The role of y capacitor in switching power supply
Y capacitors are used for common mode filtering and are connected between L and ground or N and ground to filter out common mode signals from L to ground or N to ground. (Y capacitors are usually used symmetrically) They are both safety capacitors
At the input end of the AC power supply, it is generally necessary to add three safety capacitors to suppress EMI conducted interference.
The AC power input is divided into three terminals: live wire (L), neutral wire (N), and ground wire (G). The capacitance connected in parallel between the live wire and ground wire, as well as between the neutral wire and ground wire, is generally referred to as the Y capacitance. The connection position of these two Y capacitors is crucial and must comply with relevant safety standards to prevent electronic device leakage or shell electrification, which may endanger personal safety and life. They all belong to safety capacitors, so it is required that the capacitance value cannot be too large, and the withstand voltage must be high. In general, machines operating in subtropical regions are required to have a ground leakage current of no more than 0.7mA; Working on temperate machines, it is required that the leakage current to the ground should not exceed 0.35mA. Therefore, the total capacity of Y capacitor generally cannot exceed 4700PF (472).
It is particularly pointed out that as a safety capacitor, Y capacitor must obtain certification from a safety testing agency. The appearance of Y capacitors is mostly orange or blue, and they are generally marked with safety certification marks (such as UL, CSA, etc.) and the words "withstand voltage AC250V or AC275V". However, its true DC withstand voltage can reach over 5000V. It must be emphasized that Y capacitors should not be replaced by ordinary capacitors with nominal withstand voltage of AC250V or DC400V.
The capacitance in parallel between the live line and the zero line suppression is generally referred to as the X capacitance. Due to the critical location of the capacitor connection, it is also necessary to comply with relevant safety standards. The X capacitor is also one of the safety capacitors. According to actual needs, the capacitance value of capacitor X is allowed to be larger than that of capacitor Y, but at this time, a safety resistor must be connected in parallel at both ends of capacitor X to prevent the power line plug from being charged for a long time due to the charging and discharging process of the capacitor when unplugging and plugging the power line. According to safety standards, when the power cord of a working machine is unplugged, the voltage (or ground potential) charged at both ends of the power cord plug must be less than 30% of the original rated working voltage within two seconds.
As one of the safety capacitors, X capacitor also requires certification from safety testing institutions. X capacitors are generally marked with safety certification marks and the words "withstand voltage AC250V or AC275V". However, their true DC withstand voltage can reach 2000V or above. When using them, do not use ordinary capacitors with nominal withstand voltage AC250V or DC400V as substitutes.
Usually, X capacitors are made of polyester film capacitors with high ripple currents. This type of capacitor has a large volume, but it also allows for a large current for instantaneous charging and discharging, and its internal resistance is correspondingly small. The indicators of ripple current in ordinary capacitors are very low, and the dynamic internal resistance is high. Replacing X capacitor with ordinary capacitor not only fails to meet the standard for capacitor withstand voltage, but also makes it difficult for ripple current indicators to meet the requirements.
