EMI interference sources of
Switching Power Supply are mainly reflected in power switch tube, rectifier diode, high-frequency transformer, etc. the interference of external environment to switching
Power Supply mainly comes from grid jitter, lightning strike, external radiation, etc. For the research on EMI of switching power supply, some start from the mechanism of EMI generation, some from the impact of EMI, put forward many practical and valuable programs.
Mechanism of electromagnetic interference in switching power supply
According to the types of noise interference sources, the interference generated by switching power supply can be divided into peak interference and harmonic interference; if divided by coupling path, it can be divided into conducted interference and radiation interference. Now, it is explained in terms of noise interference sources:
1. Interference caused by diode reverse recovery time
When the rectifier diode in the high-frequency rectifier circuit is turned on by the reverse bias voltage, the current will reverse flow in a period of time before the carrier disappears due to the accumulation of more carriers in the PN junction when the rectifier diode in the high-frequency rectifier circuit is turned off due to the reverse bias voltage. As a result, the reverse recovery current of the carrier disappearance decreases sharply and the current changes greatly (di / DT).
2. Harmonic interference produced by switch tube working
A large pulse current flows through the power switch when it is on. For example, the input current waveform of forward converter, push-pull converter and bridge converter is approximately rectangular wave under resistive load, which contains abundant high-order harmonic components. When zero current and zero voltage switch is used, the harmonic interference will be very small. In addition, during the cut-off period of the power switch, the current mutation caused by the leakage inductance of the high-frequency transformer winding will also produce peak interference.
3. Interference caused by AC input circuit
The input rectifier tube of switching power supply without power frequency transformer will cause high frequency attenuation oscillation and interference during reverse recovery. The peak interference and harmonic interference energy generated by switching power supply are called conducted interference when they are transmitted through the input and output lines of switching power supply; while the energy of harmonic and parasitic oscillation will generate electric field and magnetic field in space when they propagate through input and output lines. The interference generated by electromagnetic radiation is called radiation interference.
4. Other reasons
The parasitic parameters of components and the schematic design of switching power supply are not perfect. The wiring of printed circuit board (PCB) is usually arranged manually, which has great randomness. The near-field interference of PCB is large, and the installation, placement and orientation of devices on the printed board are unreasonable, which will cause EMI interference.
Best regards,
Shirley Liang
