The interference generated by the switching power supply is divided into two types according to the type of noise interference source. It can be divided into two types: peak interference and harmonic interference. If divided according to the coupling path, it can be divided into conduction interference and radiation interference. The fundamental way to prevent the interference generated by the power supply from harming the electronic system and the power grid is to weaken the source of the noise or to cut off the coupling path between the power supply noise and the electronic system and the power grid.
The high switching action of power switching devices is the main cause of electromagnetic interference (EMI) in switching power supplies. The increase in switching frequency on the one hand reduces the size and weight of the power supply, and on the other hand leads to more serious EMI problems. When the switching power supply is working, its internal voltage and current waveforms rise and fall in a very short time. Therefore, the switching power supply itself is a noise generating source. The interference generated by the switching power supply is divided into two types according to the type of noise interference source. It can be divided into two types: peak interference and harmonic interference. If divided according to the coupling path, it can be divided into conduction interference and radiation interference. The fundamental way to prevent the interference generated by the power supply from harming the electronic system and the power grid is to weaken the source of the noise or to cut off the coupling path between the power supply noise and the electronic system and the power grid. Now explain by noise interference source:
Interference caused by diode reverse recovery time
The AC input voltage changes to a sinusoidal ripple voltage through the power diode rectifier bridge, and becomes DC after being smoothed by the capacitor, but the waveform of the capacitor current is not a sine wave but a pulse wave. It can be seen from the current waveform that the current contains higher harmonics. A large amount of current harmonic components flow into the grid, causing harmonic pollution to the grid. In addition, since the current is a pulse wave, the power input power factor is lowered.
When the rectifier diode in the high-frequency rectification circuit is conducting in the forward direction, a large forward current flows, and when it is turned off by the reverse bias voltage, since there are more carriers accumulated in the PN junction, the current is carried. During a period of time before the sub-disappearance, the current flows in the opposite direction, causing the reverse recovery current of the disappearance of the carrier to drastically decrease and a large current change (di/dt) occurs.
Harmonic interference generated when the switch is working
The power switch tube flows a large pulse current when it is turned on. For example, the input current waveforms of the forward, push-pull, and bridge converters are approximately rectangular waves at resistive loads, which are rich in higher harmonic components. This harmonic interference will be small when using zero current, zero voltage switching. In addition, during the off period of the power switch tube, the current caused by the leakage inductance of the high-frequency transformer winding is abrupt, and spike interference is also generated.
Interference from the AC input loop
The switching power supply input rectifier of the power-free transformer will cause high-frequency attenuation oscillation to interfere during reverse recovery. The interference caused by the switching power supply and the interference energy generated by the switching power supply's input and output lines are called conducted interference; and the energy of harmonics and parasitic oscillations is transmitted through the input and output lines. Generate electric and magnetic fields. This interference caused by electromagnetic radiation is called radiation interference.
other reasons
The parasitic parameters of the components, the schematic design of the switching power supply is not perfect, the printed circuit board (PCB) traces are usually manually arranged, with great randomness, the near-field interference of the PCB is large, and the mounting of the devices on the printed board, Improper placement and orientation can cause EMI interference. This increases the difficulty of extracting PCB distribution parameters and estimating near-field interference.
The oscillation generated at 0.15 MHz is the interference caused by the 3rd harmonic of the switching frequency.
The oscillation generated at 0.2 MHz is the interference caused by the fourth harmonic of the switching frequency and the superposition of the Mosfet oscillation 2 (190.5 KHz) fundamental wave; therefore, this part is stronger.
The oscillation generated at 0.25 MHz is the interference caused by the 5th harmonic of the switching frequency;
The oscillation generated at 0.35 MHz is the interference caused by the 7th harmonic of the switching frequency;
The oscillation generated at 0.39 MHz is the interference caused by the 8th harmonic of the switching frequency and the superposition of the Mosfet oscillation 2 (190.5KHz) fundamental wave;
The oscillation generated at 1.31 MHz is the interference caused by the fundamental wave of Diode oscillation 1 (1.31 MHz);
The oscillation generated at 3.3 MHz is the interference caused by the fundamental wave of Mosfet oscillation 1 (3.3 MHz).
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