Solution to Electromagnetic Compatibility of Switching Power Supply
From the three elements of electromagnetic compatibility, to solve the electromagnetic compatibility of switching power supply, we can start from three aspects.
1) Reduce the interference signal generated by the interference source;
2) Cut off the propagation path of the interference signal;
3) Enhance the anti-interference ability of the interfered object.
When solving the electromagnetic compatibility inside the switching power supply, the above three methods can be used comprehensively, based on the cost-benefit ratio and the ease of implementation. The external interference generated by switching power supply, such as power line harmonic current, power line conduction interference, electromagnetic field radiation interference, etc., can only be solved by reducing the interference source. On the one hand, it can enhance the design of input and output filter circuits, improve the performance of active power factor correction (apfc) circuits, reduce the voltage and current change rates of switching tubes and rectifier freewheeling diodes, and adopt various soft switching circuit topologies and control methods wait. On the other hand, strengthen the shielding effect of the casing, improve the gap leakage of the casing, and perform good grounding treatment. For external anti-interference ability, such as surge and lightning strike, the lightning protection ability of AC input and DC output ports should be optimized. Usually, for the combined lightning strike waveform of 1.2/50μs open-circuit voltage and 8/20μs short-circuit current, due to the small energy, it can be solved by combining zinc oxide varistors and gas discharge tubes. For electrostatic discharge, usually in the small signal circuit of the communication port and the control port, use the tvs tube and the corresponding grounding protection, increase the electrical distance between the small signal circuit and the chassis, or choose a device with anti-static interference to solve it. The fast transient signal contains a very wide frequency spectrum, and it is easy to pass into the control circuit in the form of common mode. The same method of anti-static is used to reduce the distributed capacitance of the common-mode inductance and strengthen the common-mode signal filtering of the input circuit (such as adding Common mode capacitors or insertion loss ferrite cores, etc.) to improve the immunity of the system.
To reduce the internal interference of the switching power supply, realize its own electromagnetic compatibility, and improve the stability and reliability of the switching power supply, we should start from the following aspects: pay attention to the correct division of the pcb wiring of the digital circuit and the analog circuit, and the digital circuit and the analog circuit Correct decoupling of the power supply; pay attention to single-point grounding of digital circuits and analog circuits, single-point grounding of high-current circuits and low-current circuits, especially current and voltage sampling circuits, to reduce common resistance interference and the influence of ground rings; pay attention to phase when wiring The distance between adjacent lines and the nature of the signal can avoid crosstalk; reduce the impedance of the ground wire; reduce the area surrounded by the high-voltage and high-current circuit, especially the primary side of the transformer, the switch tube, and the power filter capacitor circuit; reduce the output rectifier circuit and the continuous circuit. The area surrounded by the current diode circuit and the DC filter circuit; reduce the leakage inductance of the transformer and the distributed capacitance of the filter inductance; use a filter capacitor with a high resonance frequency, etc.
The data line and address line of MCU and LCD display have a high operating frequency, which is the main interference source of radiation; the small signal circuit is the weakest link against external interference, and appropriately increase TVS with high anti-interference ability and high-frequency capacitors, iron Oxygen magnetic beads and other components to improve the anti-interference ability of small-signal circuits; small-signal circuits that are close to the chassis should be properly insulated and withstand voltage treatment. The heat sink of the power device and the electromagnetic shielding layer of the main transformer should be properly grounded. Comprehensive consideration of various grounding measures will help improve the electromagnetic compatibility of the whole machine. The large-area grounding between each control unit is shielded with a grounding plate, which can improve the stability of the internal operation of the switching power supply.
On the frame of the rectifier, it is necessary to consider the electromagnetic coupling between the rectifiers, the layout of the ground wire of the whole machine, the correct relationship between the AC input neutral wire, the ground wire and the DC ground wire, the lightning protection ground wire, and the correct distribution of the electromagnetic compatibility level, etc. .
In the internal and external interference and anti-interference of the switching power supply, the common mode signal has a very complicated relationship with the working mode of the switching device, the installation of the radiator, and the connection between the pcb board and the chassis of the whole machine. Then it can be converted into a differential mode signal. The easiest way to solve common mode interference is to solve the problems between each circuit unit, the port of the whole machine, and the chassis. Whole machine shielding is difficult to implement and the cost is high, and this measure is only adopted when there is no alternative.
