The way a switching power supply operates There are three switching power supply requirements.
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.
Electromagnetic Compatibility Design for Electromagnetic Disturbance of High Frequency Switching Power Supply
A power filter is added to the entrance of the switching power supply to suppress the high-order harmonics generated by the switching power supply.
Adding ferrite magnetic rings on the input and output power lines can suppress the high-frequency common mode in the power line on the one hand, and reduce the disturbance energy radiated through the power line on the other hand.
The power line should be as close as possible to the ground line to reduce the loop area of differential mode radiation; separate the input AC power line and output DC power line to reduce the electromagnetic coupling between input and output; the signal line should be far away from the power line and close to the ground Line routing, and the routing should not be too long to reduce the loop area of the loop; the width of the lines on the PCB board should not change suddenly, and the corners should use arc transitions, and try not to use right angles or sharp corners.
Install decoupling capacitors on the chip and MOS switch tubes, and its position is as close as possible to the power and ground pins connected in parallel to the device.
Due to the Ldi/dt of the grounding wire, the PCB board and the chassis are indirectly connected by copper pillars. For those that are not suitable for copper pillar connection, use thicker wires and ground them nearby.
Add an RC snubber circuit at both ends of the switch tube and the output rectifier diode to absorb the surge voltage.
