How to control differential-mode conducted interference in frequency converters

Controlling differential-mode conducted interference in frequency converters hinges on the filtering design at the power input side and the suppression of internal switching noise. Differential-mode interference exists between the LN or LL lines of the input power supply, with the spectrum concentrated at the switching frequency and its low harmonics. Control measures begin with designing an effective differential-mode filter circuit at the AC input, typically using an L-type or π-type filter composed of a differential-mode inductor and an X capacitor. The inductance of the differential-mode inductor needs to be calculated based on the switching frequency and required attenuation, taking into account its rated current and saturation characteristics. The capacitance of the X capacitor must balance the filtering effect with the inrush current. Secondly, suppressing internal noise sources is crucial. Optimizing the configuration of the DC bus capacitors, using low-ESR/ESL electrolytic capacitors in parallel with film capacitors, provides a low-impedance path for high-frequency switching current, reducing bus voltage ripple and thus lowering differential-mode noise reflected to the input. Optimizing the IGBT drive reduces switching overshoot. On the PCB layout, the power loop area formed by the input rectifier bridge, DC bus capacitors, and IGBTs must be minimized. Determining out-of-range frequencies through pre-testing allows for targeted adjustment of filter parameters or increasing the filter order. Eintech Electronics offers high-current differential-mode inductors and safety-rated X capacitors suitable for frequency converters. Combined with a well-designed internal system, these inductors effectively control differential-mode conducted interference to within standard limits.