Switching power supply electromagnetic interference generation mechanism and inhibition technology
Switching power supply electromagnetic interference suppression
The three elements of electromagnetic interference are the source of interference, propagation path and disturbed equipment. Thus, the suppression of electromagnetic interference should be hand in hand from these three aspects. The purpose of suppressing the source of interference, eliminating the coupling and radiation between the source of interference and the disturbed equipment, and improving the immunity of the disturbed equipment so as to improve the electromagnetic compatibility performance of the switching power supply.
Use of filters to suppress electromagnetic interference
Filtering is an important method of suppressing electromagnetic interference, which can effectively inhibit electromagnetic interference in the power grid into the equipment, but also inhibit electromagnetic interference within the equipment into the power grid. The installation of switching power supply filters in the switching power supply input and output circuits can not only solve the problem of conducted interference, but also an important weapon to solve the radiation interference. Filter suppression technology is divided into 2 ways: passive filtering and active filtering.
Passive filtering technology
Passive filter circuit is simple, low cost, reliable performance, is an effective way to suppress electromagnetic interference. Passive filter consists of inductance, capacitance, resistance components, and its direct role is to solve the conduction emission. The schematic structure diagram of the passive filter applied in the switching power supply is shown in Figure 1.
Due to the large capacity of the filter capacitor in the original power supply circuit, a pulse spike current is generated in the rectifier circuit, and this current consists of a very large number of high harmonic currents, which interfere with the power grid; in addition, the conduction or cut-off of the switching tubes in the circuit, and the primary coil of the transformer will generate pulsating currents. Due to the high rate of change of the current, the surrounding circuits will produce different frequencies of induced currents, including differential mode and common mode interference signals, these interference signals can be transmitted through the two power lines to the rest of the grid and interfere with other electronic equipment. Differential mode filtering part of the figure can reduce the switching power supply within the differential mode interference signal, but also can greatly attenuate the electromagnetic interference signal generated by the equipment itself when the work is transmitted to the power grid. And according to the law of electromagnetic induction, E = Ldi / dt, where: E is the voltage drop at both ends of the L; L is the inductance; di / dt for the current rate of change. Obviously the smaller the rate of change of current is required, the larger the inductance is required.
Pulse current circuit through electromagnetic induction other circuits and the earth or chassis composed of the circuit generated by the interference signal for the common-mode signal; switching power supply circuit between the collector of the switching tube and other circuits to produce a very strong electric field, the circuit will produce displacement current, and this displacement current also belongs to the common-mode interference signal. Mode filter is used to suppress common mode interference, so that it is attenuated.
Active Filtering Technology
Active filtering is an effective method to suppress common mode interference. The method from the noise source and take measures (shown in Figure 2), the basic idea is to try to take out of the main circuit with electromagnetic interference signal size and phase of the opposite compensation signal to balance the original interference signal, in order to achieve the purpose of reducing the level of interference. As shown in Figure 2, the use of transistor current amplification, by folding the emitter's current to the base, in the base loop to filter. r1, C2 composed of the filter so that the base ripple is very small, so that the emitter's ripple is also very small. Since the capacity of C2 is smaller than C3, the size of the capacitor is reduced. This approach is only suitable for low-voltage low-power power supply. In addition, attention should be paid to frequency characteristics, voltage withstand, current rating, impedance characteristics, shielding and reliability when designing and selecting filters. The filter should be installed in the proper location and the installation method should be correct in order to have the expected filtering effect on the interference.
