Interference factors of switching power supply: internal and external interference factors

2021-03-26 13:55:46

 

Internal interference of switching power supply: the EMI generated by switching power supply is mainly the high-order harmonic current interference generated by the basic rectifier and the peak voltage interference generated by the power conversion circuit.

 

Basic rectifier: The basic rectifier process is the most common cause of EMI. This is because ac power frequency sine wave through the current of the rectifier is no longer a single frequency, and dc component and the frequency of different harmonics, harmonic can produce conduction and radiation along the transmission line, make the front current distortion, on the one hand, make the pick up in the front of current waveform distortion on the power cord, radio frequency interference on the other hand, through the power cord.

 

Power conversion circuit: the power conversion circuit is the core of the switching regulator power supply, which is rich in harmonics. The main components that produce such pulse interference are:

 

1) There is a distributed capacitance between the switch tube, the switch tube and its radiator and the leads inside the housing and the power supply. When the switch tube flows through a large pulse current, the waveform contains many high-frequency components; At the same time, the switching power supply using the storage time of power tube parameters such as switching devices, high-current output stage, switch rectifier diode reverse recovery time, can cause instantaneous short circuit loop, have a great short-circuit current, in addition, the load switch tube is a high frequency transformer or energy storage inductors, at the instant of the switch tube conduction, appeared a lot of transformer inrush current, produce peak noise.

 

2) High-frequency transformer, the transformer in the switching power supply, used for isolation and voltage change, but due to the reason of leakage inductance, it will produce electromagnetic induction noise; At the same time, the distributed capacitance between the transformer layers will transfer the high-order harmonic noise of the primary side to the secondary at high frequency, and the transformer forms another high-frequency path to the distributed capacitance of the shell, so that the electromagnetic field generated around the transformer is more likely to be coupled to other leads to form noise.

 

3) rectifier diode, rectifier diode used as a high frequency rectifier, due to the reverse recovery time factors, often forward current accumulation of charge in the reverse voltage can not be immediately eliminated. Once the slope of this reverse current recovery is too large, the inductance flowing through the coil will produce a peak voltage, under the influence of transformer leakage inductance and other distribution parameters will produce a strong high frequency interference, the frequency can be tens of MHz.

 

4) Capacitor, inductor and conductor switching power supply work at a higher frequency, which will change the characteristics of low-frequency components, resulting in noise.

 

External interference of switching power supply: External interference of switching power supply can exist in "common mode" or "differential mode" mode. The type of interference can vary from very short duration spikes to complete loss of power. They also include voltage changes, frequency changes, waveform distortion, persistent noise or clutter, and transients.