Why is capacitance Y generally not more than 0.1uF?

Y Capacitor

The Y capacitor is a kind of safety capacitor. In such cases, a safety capacitor is used: that is, after failure of capacitor, it will not cause an electric shock and will not endanger personal safety. This is because of difference between safety capacitors and other ordinary capacitors. Ordinary capacitors will keep a certain residual voltage for a long time after power is turned off. Touching them with hands may cause electric shock, but safety capacitors will not.

Most of Y capacitors are blue, but some are yellow. Since they are safety capacitors, Y capacitors are usually marked with appropriate certifications such as CQC, VDE, UL and other certifications, as shown in fig. 1.

The Y capacitor is usually connected between neutral line and ground or live line and ground, as shown in Figure 2, is a class I EMI filter circuit, Y1 and Y2 are Y capacitors, usually two connected in series.

In terms of resistance to high pressure, it is divided into three classes: Y1, Y2 and Y4, of which:

High voltage capacitor Y1 >8 kV;

5kV

Capacitor

2.5kV

Leakage current

For switching power supplies, capacitor Y is usually connected between primary side (primary) and secondary side (secondary), as shown in fig. 3. Capacitor Y can provide a return path for interference current from primary side to secondary side.

So, why can't Y be too large?

We know that Y capacitor is very useful for EMC. Because it is a common mode capacitor, it must be grounded, so leakage current occurs. Take above EMI filter circuit as an example:

Capacitor leakage current Y I=2πfCU

Where:

f — network frequency equal to 50 Hz;

C - total capacitance Y 4700pF+4700pF=9400pF;

The voltage between U and ground terminal here is 110V.

Leakage current can be calculated as high as 0.32 mA.

And it will be found that leakage current is proportional to capacitance, that is, larger capacitance, greater leakage current. For appliances powered by AC 220 V/50 Hz, many countries stipulate that leakage current should not exceed 1 mA; other electrical appliances such as portable and mobile devices also have different leakage current requirements. Therefore, if power supply must be connected to Y capacitor, capacitance is generally not very large, and sometimes Y capacitor is even removed, so capacitance of Y capacitor that we often see is basically not more than 0.1uF. .