Handling of Spanning Partition and Slotting in EMC Design

Handling of Spanning Partition and Slotting in EMC Design

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1. The formation of slots in the PCB design process

The formation of slots in the PCB design process includes:

1. Slotting caused by the division of the power supply or ground plane; when there are multiple different power supplies or grounds on the PCB board, it is generally impossible to allocate a complete plane for each power supply network and ground network. Or divide power or ground on multiple planes. Slots are formed between different divisions on the same plane.
2. The through holes are too dense to form slots (through holes include pads and vias); when the through holes pass through the ground or power plane without electrical connection, some space needs to be left around the through holes for electrical isolation; When the distance between the holes is too close, the spacer rings will overlap to form a slot.

Second, the impact of slotting on the EMC performance of the PCB version

Slotting will have a certain impact on the EMC performance of the PCB. This impact may be negative or positive. First of all, we need to understand the surface current distribution of high-speed signals and low-speed signals. At low speeds, current flows along the path with the lowest resistance. The figure below shows the situation where the return signal returns from the ground plane to the source when the low-speed current flows from A to B. At this time, the surface current distribution is wider.

In the case of high speed, the effect of the inductance on the signal return path will exceed the effect of the resistance. The high-speed return signal will flow along the path with the lowest impedance. At this time, the distribution of the surface current is very narrow, and the return signal is concentrated under the signal line in a bundle.

When there are incompatible circuits on the PCB, it is necessary to "separate the ground", that is, set the ground plane separately according to different power supply voltages, digital and analog signals, high-speed and low-speed signals, high-current and low-current signals. From the distribution of high-speed signal and low-speed signal return given above, it can be easily understood that grounding can prevent the superposition of return signals of incompatible circuits and prevent common ground line impedance coupling.

But regardless of high-speed signal or low-speed signal, when the signal line crosses the slot on the power plane or ground plane, it will bring many serious problems, including:

1. Increase the current loop area and loop inductance, making the output waveform easy to oscillate;
2. For high-speed signal lines that require strict impedance control and are routed according to the stripline model, the stripline model will be damaged due to the slotting of the upper or lower plane or the upper and lower planes, resulting in discontinuity of impedance and serious signal integrity. Sexual problems
3. Increase the radiation emission into space, while being vulnerable to the interference of space magnetic field;
4. The high-frequency voltage drop on the loop inductance constitutes a common-mode radiation source, and common-mode radiation is generated through an external cable;
5. Increase the possibility of high-frequency signal crosstalk with other circuits on the board (as shown below).

Three, PCB design to the processing of the slot

The treatment of slotting should follow the following principles:

1. For high-speed signal lines that require strict impedance control, the trajectory is strictly prohibited to cross the split line to avoid discontinuity in impedance and cause serious signal integrity problems;
2. When there are incompatible circuits on the PCB, the ground should be divided, but the ground should not cause the cross-divided routing of high-speed signal lines, and try not to cause the cross-divided routing of low-speed signal lines;
3. When it is unavoidable to cross the slotted line, it should be bridged;

The connector (external) should not be placed on the ground barrier. If there is a large potential difference between point A and point B on the ground in the figure, it is possible to generate common mode radiation through the external cable;

During the PCB design of high-density connectors, unless there are special requirements, it should generally be ensured that the ground network surrounds each pin. The ground network can also be evenly arranged during the pin layout to ensure the continuity of the ground plane and prevent Grooving;