How to protect against electrical fast pulses during EFT in HMI (Human Machine Delivery) systems?

Protecting HMIs from EFT (Electronic Fast Transmission) interference requires designing for its rapid rise time and repetitive pulse characteristics. EFT interference mainly enters the device through power lines and signal lines. Power line protection is crucial; a combination of devices capable of absorbing fast pulse energy should be placed at the AC or DC power input, such as a varistor (e.g., 7D390K), a TVS diode (e.g., SMBJ24CA), and a common-mode inductor (e.g., CMZ7060A-701T). The varistor absorbs most of the energy, the TVS diode clamps the voltage, and the common-mode inductor suppresses common-mode components. For internal branches of the DC power supply, an additional layer of protection can be added at each sub-power input, such as small-sized TVS diodes and ferrite beads.

For signal line protection, using isolated transceiver chips at communication ports is an effective method, such as isolated RS485 chips. If isolation is not available, high-speed TVS diodes or dedicated ESD/TVS arrays, such as ESDSVLC05-4, should be used on the signal lines, along with common-mode chokes. In terms of PCB layout, protection circuits must be placed close to the interface, and the ground wires of protection devices should be short and thick, directly connected to the chassis ground or a separate protective ground plane to provide a fast discharge path for pulse current. By adopting the combination of EFT protection devices and optimized circuit layout from EENTE Electronics, the HMI's immunity to EFT interference can be significantly improved, allowing it to successfully pass relevant tests.