1 PCB layer design ideas:
The core of the PCB stack-up EMC planning and design idea is to reasonably plan the signal reflow path, to minimize the reflow area of the signal from the mirror layer of the single board, and to cancel or minimize the magnetic flux.
1.Mirror board layer
The mirror layer is a complete copper plane layer (power layer, ground layer) near the signal layer inside the PCB. It has the following functions:
(1) Reducing the noise of reflow: The mirror layer can provide a low-impedance path for the reflow 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 presence of the mirror layer reduces the area of the closed loop formed by signals and reflow, and reduces EMI;
(4) Impedance control to prevent signal reflection.
(3) Reduce crosstalk: It helps to control crosstalk between signal traces in high-speed digital circuits. By changing the height of signal lines from the mirror layer, you can control crosstalk between signal lines. The smaller the height, the smaller the crosstalk;
2.Selection of mirror layer
(1) Both the power and ground planes can be used as reference planes, and have a certain shielding effect on the internal wiring;
(3) From the perspective of shielding, the ground plane is generally grounded and used as a reference level reference point. Its shielding effect is much better than the power plane;
(4) When selecting the reference plane, the plane should be preferred, and the power plane should be selected secondly.
(2) Relatively speaking, the power plane has a high characteristic impedance, and there is a large potential difference from the reference level, and the high-frequency interference on the power plane is relatively large;
2 Flux cancellation principle:
According to Maxwell's equation, all electrically and magnetic interactions 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 a 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. At this time, they overlap each other. The effect of flux cancellation is obtained.
3 The essence of magnetic flux cancellation is the control of the signal return path. The specific diagram is as follows:
How to use the right-hand rule to explain the magnetic flux cancellation effect when the signal layer and the ground layer are adjacent, the explanation is as follows:
(1) 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.
(2) When there are two parallel wires that are close to each other, as shown in the figure below, the current of one conductor flows outward and the current of the other conductor flows in. If the current flowing through these two conductors is a signal current, respectively And its return current, then these two currents are equal in magnitude and opposite directions, so their magnetic fields are also equal in magnitude and opposite directions, so they can cancel each other out.
Six-layer board design example
analysis:
(1) Since the signal layer is adjacent to the reflow reference plane and S1, S2, and S3 are adjacent to the ground plane, it 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 small. It has the best magnetic flux cancellation effect and low power plane impedance.
(3) The main power supply and its corresponding ground are located on the 4th and 5th floor. When the layer thickness is set, increase the distance between S2-P and reduce the interval between P-G2 (respectively reduce the interval between G1-S2 layers. Pitch) to reduce the impedance of the power plane and reduce the impact of the power on S2.
2. When the cost requirements are high;
analysis:
(1) In this structure, because the signal layer is adjacent to the reflow reference plane and S1 and S2 are close to the ground plane, it has the best magnetic flux cancellation effect;
(2) Since the power plane passes through S3 and S2 to the GND plane, poor magnetic flux cancellation effect and high power plane impedance
(3) The wiring layers S1 and S2 are preferred, followed by S3 and S4.
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, and the magnetic flux cancellation effect is poor.
6 Summary
Specific principles of PCB layer design:
(1) The complete ground plane (shield) is below the component surface and the welding surface;
(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 key signal wiring layers such as high frequency, high speed, and clock must have an adjacent ground plane.
I hope that through the brief introduction of this article, we can help everyone to deepen their understanding of PCB layer design and do a good PCB optimization design in advance, once and for all.
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