The most comprehensive analysis of X/Y safety capacitors in power supply filtering

Most people know a little about the safety capacitors, but in fact the safety capacitors are made up of X capacitors and Y capacitors. It mainly plays the role of power supply filtering in the circuit. Filtering common mode and differential mode interference respectively. This article mainly introduces the X capacitor and Y capacitor in the safety capacitor, and explains the basic knowledge in detail.

Typically, we need three safe capacitors to suppress conducted interference from EMI in the AC input. The AC power input is divided into three terminals: FireWire (L) / Neutral (N) / Ground (G). Capacitors that are connected between the live and ground lines and between the neutral and ground lines are generally referred to as Y capacitors. The position of the two Y capacitors is critical, and it is necessary to comply with relevant safety standards to prevent electrical equipment leakage or the chassis from being charged, which may endanger personal safety and life. They are all safety capacitors, which require that the capacitance value not be too large and the withstand voltage must be high. Under normal circumstances, the machine operating in the subtropical zone requires that the earth leakage current should not exceed 0.7mA; working in a temperate machine requires that the earth leakage current should not exceed 0.35mA. Therefore, the total capacity of the Y capacitor generally cannot exceed 4700 PF (472).

In particular, as a Y capacitor for a safety capacitor, it is required to obtain certification from a safety inspection mechanism. The appearance of Y capacitors is mostly orange or blue. Generally, they are 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 used arbitrarily with a common capacitor such as a nominal withstand voltage of 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 comply with relevant safety standards. The X capacitor is also one of the safety capacitors. According to the 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 with the X capacitor to prevent the power cord from being inserted and removed due to the charging and discharging process of the capacitor. The power cord plug is energized for a long time. The safety standard stipulates that when the working power cord is unplugged, the voltage charged at both ends of the power cord plug (or ground potential) must be less than 30% of the rated operating voltage within two seconds.

The X capacitor, which is one of the safety capacitors, is also required to be certified by a safety inspection mechanism. X capacitors are generally marked with a safety certification mark and withstand voltage AC250V or AC275V, but the true DC withstand voltage is as high as 2000V or more. Do not use the ordinary capacitors such as AC250V or DC400V at random.

In general, X-capacitors are mostly made of polyester film-type capacitors with a large ripple current. This type of capacitor is bulky, but it allows a large amount of current to be charged and discharged instantaneously, and its internal resistance is correspondingly 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 capacitor withstand voltage can not meet the standard, the ripple current index is also difficult to meet the requirements.

According to IEC 60384-14, capacitors are divided into X capacitors and Y capacitors:

1. X capacitor refers to a capacitor across the L-N;

2. Y capacitor refers to a capacitor across L-G/N-G;

(L=Line, N=Neutral, G=Ground)

The X capacitor is further divided into X1, X2, and X3. The main differences are:

1. X1 withstand voltage is greater than 2.5 kV, less than or equal to 4 kV;

2. X2 high pressure is less than or equal to 2.5 kV;

3. X3 high pressure is less than or equal to 1.2 kV

The Y capacitor is divided into Y1, Y2, Y3, and Y4. The main differences are: (DC voltage level)

1. Y1 high pressure is greater than 8 kV;

2. Y2 high pressure is greater than 5 kV;

3. Y3 high pressure n/a

4. Y4 high pressure is greater than 2.5 kV

They are used in the power supply filter to filter the power supply and filter the common mode and differential mode interference.

Safety capacitors are used in applications where the capacitor does not cause an electric shock and does not endanger personal safety. It includes X and Y capacitors.

The X capacitor is a capacitor that is connected between the two lines of the power line (LN). Generally, a metal film capacitor is used. The Y capacitor is a capacitor that is connected between the two lines of the power line and the ground (LE, NE), and is generally in pairs. . Based on the limitation of leakage current, the value of Y capacitor should not be too large. Generally, the X capacitor is uF grade, and the Y capacitor is nF grade. The X capacitor suppresses differential mode interference and the Y capacitor suppresses common mode interference.

Safety capacitor safety level:

Safety Capacitance Safety Level Peak pulse voltage overvoltage level allowed in applications (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 additional insulation <150V

The capacitance of the Y capacitor must be limited to achieve the purpose of controlling the leakage current flowing through it under the rated frequency and rated voltage and the impact on the EMC performance of the system. GJB151 specifies that the capacity of the Y capacitor should be no more than 0.1uF. In addition to the corresponding grid voltage withstand voltage, the Y capacitor also requires sufficient safety margin for electrical and mechanical performance to avoid breakdown and short circuit under extreme environmental conditions. The pressure resistance of the Y capacitor is correct. Protecting personal safety is of great significance.

Add a little more to choose where to pay attention:

1. Capacitor for suppressing power supply electromagnetic interference

When capacitors are used in power-supply circuits to eliminate noise, not only normal voltages, but also abnormal pulse voltages (such as lightning) must be considered, which can cause the capacitor to smoke or ignite. Therefore, the safety standards for cross-line capacitors are strictly regulated in different countries, so it is necessary to use safety-certified capacitors.

2. DC capacitors are not allowed to be used as jumper capacitors:

Capacitors for X2 class suppression power supply electromagnetic interference should be suitable for applications where the capacitor does not cause a shock hazard when the capacitor fails. For example, the power supply can be connected to the line and can withstand a pulse voltage of 2.5kV.

Capacitors for Y2 class suppression power supply electromagnetic interference should be applied to the case where the capacitor does not cause a shock hazard when the capacitor fails. When the power supply is connected across the line, it can withstand a 5kV pulse voltage shock without causing breakdown.

X capacitors, polystyrene (thin film ethylene) capacitors, and polystyrene have high withstand voltage and are suitable for high voltage pulse absorption of EMI circuits.

Capacity calculation: general two-stage X capacitor, 0.47uF for the first stage, 0.1uF for the second base, and 0.47uF for the single stage. There is no convenient calculation method at present. (The size of the capacitor capacity is not directly related to the power of the power supply)

The X capacitor refers to the peak voltage, not the rated voltage. The X capacitor is generally MPX (metallized polypropylene mold capacitor). Their rated voltage is also several hundred volts, such as an X2 labeled MPX-X2-250VAC/275VAC.

What is a safety capacitor and XI X2 X3 Y1 Y2 Y3 Y4:

Safety capacitors are used in applications where the capacitor does not cause an electric shock and does not endanger personal safety. It includes two types of X capacitors and Y capacitors. The x capacitor is a capacitor that is connected between the two lines of the power line (LN). Generally, a metal film capacitor is used. The Y capacitor is connected between the two lines of the power line and the ground (LE). , NE) capacitors are generally present in pairs. Based on the limitation of leakage current, the value of Y capacitor should not be too large. Generally, the X capacitor is uF grade, and the Y capacitor is nF grade. The X capacitor suppresses differential mode interference and the Y capacitor suppresses common mode interference.