How to design surge protection for I/O modules?

Surge protection design for I/O modules requires a tiered protection principle based on the expected surge level and port type. The first level of protection is typically installed at the system inlet, using high-current-carrying gas discharge tubes or varistors, such as the 2R series GDT or 14D series varistors, to dissipate most of the surge energy. The second level of protection is installed at the module port, using TVS diodes or varistors, such as the SMBJ or 5KP series, to clamp residual voltage. The two levels of protection should be decoupled through inductors, resistors, or fuses to ensure the first level operates first. For signal ports, the capacitance of the protection device must be compatible with the signal bandwidth; for example, a low-capacitance TVS array ESDLC series should be selected. The ground of the protection device should be connected to a dedicated protective ground, which is connected to the signal ground through a high-voltage capacitor or gap. On the PCB, the surge current path should be short and straight to avoid excessively long ground lines on the protection device, which could cause voltage drops. After design, a combined waveform test must be performed according to IEC 61000-4-5 to verify the protection effect under different waveforms and to check whether the protection device is damaged after the test.