How to avoid resonance in multi-stage filtering of frequency converters

Avoiding resonance in multi-stage filter circuits of a frequency converter is crucial for ensuring stable filtering performance and system reliability. When multiple stages such as input EMI filters, DC bus LC filters, and output filters are cascaded, impedance mismatch may cause resonance peaks at specific frequencies, amplifying noise and even inducing oscillations. Methods to avoid this include: designing each filter stage with staggered resonant frequencies; for example, the input filter for 150kHz-30MHz, the DC bus filter for the switching frequency, and the output filter for higher frequencies. Adding damping resistors between stages, such as paralleling a resistor across the filter inductor or connecting a small resistor in series with the filter capacitor, can dissipate resonance energy.

Using magnetic core materials with loss characteristics, such as certain ferrite beads, can suppress resonance due to their high-frequency losses. Establishing an equivalent circuit model of the multi-stage filter using simulation software, performing frequency domain analysis, and predicting and optimizing potential resonance points are also important. In actual testing, if an abnormally high noise level is detected at a certain frequency, resonance should be suspected and can be verified by temporarily adjusting the capacitor or inductor values. For the output LC filter, the interaction with the motor cable and motor impedance must be considered, which may require adding a damping resistor network at the filter output. Eintech's filter design service can provide simulation support to help customers optimize multi-stage filtering schemes and avoid resonance risks.