Detailed explanation of X and Y safety capacitors

手可摘棉花

Detailed explanation of X and Y safety capacitors [Copy link]

At the input end of the AC power supply, three capacitors are generally needed to suppress EMI conduction interference. The input of the AC power supply can generally be divided into three wires: live wire (L)/neutral wire (N)/ground wire (G).

1. The capacitors connected in parallel between the live wire and the ground wire and between the neutral wire and the ground wire are generally called Y capacitors . The connection positions of these two Y capacitors are critical and must comply with relevant safety standards to prevent leakage of electronic equipment or charging of the casing, which may endanger personal safety and life. Therefore, they are all safety capacitors, requiring that the capacitance value cannot be too large and the withstand voltage must be high. Generally, for machines working in the subtropical zone, the leakage current to the ground must not exceed 0.7mA; for machines working in the temperate zone, the leakage current to the ground must not exceed 0.35mA. Therefore, the total capacity of Y capacitors generally cannot exceed 4700pF .

Special note: Y capacitors are safety capacitors and must be certified by a safety testing agency. The withstand voltage of Y capacitors is generally marked with a safety certification mark and AC 250V or AC 275V, but its actual DC withstand voltage is as high as 5000V or more. Therefore, Y capacitors cannot be replaced with ordinary capacitors with a nominal withstand voltage of AC 250V or DC 400V.

2. The capacitor connected in parallel between the live wire and the neutral wire is generally called an X capacitor . Since the location of this capacitor connection is also critical, it also needs to comply with safety standards. Therefore, the X capacitor is also one of the safety capacitors. 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 the power cord is plugged in and out. The safety standard stipulates that when the power cord of the machine in operation is unplugged, within two seconds, the voltage (or ground potential) at both ends of the power cord plug must be less than 30% of the original rated working voltage. Similarly, the X capacitor is also a safety capacitor and must be certified by a safety testing agency. The withstand voltage of the X capacitor is generally marked with a safety certification mark and AC 250V or AC 275V, but its actual DC withstand voltage is as high as 2000V or more. When using it, do not use ordinary capacitors with a nominal withstand voltage of AC 250V or DC 400V as a substitute.

X capacitors are usually polyester film capacitors with relatively large ripple current . Such capacitors are usually large in size, but the current they allow for instantaneous charge and discharge is also large, while their internal resistance is relatively small. The ripple current index of ordinary capacitors is very low, and the dynamic internal resistance is relatively high. When ordinary capacitors are used to replace X capacitors, in addition to the fact that the withstand voltage condition cannot be met, the general ripple current index is also difficult to meet the requirements.

In fact, it is unlikely to completely filter out the conducted interference signal by relying solely on the Y capacitor and the X capacitor. This is because the spectrum of the interference signal is very wide, basically covering the frequency range of tens of KHz to hundreds of MHz, or even thousands of MHz. Usually, filtering low-end interference signals requires large-capacity filter capacitors, but due to safety restrictions, the capacity of the Y capacitor and the X capacitor cannot be large; for filtering high-end interference signals, the filtering performance of large-capacity capacitors is extremely poor, especially the high-frequency performance of polyester film capacitors is generally poor, because it is produced by winding technology, and the high-frequency response characteristics of polyester film dielectrics are far different from those of ceramics or mica. Generally, polyester film dielectrics have an adsorption effect, which will reduce the operating frequency of the capacitor. The operating frequency range of polyester film capacitors is about 1MHz, and its impedance will increase significantly if it exceeds 1MHz.

3. Therefore, in order to suppress the conducted interference generated by electronic equipment, in addition to selecting Y capacitors and X capacitors, multiple types of inductor filters should be selected at the same time to combine them to filter out the interference.

Inductor filters are mostly low-pass filters, but there are many specifications and types of inductor filters, such as differential mode, common mode, high frequency, low frequency, etc. Each inductor is mainly used to filter out interference signals of a small frequency range, and has little effect on filtering interference signals of other frequencies. Usually, an inductor with a large inductance has a large number of coil turns, so the distributed capacitance of the inductor is also large. High-frequency interference signals will be bypassed through the distributed capacitance. Moreover, the operating frequency of the magnetic core with a high magnetic permeability is relatively low. At present, the operating frequency of the inductor filter cores used in large quantities is mostly below 75MHz. For occasions with relatively high operating frequency requirements, high-frequency toroidal magnetic cores must be selected. The magnetic permeability of high-frequency toroidal magnetic cores is generally not high, but the leakage inductance is particularly small, such as amorphous alloy cores, Permalloy, etc.

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