How are the differential mode filter parameters for industrial servo systems calculated?

Servo differential mode filter parameter calculation is based on the target attenuation, system impedance, and interference frequency. The calculation steps are as follows: First, determine the differential mode interference frequency f_dm that needs to be attenuated (e.g., the 150th harmonic of a 2kHz switching frequency at 300kHz). Calculate the required insertion loss IL at this frequency (e.g., 20dB). Using the formula IL=10log10[1+(2πf_dm L_dm / R_s)^2], where R_s is the source impedance (usually taken as 50Ω), deduce the required differential mode inductance L_dm. For example, to achieve 20dB attenuation at 300kHz, L_dm≈80μH. Based on this, select a PWR series power inductor (e.g., PWR6045R8R2M0T, 8.2μH) and a capacitor C_x. The value of C_x is determined by the filter's cutoff frequency f_c, where f_c = 1/(2π√(L_dm C_x)). Typically, f_c is set to 1/5 to 1/10 of f_dm. In practice, a 0.47μF X2 capacitor is selected. After calculation, the actual S21 curve of the filter needs to be verified using a network analyzer to ensure that the attenuation requirements are met at the target frequency and that there are no resonance peaks.