EMC/EMI Control Technology in PCB Design
This paper introduces the design of EMI control technology PCB in digital circuit With the improvement of the integration of integrated circuit devICes, the gradual miniaturization of equipment and the growth rate of equipment, the electromagnetic interference problem in electronic products has become more serious From the perspective of system equipment EMC/electromagnetic interference design, dealing with EMC/electromagnetic interference problem PCB equipment design stage is an effective and economic means to make system equipment meet electromagnetic compatibility standards
1 Principles of EM generation and suppression
EMI arises from sources of electromagnetic interference that transfer energy to sensitive systems through coupling paths. It includes three basic forms: conductor conduction or common grounding, space radiation, or near-field coupling The harm of electromagnetic interference is to reduce the quality of transmitted signals, cause interference or even damage to circuits or equipment, and make the equipment fail to meet the technical index requirements specified in the electromagnetic compatibility standards In order to suppress electromagnetic interference, the electromagnetic interference design of digital circuit shall be carried out according to the following principles: according to the relevant electromagnetic compatibility/electromagnetic interference technical specifications, the quota shall be divided into single board circuits and controlLED by stages From the three elements of electromagnetic interference, namely, interference source, energy coupling path and sensitive system, control them to make the circuit have a flat frequency response and ensure the normal and stable operation of the circuit Start from the front-end design of the equipment, pay attention to the EMC/EMI design, and reduce the design cost
2. Digital circuit EMI control technology PCB board
When dealing with various forms of electromagnetic interference, specific problems must be analyzed In PCB digital circuit design, EMI control can be carried out from the following aspects
2.1 Device Selection
In EMI design, the speed of the selected equipment should be considered first Any circuit that replaces equipment with a rise time of 5ns with equipment with a rise time of 2ns. 5ns rise time will increase EMI by about 4 times The radiation intensity of electromagnetic interference is proportional to the square of frequency, and EMI frequency (fknee) is also called the EMI emission bandwidth, which 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) The frequency range of this radiated EMI is 30MHz to several GHz. In this frequency band, the wavelength is very short, and even the very short wiring on the circuit board can become a transmitting antenna When EMI is high, the circuit will often lose its normal function Therefore, in terms of equipment selection, on the premise of ensuring circuit efficiency requirements, low-speed chips should be used as far as possible, and appropriate driving/receiving circuits should be used In addition, because the lead pin of the device has parasitic inductance and parasitic capacitance, in high-speed design, the influence of device packaging form on the signal cannot be ignored, because it is also an important factor of electromagnetic interference radiation Generally, the parasitic parameters of SMD devices are SMAller than those of plug-in devices, and the parasitic parameters of BGA packages are smaller than those of QFP packages
2.2 Connector selection and signal terminal definition
Connector is the key link of high speed signal transmission In the terminal design of the connector, more grounding pins can be arranged to reduce the distance between the signal and the ground, reduce the effective signal loop area that generates radiation in the connector, and provide a low impedance return path If necessary, consider isolating some critical signals with ground pins
2.3 Laminate Design
Under the premise of cost permitting, increase the number of ground plane and place the signal layer next to the ground plane to reduce EMI radiation For high-speed PCB boards, the power supply and ground plane are tightly coupled to reduce the power supply impedance and thus reduce electromagnetic interference
According to the signal current flow, reasonable layout can reduce the interference between signals Correct layout is the key to control electromagnetic interference The basic principle of layout is: analog signal is easy to be interfered by digital signal, and analog circuit should be separated from digital circuit; The clock line is the main source of interference and radiation. In summary, it should be far away from sensitive circuits and keep the clock track short; The use of power consumption circuits in the central area of the circuit board shall be avoided as far as possible, and the effects of heat dissipation and radiation shall be considered at the same time; The connector shall be arranged at one side of the circuit board as far as possible, 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 supply pin; Multi power devices should be placed across the boundary of power division area to effectively reduce the impact of plane division on EMI; the return plane (path) is not divided.
Impedance control: High speed signal lines exhibit the characteristics of transmission lines
Classify the signals, and separate the interference source from the sensitive system as much as possible according to the EMI radiation intensity and sensitivity of different signals (analysis signal, clock signal, I/O signal, bus, power supply, etc.) to reduce coupling Strictly control the track length, number of vias, cross partition, termination, routing layer, return path, etc of clock signals (especially high-speed clock signals).
Signal loop, that is, the loop formed by the signal flowing out to the signal flowing in, is the key to EMI control. PCB must be controlled during wiring Understand the flow direction of each key signal, and arrange the key signal near the return path to ensure its loop area For low-frequency signals, the path that allows current to flow through the resistor; For high-frequency signals, the path that causes high-frequency current to flow through the inductor is not a resistor The EMI radiation intensity (E) is significant to the current, the area of current loop and the square of frequency (where I is the current, A is the loop area, f is the frequency, r is the distance to the center of the loop, and k is a constant.) Herebefore, when the inductor loop is just below the signal line, the area of the current loop can be reduced, thereby reducing the EMI radiation energy Key signals shall not pass through the segmented area The high-speed differential signal track shall be closely coupled as far as possible Ensure that the stripline, microstrip line and their datum meet the requirements The lead of decoupling capacitor shall be short and wide All signal traces shall be as far away from the edge of the circuit board as possible For the multipoint connection network, select an appropriate topology to reduce signal reflection and EMI transmission
2.6 Split Processing of Power Plane
For the division of the power supply layer, when there is one or more sub power supplies on the main power supply plane, ensure the continuity of each power supply area and sufficient copper foil width The dividing line need not be too wide, usually 20 to 50 mils wide, to reduce gap radiation For dividing the ground layer, the integrity of the ground layer shall be maintained to avoid splitting If it is necessary to separate, distinguish digital ground, analog ground and noise ground, and connect them to external ground through the common ground point at the outlet In order to reduce the edge radiation of the power supply, the power/ground plane should follow the 20H design principle, that is, the size of the ground plane is 20H larger than the size of the power plane, reducing the edge field radiation intensity by 70%
3. Other control methods for EMI
3.1 Power System Design
Design a low impedance power system to ensure that the impedance of the power distribution system in the sub-fknee frequency range is lower than the target impedance. Use filters to control conducted disturbances Decoupling of power supply In EMI design, reasonable decoupling capacitor can make the chip work reliably, reduce high-frequency noise in the power supply, and reduce electromagnetic interference Due to the influence of wire inductance and other parasitic parameters, the power supply and its power line respond slowly, which will lead to insufficient transient current required by the driver in high-speed circuit By properly designing bypass or decoupling capacitors and distributed capacitors in the power layer, the energy storage effect of capacitors can be used to quickly supply current to equipment before the power response Proper capacitive decoupling provides a low impedance power path, which is the key to reduce common mode EMI
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