At industrial communication sites, engineers are most concerned about transient overvoltages and overcurrents caused by surges in the communication network. These surges can lead to incorrect signals being sent across the bus or even cause system failure. To prevent such incidents, it is essential to implement proper protection measures during the early design phase. This article will explore effective strategies for surge protection in bus communication systems.
Surges, whether from lightning strikes, switching operations, or other electrical disturbances, can create both common-mode and differential-mode voltages. Common-mode surges are typically associated with lightning or high-current switching events, while differential-mode surges often result from nearby high-voltage lines or poor insulation between data cables and power lines. Although differential-mode surges may have lower peak voltages and currents, they can persist longer within the communication network, potentially causing damage to sensitive components.
Optocouplers and magnetically coupled devices are rated for common-mode voltage, which represents the isolation capability between the input and output sides. If this rating is exceeded, the device may be damaged. However, differential-mode voltage tolerance is determined by the circuit design itself. A differential-mode surge exceeding the circuit’s limits can damage the front end without affecting the back end.
To protect against these threats, two main approaches are commonly used: isolation and diversion. Isolation involves using optocouplers or magnetic couplers to separate the input and output signals. This method can suppress common-mode surges but not differential-mode ones. Diversion methods, on the other hand, involve grounding the system at a single point and using devices like TVS diodes, varistors, or gas discharge tubes (GDTs) to divert harmful currents away from sensitive components.
Combining both isolation and diversion techniques provides more comprehensive protection. The diversion device can handle both common-mode and differential-mode surges, while the isolation device protects the host equipment from common-mode surges. This dual approach ensures better reliability in harsh environments.
For example, in CAN interface protection, an optocoupler or magnetic coupler is often used in conjunction with a TVS or GDT. This setup helps meet IEC61000-4-5 standards, ensuring that the system can withstand surges up to ±4kV. In some cases, integrated modules like the SP00S12 offer a compact and efficient solution, combining isolation and surge suppression in a single package.
These modules are particularly useful in applications such as CAN and RS-485 communication, where space is limited and performance must remain consistent under various conditions. They simplify design, reduce PCB footprint, and improve overall system reliability.
In summary, each surge protection method has its advantages and limitations. While discrete solutions provide flexibility, module-based solutions offer simplicity and efficiency. Testing according to IEC61000-4-5 standards ensures that the protection meets real-world requirements, providing peace of mind for engineers working in industrial environments.
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