Causes of electromagnetic compatibility caused by switching power supplies
The 24V switching power supply operates in a switching state of high voltage and large current, and the reasons for electromagnetic compatibility problems are quite complicated. From the electromagnetic compatibility of the whole machine, there are mainly common impedance coupling, line-to-line coupling, electric field coupling, and magnetic field coupling electromagnetic wave coupling. The three elements of electromagnetic compatibility are: interference source, propagation path and interfered object. Common impedance coupling is mainly the common electrical impedance between the interference source and the interfered object, through which the interfering signal enters the interfered object. Line-to-line coupling is mainly the mutual coupling of wires or PCB lines that generate interference voltages and interference currents due to parallel wiring.
Electric field coupling is mainly due to the existence of potential difference and the coupling of the induced electric field to the disturbed body. Magnetic field coupling is mainly the coupling of the low-frequency magnetic field generated near the high-current pulse power line to the disturbing object. The electromagnetic field coupling is mainly caused by the high-frequency electromagnetic waves generated by pulsating voltage or current, which radiate outward through space and cause coupling to the corresponding disturbed body. In fact, each coupling method cannot be strictly distinguished, but the focus 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. The switching voltage and switching current are close to square waves. From spectrum analysis, it is known that the square wave signal contains rich high-order harmonics. The spectrum of this high-order harmonic 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 condition of the main power switching device, high-frequency and high-voltage peak harmonic oscillations often occur when turning on or off at high frequencies. This harmonic oscillation generates high-order Harmonics are introduced into the internal circuit through the distributed capacitance between the switch tube and the radiator or radiated to the space through the radiator and transformer.
It is used in rectification and freewheeling diodes, and is also an important cause of high-frequency interference. Because the rectifier and freewheeling diodes work in a high-frequency switching state, due to the existence of the diode's lead parasitic inductance, junction capacitance, and the influence of the reverse recovery current, they work under very high voltage and current change rates, and produce high-frequency oscillation. Because the rectifier and freewheeling diodes are generally close to the power output line, the high-frequency interference they generate is easily transmitted through the DC output line.
In order to improve the power factor of the 24V switching power supply, active power factor positive circuits are used. At the same time, in order to improve the efficiency and reliability of circuits and reduce the electrical stress of power devices, soft switching technology is widely used. 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, most soft-switching lossless absorption circuits use L and C for energy transfer, and use the unidirectional conduction performance of diodes to achieve unidirectional energy conversion. Therefore, the diodes in the resonant circuit have become a major source of electromagnetic interference.
In 24V switching power supplies, energy storage inductors and capacitors are generally used to form L and C filter circuits to 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 inductor coil, the self-resonant frequency of the inductor coil is reduced, causing a large number of high-frequency interference signals to pass through the inductor coil and propagate outward along the AC power line or DC output line. As the frequency of the interference signal increases, the capacitance and filtering effect of the filter capacitor continue to decrease due to the effect of the lead inductance. Until the resonance frequency is above the resonant frequency, the capacitor completely loses its function and becomes inductive. Improper use of filter capacitors and too long leads are also causes of electromagnetic interference.
Due to the high power density and high degree of intelligence of the 24V switching power supply, it is equipped with an MCU microprocessor. Therefore, it can range from voltage signals as high as nearly kilovolts to voltage signals as low as a few volts; from high-frequency digital signals to low-frequency analog signals. The field distribution inside the signal and power supply is quite complex. Unreasonable PCB wiring, unreasonable structural design, unreasonable power line input filtering, unreasonable input and output power line wiring, and unreasonable design of the CPU and detection circuits will all lead to unstable system operation or problems such as electrostatic discharge and rapid electrical transients. Variable pulse bursts, lightning strikes, surges and conduction interference, radiation interference and reduction in the immunity to radiated electromagnetic fields.
