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

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 leakage of the electronic equipment or the casing is charged, 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 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, and is 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 the Y capacitor must not be used as a substitute for 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. However, 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 machine power cord is unplugged, the voltage charged across the power cord plug (or ground potential) must be less than 30% of the rated operating voltage within two seconds.