Electromagnetic Compatibility Design Scheme of High Frequency Stabilized Power Supply
If the electromagnetic disturbance (EMI) problem existing in the high-frequency switching power supply itself is not handled well, it will not only easily cause pollution to the power grid, directly affect the normal operation of other electrical equipment, but also easily form electromagnetic pollution into the space, resulting in high Electromagnetic Compatibility (EMC) issues of frequency switching power supplies. This paper focuses on the analysis of the electromagnetic disturbance exceeding the standard of the 1200W (24V/50A) high-frequency switching power supply module used in the railway signal power supply screen, and proposes improvement measures.
The electromagnetic disturbance generated by high-frequency switching power supply can be divided into two categories: conduction disturbance and radiation disturbance. The conduction disturbance propagates through the AC power supply, and the frequency is lower than 30MHz; the radiation disturbance propagates through the space, and the frequency is 30-1000MHz.
Analysis of Electromagnetic Disturbance Source of High Frequency Switching Power Supply
The rectifier, power tube Q1 in the circuit, power tubes Q2~Q5, high frequency transformer T1, and output rectifier diodes D1~D2 in the circuit in Figure 1b are the main sources of electromagnetic disturbance when the high frequency switching power supply is working. Specifically analyse as below.
The high-order harmonics generated by the rectification process of the rectifier will generate conducted and radiated disturbances along the power line.
The switching power tube works in the high-frequency on and off state. In order to reduce the switching loss, improve the power density and overall efficiency of the power supply, the switching tube is turned on and off faster and faster, generally in a few microseconds, and the switching tube takes Turning on and off at such a speed forms surge voltage and surge current, which will generate high-frequency and high-voltage peak harmonics, and form electromagnetic disturbances to the space and AC input lines.
While the high-frequency transformer T1 performs power conversion, it generates an alternating electromagnetic field and radiates electromagnetic waves into space, forming radiation disturbance. The distributed inductance and capacitance of the transformer oscillate, and are coupled to the AC input loop through the distributed capacitance between the primary and secondary stages of the transformer, forming conduction disturbance.
When the output voltage is relatively low, the output rectifier diode works in a high-frequency switching state, which is also a source of electromagnetic disturbance.
Due to the parasitic inductance of the lead wires of the diode, the existence of the junction capacitance and the influence of the reverse recovery current, it works at a very high rate of voltage and current change. The longer the reverse recovery time of the diode, the greater the impact of the peak current. , the stronger the disturbance signal, resulting in high-frequency attenuation oscillation, which is a 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 grounding lines to form conduction disturbances. Radiated disturbances are caused by disturbing signals radiating through conductors and devices or through interconnecting lines acting as antennas.
