Solutions to electromagnetic compatibility
From the three elements of electromagnetic compatibility, to solve the electromagnetic compatibility problem of switching power supplies, we can start from three aspects:
Firstly, reduce the disturbance signals generated by disturbance sources;
Secondly, cut off the transmission route of harassment signals;
Thirdly, enhance the ability of harassed individuals to resist harassment. When solving the internal compatibility of switching power supplies, the above three methods can be comprehensively utilized, based on cost-effectiveness ratio and ease of implementation. Therefore, external disturbances generated by switching power supplies, such as harmonic currents in power lines, conducted disturbances in power lines, and electromagnetic field radiation disturbances, can only be solved by reducing the disturbance source.
On the one hand, it can enhance the design of input/output filtering circuits, improve the performance of APFC circuits, reduce the voltage and current change rate of switch tubes, rectifiers, and freewheeling diodes, and adopt various soft switching circuit topologies and control methods; On the other hand, strengthen the shielding effect of the casing, improve the gap leakage of the casing, and carry out good grounding treatment. And for external anti-interference capabilities (such as surges and lightning strikes), the lightning protection capabilities of the AC input and DC output ports should be optimized. Usually, for 1.2/50 μ Open circuit voltage and 8/20 μ The combined lightning strike waveform of short circuit current is usually solved by combining zinc oxide varistors and gas square tubes due to their small energy. For electrostatic discharge, TVS tubes and corresponding grounding protection are usually used in the small signal circuits of communication and control ports, and the electrical distance between the signal circuit and the casing is increased to solve or select devices with anti-static interference. The fast transient signal contains a wide spectrum and is easily transmitted into the control circuit in a common mode manner. The same method as 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 a common mode capacitor or inserting a loss type ferrite magnetic ring) to improve the anti-interference performance of the system.
To reduce the internal disturbance of switching power supplies, achieve their own electromagnetic compatibility, and improve the stability and reliability of switching power supplies, the following aspects should be taken into consideration:
① Pay attention to the correct zoning of digital circuit and module circuit PCB wiring;
② Decoupling of digital circuit and analog circuit power supply;
③ Single point grounding of digital circuits and analog circuits, single point grounding of high current circuits and small current, especially current and voltage sampling circuits, to reduce common resistance disturbance and reduce the impact of ground loop grounding. When wiring, attention should be paid to the spacing between adjacent lines and signal properties to avoid crosstalk, reduce the area surrounded by the output whole current circuit, continuous current diode circuit, and branch filter circuit, and reduce the distribution capacitance of transformer leakage and filter inductance, Use filter capacitors with high resonant frequencies, etc.
