Analysis of electromagnetic interference source of High Frequency Switching Power Supply
The rectifier and power transistor Q1 in the circuit, power transistors Q2 to Q5, high-frequency transformer T1 and output rectifier diodes D1 to D2 in the circuit of Figure 1b are the main sources of electromagnetic disturbance when the high-frequency switching power supply works, and the detailed analysis is as follows.
High-order harmonics generated in the rectification process of rectifier will cause conduction disturbance and radiation disturbance along the power line.
The switching power tube works in the state of high-frequency on and off. In order to reduce the switching loss and improve the power density and overall efficiency of the power supply, the switching power tube is turned on and off faster and faster, generally in a few microseconds, and the switching power tube is turned on and off at this speed, resulting in surge voltage and surge current, which will produce peak harmonics of high frequency and high voltage and electromagnetic disturbance to space and AC input lines.
When the high-frequency transformer T1 performs power conversion, it generates alternating electromagnetic fields, which radiate electromagnetic waves into space, resulting in radiation disturbance. The distributed inductance and capacitance of the transformer generate oscillation, which is coupled to the AC input loop through the distributed capacitance between the primary stages of the transformer, forming conduction disturbance.
When the output voltage is relatively low, the output rectifier diode works in the high-frequency switching state, which is also a kind of electromagnetic interference source.
Because of the parasitic inductance of the lead, the junction capacitance and the influence of the reverse recovery current, the diode works at a high voltage and current change rate. The longer the reverse recovery time of the diode, the greater the influence of the spike current and the stronger the disturbance signal, resulting in high-frequency attenuation oscillation, which is a kind of differential mode conduction disturbance.
All these generated electromagnetic signals are transmitted to the external power supply through metal wires such as power lines, signal lines and ground wires to form conduction disturbance. Disturbance signals radiated through wires and devices or through interconnection lines serving as antennas cause radiation disturbance.
