Electromagnetic compatibility of switch mode power supplies
Communication switching power supply due to work in the switching state of high voltage and high current, the electromagnetic compatibility caused by the problem is quite complex. From the electromagnetic compatibility of the machine, there are mainly common impedance coupling, line coupling, electric field coupling, magnetic field coupling and electromagnetic wave coupling. Electromagnetic compatibility produces three elements: the source of the interference, the propagation path and the disturbed body. Common impedance coupling is mainly interference source and the interfered body in the electrical existence of a common impedance, through the impedance of the interference signal into the interfered object. Line coupling is mainly generated by the interference voltage and interference current wire or pcb line, due to parallel wiring and mutual coupling. Electric field coupling is mainly due to the existence of potential difference, the induced electric field generated by the coupling of the disturbed body. Magnetic field coupling is mainly the coupling generated by the low-frequency magnetic field near the pulsed power line of the high current to the interference object. Electromagnetic wave coupling, on the other hand, is mainly due to the high-frequency electromagnetic waves generated by pulsating voltages or currents, which are radiated outward through space and produce coupling to the corresponding disturbed body. In fact, each kind of coupling mode can not be strictly distinguished, just focus on different things.
In the switching power supply, the main power switching tube in a very high voltage, high-frequency switching mode of operation, switching voltage and switching current are square wave, the square wave contains a high harmonic spectrum up to more than 1,000 times the frequency of the square wave. At the same time, due to the leakage inductance and distribution capacitance of the power transformer, as well as the main power switching device is not ideal, in the high-frequency switching on or off, often produces high-frequency and high-voltage spike harmonic oscillation, the harmonic oscillation generated by the high harmonics, through the distribution capacitance between the switching tube and the heatsink into the internal circuit or through the heatsink and transformer to the space radiation. Switching diodes used for rectification and renewal are also an important cause of high-frequency interference. Because the rectifier and current-renewal diodes work in the high-frequency switching state, due to the diode's lead parasitic inductance, junction capacitance and the existence of reverse recovery current, so that it works at a very high rate of change of voltage and current, and produce high-frequency oscillation. Because of the rectifier and current diode is generally closer to the power supply output line, the high-frequency interference generated by the most likely to be transmitted through the DC output line.
Communication switching power supply in order to improve the power factor, are used in active power factor correction circuit. At the same time, in order to improve the efficiency and reliability of the circuit, reduce the electrical stress of the power device, a large number of soft switching technology. Among them, zero-voltage, zero-current or zero-voltage-zero-current switching technology is most widely used. This technology greatly reduces the electromagnetic interference generated by the switching device. However, the soft-switching lossless absorption circuit more than the use of l, c for energy transfer, the use of diode unidirectional conductivity to achieve unidirectional conversion of energy, and thus, the resonant circuit in the diode has become a major source of electromagnetic interference interference.
Communication switching power supply, the general use of energy storage inductors and capacitors to form l, c filter circuit to achieve the differential mode and common mode interference signal filtering, as well as AC square wave signal converted to a smooth DC signal. Due to the distributed capacitance of the inductor coil, it leads to a reduction of the self-resonant frequency of the inductor coil, which results in a large number of high-frequency interfering signals passing through the inductor coil and propagating outward along the AC power supply line or DC output line. Filter capacitors, with the rise of the frequency of the interference signal, due to the role of the lead inductance, resulting in a continuous decline in capacitance and filtering effect, until it reaches the resonant frequency above, the complete loss of capacitance and become inductive. Incorrect use of filter capacitors and lead too long, is also a cause of electromagnetic interference.
