Electromagnetic Compatibility Design Scheme for High Frequency Switching Power Supply
If the electromagnetic interference (EMI) problem existing in the high-frequency switching power supply itself is not handled properly, it is not only easy to cause pollution to the power grid, directly affecting the normal operation of other electrical equipment, but also easy to form electromagnetic pollution in the incoming space, resulting in the electromagnetic compatibility (EMC) problem of the high-frequency switching power supply. This article focuses on analyzing the electromagnetic interference exceeding the standard in the 1200W (24V/50A) high-frequency switching power supply module used in railway signal power supply screens, and proposes improvement measures.
The electromagnetic disturbances generated by high-frequency switching power supplies can be divided into two categories: conducted disturbances and radiated disturbances. Conducted disturbances propagate through AC power sources with frequencies below 30MHz; Radiation disturbance propagates through space, with frequencies ranging from 30 to 1000MHz.
Analysis of Electromagnetic Disturbance Sources in High Frequency Switching Power Supplies
Switching power transistors work 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 the switch transistor is becoming faster and faster, usually in a few microseconds. The switch transistor opens and closes at this speed, forming surge voltage and surge current, which will generate high-frequency and high-voltage peak harmonics and electromagnetic interference on space and AC input lines.
At the same time as the high-frequency transformer T1 performs power transformation, it generates alternating electromagnetic fields, radiating electromagnetic waves into space, forming radiation disturbances. The distributed inductance and capacitance of the transformer generate oscillations, which are coupled to the AC input circuit through the distributed capacitance between the primary stages of the transformer, forming conductive 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's lead, as well as the influence of reverse recovery current, it operates at high voltage and current change rates. The longer the diode's reverse recovery time, the greater the impact of peak current, and the stronger the disturbance signal, resulting in high-frequency attenuation oscillation, which is a differential mode conduction disturbance.
All generated electromagnetic signals are transmitted to external power sources through metal wires such as power lines, signal lines, and grounding wires, forming conductive disturbances. Radiated disturbances are caused by interference signals radiated through wires and devices or by interconnecting wires acting as antennas.
3. Electromagnetic Compatibility Design for High Frequency Switching Power Supply Electromagnetic Disturbance
Add a power filter at the entrance of the switching power supply to suppress the high-order harmonics generated by the switching power supply.
Adding ferrite magnetic rings to the input and output power lines can suppress high-frequency common mode within the power lines and reduce the disturbance energy radiated through the power lines.
The power line should be as close as possible to the ground wire to reduce the loop area of differential mode radiation; Route the input AC power line and output DC power line separately to reduce electromagnetic coupling between the input and output; The signal line should be routed away from the power line, close to the ground wire, and not too long to reduce the loop area of the circuit; The width of the lines on the PCB board should not change abruptly, and the corners should be transitioned with arcs, avoiding right angles or sharp corners as much as possible.
Install decoupling capacitors on the chip and MOS switch tubes as close as possible to the power and ground pins parallel to the device.
Due to the presence of Ldi/dt in the grounding wire, the PCB board and chassis are indirectly connected by copper pillars. For those that are not suitable for copper pillar connection, thicker wires are used and grounded nearby.
Add RC absorption circuits at both ends of the switch tube and output rectifier diode to absorb surge voltage.
