Causes of Electromagnetic compatibility Caused by Switching Power Supply
The reason of Electromagnetic compatibility problem caused by 24V switching power supply working in the switching state of high voltage and high current is quite complex. From the perspective of Electromagnetic compatibility of the whole machine, there are mainly common impedance coupling, line to line coupling, electric field coupling, magnetic field coupling and electromagnetic wave coupling. The three elements of electromagnetic compatibility are: scratching source, Pathogen transmission and disturbed body. Common resistance coupling mainly refers to the common impedance between the disturbance source and the disturbed object electrically, through which the disturbance signal enters the disturbed object. Line to line coupling mainly refers to the mutual coupling between wires or PCB wires that generate scratch voltage and scratch current due to parallel wiring.
Electric field coupling is mainly due to the presence of potential differences, resulting in the coupling of induced electric fields to the disturbed body. Magnetic field coupling mainly refers to the coupling of low-frequency magnetic fields generated near high current pulse power lines to the disturbance object. Electromagnetic field coupling is mainly caused by high-frequency electromagnetic waves generated by pulsating voltage or current, which radiate outward through space and couple the corresponding disturbed body. In fact, each coupling method cannot be strictly distinguished, only with different focuses.
In a 24V switching power supply, the main power switch operates in a high-frequency switching mode at a high voltage. The switching voltage and current are close to square waves. From spectrum analysis, it is known that the square wave signal contains rich high-order harmonics, which can reach a frequency spectrum of over 1000 times the square wave frequency. At the same time, because the Leakage inductance and distributed capacitance of the power transformer, as well as the working state of the main power switching device, are not ideal, when the high frequency is turned on or off, high frequency and high voltage peak harmonic oscillation is often generated. The high-order harmonics generated by the harmonic oscillation are transmitted into the internal circuit through the distributed capacitance between the switch tube and the radiator or radiated into the space through the radiator and the transformer.
Used for rectification and continuous current diodes, it is also an important reason for generating high-frequency disturbances. Because the rectifier and Flyback diode work in the high-frequency switching state, and because of the existence of the parasitic inductance of the diode's lead, the junction capacitance, and the influence of the reverse recovery current, they work at a very high voltage and current change rate, and produce high-frequency oscillation. Since the rectifier and Flyback diode are generally close to the power output line, the high-frequency interference generated by them is most likely to be transmitted through the DC output line.
In order to improve the power factor of 24V switching power supplies, active power factor correction circuits are adopted. At the same time, in order to improve the efficiency and reliability of the circuit and reduce the electrical stress of power devices, a large number of soft switching technologies have been adopted. Among them, zero voltage, zero current, or zero current switching technology is the most widely used. This technology greatly reduces the electromagnetic interference generated by switching devices. However, soft switching lossless absorption circuits mostly utilize L and C for energy transfer, utilizing the unidirectional conductivity of diodes to achieve unidirectional energy conversion. Therefore, the diodes in this resonant circuit become a major source of electromagnetic disturbance.
In 24V switching power supplies, energy storage inductors and capacitors are generally used to form L and C filtering circuits to filter differential and common mode disturbance signals, and to convert AC square wave signals into smooth DC signals. Due to the distributed capacitance of the inductance coil, the inductance coil
