Communication Switching Power Supply Characteristics and Electromagnetic Interference Mechanism
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
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 (di/dt) .
2 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.
2.1 Use filter 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 prevent the electromagnetic interference in the equipment from entering the power grid. Installing switching power supply filters in the input and output circuits of switching power supplies can not only solve the problem of conduction interference, but also an important weapon to solve radiation interference. Filter suppression technology is divided into two ways: passive filtering and active filtering.
2.1.1 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. switch electricity
