source:Industry News Popular:adapter release time:2021-06-22 15:17:44 Article author:sznbone
The inability to shield switching devices and the inability to provide radio frequency shielding to the transformer is the main cause of conducted common mode interference.
Due to the limitation of the size of the decoupling capacitor, these interference components are also the most difficult to eliminate in the filter.
Differential mode or series mode noise is more easily bypassed by electrolytic energy storage capacitors, and relatively large decoupling capacitors C3 and C4 are allowed to be connected to both ends of the AC power supply.
Common-mode radio frequency interference currents are introduced into the nearby ground plane (usually the chassis or casing of the power supply) by insulation leakage and parasitic electrostatic coupling or electromagnetic coupling, as shown in Figure 1.3 C~C. These parasitic currents will pass through. Coupling capacitors C1 and C flow back to the input AC power line to form a loop.
Because the source voltage and source impedance are both large, this loop current drive source can be approximated as a constant current source. Therefore, the voltage across the decoupling capacitors C1 and C2 can be approximated as a voltage source, which is proportional to the current amplitude of the interference harmonic frequency and the capacitor impedance:
Vhi=IiXc
In the formula, Vhi=harmonic interference voltage;
Ii=harmonic frequency interference current;
Xc=C1 or C2 harmonic frequency reactance.
It is assumed that the insulation leakage current is negligible.
The voltage source Vhi drives the current to flow through the series inductors L1, L2, and L3, and then to the output line and return via the ground line. It is the external component of the RF current that causes the external interference, and because of this it is limited by the standard and must be minimized.
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