Reasons for electromagnetic compatibility caused by switching power supply
The reasons for electromagnetic compatibility issues caused by 24V switching power supplies operating in high voltage and high current switching states are quite complex. In terms of electromagnetic compatibility 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. The three elements generated by electromagnetic compatibility are: disturbance source, propagation path, and disturbed object. Common impedance coupling is mainly the common impedance between the disturbance source and the disturbed object in the electrical field, through which the disturbance signal enters the disturbed object. Inter line coupling mainly refers to the mutual coupling between wires or PCB lines that generate disturbance voltage and disturbance current due to parallel wiring.
Electric field coupling is mainly due to the existence of potential difference, which generates induced electric field coupling with 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 due to the high-frequency electromagnetic waves generated by pulsating voltage or current, which radiate outward through space and couple with the corresponding disturbed body. In fact, each coupling method cannot be strictly distinguished, only the emphasis is different.
In a 24V switching power supply, the main power switching transistor operates in a high-frequency switching mode at high voltages. 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, and the spectrum of these harmonics can reach more than 1000 times the frequency of the square wave. At the same time, due to the leakage inductance and distributed capacitance of the power transformer, as well as the non ideal working state of the 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 these harmonic oscillations are transmitted to 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.
Used for rectification and freewheeling diodes, it is also an important cause of high-frequency disturbance. 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. Due to the proximity of rectifier and freewheeling diodes to the power output line, the high-frequency disturbances they generate are easily transmitted through the DC output line.
