Introduction to the role of switching power supply y capacitor
Y capacitors are used for common mode filtering, which 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)/ground wire (G). The capacitors connected in parallel between the live wire and the ground wire, as well as between the neutral wire and the ground wire, are generally referred to as Y capacitors. The connection position of these two Y capacitors is crucial and must comply with relevant safety standards to prevent electronic device leakage or chassis electrification, which can easily endanger personal safety and life. They all belong to safety capacitors, so it is required that the capacitance value should not be too large, and the withstand voltage must be high. In general, machines working in subtropical regions require a ground leakage current not exceeding 0.7mA; Working on temperate machines, it is required that the ground leakage current should not exceed 0.35mA. Therefore, the total capacity of Y capacitors generally cannot exceed 4700PF (472).
Special note: As a safety capacitor, Y capacitor must obtain certification from a safety testing agency. The appearance of Y capacitors is mostly orange or blue, usually marked with safety certification marks (such as UL, CSA, etc.) and the words "withstand voltage AC250V or AC275V". However, its true DC withstand voltage is over 5000V. It must be emphasized that ordinary capacitors with nominal withstand voltage of AC250V or DC400V should not be used as substitutes for Y capacitors.
The capacitor connected in parallel between the live and neutral suppression lines is generally referred to as an X capacitor. Due to the critical location of the capacitor connection, it also needs to comply with relevant safety standards. X capacitor is also one of the safety capacitors. According to actual needs, the capacitance of the X capacitor is allowed to be larger than that of the Y capacitor, but a safety resistor must be connected in parallel at both ends of the X capacitor to prevent the power cord plug from being charged for a long time due to the charging and discharging process of the capacitor when unplugging or unplugging the power cord. According to safety standards, when the power cord of a working machine is unplugged, the voltage (or ground potential) 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, the X capacitor also requires certification from a safety testing agency. 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 more. When using them, do not use ordinary capacitors with nominal withstand voltage AC250V or DC400V as substitutes.
Usually, X capacitors are mostly made of polyester film capacitors with high ripple current. This type of capacitor has a large volume, but it also allows for a large current for instantaneous charging and discharging, while 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 of capacitor withstand voltage, but also makes it difficult to meet the requirements of ripple current index.
