How can onboard and external filters in EPS systems be used in conjunction?

The onboard filter and external (or rack-mount) filter of the EPS need to work together to form a graded protection system. Onboard filters (such as decoupling capacitors near the power IC, small ferrite beads in the power circuit, and surface-mount common-mode inductors) are responsible for the first attenuation at the noise source, suppressing the generation and diffusion of high-frequency noise at the chip level and within the board. External filters (independent EMI filter modules or rack-mount filters) handle the noise remaining after onboard filtering and external interference introduced through cables, providing high attenuation at the system level to meet standard limits.

Key points for compatibility:

1. Impedance matching: The onboard filter output impedance should be higher, and the external filter input impedance should be lower to avoid mismatch.

2. Frequency emphasis: Onboard filters should prioritize suppressing VHF noise >10MHz, while external filters should prioritize mid-to-low frequency noise <10MHz.

3. Grounding coordination: Both should be connected to the same low-impedance "clean ground" reference point.

4. Cable handling: Incoming and outgoing cables must be shielded, and the shielding layer must be properly grounded at the filter inlet.