Characteristics of switching power supplies and technical analysis of electromagnetic interference suppression
With the development of modern electronic technology and power devices, switch mode 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. They are particularly widely used in program-controlled switching, optical data transmission wireless base stations, cable television systems, and IP networks, and are the core driving force for the normal operation of information technology equipment. However, communication switch power supplies generally use pulse width modulation (PWM) technology, and their switching devices operate in a high-frequency on-off state. Due to the fast transient process of high frequency itself being an electromagnetic interference source, the electromagnetic interference (EMI) signals generated by it have a wide frequency range and a certain amplitude. Through conduction and radiation, it can pollute the electromagnetic environment and cause interference to communication equipment and electronic products. In addition, communication switch power supplies should have strong resistance to electromagnetic interference, especially to lightning strikes, surges, grid voltage, electric fields, magnetic fields, electromagnetic waves, electrostatic discharge, pulse trains, voltage drops, radio frequency electromagnetic field conduction immunity, radiation immunity, conduction emission, radiation emission and other projects that need to meet the relevant EMC standards.
There are four basic characteristics of a switching power supply:
① The location is relatively clear. Mainly focused on power switching devices, diodes, and the heat sinks and high-frequency transformers connected to them;
② The energy conversion device operates in a switch state. As a switching power supply is an energy conversion device that operates in a switching state, its voltage and current change rates are high, resulting in significant interference intensity;
③ The wiring of power printed circuit boards (PCBs) is usually arranged manually. This arrangement gives it a great deal of flexibility, increasing the difficulty of extracting PCB distribution parameters and predicting and evaluating near-field interference;
④ The switching frequency is high, ranging from tens of thousands of Hz to several megahertz, and the main forms of interference are conducted interference and near-field interference.
Electromagnetic interference generated by switch circuits
The switching circuit is the core of a switching power supply, mainly composed of switching tubes and high-frequency transformers. The dv/dt generated by it is a pulse with a large amplitude, a wide frequency band, and rich harmonics. The main reasons for this pulse interference are twofold: on the one hand, the load on the switching tube is the primary coil of the high-frequency transformer, which is an inductive load. At the moment when the switch tube is turned on, a large surge current is generated in the primary coil, and high surge peak voltage appears at both ends of the primary coil; At the moment when the switch tube is disconnected, due to the leakage flux of the primary coil, a portion of the energy is not transmitted from the primary coil to the secondary coil. The energy stored in the inductor will form a decay oscillation with a peak with the capacitance and resistance in the collector circuit, which will be superimposed on the turn off voltage to form a turn off voltage peak. This type of power supply voltage interruption will generate the same transient magnetization impulse current as when the primary coil is connected, and this noise will be conducted to the input and output terminals, forming conducted interference. On the other hand, the high-frequency switching current loop composed of the primary coil, switching tube, and filtering capacitor of the pulse transformer may generate significant spatial radiation, forming radiation interference.
