Generation mechanism and suppression technology of electromagnetic interference in switching power supply
Suppression of electromagnetic interference of switching power supply
The three elements of electromagnetic interference are interference source, propagation path and disturbed equipment. Therefore, the suppression of electromagnetic interference should be based on these three aspects. Restrain the interference source, eliminate the coupling and radiation between the interference source and the disturbed equipment, and improve the anti-interference ability of the disturbed equipment, thus improving the electromagnetic compatibility of the switching power supply.
Using filter to suppress electromagnetic interference
Filtering is an important method to suppress electromagnetic interference, which can effectively prevent electromagnetic interference from entering the equipment in the power grid, and can also prevent electromagnetic interference in the equipment from entering the power grid. Installing switching power supply filters in the input and output circuits of switching power supply can not only solve the problem of conduction interference, but also be an important weapon to solve radiation interference. Filter suppression technology can be divided into passive filtering and active filtering.
1 passive filtering technology
Passive filter circuit is simple, low cost and reliable, which is an effective way to suppress electromagnetic interference. Passive filter is composed of inductance, capacitance and resistance elements, and its direct function is to solve conducted emission.
Because of the large capacity of the filter capacitor in the original power supply circuit, the pulse peak current will be generated in the rectifier circuit, which is composed of a lot of higher harmonic currents, which will interfere with the power grid. In addition, the on or off of the switch tube in the circuit and the primary coil of the transformer will produce pulsating current. Due to the high current change rate, induced currents with different frequencies will be generated to the surrounding circuits, including differential mode and common mode interference signals, which can be conducted to other lines of the power grid and interfere with other electronic equipment through two power lines. The differential mode filtering part in the figure can reduce the differential mode interference signal in the switching power supply and greatly attenuate the electromagnetic interference signal generated by the equipment itself when it works and transmit it to the power grid. According to the law of electromagnetic induction, E=Ldi/dt is obtained, where e is the voltage drop across l; L is inductance; Di/dt is the current change rate. Obviously, the smaller the required current change rate, the greater the required inductance.
The interference signal generated by the pulse current loop through electromagnetic induction of other circuits and the loop formed by the earth or the casing is a common-mode signal; In the switching power supply circuit, a strong electric field is generated between the collector of the switching tube and other circuits, and the circuit will generate displacement current, which is also a common-mode interference signal. Figure 1* Mode filter is used to suppress common-mode interference and attenuate it.
Active filtering technology
Active filtering technology is an effective method to suppress common-mode interference. The basic idea of this method is to take out a compensation signal with the same size and opposite phase as the electromagnetic interference signal from the main loop to balance the original interference signal, so as to reduce the interference level. As shown in fig. 2, the current of the emitter is converted to the base by the current amplification of the transistor, and it is filtered in the base loop. The filter composed of R1 and C2 makes the base ripple very small, so the emitter ripple is also very small. Since the capacity of C2 is smaller than that of C3, the volume of the capacitor is reduced. This method is only suitable for low-voltage and low-power power supply. In addition, attention should be paid to frequency characteristics, withstand voltage performance, rated current, impedance characteristics, shielding and reliability when designing and selecting filters. The installation position and method of the filter should be appropriate, so as to play the expected filtering role for interference.
