Expounding the External Interference of Switching Power Supply
The external interference of switching power supply can exist in "common mode" or "differential mode". The type of interference can vary from short-duration peak interference to complete power loss. It also includes voltage change, frequency change, waveform distortion, persistent noise or clutter and transients.
Electrical fast transient and surge shock wave are the main factors that can transmit through the power supply and cause damage to the equipment or affect its work. As long as the power supply equipment itself does not stop vibration and output voltage drops, the interference such as electrostatic discharge will not affect the electrical equipment caused by the power supply.
Power conversion circuit: The power conversion circuit is the core of switching regulated power supply, which has wide production band and rich harmonics. The main components that produce this pulse interference are:
1) There is a distributed capacitance between the switch tube and its radiator, the shell and the lead wire inside the power supply. When the switch tube flows with excessive pulse current (generally rectangular wave), the waveform contains many high-frequency components; At the same time, the device parameters used in the switching power supply, such as the storage time of the switching power tube, the high current of the output stage and the reverse recovery time of the switching rectifier diode, will cause the loop to be short-circuited instantly, resulting in a large short-circuit current. In addition, the load of the switching tube is a high-frequency transformer or an energy storage inductor, and at the moment when the switching tube is turned on, a large inrush current appears in the primary of the transformer, resulting in peak noise.
2) The transformer in the switching power supply of high-frequency transformer is used for isolation and transformation, but due to leakage inductance, it will produce electromagnetic induced noise; At the same time, at high frequency, the distributed capacitance between transformer layers will transfer the high-order harmonic noise of the primary side to the secondary, and the distributed capacitance of the transformer to the shell forms another high-frequency path, which makes the electromagnetic field generated around the transformer more easily coupled to other leads to form noise.
3) When the rectifier diode on the secondary side of the rectifier diode is used for high-frequency rectification, due to the factor of reverse recovery time, the charge accumulated by the forward current can not be eliminated immediately when the reverse voltage is applied (because of the existence of carriers, there is still current flowing). Once the slope of this reverse current is too large, the inductance flowing through the coil will produce a peak voltage, which will produce strong high-frequency interference under the influence of transformer leakage inductance and other distribution parameters, and its frequency can reach tens of MHz.
4) Capacitor, inductor and wire switching power supply will change the characteristics of low-frequency components because it works at higher frequency, thus generating noise.
