At the AC input, it is generally necessary to add three capacitors to suppress EMI conducted interference.
The input of the AC power supply can generally be divided into three lines: FireWire (L) / Neutral (N) / Ground (G). The capacitor connected between the live and ground lines and between the neutral and ground lines is generally referred to as a Y capacitor. The position of the two Y capacitors is relatively critical, and must comply with relevant safety standards to prevent leakage of electronic equipment or the casing is charged, which is likely to endanger personal safety and life. Therefore, they are all safety capacitors, and the capacitance value cannot be too large. The withstand voltage must be high. Generally, a machine operating in a subtropical zone requires that the earth leakage current should not exceed 0.7 mA; for a temperate machine, the earth leakage current should not exceed 0.35 mA. Therefore, the total capacity of the Y capacitor generally cannot exceed 4700pF.
Special note: The Y capacitor is a safe capacitor and must be certified by a safety inspection agency. The voltage tolerance of Y capacitors is generally marked with a safety certification mark and AC250V or AC275V, but its true DC withstand voltage is as high as 5000V or more. Therefore, the Y capacitor cannot be used arbitrarily with a nominal capacitor such as AC250V or DC400V.
A capacitor connected in parallel between the live line and the neutral line is generally referred to as an X capacitor. Since the location of this capacitor connection is also critical, it also needs to meet 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 the Y capacitor, but a safety resistor must be connected in parallel with 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. The safety standard stipulates that when the working power cord is unplugged, the voltage applied to both ends of the power cord plug (or ground potential) must be less than 30% of the original rated operating voltage within two seconds.
Similarly, the X capacitor is also a safety capacitor and must be certified by a safety inspection agency. The X capacitor's withstand voltage is generally marked with the safety certification mark and the words AC250V or AC275V, but its true DC withstand voltage is as high as 2000V or more. Do not use the nominal voltage AC250V or DC400V ordinary capacitor when using it. Substitute.
X capacitors generally use polyester film capacitors with large ripple current. These capacitors are generally large in size, but they allow a large amount of current for instantaneous charge and discharge, and their internal resistance is relatively small. The common capacitor ripple current has a low index and a high dynamic internal resistance. Replacing the X capacitor with a common capacitor, in addition to the withstand voltage conditions can not be met, the general ripple current index is also difficult to meet the requirements.
In fact, it is not possible to rely solely on the Y and X capacitors to completely filter out conducted interference signals. Because the spectrum of the interfering signal is very wide, it covers the frequency range from tens of KHz to several hundred MHz, even thousands of MHz. Generally, the filtering of the low-end interference signal requires a large-capacity filter capacitor, but due to the safety conditions, the capacity of the Y-capacitor and the X-capacitor cannot be used large; the filtering of the high-end interference signal, the filtering of the large-capacity capacitor The performance is extremely poor, especially the high-frequency performance of the polyester film capacitor is generally poor, because it is produced by the winding process, and the high-frequency response characteristics of the polyester film medium is far from the ceramic or mica, generally The polyester film media has an adsorption effect, which reduces the operating frequency of the capacitor. The operating frequency range of the polyester film capacitor is about 1 MHz, and the impedance will increase significantly beyond 1 MHz.
Therefore, in order to suppress the conducted interference generated by the electronic device, in addition to the Y capacitor and the X capacitor, multiple types of inductive filters are also selected, and the interference is filtered together. Inductance filters are mostly low-pass filters, but there are many types of inductive filters, such as differential mode, common mode, and high frequency and low frequency. Each type of inductor is mainly used for filtering out interference signals of a certain small frequency, and the filtering effect of interference signals of other frequencies is not large. Generally, an inductor with a large inductance has a large number of turns, and the distributed capacitance of the inductor is also large. The high frequency interference signal will be bypassed by the distributed capacitance. Moreover, a magnetic core with a high magnetic permeability has a lower operating frequency. At present, the operating frequency of a large number of inductive filter cores is mostly below 75 MHz. For applications where the operating frequency is relatively high, high-frequency toroidal cores must be used. The magnetic permeability of high-frequency toroidal cores is generally not high, but the leakage inductance is particularly small, such as amorphous alloy cores, permalloys, etc.
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