How to avoid resonance in multi-stage power supply filtering for Human-Machine Interface (HMI)?

To avoid resonance in multi-stage power supply filter circuits in an HMI (High-Milk Media Filter), the key lies in understanding the resonance characteristics of LC (Low-Loss) circuits and designing them appropriately. When two or more LC filters are cascaded, impedance mismatch between the preceding and following stages can generate resonance peaks at specific frequencies, amplifying noise. First, when designing each filter stage, its resonant frequency should be kept far away from the noise band and operating frequency that need to be suppressed. Cascading LC filters with different resonant frequencies can be used; for example, the first stage can target lower frequencies, and the second stage can target higher frequencies. Inserting damping resistors between stages is an effective way to suppress resonance.

A resistor can be connected in parallel with the filter inductor or in series with the filter capacitor to dissipate resonance energy. Using magnetic beads or inductors made of lossy core materials, whose high-frequency loss characteristics help to smooth out resonance peaks. Frequency domain analysis of the multi-stage filter network using simulation software can predict and optimize resonance points in advance. In actual testing, if an abnormally high conducted noise level is found at a certain frequency, resonance should be suspected, and can be verified and adjusted by temporarily adding or removing capacitors or changing inductor values. The PBZ series ferrite beads from Eintech Electronics have inherent resistance, which provides a certain degree of damping while filtering. This helps to build a stable multi-stage filter network, avoids resonance problems, and ensures the reliability of the filtering performance.