Introduction of Electromagnetic Compatibility Design Method for Switching Power Supply
Due to the advantages of small size and high power factor, switching power supply is widely used in communication, control, computer and other fields. However, due to electromagnetic interference, its further application is limited to a certain extent. This paper will analyze the various mechanisms of electromagnetic interference of switching power supply, and on the basis of it, propose the electromagnetic compatibility design method of switching power supply.
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 to DC, and then converted to high frequency, and finally output through the rectification and filtering 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 leakage inductance of the transformer and the peak caused by the reverse recovery current of the output diode are also potential strong interference sources.
1 Internal sources of interference
● switch circuit
The switch circuit is mainly composed of a switch tube and a high frequency transformer. There is distributed capacitance between the switch tube and its heat sink, the casing and the internal leads of the power supply. The du/dt generated by it has relatively large pulses, wide 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 is turned off, the leakage inductance of the high-frequency transformer generates a counter electromotive force E=-Ldi/dt, and its value is proportional to the current change rate of the collector and proportional to the leakage inductance, superimposed on the off On the cut-off voltage, a turn-off voltage peak is formed, thereby forming a conduction interference.
● Rectifier diodes for rectifier circuits
There is a reverse current when the output rectifier diode is cut off, and the time it returns 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, and generate strong high-frequency interference, the frequency can reach tens of megahertz.
● Spurious parameters
Due to working at a higher frequency, the characteristics of low-frequency components in the switching power supply will change, resulting in noise. At high frequencies, the stray parameters have a great influence on the characteristics of the coupling channel, and the distributed capacitance becomes the channel of electromagnetic interference.
2 External sources of interference
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 power grid is directly connected to the rectifier bridge and the filter circuit, in half a cycle, only the peak time of the input voltage has input current, resulting in a very low input power factor of the power supply (about 0.6). Moreover, this current contains a large number of current harmonic components, which will cause harmonic "pollution" to the grid.
