Characteristics of Communication Switching Power Supplies and Techniques for Suppressing Electromagnetic Interference
With the development of modern electronic technology and power devices, switching power supply with its small size, light weight, high performance, high reliability and other characteristics are widely used in communication systems, automatic control, household appliances and other fields, especially widely used in program-controlled switching, optical data transmission wireless base stations, cable television systems and IP networks, is the core power of the normal operation of information technology equipment. However, communication switching power supply generally use pulse width modulation (PWM) technology, its switching devices work in the high-frequency on and off state, due to the high-frequency fast transient process itself is a source of electromagnetic interference, it produces electromagnetic interference (EMI) signals have a wide range of frequencies, but also have a certain amplitude, by conduction and radiation will contaminate the electromagnetic environment, the communication equipment and electronic products cause interference. In addition, communication switching power supply to have a strong anti-electromagnetic interference ability, especially for lightning, surge, grid voltage, electric field, magnetic field, electromagnetic wave, electrostatic discharge, burst, voltage drop, RF electromagnetic field conductive immunity, radiated immunity, conductive emission, radiated emissions, and other items need to satisfy the provisions of the relevant EMC standards.
1, electromagnetic interference generated by the switching circuit
Switching circuit is the core of the switching power supply, mainly composed of switching tubes and high-frequency transformers, which produces dv/dt is a pulse with a large amplitude, wide bandwidth and rich in harmonics. The main reason for this pulse interference is twofold: on the one hand, the switching tube load is the primary coil of the high-frequency transformer, which is an inductive load. In the switching tube conduction moment, the primary coil produces a large inrush current, and in the primary coil at both ends of the high surge spike voltage; in the switching tube disconnect instantaneous, due to the leakage flux of the primary coil, resulting in a part of the energy is not transferred from the primary coil to the secondary coil, stored in the inductance of this part of the energy will be and the collector circuit in the formation of capacitance, resistance with a spike of the attenuation of the oscillations, superimposed on the shutdown voltage, the formation of shutdown voltage spike. This will be superimposed on the turn-off voltage to form a turn-off voltage spike. This power supply voltage interruption will produce the same magnetization impact current transient with the primary coil on, this noise will be transmitted to the output of the output, the formation of conduction interference. Another aspect of the pulse transformer primary coil, switching tubes and filter capacitors constitute a high-frequency switching current loop may produce a large space radiation, forming radiation interference.
2, the diode's reverse recovery time caused by the interference of high-frequency rectifier circuit in the rectifier diode forward conduction when there is a large forward current flow, in its reverse bias voltage and turn to cut-off, due to the PN junction in the accumulation of more carriers, and thus in the carriers before the disappearance of the period of time, the current will be reversed flow, resulting in the disappearance of carriers in the reverse recovery of the current is reduced drastically and the occurrence of a large change in the current.
Electromagnetic interference suppression measures
The three elements of electromagnetic interference are interference source, propagation path and disturbed equipment. Thus, the suppression of electromagnetic interference should be handled 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 two ways: passive filtering and active filtering.
Passive filtering technology
Due to the large capacity of filter capacitors in the original power supply circuit, the rectifier circuit will produce pulse spike current, which consists of a very large number of high harmonic currents, causing interference to the power grid; in addition, the switching tube's conduction or cut-off in the circuit and the transformer's primary coils will produce pulsating currents. Due to the high rate of change of the current, the surrounding circuit 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 differential mode interference signal inside the switching power supply, 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, E is the voltage drop across L, L is the inductance, di/dt for the current rate of change. Obviously, the smaller the rate of change of current is, the larger the inductance is required.
