Features of communication switching power supplies
With the development of modern electronic technology and power devices, switching power supplies are widely used in communication systems, automatic control, household appliances and other fields due to their small size, light weight, high performance, and high reliability, especially in program-controlled Switching, optical data transmission, wireless base stations, cable TV systems and IP networks are the core power for the normal operation of information technology equipment. However, the communication switching power supply generally adopts pulse width modulation (PWM) technology, and its switching devices work in the high-frequency on-off state. Since the high-frequency fast transient process itself is the source of electromagnetic interference, the electromagnetic interference (EMI) signal it generates It has a wide frequency range and a certain amplitude. It will pollute the electromagnetic environment through conduction and radiation, and cause interference to communication equipment and electronic products. In addition, the communication switching power supply must have a strong ability to resist electromagnetic interference, especially for lightning strikes, surges, grid voltage, electric field, magnetic field, electromagnetic wave, electrostatic discharge, pulse train, voltage drop, radio frequency electromagnetic field conduction immunity, radiation Items such as immunity, conducted emission, and radiated emission need to meet the requirements of relevant EMC standards.
In China, in the 1980s and 1990s, in order to strengthen the control of current domestic electromagnetic pollution, some standards corresponding to international standards such as CISPR standards and IEC801 were formulated. Since China Compulsory Certification (ChinaCompulsoryCertification)-K was enforced on August 1, 2003, "EMC fever" has been set off. The research and control of electromagnetic interference at close range has attracted more and more attention from electronic researchers. A new hotspot in the field of research. This paper will systematically discuss the relevant suppression technology for the generation mechanism of the electrical interference of the communication switching power supply.
1 Characteristics of communication switching power supply and mechanism of electromagnetic interference
1.1 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.
1.2 The mechanism of electromagnetic interference
1.2.1 Electromagnetic interference generated by 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.
1.2.2 Interference caused by the reverse recovery time of the diode When the rectifier diode in the high-frequency rectification circuit is forward-conducting, a large forward current flows through it. When it is reverse-biased and turned to cut-off, due to the presence of More carriers accumulate, so the current will flow in the opposite direction for a period of time before the carriers disappear, resulting in a sharp decrease in the reverse recovery current of the carrier disappearance and a large current change
