Generation and suppression of electromagnetic interference in switching power supply
Electromagneticcompatibility EMC is the abbreviation of English electromagnetic compatibility. It includes two meanings. First, the electromagnetic radiation generated by the equipment in the work must be limited to a certain level. Second, the equipment itself must have certain anti-interference ability. It must have three elements: interference source, coupling channel and sensitive body. The suppression of interference by switching power supply for electronic circuits is of great significance to ensure the normal and stable operation of electronic systems. By analyzing the interference sources and coupling channels in switching power supply, this paper puts forward effective measures to suppress interference. The design and manufacturing method of switching power supply transformer are put forward.
Interference sources and coupling channels in switching power supply
The switching power supply first rectifies the power frequency alternating current into direct current, then changes it into high frequency through the control of the switching tube, and then outputs it through the rectification filter circuit to obtain a stable direct current voltage, so it contains a lot of harmonic interference. At the same time, due to the leakage inductance of the transformer and the peak caused by the reverse recovery current of the output diode, electromagnetic interference of different degrees will be produced. The interference in switching power supply is mainly concentrated on components with large voltage and current changes (that is, large dv/dt or di/dt), especially switching tubes, output diodes and high-frequency transformers. At the same time, the stray capacitance will conduct the noise of the power grid to the power supply of the electronic system and interfere with the work of the electronic circuit. Here, let's analyze the causes of several kinds of interference and their coupling paths.
The filtering interference generated by the output rectifier filter circuit generally adopts bridge rectifier and capacitor filter circuit at the output end of switching power supply. Because of the nonlinearity of rectifier diode and the energy storage function of filter capacitor, the output current becomes a periodic peak current with short time and high peak value. This distorted input current, besides the fundamental wave, also contains rich higher harmonic components.
Interference generated by switching circuit
The core of switching circuit is also one of the main interference sources, which is mainly composed of switching tube and high frequency transformer. The dv/dt generated by the switch tube has a large pulse, a wide frequency band and rich harmonics. The main reasons for this pulse interference are:
(1) At the moment when the switch tube is turned on, the primary winding of the transformer generates a great inrush current, and a high surge peak voltage appears at both ends of the primary winding; At the moment when the switch is turned off, a part of energy is not transmitted from the primary coil to the secondary coil due to the leakage flux of the primary coil, and this part of energy stored in the leakage inductance will form an attenuation oscillation with a peak with the inter-electrode capacitance and resistance of the switch tube itself, and will be superimposed on the turn-off voltage of the switch tube to form a turn-off peak voltage. This noise will be conducted to the input and output terminals, forming conducted interference.
(2) When the output diode is turned on in the forward direction, the charge in the PN junction is accumulated, and when the diode is applied with a reverse voltage, the accumulated charge will disappear and a reverse current will be generated. Because the frequency of V in the secondary rectifier circuit is very high when switching, that is, the time from on to off is very short, and the surge of reverse current will occur if the stored charge disappears in a short time. Due to the existence of distributed capacitance and distributed inductance in DC output, the interference caused by surge becomes high frequency attenuation and oscillation reduction.
(3) The high-frequency switching current loop composed of high-frequency transformer primary coil, switching tube and filter capacitor may generate large space radiation and form radiation interference.
Interference coupling channel
Due to the stray capacitance between the primary windings of the transformer, the common-mode interference generated by the switching circuit propagates through the transformer in the primary and secondary sides. Comparatively speaking, the differential mode interference path is simple and easy to handle. This paper mainly introduces the generation and suppression of common mode interference.
