Cause analysis of electromagnetic interference in switching power supply
Switching power supply can be divided into full bridge, half bridge, push-pull and so on according to the main circuit type, but no matter which type of switching power supply will produce strong noise when working. They are conducted out in common mode or differential mode through the power line, and also radiate to the surrounding space. Switching power supply is also sensitive to external noise invaded by power grid, and it is transmitted to other electronic equipment to cause interference.
After AC power is input into the switching power supply, it is rectified into DC voltage Vi by bridge rectifiers V1 ~ V4, which is applied to the primary L1 of the high-frequency transformer and the switching tube V5. The base of the switching tube V5 inputs a high-frequency rectangular wave of tens to hundreds of kilohertz, and its repetition frequency and duty ratio are determined by the requirements of the output DC voltage VO. The pulse current amplified by the switch tube is coupled to the secondary circuit by the high frequency transformer. The ratio of primary turns of high frequency transformer is also determined by the requirement of output DC voltage VO. The high-frequency pulse current is rectified by diode V6 and filtered by C2 to become DC output voltage VO. Therefore, switching power supply will produce noise in the following links, forming electromagnetic interference.
(1) The high-frequency switching current loop composed of the high-frequency transformer primary L1, the switching tube V5 and the filter capacitor C1 may generate large space radiation. If the capacitor filter is insufficient, the high-frequency current will be conducted to the input AC power supply in a differential mode.
(2) The high-frequency transformer secondary L2, rectifier diode V6 and filter capacitor C2 also form a high-frequency switching current loop, which will generate space radiation. If the capacitor filter is insufficient, the high-frequency current will be mixed with the output DC voltage in the form of differential mode for external conduction.
(3) There are distributed capacitors Cd between the primary and secondary of the high-frequency transformer, and the high-frequency voltage of the primary will be directly coupled to the secondary through these distributed capacitors, resulting in common-mode noise in the same phase on the two output DC power lines of the secondary. If the impedance of the two wires to the ground is unbalanced, it will also turn into differential mode noise.
(4) The output rectifier diode V6 will generate reverse surge current. When the diode is turned on in the forward direction, the charge in the PN junction will accumulate, and when the diode is applied with reverse voltage, the accumulated charge will disappear and produce reverse current. Because the switching current needs to be rectified by the diode, the time for the diode to turn from on to off is very short, and the surge of reverse current occurs in order to make the stored charge disappear in a short time. High frequency attenuation oscillation is caused by distributed inductance, distributed capacitance and surge in DC output line, which is a kind of differential mode noise.
(5) The load of the switch tube V5 is the primary coil L1 of the high-frequency transformer, which is an inductive load. Therefore, when the switch is turned on and off, there will be a high surge peak voltage at both ends of the tube, and this noise will be conducted to the input and output terminals.
(6) There is a distributed capacitor CI between the collector of the switching tube V5 and the radiator K, so the high-frequency switching current will flow to the radiator K through CI, then to the chassis ground, and finally to the protective ground PE of the AC power line connected to the chassis ground, thus generating common-mode radiation. Power lines L and N have a certain impedance to PE. If the impedance is unbalanced, common-mode noise will be converted into differential-mode noise.
