Electromagnetic compatibility design solution for high frequency switching power supplies
If the high-frequency switching power supply itself exists in the electromagnetic interference (EMI) problem is not handled well, not only easy on the power grid pollution, directly affecting the normal work of other electrical equipment, and into the space is also easy to form electromagnetic pollution, which gives rise to a high-frequency switching power supply electromagnetic compatibility (EMC) problems. This paper focuses on the railroad signal power supply screen using 1200W (24V/50A) high-frequency switching power supply module of electromagnetic harassment exceeding the standard problem analysis, and put forward improvement measures.
High-frequency switching power supply electromagnetic nuisance can be divided into two categories: conducted nuisance and radiated nuisance. Conducted nuisance through the AC power supply propagation, the frequency is lower than 30MHz; radiated nuisance through the space propagation, the frequency of 30 ~ 1000MHz.
Analysis of high-frequency switching power supply electromagnetic harassment source
In the circuit of the rectifier, power tube Q1, in Figure 1b circuit of the power tube Q2 ~ Q5, high-frequency transformer T1, output rectifier diodes D1 ~ D2 are high-frequency switching power supply work to produce electromagnetic harassment of the main sources of harassment, specifically analyzed as follows.
Rectifier rectification process generates high harmonics along the power line to generate conducted and radiated nuisance.
The switching power tube works in the state of high-frequency conduction and cut-off, in order to reduce the switching loss and improve the power density and overall efficiency of the power supply, the switching tube turns on and off faster and faster, generally in a few microseconds, the switching tube turns on and off at such a speed, which forms the inrush voltage and inrush current, and it will generate high-frequency and high-voltage spike harmonics, which will form electromagnetic nuisance to the space and the AC input line.
High Frequency transformer T1 carries out power conversion at the same time, generating an alternating electromagnetic field, radiating electromagnetic waves into space, forming a radiation nuisance. The distributed inductance and capacitance of the transformer generate oscillations and are coupled to the AC input circuit through the distributed capacitance between the initial stages of the transformer, forming conducted nuisance.
In the case of a relatively low output voltage, the output rectifier diode operates in a high-frequency switching state, which is also a source of electromagnetic harassment.
Due to the diode lead parasitic inductance, junction capacitance and the presence of reverse recovery current, so that it works in a very high rate of change of voltage and current, the longer the diode reverse recovery time, then the greater the impact of the spike current, the stronger the nuisance signal, which produces high-frequency attenuation of oscillations, which is a kind of differential-mode conducted harassment.
All of these electromagnetic signals generated through the power cord, signal wire, ground wire and other metal wires transmitted to the external power supply to form a conducted nuisance. Through the conductor and device radiation or by acting as an antenna interconnect line radiation of the nuisance signal caused by radiation harassment.
3, for high-frequency switching power supply electromagnetic harassment of electromagnetic compatibility design
Switching power supply inlet plus power filter, inhibit the high harmonics generated by the switching power supply.
Input and output power lines with a ferrite ring, on the one hand, to suppress the high-frequency common-mode power line, on the other hand, to reduce the nuisance energy radiated through the power line.
Power line as close as possible to the ground to reduce the loop area of the differential mode radiation; the input AC power line and output DC power line separate alignment, reducing the electromagnetic coupling between input and output; signal lines away from the power line, close to the ground line alignment, and the alignment should not be too long, in order to reduce the loop area of the circuit; PCB board line width can not be changed abruptly, the corner of the corner using the rounded transition, as far as possible, do not use the right angle or sharp corners.
Chip and MOS switching tube installation decoupling capacitor, its location as close as possible to the power supply connected in parallel in the device and the ground pin.
Due to the existence of Ldi/dt grounding conductor, the PCB board and chassis indirectly using copper post connection, not suitable for copper post connection using thicker wires, and close to ground.
Add RC absorption circuits on both ends of the switching tube as well as the output rectifier diode to absorb surge voltage.
