The causes of electromagnetic compatibility caused by switching power supply
The 24V switching power supply works in the switching state of high voltage and high current, and the causes of electromagnetic compatibility problems are quite complicated. From the electromagnetic compatibility of the whole machine, there are mainly several kinds of common impedance coupling, line coupling, electric field coupling and magnetic field coupling electromagnetic wave coupling. The three elements of electromagnetic compatibility are: the source of disturbance, the propagation path and the disturbed object. Common impedance coupling is mainly the common impedance between the source and the object, through which the signal can enter the object. Line-to-line coupling mainly refers to the mutual coupling of wires or PCB lines that generate disturbing voltage and disturbing current due to parallel wiring.
Electric field coupling is mainly due to the existence of potential difference, and the induced electric field is coupled to the disturbed object. Magnetic field coupling is mainly the coupling of the low-frequency magnetic field generated near the pulse power line of large current to the scratching object. The electromagnetic field coupling is mainly due to the high-frequency electromagnetic wave generated by pulsating voltage or current, which radiates outward through space and couples the corresponding disturbed object. In fact, each coupling mode cannot be strictly distinguished, but the emphasis is different.
In the 24V switching power supply, the main power switch tube works in a high-frequency switching mode at a very high voltage, and the switching voltage and switching current are close to square waves. From the spectrum analysis, it is known that the square wave signal contains rich higher harmonics, and the spectrum of the higher harmonics can reach more than 1000 times of the square wave frequency. At the same time, due to the leakage inductance and distributed capacitance of the power transformer and the non-ideal working state of the main power switching device, high-frequency and high-voltage peak harmonic oscillation often occurs when high frequency is turned on or off, and the higher harmonics generated by this harmonic oscillation are transmitted into the internal circuit through the distributed capacitance between the switch tube and the radiator or radiated into space through the radiator and transformer.
Diodes used for rectification and freewheeling are also an important cause of high frequency interference. Because the rectifier and freewheeling diodes work in the high-frequency switching state, due to the existence of lead parasitic inductance, junction capacitance and the influence of reverse recovery current, they work at a very high voltage and current change rate and produce high-frequency oscillation. Because the rectifier and freewheeling diode are generally close to the power output line, the high-frequency interference generated by them is easy to be transmitted through the DC output line.
In order to improve the power factor of 24V switching power supply, 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 are adopted. Among them, zero voltage, zero current or zero current switching technology is widely used. This technology greatly reduces the electromagnetic interference generated by switching devices. However, most soft-switching lossless absorption circuits use L and C to transfer energy, and use the unidirectional conductivity of diodes to realize unidirectional energy conversion. Therefore, the diodes in this resonant circuit become a major source of electromagnetic interference.
In 24V switching power supply, L and C filter circuits are generally composed of energy storage inductors and capacitors, which can filter differential mode and common mode interference signals and 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, so that a large number of high-frequency interference signals pass through the inductance coil and propagate outward along the AC power line or DC output line. With the increase of disturbing signal frequency, the capacitance and filtering effect of the filter capacitor are continuously decreased due to the effect of lead inductance, until it is above the resonant frequency, it completely loses its function and becomes inductive. Incorrect use of filter capacitors and too long leads are also a cause of electromagnetic interference.
Because of the high power density and high intelligence of 24V switching power supply with MCU microprocessor, the voltage signal from high to nearly thousand volts is as low as several volts. From high-frequency digital signals to low-frequency analog signals, the field distribution inside the power supply is quite complicated. Unreasonable wiring of PCB, unreasonable structural design, unreasonable input filtering of power cord, unreasonable wiring of input and output power cord, and unreasonable design of CPU and detection circuit will all lead to unstable system work or reduced immunity of radiated electromagnetic fields, such as electrostatic discharge, electrical fast transient, lightning strike, surge and conduction interference, radiation interference.
