The role of capacitors in the switching power supply y
Y capacitor is used for common mode filtering, it is connected to the L on the ground or N on the ground between, filtering out the L on the ground or N on the ground of the common mode signal, (Y capacitor is usually used symmetrically) they are safety capacitors.
In the AC power input, generally need to add 3 safety capacitors to suppress EMI conducted interference.
The AC power input is divided into three terminals: Fire (L) / Zero (N) / Ground (G). The capacitors connected in parallel between the Fire and Ground wires and between the Zero and Ground wires are generally referred to as Y-capacitors. These two Y capacitors are connected to the location of the more critical, must be required to comply with the relevant safety standards, in order to prevent leakage caused by electronic equipment or casing charged, easy to endanger personal safety and life. They all belong to the safety capacitors, which requires the capacitance value can not be large, and the withstand voltage must be high. In general, work in the subtropical machine, the requirements of the earth leakage current can not exceed 0.7mA; work in the temperate zone machine, the requirements of the earth leakage current can not exceed 0.35mA. Therefore, the total capacity of the Y capacitor generally can not be more than 4700PF (472).
Special note: as a safety capacitor Y capacitors, the requirement must be certified by the safety testing agency, Y capacitors are mostly orange or blue in appearance, generally marked with safety certification mark (such as UL, CSA and other signs) and withstand voltage AC250V or AC275V words. However, its true DC withstand voltage up to 5000V or more. It must be stressed that the Y capacitor shall not be used arbitrarily nominal withstand voltage AC250V or DC400V or such ordinary capacitors to substitute.
In parallel between the fire wire and zero line suppression of capacitance, generally known as X capacitance. Because this capacitor is connected to the position is also more critical, also need to comply with the relevant safety standards. x capacitor also belongs to one of the safety capacitors. According to the actual need, X capacitance is allowed to be larger than the capacitance of Y capacitance, but at this time must be connected in parallel with a safety resistor at both ends of the X capacitance, used to prevent the power cord unplugging, due to the capacitor's charging and discharging process, resulting in a long period of time the power cord plug with electricity. Safety standards require that when the power cord is unplugged from a working machine, within two seconds, the voltage (or potential to ground) at both ends of the power cord plug must be less than 30 per cent of the original rated operating voltage.
As one of the safety capacitors X capacitors, also requires that must obtain the certification of the safety testing agencies. X capacitors are generally marked with safety certification mark and withstand voltage AC250V or AC275V words, but its real DC withstand voltage up to 2000V or more, do not feel free to use the use of nominal withstand voltage AC250V or DC400V and so on the ordinary capacitors to substitute.
Usually, the X capacitor is used more ripple current polyester film capacitors. This type of capacitor, the volume is larger, but it allows instantaneous charging and discharging current is also very large, and its internal resistance is correspondingly small. Ordinary capacitor ripple current indicators are very low, dynamic internal resistance is high. Using ordinary capacitors instead of X capacitors, in addition to the capacitance withstand voltage can not meet the standard, ripple current indicators are also difficult to meet the requirements.
