Analysis of Electromagnetic Nuisance Sources of High Frequency Switching Power Supplies
In the circuit of the rectifier, power tube Q1, in Figure 1b circuit of the power tube Q2 ~ Q5, high-frequency transformer T1, output rectifier diodes D1 ~ D2 are high-frequency switching power supply work to generate electromagnetic harassment of the main source of harassment, specific analysis is as follows.
Rectifier rectification process generates high harmonics along the power line to generate conducted and radiated nuisance.
The switching power tube works in the state of high-frequency conduction and cut-off, in order to reduce the switching loss and improve the power density and overall efficiency of the power supply, the switching tube turns on and off faster and faster, generally in a few microseconds, the switching tube turns on and off at such a speed, which forms the inrush voltage and inrush current, and it will generate high-frequency and high-voltage spike harmonics, which will form electromagnetic nuisance to the space and the AC input line.
The high-frequency transformer T1 performs power conversion while generating an alternating electromagnetic field that radiates electromagnetic waves into space, forming a radiation nuisance. The distributed inductance and capacitance of the transformer generates oscillations and is coupled to the AC input circuit through the distributed capacitance between the initial stages of the transformer, forming a conducted nuisance.
In the case of a relatively low output voltage, the output rectifier diode operates in a high-frequency switching state, which is also a source of electromagnetic harassment.
Due to the diode's lead parasitic inductance, junction capacitance, and the influence of the reverse recovery current, so that it works at a very high rate of change of voltage and current, the longer the diode reverse recovery time, the greater the impact of the spike current, the stronger the nuisance signal, which produces high-frequency attenuation of oscillations, which is a kind of differential-mode conducted nuisance.
All of these electromagnetic signals generated through the power line, signal line, grounding wire and other metal wires transmitted to the external power supply to form a conducted nuisance. Nuisance signals radiated through wires and devices or through interconnecting wires acting as antennas cause radiated nuisance.
