How are EPS surge protection categorized?

Surge protection for EPS (e.g., IEC61000-4-5) requires a graded (multi-level) protection design to discharge and clamp energy step by step. A typical three-level protection is as follows:

1. Level 1 (Coarse Protection): At the very front end of the system (e.g., AC mains input, DC battery input), use a gas discharge tube (GDT) or a high-current-carrying varistor (MOV, e.g., 14D, 20D series) to discharge most of the lightning strike or operational overvoltage energy.

2. Level 2 (Intermediate Protection): After the power filter inside the cabinet or before the DC/DC module, use a varistor with slightly lower current-carrying capacity (e.g., 7D series) or a TVS diode (e.g., SMCJ, 5KP series) to further clamp residual voltage.

3. Level 3 (Fine Protection): At the front end of sensitive circuits (e.g., control board power supply, communication interface), use a TVS diode (e.g., SMBJ, P6KE series) or a semiconductor discharge tube (TSS) to ultimately clamp the voltage to the safe operating range of the IC. Inductors or resistors are typically used to decouple between different stages to achieve coordinated energy release.