Introduction to electromagnetic compatibility of switching power supplies
The reasons for electromagnetic compatibility issues caused by switching power supplies operating in high voltage and high current switching states are quite complex. In terms of the electromagnetic properties of the whole machine, there are mainly several types: common impedance coupling, line to line coupling, electric field coupling, magnetic field coupling, and electromagnetic wave coupling. Common impedance coupling mainly refers to the common impedance between the source of disturbance and the object of disturbance in the electrical field, through which the disturbance signal enters the object of disturbance. Inter line coupling mainly refers to the mutual coupling between wires or PCB lines that generate interference voltage and current due to parallel wiring. Electric field coupling is mainly due to the existence of potential difference, which generates induced electric field coupling on the disturbed body. Magnetic field coupling mainly refers to the coupling of low-frequency magnetic fields generated near high current pulse power lines to disturbed objects. Electromagnetic field coupling is mainly caused by the high-frequency electromagnetic waves generated by pulsating voltage or current radiating outward through space, resulting in coupling with the corresponding disturbed body. In fact, each coupling method cannot be strictly distinguished, only the emphasis is different.
In a switching power supply, the main power switching transistor operates in a high-frequency switching mode at high voltages, and the switching voltage and current are close to square waves. From spectral analysis, it is known that the square wave signal contains rich high-order harmonics. The spectrum of this high-order harmonic can reach over 1000 times the frequency of a square wave. At the same time, due to the leakage inductance and distributed capacitance of power transformers, as well as the non ideal working state of main power switching devices, high-frequency and high-voltage peak harmonic oscillations are often generated when turning on or off at high frequencies. The high-order harmonics generated by the harmonic oscillation are transmitted into the internal circuit through the distributed capacitance between the switching tube and the heat sink, or radiated into space through the heat sink and transformer. Switching diodes used for rectification and freewheeling are also an important cause of high-frequency disturbances. Due to the operation of rectifier and freewheeling diodes in high-frequency switching state, the parasitic inductance and junction capacitance of the diode leads, as well as the influence of reverse recovery current, cause them to operate at high voltage and current change rates, and generate high-frequency oscillations. Rectifiers and freewheeling diodes are generally located close to the power output line, and the high-frequency disturbances they generate are most likely to be transmitted through the DC output line. Switching power supplies use active power factor correction circuits to improve power factor. Meanwhile, in order to improve the efficiency and reliability of the circuit and reduce the electrical stress on power devices, a large number of soft switching technologies have been adopted. Among them, zero voltage, zero current, or zero voltage/zero current switching technology is the most widely used. This technology greatly reduces the electromagnetic interference generated by switching devices. However, most soft switch lossless absorption circuits use L and C for energy transfer, and utilize the unidirectional conductivity of diodes to achieve unidirectional energy conversion. Therefore, the diodes in this resonant circuit become a major source of electromagnetic interference.
Switching power supplies generally use energy storage inductors and capacitors to form L and C filtering circuits to filter differential and common mode interference signals. Due to the distributed capacitance of the inductor coil, the self resonant frequency of the inductor coil decreases, resulting in a large amount of high-frequency interference signals passing through the inductor coil and propagating outward along the AC power line or DC output line. As the frequency of the interference signal increases, the effect of the lead inductance on the filtering capacitor leads to a continuous decrease in capacitance and filtering effect, and even changes in capacitor parameters, which is also a cause of electromagnetic interference.
