Characteristics of Communication Switching Power Supply and Mechanism of Electromagnetic Interference
Basic characteristics of switching power supply
There are four basic characteristics of switching power supply:
①The location is relatively clear. Mainly focus on power switching devices, diodes, radiators and high-frequency transformers connected to them;
②The energy conversion device works in the switching state. Because the switching power supply is an energy conversion device that works in the switching state, its voltage and current change rate is very high, and the interference intensity generated is relatively large;
③ Power printed circuit board (PCB) wiring is usually arranged manually. This arrangement makes it very random, which increases the difficulty of extracting PCB distribution parameters and predicting and evaluating near-field interference;
④ The switching frequency is large, ranging from tens of thousands of Hz to several megahertz. The main forms of interference are conduction interference and near-field interference.
The mechanism of electromagnetic interference
EMI from switching circuits
The switching circuit is the core of the switching power supply. It is mainly composed of a switching tube and a high-frequency transformer. The dv/dt generated by it is a pulse with a relatively large amplitude, a wide frequency band and rich harmonics. There are two main reasons for this pulse interference: on the one hand, the switch tube load is the primary coil of a high-frequency transformer, which is an inductive load. At the moment when the switch tube is turned on, the primary coil generates a large inrush current, and a high surge peak voltage appears at both ends of the primary coil; when the switch tube is turned off, due to the leakage flux of the primary coil, a part of the energy If there is no transmission from the primary coil to the secondary coil, this part of the energy stored in the inductor will form an attenuating oscillation with a spike with the capacitance and resistance in the collector circuit, which is superimposed on the turn-off voltage to form a turn-off voltage spike. This power supply voltage interruption will produce the same magnetizing inrush current transient as when the primary coil is turned on, and this noise will be conducted to the input and output terminals to form conducted interference. On the other hand, the high-frequency switching current loop formed by the primary coil of the pulse transformer, the switching tube and the filter capacitor may generate large space radiation and form radiation interference.
Electromagnetic Interference Suppression Measures
The three elements that form electromagnetic interference are interference source, propagation path and disturbed equipment. Therefore, the suppression of electromagnetic interference should be done from these three aspects.
The purpose of suppressing the interference source, eliminating the coupling and radiation between the interference source and the disturbed device, and improving the anti-interference ability of the disturbed device, thereby improving the electromagnetic compatibility performance of the switching power supply.
Use filters to suppress electromagnetic interference
Filtering is an important method to suppress electromagnetic interference. It can effectively suppress the electromagnetic interference in the power grid from entering the equipment, and can also suppress the electromagnetic interference in the equipment from entering the power grid. Installing a switching power supply filter in the input and output circuits of the switching power supply can not only solve the problem of conduction interference, but also an important weapon to solve the radiation interference. Filter suppression technology is divided into two ways: passive filtering and active filtering.
Passive Filtering Technology
The passive filter circuit is simple, low in cost, reliable in performance, and is an effective way to suppress electromagnetic interference. Passive filters are composed of inductors, capacitors, and resistors, and their direct role is to solve conducted emissions.
Due to the large capacity of the filter capacitor in the original power supply circuit, a pulse peak current will be generated in the rectifier circuit. This current is composed of a lot of high-order harmonic current, which will interfere with the power grid; The primary coil will generate pulsating current. Due to the high rate of current change, induced currents of different frequencies will be generated on the surrounding circuits, including differential mode and common mode interference signals, which can be conducted to other lines of the power grid through 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 greatly attenuate the electromagnetic interference signal generated by the equipment itself when it 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 rate of change of current. Obviously, the smaller the rate of change of the current is required, the larger the inductance is required.
