Role of X and Y capacitors in switching power supplies
Switching power supply module of electromagnetic interference has been an important solution point, from the principle of electromagnetic interference mainly comes from two aspects, respectively, the conduction interference and radiation interference.
Conducted interference and radiated interference.
Due to the existence of parasitic parameters in the circuit, as well as the switching and switching off of the switching power supply in the FM switching device, the switching power supply in the utility AC input generates a large common mode interference and differential mode interference.
common mode interference and differential mode interference. Due to changes in the current in the conductor will produce changes in the space around the magnetic field, and changes in the magnetic field to produce changes in the electric field, the amplitude and frequency of this change in the current to determine the size of the electromagnetic generated as well as the scope of its role. In order to reduce and resist these electromagnetic interference on the power grid as well as electronic equipment to produce harm, engineers and technicians in the circuit design added X capacitance and Y capacitance.
X capacitor is used to eliminate differential mode interference. It mainly acts as a filter, matching with the common mode inductor, and is connected in parallel at both ends of the input to filter out the differential mode signal between the L and N lines. Usually
Often choose to resist ripple current is relatively large polyester film capacitors, the volume is larger to allow instantaneous charging and discharging of the current is relatively large, and its internal resistance is correspondingly small. In addition, X-capacitor
Capacitor will also use plastic square high-voltage CBB capacitors, CBB capacitors not only have better electrical performance, and parallel with the input of the power supply can effectively reduce the impact of high-frequency pulses on the power supply.
Generally two pins across the zero line and between the fire line, suitable for high-frequency, DC, AC, coupling, across the pulse circuit, can withstand the impact of over-voltage, generally used in parallel with the resistor, the purpose is to play a role in the discharge of charge.
Y capacitor is used to eliminate common mode interference. Respectively across the power line between the two lines and ground (L-E, N-E) capacitors, generally appear in pairs, filtering out the high harmonics, preventing interference and improving the quality of the output voltage. Based on the leakage current limitation, the Y capacitor value cannot be too large, and the Y capacitor is usually made of high-voltage ceramic chip.
Many isolated switching power supply in the primary and secondary Y capacitor is added in order to provide a loop for the common mode current of the secondary to the primary, to reduce the impact of common mode current on the output. Y capacitor is connected in series between the high-voltage ground and the low-voltage ground, and sometimes two Y capacitors are connected in series in order to increase the withstand voltage between the high-voltage ground and the low-voltage ground, and sometimes there is a lack of withstand voltage, resulting in the safety capacitor can not hit the withstand voltage, can be used as the Y capacitor. Choose high-voltage ceramic capacitors as Y capacitors, Y capacitors are usually connected to four cases.
① input, and common mode inductance to form a filter, L and N were added to the PE.
② energy storage capacitor positive and negative end of the PE plus (as shown in Figure 4)
③Output terminal to PE plus
④Transformer primary and secondary jumper
X capacitor and Y capacitor belong to the same safety capacitor. When the safety capacitor fails, it will not produce electric shock and will not endanger personal safety. Safety capacitors are usually used in anti-interference circuit to play the role of filtering, the discharge of the safety capacitor is not the same as ordinary capacitors, ordinary capacitors in the external power supply is disconnected after the charge will be retained for a long time. If you use X-capacitor and Y-capacitor used in switching power supply are different, they are not interchangeable. They are not interchangeable.
