How to balance safety capacitors and filtering in a smart controller PAC?

In the design of a PAC (Programmable Controller), safety capacitors (X/Y capacitors) not only provide safety isolation but are also key components of the filter network. The X capacitor is connected between the L and N lines for differential-mode filtering; its capacitance value must consider leakage current limitations and is typically 0.1μF-1μF, used in conjunction with the CMZ7060A-701T common-mode inductor. The Y capacitor is connected between L/G or N/G for common-mode filtering; its capacitance value generally does not exceed 4.7nF to control the leakage current to ground to less than 0.25mA. In the filter network design, the placement of the safety capacitors should be...

Key points: The X capacitor should be placed before the common-mode inductor, and the Y capacitor after it, to form an effective π-type filter; at the same time, the safety capacitors must be products certified by IEC60384-14 to ensure withstand voltage and failure mode safety; for the 24VDC power supply of the PAC, X2Y three-terminal capacitors can be used to replace the traditional combination, simplifying the design; actual tests show that the design that takes into account both safety and filtering can make the insulation resistance of the PAC greater than 100MΩ, while reducing conducted interference by more than 30dB, meeting the GB4943.1 safety standard and EN55032 EMC standard;