Generation and suppression of electromagnetic interference (EMI) in switching power supplies
Switching power supply first rectifies AC power into DC power, which is then controlled by switching tubes to become high-frequency. Finally, it is output through rectification and filtering circuits to obtain stable DC voltage. Therefore, it contains a large amount of harmonic interference. Meanwhile, due to the leakage inductance of the transformer and the peak caused by the reverse recovery current of the output diode, varying degrees of electromagnetic interference will be generated. The interference in switch mode power supplies is mainly concentrated on components with large voltage and current changes (i.e. dv/dt or di/dt), especially switch tubes, output diodes, and high-frequency transformers. Meanwhile, stray capacitance can transmit noise from the power grid to the power supply of the electronic system, causing interference to the operation of electronic circuits. Here we will analyze the causes of several interferences and their coupled paths.
The filtering interference generated by the output rectification and filtering circuit of the switch power supply commonly uses bridge rectification and capacitor filtering circuits at the output end. Due to the nonlinearity of rectifier diodes and the energy storage effect of filtering capacitors, the output current becomes a periodic peak current with a short time and high peak value,. This distorted input current contains not only fundamental components but also abundant higher-order harmonic components.
Interference generated by switch circuit 2
The core of the switch circuit is also one of the main sources of interference, which is mainly composed of switch tubes and high-frequency transformers. The dv/dt generated by the switching tube has large pulses, a wide frequency band, and abundant harmonics. The main reason for this pulse interference is:
(1) At the moment when the switch tube is turned on, a large surge current is generated in the primary coil of the transformer, and high surge peak voltage appears at both ends of the primary coil; At the moment when the switch is turned off, due to the leakage flux of the primary coil, a portion of the energy is not transmitted from the primary coil to the secondary coil. The energy stored in the leakage inductance will form a decay oscillation with a peak with the inter pole capacitance and resistance of the switch tube itself, which will be superimposed on the turn off voltage of the switch tube, forming a turn off peak voltage. This noise will be transmitted to the input and output terminals, forming conducted interference.
(2) When the output diode is conducting in the forward direction, the charge inside the PN junction is accumulated. When the diode is subjected to a reverse voltage, the accumulated charge disappears and a reverse current is generated. Due to the high frequency of V in the secondary rectification circuit during switching, the time for it to transition from conduction to cutoff is very short. In order to make the stored charge disappear in a short period of time, a surge of reverse current is generated. Due to the presence of distributed capacitance and distributed inductance in DC output, the interference caused by surges becomes high-frequency attenuation and oscillation reduction.
(3) The high-frequency switching current loop composed of the primary coil, switching tube, and filtering capacitor of the high-frequency transformer may generate significant spatial radiation, forming radiation interference.
