Electromagnetic Compatibility of the Switch Mode Power Supply
The reasons for electromagnetic compatibility problems caused by switching power supplies operating under high voltage and large current switching conditions are quite complex. In terms of the electromagnetic properties of the whole machine, there are mainly common impedance coupling, line-to-line coupling, electric field coupling, magnetic field coupling and electromagnetic wave coupling. Common impedance coupling is mainly the common electrical impedance between the harassment source and the harassed body. Through this impedance, the harassment signal enters the harassed body. Line-to-line coupling is mainly the mutual coupling of wires or PCB lines that generate disturbance voltages and disturbance currents due to parallel wiring. Electric field coupling is mainly due to the existence of potential difference, which causes the induced electric field to cause field coupling 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 harassing objects. Electromagnetic field coupling is mainly due to the high-frequency electromagnetic waves generated by pulsating voltage or current that 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 switching power supply, the main power switch tube operates in a high-frequency switching mode at a very high voltage. The switching voltage and switching current are both 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 non-ideal working conditions of the leakage inductance and distributed capacitance of the power transformer and the main power switching device, high-frequency and high-voltage peak harmonic oscillations often occur when high frequencies are turned on or off. The higher harmonics generated by this harmonic oscillation 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. Switching diodes used for rectification and freewheeling are also an important cause of high-frequency disturbance. Because the rectifier and freewheeling diodes work in a high-frequency switching state, the diode's lead parasitic inductance, junction capacitance, and the influence of reverse recovery current make it work at a very high voltage and current change rate, and produce high-frequency oscillations. . Rectifier and freewheeling diodes are generally close to the power output line, and the high-frequency disturbance they generate is most likely to be transmitted through the DC output line. In order to improve the power factor, switching power supplies adopt active power factor correction circuits. At the same time, in order to improve the efficiency and reliability of circuits and reduce the electrical stress on power devices, soft switching technology is widely used. Among them, zero voltage, zero current or zero voltage/zero current switching technology is the most widely used. This technology greatly reduces the electromagnetic disturbance generated by switching devices. However, most soft-switching lossless absorption circuits use L and C for energy transfer, and use the unidirectional conductive performance of diodes to achieve unidirectional energy conversion. Therefore, the diodes in the resonant circuit become a major source of electromagnetic disturbance.
Switching power supplies generally use energy storage inductors and capacitors to form L and C filter circuits to filter differential mode and common mode disturbance 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 disturbance signals to pass through the inductor coil and propagate outward along the AC power line or DC output line. As the frequency of the disturbance signal of the filter capacitor increases, the effect of the lead inductance causes the capacitance and filtering effect to continuously decrease, and even causes the capacitor parameters to change, which is also a cause of electromagnetic disturbance.
