No projeto EMC de PCB, a primeira preocupação é a configuração da camada; As camadas da placa são compostas de fonte de alimentação, camada de solo e camada de sinal. In EMC design of products, besides the selection of components and circuit design, good PCB design is also a very important factor.

The key to the EMC design of PCB is to minimize the backflow area and make the backflow path flow in the direction we designed. The layer design is the basis of PCB, how to do a good job of PCB layer design to make THE EMC effect of PCB optimal?

I. Idéias de design de camada de PCB

The core of PCB laminated EMC planning and design is to reasonably plan signal backflow path to minimize the backflow area of signal from the board mirror layer, so as to eliminate or minimize magnetic flux.

Single board mirroring layer

A camada de espelho é uma camada completa de camada plana revestida de cobre (camada de fonte de alimentação, camada de aterramento) adjacente à camada de sinal dentro do PCB. As principais funções são as seguintes:

(1) Reduzir o ruído de refluxo: a camada de espelho pode fornecer um caminho de baixa impedância para o refluxo da camada de sinal, especialmente quando há um grande fluxo de corrente no sistema de distribuição de energia, o papel da camada de espelho é mais óbvio.

(2) redução de EMI: a existência da camada de espelho reduz a área do loop fechado formado pelo sinal e refluxo e reduz a EMI;

(3) reduce crosstalk: help to control the crosstalk problem between signal lines in high-speed digital circuit, change the height of the signal line from the mirror layer, you can control the crosstalk between signal lines, the smaller the height, the smaller the crosstalk;

(4) Impedance control to prevent signal reflection.

Selection of mirror layer

(1) Tanto a fonte de alimentação quanto o plano de aterramento podem ser usados ​​como o plano de referência e têm um certo efeito de blindagem na fiação interna;

(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 the reference point of the reference level, and its shielding effect is far better than that of the power plane;

(4) Ao selecionar o plano de referência, o plano de terra deve ser preferido, e o plano de potência deve ser selecionado em segundo

Two, magnetic flux cancellation principle

According to Maxwell’s equations, all electrical and magnetic action between separate charged bodies or currents is transmitted through the intermediate region between them, whether it is a vacuum or solid matter. In a PCB, the flux is always propagated in the transmission line. If the rf backflow path is parallel to the corresponding signal path, the flux on the backflow path is in the opposite direction to that on the signal path, then they are superimposed on each other, and the effect of flux cancellation is obtained.

A natureza do cancelamento do fluxo magnético

The essence of flux cancellation is the control of signal backflow path, as shown in the following diagram:

The right hand rule explains the magnetic flux cancellation effect

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 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) quando houver dois próximos um do outro e paralelos ao fio, conforme mostrado na figura abaixo, um dos condutores de eletricidade para drenar e o outro um condutor de eletricidade para fluir, se a corrente elétrica passar pelo fio são atuais e seu sinal de corrente de retorno, então as duas direções opostas da corrente são iguais, então seus campos magnéticos são iguais, mas a direção é oposta,Então, eles se cancelam.

Five, six board design examples

Para seis camadas, o plano 3 é o preferido

Análise:

(1) Como a camada de sinal é adjacente ao plano de referência de refluxo e S1, S2 e S3 são adjacentes ao plano de massa, o melhor efeito de cancelamento de fluxo magnético é alcançado. Portanto, S2 é a camada de roteamento preferida, seguida por S3 e S1.

(2) The power plane is adjacent to the GND plane, the distance between the planes is very small, and it has the best magnetic flux cancellation effect and low power plane impedance.

(3) The main power supply and its corresponding floor cloth are located at layer 4 and 5. When layer thickness is set, the spacing between S2-P should be increased and the spacing between P-G2 should be reduced (the spacing between layer G1-S2 should be correspondingly reduced), so as to reduce the impedance of the power plane and the influence of the power supply on S2.

For six layers, option 4

Análise:

Scheme 4 is more suitable than Scheme 3 for local, small number of signal requirements, which can provide an excellent wiring layer S2.

Worst EMC effect, plan 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 cancellation effect is poor.

conclusão

Specific principles of PCB layer design:

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

(2) Tente evitar a adjacência direta de duas camadas de sinal;

(3) Todas as camadas de sinal são adjacentes ao plano de terra, tanto quanto possível;

(4) A camada de fiação de alta frequência, alta velocidade, relógio e outros sinais principais devem ter um plano de aterramento adjacente.