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 EMC requirements as much as possible without causing too much cost pressure? How to improve electromagnetic compatibility performance in PCB design? This article has the answers to these EMC issues that give you headaches~
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 increasing the number of ground layers to enhance the shielding effect and adding ferrite beads, chokes and other high-frequency harmonic suppression devices. In addition, it is usually necessary to match the shielding structure of other institutions to make the entire system pass EMC requirements. The following only provides a few tips for reducing the electromagnetic radiation effects produced by circuits in the design of PCB boards:
1. Try to use devices with slower signal slopes to reduce the high-frequency components generated by the signal. Pay attention to the placement of high-frequency components and do not place them too close to external connectors.
2. Pay attention to the impedance matching of high-speed signals, wiring layers and return current paths to reduce high-frequency reflection and radiation.
3. Place sufficient and appropriate decoupling capacitors on the power pins of each device to alleviate noise on the power layer and ground layer. 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 layer, and the ground of the connector can be connected to the chassis ground nearby.
5. Ground protection/shunt traces can be appropriately used next to some particularly high-speed signals, but attention should be paid to their impact on the characteristic impedance of the wiring.
6. The power layer is 20H smaller than the ground layer. H is the distance between the power layer and the ground layer.
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 jump rate of the upper and lower edges of the control signal line.
2. Try to provide some form of damping (high-frequency capacitor, reverse diode, etc.) for relays.
3. The signals entering the printed board must be filtered, and the signals from the high-noise area to the low-noise area must also be filtered. At the same time, use series terminal resistors to reduce signal reflection.
4. The useless end of the MCU must be connected to the power supply through the corresponding matching resistor. Either be grounded or defined as an output terminal. The terminals on the integrated circuit that should be connected to power and ground must be connected and should not be left floating.
5. Do not leave unused gate circuit input terminals floating, but connect them to power or ground through corresponding matching resistors. The positive input terminal of the unused op amp is connected to ground, 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 next 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.
How to reduce EMI problems by arranging stackup?
First of all, EMI must be considered from the system, and PCB alone cannot solve the problem. For EMI, lamination mainly provides the shortest return path for signals, reduces the coupling area and suppresses differential mode interference. In addition, the ground layer and the power layer are closely coupled and appropriately extended than the power layer, which is beneficial to suppressing common mode interference.
What is the difference between magnetic cores used for electromagnetic interference suppression and those traditionally used as inductors? What will happen if the two are used incorrectly?
The materials traditionally used as inductor cores have very small losses, and the inductors made with this type of magnetic core have very small losses. The magnetic core used for electromagnetic interference suppression has a large loss, and the inductor made with this magnetic core has a large loss, and its characteristics are closer to that of a resistor. If both are used incorrectly, the intended purpose will not be achieved. If the magnetic core used for electromagnetic interference suppression is used in an ordinary inductor, the Q value of the inductor will be very low, which will cause the resonant circuit to fail to meet the requirements, or the signal that needs to be transmitted will suffer too much loss. If the ordinary magnetic core used for making inductors is used for electromagnetic interference suppression, the interference at a certain frequency may be enhanced due to resonance between the inductor and the parasitic capacitance in the circuit.
Knowledge expansion: PCB EMC design layout and wiring experience
1. Overall layout
1) High-speed, medium-speed and low-speed circuits should be separated;
2) Keep high current, high voltage, and strong radiation components away from weak current, low voltage, and sensitive components;
3) Analog, digital, power supply, and protection circuits should be separated;
4) Multi-layer board design with separate power and ground planes;
5) Heat-sensitive components (including liquid dielectric capacitors, crystal oscillators) should be kept as far away from high-power components, radiators and other heat sources as possible.
2. Overall wiring
1) Route key signal lines to avoid cross-segmentation;
2) Avoid “U” or “O” shapes when routing key signal lines;
3) Whether the key signal lines are artificially long;
4) Whether the key signal line is more than 400mil away from the edge and interface;
5) Buses with the same function should run in parallel, and other signals should not be intertwined in the middle;
6) Whether there are traces underneath the crystal oscillator;
7) Whether the wiring is routed under the switching power supply;
8) Receiving and sending signals must be done separately and cannot cross each other.
