Causes of electromagnetic interference in switching power supply
Electromagneticlnterference (EMI) is a kind of performance damage of electronic system or subsystem caused by unexpected electromagnetic disturbance. It consists of three basic elements: interference source, that is, equipment that generates electromagnetic interference energy; Coupling channel, that is, the channel or medium for transmitting electromagnetic interference; Sensitive equipment, that is, devices, equipment, subsystems or systems damaged by electromagnetic interference. Based on this, the basic measures to control electromagnetic interference are: suppressing interference sources, cutting off the path of disaster, reducing the response of sensitive equipment to interference or increasing the electromagnetic sensitivity level.
According to the working principle of switching power supply, it is known that switching power supply first rectifies power frequency alternating current into direct current, then inverts it into high frequency alternating current, and finally outputs it through rectification and filtering to obtain stable direct current voltage. In the circuit, the power triode and diode mainly work in the switching state, and work in the microsecond order; When triode and diode are turned on and off, the current changes greatly during the rising and falling time, which is easy to generate radio frequency energy and form interference sources. At the same time, the leakage inductance of transformer and the peak caused by the reverse recovery current of output diode will also form potential electromagnetic interference.
Switching power supply usually works at high frequency, and the frequency is above 02 kHz, so its distributed capacitance can not be ignored. On the one hand, the insulation sheet between the heat sink and the collector of the switch tube has a large contact area and a thin insulation sheet, so the distributed capacitance between them can not be ignored at high frequency, and high-frequency current will flow to the heat sink through the distributed capacitance and then to the chassis ground, resulting in common-mode interference; On the other hand, there is a distributed capacitance between the primary stages of the pulse transformer, which can directly fuse the voltage of the primary winding to the secondary winding and produce common-mode interference on the two power lines with DC output of the secondary winding.
Therefore, the interference sources in switching power supply are mainly concentrated in the components such as switching tubes, diodes and high-frequency transformers, as well as AC input and rectification output circuits.
