Selection of capacitors for power supply(III)

EMI/RFI inhibition

These capacitors are connected to the input of the power supply to mitigate electromagnetic or radio interference from semiconductors. These capacitors are vulnerable to destructive overvoltage and transient voltage due to their direct connection to the main input line. As a result, different safety standards have been introduced in various parts of the world, including the European EN132 400, the United States UL1414 and 1283, and Canada's CSA C22.2 no. 0. 0 and 8.

X- and Y- capacitors using plastic film technology (such as EPCOS B3292X / B81122) offer one of the cheapest methods of suppression. The impedance of the suppression capacitor decreases with the increase of frequency, allowing high frequency current to pass through the capacitor. X capacitors provide a "short circuit" between lines for this current, and Y capacitors provide a "short circuit" between the line and the grounding equipment.

X and Y capacitors are further classified according to the peak of the surge voltage that they can withstand. For example, an X2 capacitor with a capacitance value up to 1 μ F has a rated peak surge voltage of 2.5 KV, while an X1 capacitor with a similar capacitance value has a rated peak surge voltage of 4 KV. The appropriate level of interference suppression capacitor should be selected according to the peak voltage during the load outage.