Causes of electromagnetic interference generated by switching power supplies

Electromagnetic interference (EMI) is a performance impairment caused by an unexpected electromagnetic disturbance in an electronic system or subsystem. It consists of three basic elements: the source of interference, that is, the device that generates electromagnetic interference energy; the way of combining, that is, the path or medium that transmits electromagnetic interference; the sensitive device, that is, the device, device, subsystem or which is damaged by electromagnetic interference or system. Based on this, the basic measures for controlling electromagnetic interference are: suppressing the source of interference, cutting off the path of mismatch, and reducing the response of sensitive equipment to interference or increasing the level of electromagnetic sensitivity.

According to the working principle of the switching power supply, the switching power supply first rectifies the commercial frequency alternating current into direct current, and then inverts into high frequency alternating current, and finally rectifies and filters the output to obtain a stable direct current voltage. In the circuit, the power transistor and the diode mainly work in the state of the switch tube, and work in the microsecond order; during the turn-on and turn-off process of the triode and the diode, the current changes greatly during the rising and falling time, and the RF energy is easily generated. Source of interference. At the same time, potential electromagnetic interference is also formed due to the leakage inductance of the transformer and the spike caused by the reverse recovery current of the output diode.

Switching power supplies typically operate at high frequencies with frequencies above 02 kHz, so their distributed capacitance is not negligible. On the one hand, the insulating sheet between the heat sink and the collector of the switch tube has a large contact area and a thin insulating sheet. Therefore, the distributed capacitance between the two cannot be ignored at high frequencies, and the high-frequency current flows through the distributed capacitor. On the heat sink, it flows to the chassis ground to generate common mode interference. On the other hand, there is a distributed capacitance between the primary and secondary of the pulse transformer, which can directly confuse the primary winding voltage to the secondary winding. Common mode interference occurs on the two power lines that make DC output.

Therefore, the interference sources in the switching power supply are mainly concentrated in voltage and current changes, such as switching tubes, diodes, high-frequency transformers and other components, as well as AC input and rectification output circuit parts.