Electromagnetic interference analysis of switching power supply
The structure of the switching power supply is shown in Figure 1. First, the power frequency AC is rectified into DC, then inverted into high frequency, and finally output through the rectifier and filter circuit to obtain a stable DC voltage. Unreasonable circuit design and layout, mechanical vibration, poor grounding, etc. will cause internal electromagnetic interference. At the same time, the peaks caused by the leakage inductance of the transformer and the reverse recovery current of the output diode are also potential sources of strong interference.
1 Internal interference sources
● Switch circuit
The switching circuit is mainly composed of switching tubes and high-frequency transformers. There is distributed capacitance between the switch tube and its heat sink, the casing and the leads inside the power supply. The du/dt it generates has a larger amplitude pulse, a wider frequency band and rich harmonics. The switching tube load is the primary coil of the high-frequency transformer, which is an inductive load. When the switch tube that was originally turned on turns off, the leakage inductance of the high-frequency transformer generates a back electromotive force E=-Ldi/dt, whose value is proportional to the current change rate of the collector and the leakage inductance. On the off-voltage, a turn-off voltage spike is formed, thereby causing conduction interference.
● Rectifier diode of rectifier circuit
There is a reverse current when the output rectifier diode is turned off, and the time it takes to return to zero is related to factors such as junction capacitance. It will produce a large current change di/dt under the influence of transformer leakage inductance and other distribution parameters, resulting in strong high-frequency interference with a frequency of up to tens of megahertz.
● Spurious parameters
Due to operating at a higher frequency, the characteristics of low-frequency components in the switching power supply will change, resulting in noise. At high frequencies, spurious parameters have a great influence on the characteristics of the coupling channel, and distributed capacitance becomes a channel for electromagnetic interference.
2 External interference sources
External interference sources can be divided into power interference and lightning interference, and power interference exists in "common mode" and "differential mode". At the same time, since the AC grid is directly connected to the rectifier bridge and filter circuit, within half a cycle, there is input current only during the peak time of the input voltage, resulting in a very low input power factor of the power supply (about 0.6). Moreover, this current contains a large amount of current harmonic components, which will cause harmonic "pollution" to the power grid.
