Electromagnetic compatibility design solution for high-frequency switching power supply
If the electromagnetic interference (EMI) problem of the high-frequency switching power supply itself is not handled well, it will not only easily cause pollution to the power grid and directly affect the normal operation of other electrical equipment, but also easily form electromagnetic pollution into the space, resulting in high The electromagnetic compatibility (EMC) problem of frequency switching power supply. This article focuses on analyzing the electromagnetic disturbance exceeding the standard problem of the 1200W (24V/50A) high-frequency switching power supply module used in railway signal power supply panels, and proposes improvement measures.
The electromagnetic disturbance generated by high-frequency switching power supply can be divided into two categories: conducted disturbance and radiation disturbance. Conducted harassment is transmitted through the AC power supply, with a frequency below 30MHz; radiated harassment is transmitted through space, with a frequency between 30 and 1000MHz.
Analysis of electromagnetic disturbance sources of high-frequency switching power supply
The rectifier and power tube Q1 in the circuit, the power tubes Q2~Q5 in the circuit of Figure 1b, the high-frequency transformer T1, and the output rectifier diodes D1~D2 are all the main sources of electromagnetic disturbance when the high-frequency switching power supply is working. Specifically analyse as below.
The high-order harmonics generated during the rectification process of the rectifier will cause conduction disturbance and radiation disturbance along the power line.
The switching power tube works in a high-frequency on and off state. In order to reduce switching losses and improve power density and overall efficiency, the switching tube turns on and off faster and faster, generally within a few microseconds. Turning on and off at such a speed forms surge voltage and surge current, which generates high-frequency and high-voltage peak harmonics, causing electromagnetic disturbance to the space and AC input lines.
While high-frequency transformer T1 performs power conversion, it generates an alternating electromagnetic field and radiates electromagnetic waves into space, forming radiation harassment. The distributed inductance and capacitance of the transformer generate oscillation and are coupled to the AC input loop through the distributed capacitance between the primary and secondary stages of the transformer, forming conductive 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 diode's lead parasitic inductance, junction capacitance and the influence of reverse recovery current, it operates under very high voltage and current change rates. The longer the diode's reverse recovery time, the greater the impact of the peak current. , the stronger the disturbance signal becomes, resulting in high-frequency attenuated 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, forming conductive disturbance. Radiated disturbances are caused by disturbance signals that radiate through wires and devices or through interconnects that act as antennas.
3. Electromagnetic compatibility design for electromagnetic disturbance of high-frequency switching power supply
Add a power filter to the switching power supply inlet to suppress high-order harmonics generated by the switching power supply.
Adding a ferrite magnetic ring to the input and output power lines not only suppresses the high-frequency common mode in the power lines, but also reduces the disturbance energy radiated through the power lines.
Keep the power line as close as possible to the ground wire to reduce the loop area of differential mode radiation; route the input AC power line and the output DC power line separately to reduce the electromagnetic coupling between input and output; keep the signal line away from the power line and close to the ground The lines should be routed, and the lines should not be too long to reduce the loop area of the loop; the width of the lines on the PCB board should not be sudden, use arc transitions at the corners, and try not to use right angles or sharp corners.
Install decoupling capacitors on the chip and MOS switch tube as close as possible to the power and ground pins of the device in parallel.
Due to the existence of Ldi/dt in the ground wire, the PCB board and the chassis are indirectly connected using copper pillars. For those that are not suitable for connection with copper pillars, thicker wires should be used and grounded nearby.
Add an RC absorption circuit at both ends of the switch tube and the output rectifier diode to absorb the surge voltage.
