Characteristics of communication switching power supplies and the 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 focused on power switching devices, diodes and the heat sinks 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 a switching state, its voltage and current change rate is very high, and the intensity of the interference generated is relatively large;
③Power printed circuit board (PCB) traces are usually laid out manually. This arrangement makes it highly arbitrary and increases the difficulty of extracting PCB distribution parameters and predicting and evaluating near-field interference;
④The switching frequency is large, ranging from tens of thousands Hz to several megahertz. The main forms of interference are conduction interference and near-field interference.
Mechanism of electromagnetic interference generation
1. Electromagnetic interference generated by switching circuits
The switching circuit is the core of the switching power supply, which is mainly composed of switching tubes and high-frequency transformers. The dv/dt it generates is a pulse with a larger amplitude, a wider frequency band and rich harmonics. There are two main reasons for this pulse interference: on the one hand, the switching tube load is the primary coil of the high-frequency transformer, which is an inductive load. 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, part of the energy is lost due to the leakage magnetic flux of the primary coil. Without being transmitted from the primary coil to the secondary coil, this part of the energy stored in the inductor will form an attenuated 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 spike. This supply voltage interruption will produce the same magnetizing impulse current transient as when the primary coil is turned on. This noise will be transmitted to the input and output terminals, forming conductive interference. On the other hand, the high-frequency switching current loop composed of the primary coil of the pulse transformer, the switching tube and the filter capacitor may produce large space radiation, causing radiation interference.
2. Interference caused by the reverse recovery time of the diode. When the rectifier diode in the high-frequency rectifier circuit is forward-conducting, a large forward current flows. When it is turned off by the reverse bias voltage, due to the relatively large amount of current in the PN junction, Many carriers accumulate, so the current will flow in the reverse direction for a period of time before the carriers disappear, causing the reverse recovery current when the carriers disappear to decrease sharply and cause a large current change (di/dt).
