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This article introduces how EMC/EMI control is applied in PCB design. The content is very detailed. Interested friends can use it for reference. I hope it will be helpful to you.

"with the improvement of the integration of IC devices, the gradual miniaturization of devices and the increasing speed of devices, the EMI problem in electronic products is becoming more and more serious. from the point of view of EMC/EMI design of system equipment, dealing with the EMC/EMI problem in the PCB design stage of the equipment is the most effective and cost-effective means to make the system equipment meet the electromagnetic compatibility standard. This paper introduces the EMI control technology in digital circuit PCB design."

Generation and suppression principle of EMI

The generation of EMI is due to the fact that the electromagnetic interference source transfers energy to the sensitive system through the coupling path. It includes three basic forms: conduction through wires or common ground wires, radiation through space or near-field coupling. The harm of EMI is to reduce the quality of the transmitted signal, cause interference or even damage to the circuit or equipment, so that the equipment can not meet the technical requirements of the electromagnetic compatibility standard.

To suppress EMI, the EMI design of digital circuits should be carried out in accordance with the following principles:

● decomposes the index into veneer circuit and graded control according to the relevant EMC/EMI technical specifications.

● is controlled from three elements of EMI, namely, interference source, energy coupling path and sensitive system, so that the circuit has a flat frequency response and ensures the normal and stable operation of the circuit.

● starts from the equipment front-end design, pays attention to the EMC/EMI design, reduces the design cost.

EMI Control Technology of Digital Circuit PCB

When dealing with various forms of EMI, we must analyze the specific problems. In the PCB design of digital circuits, EMI control can be carried out from the following aspects.

Device selection

In the design of EMI, the speed of the selected device should be considered first. In any circuit, if a device with a rise time of 5ns is replaced with a device with a rise time of 2.5ns, the EMI will be increased by about 4 times. The radiation intensity of EMI is proportional to the square of frequency. The highest EMI frequency (fknee), also known as EMI transmission bandwidth, is a function of signal rise time rather than signal frequency: fknee = 0.35/Tr (where Tr is the signal rise time of the device).

This radiant EMI has a frequency range from 30MHz to several GHz. In this band, the wavelength is so short that even a very short wiring on the circuit board can become a transmitting antenna. When the EMI is high, the circuit is easy to lose its normal function. Therefore, in the device selection, under the premise of ensuring the circuit performance requirements, we should try to use the low-speed chip and adopt the appropriate drive / receiver circuit. In addition, because the lead pins of the device have parasitic inductance and parasitic capacitance, the influence of device packaging on the signal can not be ignored in high-speed design, because it is also an important factor in the generation of EMI radiation. Generally speaking, the parasitic parameters of chip devices are smaller than those of plug-in devices, and the parasitic parameters of BGA packages are smaller than those of QFP packages.

Selection of Connector and definition of signal Terminal

Connector is not only the key link of high-speed signal transmission, but also the weak link which is easy to produce EMI. More ground pins can be arranged in the terminal design of the connector to reduce the distance between the signal and the ground, reduce the effective signal loop area of radiation in the connector, and provide low impedance reflux path. If necessary, consider isolating some key signals.

Laminated design

Under the premise of cost permitting, increasing the number of ground layers and keeping the signal layer close to the ground plane layer can reduce EMI radiation. For high-speed PCB, the power layer and ground layer are coupled next to each other, which can reduce the power impedance, thus reducing the EMI.

Overall Arrangement

According to the current flow direction of the signal, the reasonable layout can reduce the interference between signals. Reasonable layout is the key to control EMI. The basic principles of layout are:

● analog signal is vulnerable to the interference of digital signal, so analog circuit should be separated from digital circuit.

The ● clock line is the main source of interference and radiation. Stay away from the sensitive circuit and keep the clock line to the shortest.

● circuits with high current and high power consumption should be arranged in the central area of the board as far as possible, and the effects of heat dissipation and radiation should be taken into account.

The ● connector is arranged as far as possible on one side of the board and away from the high frequency circuit.

The ● input / output circuit is close to the corresponding connector, and the decoupling capacitor is close to the corresponding power pin.

● fully considers the feasibility of power division by layout, and multi-power devices should be placed across the boundary of the power division area in order to effectively reduce the impact of plane segmentation on EMI.

The ● reflux plane (path) is not divided.

Cabling

● impedance control: high-speed signal lines will show the characteristics of transmission lines, which need impedance control to avoid signal reflection, overshoot and ringing, and reduce EMI radiation.

● classifies the signals, and according to the EMI radiation intensity and sensitivity of different signals (analog signal, clock signal, Imax O signal, bus, power supply, etc.), separate the interference source from the sensitive system as much as possible and reduce the coupling.

● strictly controls the routing length of clock signal (especially high-speed clock signal), the number of holes, cross-division area, termination, wiring layer, reflux path and so on.

● signal loop, that is, the loop formed by signal outflow to signal inflow, is the key of EMI control in PCB design, which must be controlled during wiring. To understand the flow direction of each key signal, the key signal should be routed close to the reflux path to ensure that the loop area is minimum.

Figure 1, signal circuit

For low-frequency signals, the current should flow through the path with the lowest resistance; for high-frequency signals, the high-frequency current should flow through the path with the lowest inductance, not the path with the least resistance (see figure 1). For differential mode radiation, the EMI radiation intensity (E) is proportional to the current, the area of the current loop and the square of the frequency. (where I is the current, An is the loop area, f is the frequency, r is the distance to the center of the loop, and k is constant. )

Therefore, when the minimum inductor reflux path is just below the signal wire, the current loop area can be reduced, thus reducing the EMI radiation energy.

The critical signal of ● must not span the segmented area.

The ● high-speed differential signal routing adopts tight coupling mode as far as possible.

● ensures that strip lines, microstrip lines and their reference planes meet the requirements.

The lead-out line of ● decoupling capacitor should be short and wide.

All ● signal lines should be as far away from the edge of the board as possible.

● for multi-point connection network, choose the appropriate topology to reduce the signal reflection and reduce the EMI radiation.

The Segmentation of Power supply plane

Segmentation of ● Power layer

When there are one or more sub-power supplies on a main power supply plane, the continuity of each power supply area and sufficient copper foil width should be ensured. The dividing line does not need to be too wide, generally 20 ~ 50 millet line width can be used to reduce gap radiation.

Segmentation of ● ground layer

The ground level should be kept intact to avoid segmentation. If it must be divided, it is necessary to distinguish between digital, analog and noisy, and connect to the outside through a common ground at the exit.

In order to reduce the edge radiation of the power supply, the power supply / ground plane should follow the 20H design principle, that is, the size of the ground plane is 20H larger than that of the power supply plane (see figure 2), so that the radiation intensity of the edge field can be reduced by 70%.

About how the application of EMC/EMI control in PCB design is shared here, I hope the above content can be of some help to you, can learn more knowledge. If you think the article is good, you can share it for more people to see.

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