How to design the EMC of the trip circuit for a power FTU?

The trip circuit's EMC design ensures reliable operation and interference immunity. It employs a fast-acting, balanced filtering approach. A 1N4007W freewheeling diode and a 470Ω/2W resistor are connected in parallel across the trip coil. The drive signal uses optocoupler isolation (TLP281) with an isolation voltage of 2500Vrms. The power supply uses an independent isolation module WRB2405S-3W. Shielded twisted-pair cable is used for the signal lines. On the PCB, the trip circuit is isolated from other circuits and uses an independent ground plane. In software, the trip command uses pulse broadening (100ms) and multiple checks. With this design, the trip action time is less than 50ms, and the immunity meets the IEC 61000-4-4 EFT 4kV and IEC 61000-4-5 surge test requirements.