What are the main sources of electromagnetic interference in the power adapter?
Electromagnetic interference sources in the power adapter are mainly switching devices, diodes and nonlinear passive components; in the switching power supply, improper printed board wiring is also a major factor causing electromagnetic interference. For power adapters, electromagnetic interference generated by the switching circuit is one of the main sources of interference in the switching power supply. The switching circuit is the core, mainly composed of switching tubes and high-frequency transformers. It produces du/dt is a pulse with a large amplitude, wide bandwidth and rich in harmonics. The main reasons for this pulse interference are
1. The load of the switching tube is the primary coil of the high-frequency transformer, which is an inductive load. At the instant of switching tube conduction, the primary coil flows through a large inrush current, and there is a high surge voltage spike at both ends of the primary coil; at the instant of switching tube disconnection, due to the leakage flux of the primary coil, resulting in a part of the energy is not transferred from the primary coil to the secondary coil, the part of the energy stored in the inductance will be stored in the capacitance of the collector circuit and transistor, resistor formation of attenuation oscillations with a spike superimposed on the off voltage, resulting in a shutdown voltage spike. voltage, forming a turn-off voltage spike. This power supply voltage interruption will produce the same magnetization impact current transient with the primary coil on, this noise will be conducted to the input and output, the formation of conduction interference, the heaviest possible breakdown of the switch.
2. Pulse transformer primary coil, switching tube and filter capacitors constitute a high-frequency switching current loop may produce a large space radiation, the formation of radiation interference. If the capacitor filter capacity is insufficient or poor high-frequency characteristics, capacitance on the high-frequency impedance will make the high-frequency current in the form of differential mode conduction to the AC power supply to form conduction interference.
Electromagnetic interference generated by the diode rectifier circuit. The reverse recovery current generated by the rectifier diode in the main circuit is much smaller than the reverse recovery current of the continuity diode (|di/dt|). As a source of electromagnetic interference, the rectifier diode reverse recovery current creates interference with high intensity and wide frequency band. The voltage jumps generated by the rectifier diode are much smaller than the voltage jumps generated by the power switching tubes in the power supply when they turn on and off. Therefore, it is possible to study the rectifier circuit as part of the EMI coupling channel without considering the effects of |dv/dt| and |di/dt| generated by the rectifier diode. dv/dt versus load size. The dv/dt generated when the power switching tube is turned on and off is the main source of interference in switching power supply. Theoretical analyses and experiments have shown that, as the load increases, the |dv/dt| value generated by the turn-off increases, while the load change has little effect on the turn-on dv/dt. Due to the different |dv/dt| generated during turn-on and turn-off, the interference pulses generated to the outside world are also different. These are the main sources of electromagnetic interference in the power adapter.
