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Introduction of X capacitor and Y capacitor

By：JASENCAN

Time：2020.09.26

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At the AC power input end, it is generally necessary to add 3 safety capacitors to suppress EMI conducted interference. The AC power input is divided into 3 terminals: live wire (L) / neutral wire (N) / ground wire (G). The capacitor connected in parallel between the live line and the neutral line suppression is generally called an X capacitor. Since the location of the 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 the capacitance 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 plugged and unplugged due to the charging and discharging process of the capacitor. Cause the power cord plug to live for a long time. The safety standard stipulates that when the power cord of the working machine is unplugged, within two seconds, the live voltage (or ground potential) at both ends of the power cord plug must be less than 30% of the original rated working voltage.
 
The capacitors connected in parallel between the live wire and the ground wire and between the neutral wire and the ground wire are generally collectively referred to as Y capacitors. The location where the two Y capacitors are connected is critical and must comply with relevant safety standards to prevent leakage of electronic equipment or electrification of the housing, which may endanger personal safety and life. They are all safety capacitors, so the capacitance value must not be too large, and the withstand voltage must be higher. Under normal circumstances, the machine working in the subtropical zone requires the ground leakage current not to exceed 0.7mA; for the temperate machine, the ground leakage current must not exceed 0.35mA. Therefore, the total capacity of Y capacitors generally cannot exceed 4700PF (472).
 
Specially point out:
As one of the safety capacitors, the X capacitor also requires the certification of a safety inspection agency. 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 it, do not use ordinary capacitors such as nominal withstand voltage AC250V or DC400V. As a safety capacitor, Y capacitors must be certified by a safety inspection 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 Y capacitors should not be replaced by ordinary capacitors such as nominal withstand voltage AC250V or DC400V.
 
Usually, X capacitors are mostly polyester film capacitors with relatively large ripple current resistance. This type of capacitor has a relatively large volume, but its allowable instantaneous charge and discharge current is also large, and its internal resistance is correspondingly small. Ordinary capacitors have low ripple current indicators and high dynamic internal resistance. Using ordinary capacitors instead of X capacitors, in addition to the capacitor withstand voltage can not meet the standard, the ripple current index is also difficult to meet the requirements.

Figure 1: Y capacitor
 
According to IEC 60384-14, capacitors are divided into X capacitors and Y capacitors.
 
1. X capacitance refers to the capacitor across L-N,
2. The Y capacitor refers to the capacitor across L-G/N-G.
(L=Line, N=Neutral, G=Ground)
 
The X capacitor is divided into X1, X2, X3, the main difference is:
 
1. X1 withstand high voltage is greater than 2.5 kV, less than or equal to 4 kV,
2. X2 withstand high voltage is less than or equal to 2.5 kV,
3. X3 withstand high voltage is less than or equal to 1.2 kV
 
The Y capacitor is divided into Y1, Y2, Y3, Y4, the main difference is: (DC voltage resistance level)
 
1. Y1 withstands high voltage greater than 8 kV,
2. Y2 withstands high voltage greater than 5 kV,
3. Y3 high pressure resistance n/a
4. Y4 withstands high voltage greater than 2.5 kV
 
They are used in the power filter to filter the power supply, and filter the common mode and differential mode interference respectively.
 
Safety capacitors are used in such occasions that after the capacitor fails, it will not cause electric shock and will not endanger personal safety. It includes X capacitors and Y capacitors.
 
The x capacitor is the capacitor connected between the two lines of the power line (LN), and metal film capacitors are generally used; the Y capacitor is the capacitor connected between the two lines of the power line and the ground (LE, NE), and usually appears in pairs . Due to the limitation of leakage current, the Y capacitor value cannot be too large. Generally, the X capacitor is uF and the Y capacitor is nF. The X capacitor suppresses differential mode interference, and the Y capacitor suppresses common mode interference.

Figure 2: X capacitor q device
Safety grade of safety capacitor
 
Safety capacitor safety class Allowable peak pulse voltage in application Overvoltage class (IEC664)
X1 ＞2.5kV ≤4.0kV Ⅲ
X2 ≤2.5kV Ⅱ
X3 ≤1.2kV ——
 
Safety capacitor safety class Insulation type Rated voltage range
 
Y1 Double insulation or reinforced insulation ≥ 250V
Y2 basic insulation or additional insulation ≥150V ≤250V
Y3 basic insulation or additional insulation ≥150V ≤250V
Y4 basic insulation or supplementary insulation ＜150V
 
The capacitance of the Y capacitor must be limited, so as to achieve the purpose of controlling the leakage current flowing through it under the action of the rated frequency and rated voltage and the impact on the EMC performance of the system.
GJB151 stipulates that the capacity of Y capacitor should not be greater than 0.1uF. In addition to complying with the corresponding grid voltage withstand voltage, Y capacitors are also required to have sufficient safety margins in electrical and mechanical properties to avoid breakdown and short-circuit phenomena under extremely harsh environmental conditions. The withstand voltage performance of Y capacitors Protecting personal safety is of great significance.
 
One more point to note when choosing:
 
1. Capacitors for suppressing electromagnetic interference of power supply
　　
When using capacitors to eliminate noise in power cross-line circuits, not only normal voltages, but also abnormal pulse voltages (such as lightning) must be considered, which may cause the capacitors to smoke or catch fire. Therefore, the safety standards of cross-line capacitors are strictly regulated in different countries, so safety-certified capacitors must be used.
 
2. It is not allowed to use DC capacitors as cross-line capacitors:
　　
The X2 type capacitor for suppressing electromagnetic interference of power supply should be suitable for occasions that will not cause electric shock when the capacitor fails, such as the power supply is connected across the line, and can withstand 2.5kV pulse voltage.
 
The Y2 type capacitor for suppressing electromagnetic interference of power supply should be suitable for occasions that will not cause electric shock when the capacitor fails. When used for cross-line connection of power supply, it can withstand 5kV pulse voltage impact without causing breakdown.
 
 
1. X capacitors, polystyrene (film vinyl) capacitors, and polystyrene have higher voltage resistance and are suitable for high voltage pulse absorption in EMI circuits.
 
2. Capacity calculation: Generally, two-stage X capacitors, 0.47uF for the first stage and 0.1uF for the second base; 0.47uF for single stage. There is no convenient calculation method yet. (The size of the capacitor is not directly related to the power of the power supply)
 
X capacitors refer to peak voltages, not rated voltages. X capacitors generally use MPX (metallized polypropylene mold capacitors) and their rated voltage is also several hundred volts. For example, an X2 is marked as MPX-X2-250VAC/275VAC.