Analysis and introduction of electromagnetic compatibility design method of switching power supply
1. Internal interference source
● Switch circuit
The switch circuit is mainly composed of switch tubes and high-frequency transformers. There is distributed capacitance between the switch tube and its heat sink, and the leads inside the housing and power supply. The du/dt generated by it has a large amplitude pulse, a wide frequency band, and rich harmonics. The load of the switch tube is the primary coil of the high-frequency transformer, which is an inductive load. When the originally conducting switch tube is turned off, the leakage inductance of the high-frequency transformer generates a back electromotive force E=- LDi/dt, which is proportional to the rate of change of the collector current and the leakage inductance, and is superimposed on the turn off voltage to form a turn off voltage peak, thereby forming conduction interference.
The rectifier diode of the rectifier circuit
When the output rectifier diode is turned off, there is a reverse current, and the time it takes to recover to zero is related to factors such as junction capacitance. It will generate significant current changes di/dt and strong high-frequency interference under the influence of transformer leakage inductance and other distributed parameters, with frequencies reaching several tens of megahertz.
● Stray parameters
Due to working at higher frequencies, the characteristics of low-frequency components in switching power supplies will change, resulting in noise. At high frequencies, stray parameters have a significant impact on the characteristics of the coupling channel, and distributed capacitance becomes a channel for electromagnetic interference.
2 External sources of interference
External interference sources can be divided into power interference and lightning interference, with power interference existing in both "common mode" and "differential mode" modes. At the same time, due to the direct connection of the AC power grid to the rectifier bridge and filtering circuit, only the peak time of the input voltage has input current during half a cycle, resulting in a very low input power factor of the power supply (approximately 0.6). Moreover, this current contains a large amount of harmonic components, which can cause harmonic "pollution" to the power grid.
