Reasons for electromagnetic compatibility caused by power supply
The reasons for electromagnetic compatibility issues caused by 24V switching power supply operating under high voltage and high current switching conditions are quite complex. In terms of electromagnetic compatibility of the entire 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 that generate electromagnetic compatibility are: the source of disturbance, the propagation path, and the subject of disturbance. 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, due to the leakage inductance and distributed capacitance of the power transformer, as well as the non ideal working state of the main power switch device, high-frequency and high-voltage peak harmonic oscillations are often generated when high-frequency is turned on or off. The high order harmonics generated by this harmonic oscillation are transmitted to the internal circuit through the distributed capacitance between the switch tube and the heat sink or radiated into space through the heat sink and transformer.
Used for rectification and continuous current diodes, it is also an important reason for generating high-frequency disturbances. Due to the rectifier and freewheeling diodes operating in high-frequency switching mode, the presence of parasitic inductance and junction capacitance in the diode leads, as well as the influence of reverse recovery current, make them operate at high voltage and current change rates, and generate high-frequency oscillations. Due to the fact that rectifier and freewheeling diodes are generally close to the power output line, the high-frequency disturbances generated by them are 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 used. 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 self resonant frequency of the inductance coil is reduced, resulting in a large number of high-frequency disturbance signals passing through the inductance coil and propagating outward along the AC power line or DC output line. As the frequency of the disturbance signal increases, the capacitance and filtering effect of the filter capacitor continue to decrease due to the effect of the lead inductance, until it completely loses the function of the capacitor and becomes inductive above the resonance frequency. Improper use of filter capacitors and excessively long leads is also a cause of electromagnetic interference.
Due to the high power density and high level of intelligence of 24V switching power supply, equipped with MCU microprocessor, the voltage signal ranges from high to nearly kilovolts to low to several volts; From high-frequency digital signals to low-frequency analog signals, the field distribution inside the power supply is quite complex. Unreasonable PCB wiring, unreasonable structural design, unreasonable input filtering of power lines, unreasonable input and output power line wiring, and unreasonable design of CPU and detection circuits can all lead to unstable system operation or reduced immunity to electromagnetic fields such as electrostatic discharge, electrical fast transient pulse groups, lightning strikes, surges, conducted disturbances, radiated disturbances, and radiated electromagnetic fields.
