Electromagnetic compatibility design scheme for high frequency switching power supply
If the electromagnetic interference (EMI) problem of the high-frequency switching power supply itself is not handled properly, it not only easily pollutes the power grid and directly affects the normal operation of other electrical equipment, but also easily forms electromagnetic pollution when transmitted into space, resulting in the electromagnetic capacitance (EMC) problem of the high-frequency switching power supply. This article focuses on the analysis of electromagnetic interference exceeding the standard in the 1200W (24V/50A) high-frequency switch power module used in railway signal power supply panels, and proposes improvement measures.
The electromagnetic interference generated by high-frequency switching power supplies can be divided into two categories: conducted interference and radiated interference. Conducted disturbances propagate through AC power sources with frequencies below 30MHz; Radiation disturbance propagates through space, with frequencies ranging from 30 to 1000 MHz.
Analysis of Electromagnetic Disturbance Sources in High Frequency Switching Power Supply
The rectifier and power transistor Q1 in the circuit, as well as the power transistors Q2 to Q5, high-frequency transformer T1, and output rectifier diodes D1 to D2 in the circuit shown in Figure 1b, are the main sources of electromagnetic interference generated during the operation of high-frequency switching power supplies. The specific analysis is as follows.
The high-order harmonics generated during the rectification process of the rectifier will generate conducted and radiated disturbances along the power line.
Switching power transistors operate in high-frequency conduction and cutoff states. In order to reduce switching losses, improve power density and overall efficiency, the opening and closing speed of switching transistors is getting faster and faster. Generally, within a few microseconds, switching transistors open and close at this speed, forming surge voltage and surge current, which will generate high-frequency and high-voltage peak harmonics, causing electromagnetic interference to space and AC input lines.
At the same time as the high-frequency transformer T1 performs power conversion, it generates an alternating electromagnetic field that radiates electromagnetic waves into space, forming radiation disturbances. The distributed inductance and capacitance of the transformer oscillate and couple to the AC input circuit through the distributed capacitance between the primary stages of the transformer, forming conducted disturbances.
When the output voltage is relatively low, the output rectifier diode operates in a high-frequency switching state and is also a source of electromagnetic interference.
Due to the parasitic inductance and junction capacitance of the diode leads, as well as the influence of reverse recovery current, it operates at high voltage and current change rates. The longer the reverse recovery time of the diode, the greater the impact of peak current and the stronger the disturbance signal, resulting in high-frequency attenuation oscillation, which is a type of differential mode conduction disturbance.
