How to control differential-mode conducted interference in industrial servo systems?

Servo differential-mode conducted interference mainly exists between the power line and the return line, caused by the voltage drop across the loop inductor due to the di/dt of the switching devices. The core of controlling differential-mode interference is to reduce the loop area and increase the high-frequency impedance.

Based on the Etymotic solution, firstly, a combination of low ESR/ESL electrolytic capacitors and C0G ceramic capacitors is connected in parallel between the positive and negative terminals of the DC bus, for example, using a PWR6045R6R8M0T power inductor (6.8μH) to form an LC filter.

Secondly, a PBZ3216E-120Z0T ferrite bead (120Ω@100MHz) is connected in series at the rectifier bridge output to suppress high-frequency noise. For PCB layout, the power loop uses tightly coupled routing to minimize the loop area. Simultaneously, an X2 safety capacitor (e.g., 0.1μF/305VAC) is added at the AC input to work in conjunction with the common-mode inductor for filtering.

Tests and verifications show that this differential-mode filter network can reduce conducted interference in the 150kHz-30MHz frequency band by more than 20dB, meeting the GB/T 17799.3 industrial environmental standard. Attention must be paid to the design of the discharge resistor for the X capacitor in the solution to ensure it meets safety regulations.