How to design I/O module transistor output EMC?

Designing an I/O module transistor output circuit with good EMC performance requires optimizing switching transients. An RC snubber circuit should be connected in parallel between the transistor collector or drain and the power supply to slow voltage rise and suppress ringing. When driving inductive loads, a freewheeling diode must be connected in parallel across the load; a fast recovery type diode such as ES3AC should be used, and it should be placed as close to the load as possible. To reduce switching noise, a small resistor can be connected in series in the transistor base or gate drive circuit to control the switching speed. Output cables should be twisted-pair and shielded if possible.

On the PCB, the transistor output loop area should be minimized, and high-current paths should be short and wide. The transistor driver's power supply should be isolated from the main power supply via a ferrite bead, and sufficient decoupling capacitors should be provided. For high-frequency noise, a ferrite bead can be connected in series on the output line. If possible, use a MOSFET driver with soft-switching characteristics. After the design is completed, conducted and radiated emission tests should be performed, focusing on the noise level at the switching frequency and its harmonics, and adjusting the snubber circuit parameters or adding filters based on the results.