How should the common-mode inductor of the power supply be matched for HMI (Human Machine Interface)?

Matching a common-mode inductor to an HMI power supply requires selection based on the frequency band of common-mode noise to be suppressed, the rated current, and the impedance characteristics. First, determine the frequency range where common-mode noise exceeds the limit through conducted emissions testing or spectrum analysis. The impedance-frequency curve of the common-mode inductor is crucial; a model with higher impedance within the exceeding frequency band should be selected. For example, the CMZ7060A series can be used for the low-frequency band of 150kHz-30MHz; the CMZ3225A series can be used for higher frequency bands. The rated current must be greater than the operating current flowing through the inductor, with a margin to prevent impedance drop due to core saturation.

The DC resistance (DCR) of the inductor must also be considered; an excessively high DCR can lead to unnecessary voltage drop and heat generation. In some high-frequency noise-rich applications, two stages of common-mode inductors may need to be connected in series to achieve a wider suppression bandwidth and higher insertion loss. Matching with safety Y capacitors must also be considered, as the capacitance value of the Y capacitors affects the common-mode resonant point of the system. In practical circuits, the common-mode inductor should be placed as close as possible to the power supply port, with the Y capacitors before and after it forming a complete filter network. Etymotic's CMZ series common mode inductors are available in a variety of sizes, current ratings, and impedance characteristics. Engineers can select the most suitable model based on specific noise spectrum and circuit parameters to achieve optimal common mode noise suppression.