How to set up differential mode filtering at the power port of an HMI (Human Machine Interface)?

When designing a differential-mode filter circuit for an HMI power port, it's crucial to define the frequency band and amplitude of the differential-mode noise to determine the filter order and component parameters. Differential-mode noise primarily occurs at the switching frequency of the power supply and its low-order harmonics. A basic first-order filter consists of an L-shaped circuit with a differential-mode inductor and an X capacitor. For greater attenuation, a second-order π-shaped circuit can be used, consisting of two inductors and one X capacitor, or one inductor and two X capacitors. The choice of inductance is critical; the required inductive reactance at the target attenuation frequency must be calculated, while considering the impact of its DC resistance (DCR) on voltage drop and the saturation current (Isat).

The X capacitor's value must balance filtering effectiveness and inrush current; thin-film capacitors are typically used. In practical designs, a high-resistance resistor can be connected in parallel across the differential-mode inductor to suppress potential resonance peaks generated by the LC circuit. During PCB layout, the differential-mode filter circuit should be concentrated at the power port inlet, and the lead wires of the inductors and capacitors should be short and thick to reduce parasitic parameters. Eintech's power inductor series and safety-certified X2 capacitors are ideal for building power port differential filters. They offer a full range of parameters, reliable performance, and help HMI products effectively suppress differential conducted emissions at the power port.