Switching power supply electromagnetic interference causes analysis
Switching power supply according to the main circuit type can be divided into full-bridge, half-bridge, push-pull and so on several kinds, but no matter what type of switching power supply in the work will produce very strong noise. They are conducted outward through the power line in common mode or differential mode, and also radiated to the surrounding space. Switching power supplies are also sensitive to external noise intruded by the power grid and transmitted through it to other electronic equipment to produce interference.
AC power input switching power supply, by the bridge rectifier V1 ~ V4 collated into a DC voltage Vi added to the high-frequency transformer primary L1 and switching tube V5. The base of the switching tube V5 is fed with a high-frequency rectangular wave of several tens to hundreds of kilohertz, whose repetition frequency and duty cycle are determined by the requirements of the output DC voltage VO. The pulse current amplified by the switching tube is coupled to the secondary circuit by a high-frequency transformer. The ratio of the number of turns of the initial stage of the high-frequency transformer is also determined by the requirements of the output DC voltage VO. The high-frequency pulse current is rectified by diode V6 and filtered by C2 into a DC output voltage VO. Therefore, the switching power supply will generate noise and form electromagnetic interference in the following links.
(1) The high-frequency switching current loop formed by the primary L1 of the high-frequency transformer, the switching tube V5 and the filtering capacitor C1 may generate large space radiation. If the capacitor filtering is insufficient, the high-frequency current will also be conducted to the input AC power supply in a differential mode.
(2) High-frequency transformer secondary L2, rectifier diode V6, filter capacitor C2 also constitute a high-frequency switching current loop will produce space radiation. If the capacitor filtering is insufficient, the high-frequency current will be mixed in the form of differential mode on the output DC voltage to the outside.
(3) high-frequency transformer primary and secondary distribution capacitance Cd, the primary high-frequency voltage through the distribution capacitance will be directly coupled to the secondary up to the secondary two output DC power line to produce the same phase of the common-mode noise. If the impedance of the two lines to ground is unbalanced, it will also be transformed into differential mode noise.
(4) output rectifier diode V6 will produce reverse inrush current. Diode in forward conduction when the PN junction charge accumulation, diode plus reverse voltage when the accumulated charge will disappear and produce reverse current. Because the switching current needs to be rectified by the diode, the diode from the conduction of the diode into the cut-off time is very short, in a short period of time to let the stored charge disappear on the reverse current generated by the surge. Due to the distributed inductance in the DC output line, distributed capacitance, the surge caused by high-frequency attenuation oscillations, which is a differential mode noise.
(5) The load of switching tube V5 is the primary coil L1 of the high-frequency transformer, which is an inductive load, so there will be a high surge spike voltage at both ends of the tube when the switch is turned on and off, and this noise will be conducted to the input and output terminals.
(6) There is a distributed capacitance CI between the collector of the switching tube V5 and the heatsink K. Therefore, the high-frequency switching current will flow through CI to the heatsink K, 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. The power lines L and N have a certain impedance to PE, and if the impedance is unbalanced, the common mode noise will be transformed into differential mode noise.
