How to Address Electromagnetic Compatibility and Interference in PCB Design
Electromagnetic compatibility (EMC) and its associated electromagnetic interference (EMI) have always been critical concerns for system design engineers. With the ongoing miniaturization of circuit boards and component packaging, coupled with OEM demands for faster systems, these challenges are especially daunting for PCB layout and design engineers.
EMC involves the generation, propagation, and reception of electromagnetic energy, which PCB designs strive to minimize. Electromagnetic energy arises from various sources that often mix, making it crucial to ensure that circuits, traces, vías, and PCB materials work harmoniously to maintain signal compatibility and prevent interference.
Conversely, EMI results from unwanted electromagnetic energy and poses destructive effects. PCB designers must mitigate EMI by minimizing the generation of such energy and reducing interference to the greatest extent possible.
Techniques to Solve EMC and EMI Issues in PCB Design
Technique 1: PCB Grounding
One of the most effective ways to reduce EMI is through PCB grounding. Start by maximizing the ground area across the PCB, which helps minimize emissions, diafonía, and noise. Extra care should be taken to connect each component to the ground point or plane, as failing to do so negates the neutralizing benefits of a reliable ground plane.
Complex PCB designs often feature multiple stable voltage levels. Ideally, each reference voltage should have a dedicated ground plane. Sin embargo, having too many ground planes can increase manufacturing costs. A balanced approach is to use three to five ground planes at strategic locations, with each plane covering multiple ground sections. This method helps control manufacturing costs while reducing EMI and EMC.
To minimize EMC, a low-impedance grounding system is essential. In multilayer PCBs, a robust ground plane is preferable to a copper balancing block or scattered ground areas, as it offers low impedance, a clear current path, and an optimal return signal source.
Signal return time is another critical factor. Signals must travel to and from their source within equivalent timeframes. Otherwise, they act like antennas, turning radiated energy into EMI. Similarmente, the traces transmitting current to and from the signal source should be as short as possible. Unequal source and return path lengths can lead to ground bounce, further contributing to EMI.
Technique 2: Distinguishing EMI Sources
Since different EMI sources vary in characteristics, a sound EMC design principle is to separate analog circuits from digital circuits. Analog circuits, which often involve higher currents, should be kept away from high-speed traces or switching signals. When possible, ground signals should be used to shield them. On multilayer PCBs, analog traces should be routed over one ground plane, while switching or high-speed traces should be over another, ensuring that signals with different characteristics remain isolated.
A low-pass filter can sometimes be employed to eliminate high-frequency noise coupled from nearby traces. Such filters help suppress noise and stabilize current flow. Separating the ground planes for analog and digital signals is equally critical. Los circuitos analógicos y los circuitos digitales exhiben características únicas., Requiere conexión a tierra independiente.. Las señales digitales deben terminar en una tierra digital., mientras que las señales analógicas deben terminar en una tierra analógica.
Los ingenieros experimentados en diseño de PCB prestan mucha atención a las señales y relojes de alta velocidad en el diseño de circuitos digitales.. Para señales de alta velocidad, Las trazas y los relojes deben ser lo más cortos posible y estar ubicados cerca de los planos de tierra.. Esto minimiza la diafonía, ruido, y radiación, manteniéndolos bajo control.
Las señales digitales también deben mantenerse alejadas de los aviones de potencia.. La proximidad entre estos planos puede inducir ruido o diafonía., debilitando la integridad de la señal.
Technique 3: Priorizar la reducción de diafonía en el diseño de trazas
El diseño de traza adecuado es crucial para garantizar un flujo de corriente fluido. Para corrientes provenientes de osciladores o dispositivos similares, Es vital separarlos de los planos de tierra o evitar el recorrido paralelo con otras trazas., trazas particularmente de alta velocidad. Las señales paralelas de alta velocidad son propensas a problemas de EMC y EMI, especialmente diafonía. Los caminos de resistencia de traza deben mantenerse lo más cortos posible, con rutas de corriente de retorno igualmente minimizadas. Las longitudes de las trazas de la ruta de retorno deben coincidir con las longitudes de las trazas de transmisión..
En contextos EMI, un rastro a menudo se etiqueta como el «agresor» mientras que el otro es el «víctima.» El acoplamiento inductivo y capacitivo debido a campos electromagnéticos puede afectar el rastro de la víctima., Inducir corrientes hacia adelante y hacia atrás que provocan ondulaciones en las señales..
En un ambiente ideal y equilibrado, las corrientes inducidas se cancelarían entre sí, eliminando la diafonía. Sin embargo, Las condiciones del mundo real rara vez permiten la perfección., por lo que es esencial minimizar la diafonía. Mantener un espacio entre pistas paralelas que sea al menos el doble del ancho de la pista puede reducir significativamente la diafonía.. Por ejemplo, si el ancho de la traza es 5 mils, El espacio entre trazas paralelas debe ser 10 milésimas o más.
Technique 4: Condensadores de desacoplamiento
Los condensadores de desacoplamiento ayudan a mitigar los efectos adversos de la diafonía. Estos deben colocarse entre los pines de alimentación y tierra de un dispositivo para garantizar una baja impedancia de CA., reducir el ruido y la diafonía. El uso de múltiples condensadores de desacoplamiento en un amplio rango de frecuencia garantiza un rendimiento óptimo.
El condensador de menor valor debe colocarse lo más cerca posible del dispositivo para minimizar los efectos inductivos en la traza.. Este condensador debe conectarse directamente al pin de alimentación o al rastreo de alimentación del dispositivo., con sus pads vinculados a vías o al plano de tierra. Para trazas más largas, multiple vias can minimize grounding impedance.
Technique 5: Avoiding 90° Angles
To reduce EMI, avoid creating 90° angles in traces, vías, or other components, as sharp angles can lead to increased radiation. At these points, capacitance increases and characteristic impedance changes, causing reflections and EMI. Use two 45° angles to route traces around corners instead.
Technique 6: Careful Use of Vias
Vias are often indispensable in PCB layouts, providing conductive connections between layers. Sin embargo, they introduce inductance and capacitance, and in some cases, reflections due to impedance changes in the traces.
Vias also extend trace lengths, requiring proper length matching. For differential pairs, avoid vias if possible. If unavoidable, ensure both traces in the pair use vias to compensate for delay in signal and return paths.
Technique 7: Cable and Physical Shielding
Cables carrying digital and analog currents often generate parasitic capacitance and inductance, leading to EMC issues. Twisted pair cables maintain low coupling levels, eliminating magnetic fields. High-frequency signals require shielded cables grounded at both ends to prevent EMI interference.
Physical shielding involves enclosing all or parts of the system in metal to block EMI from entering the PCB circuit. Such shielding acts like a grounded conductive container, reducing antenna loop size and absorbing EMI.
Technique 8: Shielding and Filtering
- Adding Shielding: Use metal shields or shielding layers to reduce EMI when necessary. High-frequency components should be isolated using shielding boxes to prevent interference with other components.
- Filters and Suppressors: Add low-pass filters to suppress high-frequency noise and suppressors to control electromagnetic interference. These measures help keep crosstalk, ruido, and radiation levels within acceptable limits.
Technique 9: Simulation and Validation
- Perform electromagnetic field and radiation analyses using simulation software after completing the PCB design to identify potential EMI issues.
- Optimize the PCB design based on simulation results to ensure compliance with EMC requirements.
By applying these techniques, engineers can design more efficient and stable circuit boards, reducing electromagnetic interference and improving overall system performance. Follow LSTPCB for more insights into PCB, PCBA, and component design tips, and enjoy free prototyping services!
