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How to improve the electromagnetic compatibility performance in PCB design?

source:News release time:2022-09-16 Hits:     Popular:PCB Assembly company

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Electromagnetic compatibility (EMC) is a headache for many PCB design engineers, including electromagnetic interference (EMI) and electromagnetic susceptibility (EMS). How to make your PCB design meet the EMC requirements as much as possible without causing too much cost pressure? How to improve electromagnetic compatibility performance in PCB design?

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01. How can PCB design meet EMC requirements as much as possible without causing too much cost pressure?


The increased cost due to EMC on the PCB is usually due to the increase of the number of ground layers to enhance the shielding effect and the addition of ferrite beads, choke coils and other devices to suppress high-frequency harmonics. In addition, it is usually necessary to match the shielding structure on other mechanisms to make the entire system pass the EMC requirements. The following are just a few tips for reducing the effect of electromagnetic radiation generated by the circuit in the design of the PCB board:


1. Use devices with slow signal slopes as much as possible to reduce the high-frequency components generated by the signal. Pay attention to the placement of high-frequency components, not too close to external connectors.


2. Pay attention to the impedance matching of high-speed signals, the trace layer and its return current path to reduce high-frequency reflection and radiation.


3. Place enough and appropriate decoupling capacitors on the power pins of each device to alleviate the noise on the power and ground layers. Pay special attention to whether the frequency response and temperature characteristics of the capacitor meet the design requirements.


4. The ground near the external connector can be properly separated from the ground, and the ground of the connector can be connected to the chassis ground nearby.


5. The ground protection/shunt traces can be properly used beside some particularly high-speed signals, but pay attention to its influence on the characteristic impedance of the traces.


6. The power layer is 20H smaller than the ground layer, and H is the distance between the power layer and the ground layer.


02. What are the circuit measures to improve electromagnetic compatibility performance in PCB design?


1. A resistor can be connected in series on the PCB trace to reduce the transition rate of the upper and lower edges of the control signal line.


2. Try to provide some form of damping (high-frequency capacitors, reverse diodes, etc.) for relays, etc.


3. The signal entering the printed board should be filtered, and the signal from the high-noise area to the low-noise area should also be filtered. At the same time, the method of serial terminal resistance is used to reduce signal reflection.


4. The useless end of the MCU should be connected to the power supply through the corresponding matching resistor. Or grounded or defined as an output terminal, the terminal that should be connected to the power supply and the ground on the integrated circuit should be connected, and should not be left floating.


5. The input terminal of the gate circuit that is not used should not be left floating, but should be connected to the power supply or ground through the corresponding matching resistor. The positive input terminal of the unused operational amplifier is grounded, and the negative input terminal is connected to the output terminal.


6. Set up a high-frequency decoupling capacitor for each integrated circuit, and add a small high-frequency bypass capacitor to each electrolytic capacitor.


7. Use large-capacity tantalum capacitors or polyester capacitors instead of electrolytic capacitors as charge and discharge energy storage capacitors on the circuit board. When using tubular capacitors, the case should be grounded.


03. How to reduce the EMI problem by arranging the stack?


First of all, EMI must be considered from the system, and PCB alone cannot solve the problem. For EMI, the stack is mainly to provide the shortest return path of the signal, reduce the coupling area and suppress the differential mode interference. In addition, the ground layer is tightly coupled with the power supply layer, and it is appropriate to extend the power supply layer, which is good for suppressing common mode interference.


04. What is the difference between the magnetic core used for electromagnetic interference suppression and the magnetic core traditionally used for inductance? What happens if the two are used incorrectly?


The material traditionally used as an inductor core has very little loss, and the loss of the inductor made with this core is very small. The magnetic core used for EMI suppression has a large loss, and the inductor made with this magnetic core has a large loss, and its characteristics are closer to resistance. If the two are used incorrectly, neither will achieve the intended purpose. If the magnetic core for electromagnetic interference suppression is used on a common inductor, the Q value of the inductor will be very low, which will make the resonant circuit fail to meet the requirements, or the loss of the signal to be transmitted will be too large. If the common magnetic core used for making inductance is used for electromagnetic interference suppression, the inductance and the parasitic capacitance in the circuit will resonate, which may increase the interference at a certain frequency.


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