In the EMC design consideration of PCB, it is first involved in the setting of the layer; the number of sheets of the single plate is composed of the number of layers of the power, the layer of the signal; in the EMC design of the product, in addition to the selection and circuit design of the components In addition, good PCB design is also a very important factor.
The key to the EMC design of PCB is to minimize reflow area, let the return path flow in the direction of our design. The design is the foundation of the PCB, how to do a good PCB layer design to make PCB's EMC effect?
1. Design Ideas for the PCB layer:
The core of the PCB laminate EMC planning and design ideas is a reasonable planning signal reflow path, minimizing the backflow area of the single-plate mirror layer, so that the magnetic flux deselects or minimizes.
1, single board mirror layer
The mirror layer is a complete layer of copper-applying plane (power layer, grounding layer) within the signal layer within the PCB. It mainly has the following role:
(1) Reduce reflow noise: The mirror layer can provide a low impedance path for signal layer reflow, especially when there is a large current flow in the power distribution system, and the mirror layer is more obvious.
(2) Reduce EMI: The presence of the mirror layer reduces the area of the closed loop formed by the signal and reflow, and reduces EMI;
(3) Reduce crosstalk: Control the crosstalk problem between signal traces in the high-speed digital circuit, change the height of the signal line from the mirror layer, so that the signal line is crosstalk, the height is, the smaller the crosstalk;
(4) Impedance control prevents signal reflection.
2, the choice of mirror layer
(1) Power supply, the ground plane can be used as a reference plane, and there is a certain shielding effect on the internal traces;
(2) Relatively, the power supply 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 supply plane is relatively large;
(3) From the angle of the shield, the ground plane is generally grounded, and as a reference level reference point, the shielding effect is far superior to the power supply plane;
(4) When the reference plane is selected, it should preferably be planar, the power supply plane is selected.
2, magnetic flux alignment principle:
According to the Maxwell equation, discrete charged belt or current, the role of all electricity between them is transmitted through the intermediate region between them, whether the intermediate region is vacuum or physical substances. In the PCB, magnetic flux is always propagated in the transmission line. If the radio frequency return path is parallel to its corresponding signal path, the magnetic flux on the backflow path is opposite to the magnetic flux on the signal path, and they superimpose each other. Then get the effect of flux cancellation.
3, the essence of the flux cancellation is the control of the signal reflow path, and the specific diagram is as follows:
How to use the right hand to explain the signal layer and the formation of the magnetic flux to the elimination effect, explained as follows:
(1) When there is current on the wire, the magnetic field is generated around the wire, and the direction of the magnetic field is determined by the right hand.
(2) When there are two wires adjacent to each other, as shown in the figure below, the current of one of the conductors flows out externally, and the current of the other conductor flows inward. If the current flowing through these two wires is signal current, respectively And its reflow current, then the two currents are the opposite direction, so their magnetic field is also equal, and the direction is opposite, so it can be canceled with each other.
5, six-layer design example
1. For six-layer board, priority consideration 3;
Analysis:
(1) Since the signal layer is adjacent, S1, S2, S3 adjacent, S1, S2, S3 have the optimum magnetic flux cancellation effect, preferably wiring layer S2, followed by second S3, S1.
(2) The power supply plane is adjacent to the GND plane, the distance between the plane is small, and the best flux cancellation effect and low power supply plane impedance.
(3) When the main power and its corresponding ground cloth are set at 4, 5 layers, the spacing between the S2-P is increased, and the intercommodation between the P-G2 (corresponding to the corresponding reduction between the G1-S2 layers) Spacing) to reduce the impedance of the power supply plane, reduce the impact of the power supply on S2.
2. When the cost is high, scheme 1 can be adopted;
Analysis:
(1) This structure, since the signal layer is adjacent to the reflow reference plane, the S1 and S2 are adjacent to the ground plane, and there is optimal magnetic flux cancellation effect;
(2) The power supply plane is required to pass the S3 and S2 to GND plane, the difference of magnetic flux cancellation effect and high power supply plane impedance;
(3) It is preferred that the wiring layers S1, S2, followed by seconds, S4.
3, for six-layer board, alternative 4
Analysis:
For situations where local, small amounts of signals are required, the solution 4 is more suitable than the scheme 3, which provides excellent wiring layer S2.
4, worst EMC effect, program 2
Analysis:
Such structures, S1 and S2 adjacent, S3 adjacent to S4, and S3 is not adjacent to the ground plane, and the magnetic flux cancels is poor.
6, summary
PCB layer design specific principles:
(1) The element surface, the welding surface is a complete flat surface (shield);
(2) Try to avoid the two signal layers directly adjacent;
(3) All signal layers are as close as possible to the plane plane;
(4) High-frequency, high speed, clock, etc. Key signal wiring layer must have an adjacent ground plane.
I hope that through this paper, you can help you can deepen the understanding of the PCB layer design, and do a good job in the optimization design of PCB, once again.
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