The role of switching power supply y capacitor
The Y capacitor is used for common-mode filtering. It is connected between L and N or N and ground to filter out the common-mode signal of L to ground or N to ground. (Y capacitors are usually used symmetrically) They are all safety capacitors.
At the input end of the AC power supply, it is generally necessary to add 3 safety capacitors to suppress EMI conduction interference.
The AC power input is divided into 3 terminals: live wire (L)/neutral wire (N)/ground wire (G). The capacitors connected in parallel between the live wire and the ground wire and between the neutral wire and the ground wire are generally referred to as Y capacitors. The positions where the two Y capacitors are connected are critical, and must comply with relevant safety standards to prevent leakage of electronic equipment or electrification of the casing, which may endanger personal safety and life. They are all safety capacitors, which require that the capacitance value cannot be too large, and the withstand voltage must be high. In general, for machines working in subtropical regions, the earth leakage current should not exceed 0.7mA; for machines operating in temperate regions, the earth leakage current should not exceed 0.35mA. Therefore, the total capacity of Y capacitors generally cannot exceed 4700PF (472).
In particular, it is pointed out that the Y capacitor as a safety capacitor must be certified by a safety testing agency. Y capacitors are mostly orange or blue in appearance, and are generally marked with safety certification marks (such as UL, CSA, etc.) and withstand voltage AC250V or AC275V. However, its true DC withstand voltage is as high as 5000V or more. It must be emphasized that the Y capacitor should not be replaced by ordinary capacitors such as AC250V or DC400V with a nominal withstand voltage.
The capacitor connected in parallel between the live line and the neutral line suppression is generally called an X capacitor. Since the location of this capacitor connection is also critical, it also needs to comply with relevant safety standards. The 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 at this time a safety resistor must be connected in parallel at both ends of the X capacitor to prevent the power cord from being damaged due to the charging and discharging process of the capacitor. Cause the power cord plug to be charged for a long time. The safety standard stipulates that when the power cord of the machine at work is unplugged, within two seconds, the charged voltage (or ground potential) at both ends of the power cord plug must be less than 30% of the original rated working voltage.
As one of the safety capacitors, X capacitors are also required to obtain certification from safety testing agencies. X capacitors are generally marked with safety certification marks and withstand voltage AC250V or AC275V, but their real DC withstand voltage is as high as 2000V or more. When using them, do not use ordinary capacitors with a nominal withstand voltage of AC250V or DC400V as substitutes.
Usually, X capacitors mostly use polyester film capacitors with relatively large ripple currents. This type of capacitor has a large volume, but it allows a large current for instantaneous charging and discharging, and its internal resistance is correspondingly small. Ordinary capacitors have very low ripple current indicators and high dynamic internal resistance. Replacing the X capacitor with an ordinary capacitor, in addition to the capacitor withstand voltage cannot meet the standard, the ripple current index is also difficult to meet the requirements.
