How to design the power supply filter circuit for an electrical force measurement switch (PMS)?

The PMS power supply filter circuit is the first line of defense against internal and external electromagnetic interference. Its design must consider differential-mode and common-mode rejection as well as safety requirements. A typical front-end filter employs a two-stage architecture: the first stage is a coarse filter, using an X2 capacitor (0.1-0.47μF/275VAC) and a differential-mode inductor PBZ3216E120Z0T (12μH) to form an LC network for differential-mode filtering; for common-mode filtering, a CMZ1211-501T common-mode inductor (500μH) and a Y2 capacitor (2.2nF) are connected to protective ground. The second stage is a board-level fine filter, adding a π-type filter at the DC/DC input front end, consisting of a PBZ1608E600Z0T ferrite bead, a 100μF electrolytic capacitor, and a 0.1μF ceramic capacitor.

Key design considerations include: safety capacitors must meet rated voltage and pulse withstand voltage requirements; the common-mode inductor saturation current must be greater than 1.5 times the maximum operating current; the filter circuit should be located close to the power supply input, and input and output lines should be separated to avoid coupling. In addition, to cope with surges, an SMCJ33CA TVS diode can be connected in parallel at the filter front end. With a properly designed filter circuit, the conducted emissions of the PMS in the 0.15-30MHz frequency band can be reduced by 10dB below the EN 55032 Class B limit, and it can easily pass the IEC 61000-4-5 surge test (common mode 2kV, differential mode 1kV).