1. Generation of electromagnetic interference from switching power supply The switching power supply usually rectifies the commercial frequency alternating current into direct current, and then turns it into a high frequency through the control of the switching tube, and then outputs through the rectifying and filtering circuit to obtain a stable direct current voltage. Power frequency rectification filtering uses high-capacity capacitor charging and discharging, high-frequency switching of switching tube, reverse recovery of output rectifier diode, etc., which produces extremely high di/dt and du/dt, forming a strong inrush current. And spike voltage, which is the most basic cause of electromagnetic interference from switching power supplies. Further, the driving waveform of the switching transistor, the MOSFET drain-source waveform, and the like are all periodic waves close to a rectangular wave shape. Therefore, the frequency is in the MHz level, and these high frequency signals interfere with the basic signals of the switching power supply, especially the signals of the control circuit. 1.1 Input Harmonic Interference of Rectifier Circuit The switching power supply input terminal usually adopts a bridge rectifier and capacitor filter circuit. The rectifier bridge can only be turned on when the ripple voltage exceeds the voltage on the input filter capacitor. The current is input from the mains supply and the filter capacitor is charged. Once the voltage on the filter capacitor is higher than the instantaneous voltage of the mains supply, the rectifier is turned off. Therefore, the current of the input circuit is pulsed and has a high efficiency of harmonic current. This is because of the nonlinear nature of the rectifier circuit, and the current on the AC side of the rectifier bridge is severely distorted. The number of harmonics on the DC side is n times. Therefore, the high-frequency harmonic current on the DC side of the rectifier circuit not only causes the circuit to generate power, but also increases the reactive power of the circuit, and the high-frequency harmonics generate conducted interference and radiation interference along the transmission line. 1.2 Interference generated by the switching circuit The switching circuit plays a key role in the switching power supply and is also one of the main sources of interference. The switch tube load is the primary coil of the high frequency transformer and is an inductive load. At the moment of conduction, the primary coil generates a large inrush current, and a high surge spike voltage appears at both ends of the primary coil; at the moment of disconnection, due to the leakage flux of the primary coil, a portion of the energy is not from the primary coil. Transmitted to the secondary coil, the energy stored in the inductor will form a peak with attenuated oscillations of the capacitor and the resistor in the collector circuit, superimposed on the turn-off voltage to form a turn-off voltage spike. If the spike has a high enough amplitude, it is very likely that the switch will break down. Aluminum Alloy LED lamps,Low Power LED lamps,LED wall washer Kindwin Technology (H.K.) Limited , https://www.szktlled.com
Electromagnetic interference suppression technology for switching power supplies
Electromagnetic compatibility (EMC) refers to the ability of an electronic device or system to function properly in its electromagnetic environment without posing unacceptable electromagnetic disturbances to anything in the environment. It includes both electromagnetic interference (EMI) and electromagnetic sensitivity (EMS). EMI refers to the external interference of electrical products. EMS refers to the ability of electrical products to resist electromagnetic interference. A device with good electromagnetic compatibility should be immune to ambient electromagnetic noise and electromagnetic interference to the surrounding environment. The three elements of electromagnetic interference are the interference source, the coupling channel, and the sensitive body. Suppressing the interference generated by the switching power supply is of great significance to ensure the normal and stable operation of the electronic system. The electromagnetic interference suppression technology mainly includes weakening the energy of the interference, isolating and weakening the noise coupling path and improving the resistance of the device to electromagnetic disturbance. This paper analyzes the causes of electromagnetic interference of switching power supply, and introduces the electromagnetic interference suppression technology and design method of switching power supply.