Measures to suppress electromagnetic interference in communication switching power supply
The three elements that form electromagnetic interference are interference sources, propagation pathways and interfered equipment. Therefore, efforts should be made to suppress electromagnetic interference from these three aspects.
The purpose is to suppress the interference source, eliminate the coupling and radiation between the interference source and the victim equipment, and improve the immunity of the victim equipment, thereby improving the electromagnetic compatibility performance of the switching power supply.
1 Use filters to suppress electromagnetic interference
Filtering is an important method to suppress electromagnetic interference. It can effectively suppress electromagnetic interference in the power grid from entering the equipment, and it can also inhibit electromagnetic interference in the equipment from entering the power grid. Installing switching power supply filters in the switching power supply input and output circuits can not only solve the problem of conducted interference, but is also an important weapon to solve the radiation interference. Filter suppression technology is divided into two methods: passive filtering and active filtering.
1.1 Passive filtering technology
Passive filter circuits are simple, low-cost, and reliable in performance, and are an effective way to suppress electromagnetic interference. Passive filters are composed of inductors, capacitors, and resistors, and their direct function is to solve conducted emissions.
Due to the large filter capacitance in the original power circuit, a pulse peak current will be generated in the rectifier circuit. This current is composed of a lot of high-order harmonic currents, causing interference to the power grid. In addition, the switching tube in the circuit is turned on or off, the transformer is The primary coil will generate pulsating current. Due to the high current change rate, different frequency induced currents will be generated in the surrounding circuits, including differential mode and common mode interference signals. These interference signals can be conducted to other lines of the power grid through the two power lines and interfere with other electronic equipment. The differential mode filtering part in the figure can reduce the differential mode interference signal inside the switching power supply, and can also greatly attenuate the electromagnetic interference signal generated when the equipment itself is working and transmitted to the power grid. According to the law of electromagnetic induction, E-Ldi/dt is obtained. E is the voltage drop across L, L is the inductance, and di/dt is the current change rate. Obviously, the smaller the required current change rate, the larger the required inductance.
The interference signal generated by the pulse current loop through electromagnetic induction and the loop composed of other circuits and the earth or the chassis is a common mode signal; a strong electric field is generated between the collector of the switch tube in the switching power supply circuit and other circuits, and the circuit will generate a displacement current , and this displacement current also belongs to the common mode interference signal. The common mode filter in the figure is used to suppress common mode interference and attenuate it.
1.2 Active filtering technology
Active filtering technology is an effective method to suppress common mode interference. This is a measure taken based on the noise source (Figure 2). The basic idea is to try to extract a compensation signal from the main loop that is equal in size and opposite in phase to the electromagnetic interference signal to balance the original interference signal to achieve the purpose of reducing the interference level. As shown in the figure, the current amplification effect of the transistor is used to convert the emitter current to the base and filter it in the base loop. The filter composed of R1 and C2 makes the base ripple very small, so that the emitter ripple is also small. Very small. Since the capacity of c2 is smaller than C3, the volume of the capacitor is reduced. This method is only suitable for low-voltage and low-power power supplies.
