At industrial communication sites, engineers are most concerned about transient overvoltages and overcurrents generated by the communication network due to surges. These can cause the bus communication network to send incorrect signals or even lead to system failure. To prevent such incidents, how should protection be integrated into the early design phase? This article will explore these issues in detail.
Surges at industrial sites often come from lightning strikes, induced thunder surges, or switching operations in power systems—especially those with heavy inductive loads. These events can create transient overvoltages that affect data bus networks, leading to signal errors or component damage. As a result, surge protection is a critical consideration in bus design. In this article, we'll discuss common methods for protecting buses against surges.
There are two main types of surge protection: common-mode and differential-mode. Surges caused by lightning or high-current switching are typically common-mode. On the other hand, differential-mode surges usually result from nearby high-voltage lines or poor insulation between data cables and high-voltage lines. While differential-mode surges may have lower voltage and current than common-mode ones, they tend to persist longer in communication networks. The nominal withstand voltage of optocouplers or magnetically coupled devices refers to the common-mode voltage, which is the voltage between the input and output. If this limit is exceeded, both sides can be damaged. Differential-mode voltages, however, depend on circuit design, and exceeding their tolerance can damage the front end without affecting the back end.
A conventional surge protection approach involves isolation and bypass techniques. Isolation uses optocouplers or magnetic couplers to separate input and output signals. If the surge voltage doesn't exceed the device's rated value, it won't be damaged, even if the surge persists for a long time. However, this method only protects against common-mode surges and not differential-mode ones. Bypass techniques involve connecting the ground of the main equipment to a single point, allowing surge energy to be safely dissipated. Additional components like TVS diodes, varistors, and gas discharge tubes (GDTs) can also be used to divert harmful currents before they reach the data port.
Combining isolation and bypass methods offers better protection. The bypass device suppresses surges that the isolation device cannot handle, while the isolation device protects the host from common-mode surges. Together, they provide more comprehensive protection for the bus system.
For example, CAN interface protection often includes an optocoupler or magnetic coupler along with a transceiver. Using an integrated transceiver module simplifies the design, reduces PCB space, and improves environmental adaptability. While common-mode protection is widely used, this article focuses on enhancing differential-mode protection. Common bypass devices include GDTs, TVS diodes, and common-mode inductors. A typical configuration places a GDT at the front of the interface for primary surge protection, followed by a TVS for secondary protection.
Zhiyuan Electronics has developed a compact signal surge protector called SP00S12. This module combines potting material with an isolation module to meet IEC61000-4-5 ±4kV standards. It is ideal for protecting communication ports in CAN and RS-485 systems. The SP00S12 can be easily integrated into various signal transmission systems to suppress surges caused by lightning or overvoltage events.
Testing is essential to ensure that surge protectors meet the required standards. According to IEC61000-4-5, a common-mode surge test involves applying a 4kV, 1.2/50μs surge to the terminals of the surge protector and measuring the output voltage. For instance, when a 3.94kV surge is applied to the SP00S12, the output voltage is reduced to 17.1V, demonstrating effective suppression. This test confirms that the device meets the necessary immunity requirements for industrial communication systems.
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