Introduction to Electromagnetic Compatibility of Switching Power Supplies
The reasons for electromagnetic compatibility issues caused by switching power supplies operating under high voltage and high current switching conditions are quite complex. In terms of the electromagnetic properties 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. Common impedance 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 disturbance voltage and current due to parallel wiring. Electric field coupling is mainly due to the presence of potential difference, which generates induced electric field coupling on the disturbed body. Magnetic field coupling mainly refers to the coupling of low-frequency magnetic fields generated near high current pulse power lines to disturbance objects. Electromagnetic field coupling is mainly due to the high-frequency electromagnetic waves generated by pulsating voltage or current radiating outward through space, resulting in coupling with the corresponding disturbed body. In fact, each coupling method cannot be strictly distinguished, only with different focuses.
In a switching power supply, the main power switch operates in a high-frequency switching mode at a high voltage, and the switching voltage and current are close to square waves. From spectrum analysis, it is known that square wave signals contain rich high-order harmonics. The spectrum of this higher-order harmonic can reach 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 turning on or off at high frequencies. 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. Switching diodes used for rectification and continuation are also an important cause of high-frequency disturbances. Due to the high-frequency switching state of the rectifier and freewheeling diodes, 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. Rectifier and freewheeling diodes are generally close to the power output line, and the high-frequency disturbances generated by them are most likely to be transmitted through the DC output line. In order to improve power factor, switching power supplies adopt active power factor correction circuits. 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 voltage/zero current switching technology is the most widely used. This technology greatly reduces the electromagnetic interference generated by switching devices. However, most soft switch lossless absorption circuits 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.
Switching power supplies generally use energy storage inductors and capacitors to form L and C filtering circuits, achieving filtering of differential and common mode disturbance 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 in the filter capacitor, the effect of the lead inductance leads to a continuous decrease in capacitance and filtering effect, and even changes in capacitor parameters, which is also a reason for electromagnetic interference.
