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Würth Elektronik 2022 9 Assuming that the current through the rectifier bridge is trapezoidal, the EMC spectrum without line filter and without further Fourier transformation can be approximately determined. The first corner point of the enclosing amplitude spectral density is needed first: The first corner frequency of the enclosing amplitude spectral density is analogous to this: The amplitude of the first harmonic can be determined from this: Caused by fast switching processes of semiconductor components, usually MOSFETs, high-frequency oscillations occur in combination with parasitic effects. In principle, the differential-mode current flows from the mains power line "L" over a rectifier bridge, then over the primary winding of the isolating transformer, over the MOSFET and over the neutral conductor "N" back to the mains. The MOSFET is mounted on a heat sink for cooling. In turn, this is connected to the protective earth conductor "PE". Capacitive coupling between the heat sink and the drain of the MOSFET occurs here and produces common mode interference. A capacitive coupled common mode current "i CM " now flows along the earth line "PE" back to the switch- mode power-supply input where it is coupled again via parasitic capacitance both on the mains power line "L" and on the neutral line "N". The common mode current "i CM " now flows (Figure 1) over both mains power lines, via the rectifier bridge to the MOSFET, where it is again parasitically coupled via the heat sink to the earth line "PE". Expected interference spectrum The rectified mains voltage is applied at the drain-source section. The peak level of the rectified mains voltage corresponds to: A switch-mode power supply with pulse frequency of 100kHz has been used as an example. For this pulse frequency, the timing signal corresponds to "T", 10µs. The pulse duration is 2µs. Based on this, the duty cycle can first be determined: Based on the assumption that the parasitic coupling capacitance "C P " between the switch mode power supply and ground is 20pF, the first harmonic common mode current can now be determined: The radio interference voltage is measured using a Line Impedance Stabilization Network (LISN) and an EMC test receiver. Due to the parallel connection of the 50Ω input impedance of the EMC test receiver and the 50Ω output impedance of the LISN, total impedance "Z" of 25Ω is produced. The measured radio interference voltage "V cm " can now be calculated: Converted to dBµV, this gives: Switch-mode power supplies result in conducted interference because they generate radio interference voltage on the mains side. "