Reasons for EMC caused by switching off the power supply
24V switching power supply work in the high voltage and high current switching state, the causes of electromagnetic compatibility problems is quite complex. Electromagnetic compatibility from the machine, there are mainly common impedance coupling, line coupling, electric field coupling, magnetic field coupling electromagnetic wave coupling several kinds. Electromagnetic compatibility produces three elements: the source of the scratch, the propagation path and the subject of the scratch body. Common impedance coupling is mainly a common impedance that exists between the source of disturbance and the disturbed body electrically, and through this impedance, the disturbed signal enters the disturbed object. Interline coupling is mainly generated by the scratch voltage and scratch current of the 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 near the pulsed power line of high current, the low-frequency magnetic field generated by the coupling of the object to be scratched. Electromagnetic field coupling, mainly due to the pulsating voltage or current generated by high-frequency electromagnetic waves, through the space to the outer radiation, the corresponding coupling to the subject of the coupling. In fact, each type of coupling can not be strictly distinguished, just focus on different things.
In 24V switching power supply, the main power switching tube in a very high voltage, high-frequency switching mode, switching voltage and switching current close to the square wave, from the spectral analysis, the square wave signal contains a wealth of high harmonics, the high harmonic spectrum up to more than 1,000 times of the square wave frequency. At the same time, due to the leakage inductance and distribution capacitance of the power transformer, as well as the non-ideal working condition of the main power switching device, high-frequency and high-voltage spike harmonic oscillations are often generated during high-frequency turn-on or turn-off, and the high-level harmonics generated by this harmonic oscillation are transmitted into the internal circuit or through the distribution capacitance between the switching tube and the heat sink through the heat sink.
The high harmonics generated by this harmonic oscillation are transmitted into the internal circuit or radiated into space through the heat sink and transformer.
Used for rectifier and current-renewal diodes, is also an important cause of high-frequency scratching. Because of 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 presence of reverse recovery current, so that it works at a very high rate of change of voltage and current, and generate high-frequency oscillation. Because the rectifier and renewal diodes are generally closer to the power supply output line, the high-frequency scratches generated by them * are easily transmitted through the DC output line.
In order to improve the 24V switching power supply power factor, are used active power factor efficiency 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-current switching technology is widely used*. This technology greatly reduces the electromagnetic scratch generated by the switching device. However, the soft switching lossless absorption circuit, most of the use of L, C for energy transfer, the use of diode unidirectional conductivity to achieve unidirectional conversion of energy, and therefore, the resonant circuit diode has become a major source of electromagnetic scratch.
24V switching power supply, the general use of energy storage inductors and capacitors, composed of L, C filtering circuit to achieve the differential mode and common mode interference signal filtering, as well as the conversion of 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 lowered, so that a large number of high-frequency scratch signals pass through the inductor coil and propagate outward along the AC power supply line or DC output line. Filter capacitors, with the rise of the frequency of the scratch signal, due to the role of the lead inductance, resulting in a continuous decline in capacitance and filtering effect, until the resonant frequency above the complete loss of the capacitor's role and become inductive. Incorrect use of filtering capacitors and long leads are also a cause of electromagnetic interference.
Due to the 24V switching power supply high power density, high degree of intelligence, with MCU microprocessor, thus, from the high voltage signal to nearly a thousand volts, to the low voltage signal to a few volts; from the high-frequency digital signals, to the low-frequency analogue signals, power supply inside the field distribution is quite complex. PCB wiring is unreasonable, structural design is unreasonable, the power line input filtering is unreasonable, the input and output power line wiring is unreasonable and the CPU, the detection circuit design is unreasonable. CPU, detection circuit design is unreasonable, will lead to system instability or such as electrostatic discharge, electrical fast transient pulse group, lightning, surge and conduction scratch, radiation scratch and radiation electromagnetic field immunity ability to reduce.
