1. The interference problem of high frequency switching power supply

　　In the current intelligent switching power supply, there are internal microprocessors or DSPs for internal monitoring and communication. The micro-processing chip has very high requirements for the power supply, and the required amplitude is quite stable, not to mention the large spikes and burrs, which cause electromagnetic interference, and the AC adaptability of the auxiliary power supply is required to be wider than the normal working range of the rectifier. When the rectifier is connected to AC input power, the monitoring part must work normally first, and perform self-checking and various conditions to determine whether the rectifier can be turned on; in case of extremely high or very low AC voltage, the rectifier has stopped working, However, the monitoring part must still work normally and maintain normal monitoring and communication.

Some power products have unreasonable resetting during the operation process. When designing the auxiliary power supply of high-power switching power supply, we analyzed it and found that the auxiliary power supply has more power under different AC input voltage and different load conditions. Problem: The AC adaption range is narrow, the load capacity is low, the working waveform is unstable and extremely asymmetric, there is magnetic bias, and the electromagnetic interference is extremely serious.

The working principle of the auxiliary power supply of general switching rectifiers is: the input AC power is rectified into high-voltage DC power, and then into a low-voltage high-frequency square wave through the conversion circuit, and then becomes the stable low-voltage DC power required by the system through the rectification filter circuit, usually by a three-terminal regulator Regulated voltage, a DC output provides a high-frequency conversion drive pulse control loop voltage feedback signal. The series resistance sampling on the main loop of the power conversion is used as the current feedback signal, and the driving pulse of the power conversion tube is generated by the control chip such as UC3844 and its peripheral circuits.

　　 (Note: AC low voltage is the measured value of the lowest input voltage when the auxiliary power supply starts to work)

It can be seen that the auxiliary transformer can no longer work normally under the condition of low AC input voltage and no current feedback. The pulse width of its waveform is different, some are wide and some are narrow, and jitter occurs, and the oscilloscope can no longer stabilize. Grab the waveform. For current feedback, the pulse width of the waveform is also wide and narrow, and the duty cycle reaches 47%, while the maximum duty cycle of UC3844 is only 50%. If the load is increased, the output voltage will decrease.

How to make the auxiliary power supply work stably under the upper limit and lower limit voltage of the AC input, and how to make the auxiliary power supply work stably and normally in the full range of load from no-load to overload, are relatively difficult, which involves several Technical problems: the withstand voltage and overload capacity of power devices; the design of high-frequency transformers; the selection of the parameters of the drive pulse control loop.

Two, the solution

　　 Through certain theoretical analysis and experimental exploration, technical personnel made corresponding improvements to the auxiliary transformer and control loop, and finally solved this problem. The solution is: adjust the turns ratio of the auxiliary transformer, change the number of primary turns Np, reduce the ratio of the primary and secondary turns, and reduce the duty cycle at low voltage, which is much less than the upper limit 45% specified by UC3844; reduce the current of UC3844 The RC filter network in the feedback link adjusts the parameters. After many experiments, the ideal parameters are finally obtained, and the filter capacitor is increased. Test the same secondary winding of the auxiliary transformer under the same conditions again.

From these 4 waveforms, it can be seen that the improved auxiliary power supply, whether in the case of extremely high or low AC input (and the starting working voltage is lower than before the improvement), or in the case of no load or heavy load, The working waveforms are more stable than before the improvement, the pulse width is symmetrical and more balanced, and the load capacity is significantly better than before the improvement. Compared with the low input voltage, the duty cycle after the improvement has decreased by 7% compared to the duty cycle before the improvement, which indicates that the output voltage of the auxiliary power supply can remain stable under the condition of increased load, and the load capacity is obviously strong. Before the improvement, the improvement of the auxiliary power supply has achieved obvious results.

3. Summary of experience

In the process of improving the auxiliary power supply, technicians have started from many aspects, including changing the PI adjustment parameters of the voltage feedback loop, changing the pulse frequency, increasing the filter capacitor after the secondary side rectification, etc., but they did not find the source of the problem. Under the conditions of input high and low voltage, light load and overload, the waveform is still jittery and the DC output voltage is unstable. When adjusting the RC filter network parameters of the current feedback link of UC3844, many experiments have been carried out to find a more suitable one. Matching parameters, it can be seen that engineers still need to verify the improved results through continuous experiments after theoretical analysis.

The above conclusions are also useful for other low-power switching power supplies that use the same circuit. Using this method to change the RC filter network parameters of the current feedback link of the control chip has also achieved obvious results. The specific parameters are different due to the difference of each circuit. The difference is different, but the direction of improvement is the same. With the development of power supply technology, the control of high-frequency switching power supplies has gradually developed from the initial analog circuit to highly integrated control devices such as microprocessors and DSPs. These devices are small in size and high in precision, but the electromagnetic interference in the switching power supply, The radiation is stronger than the working environment of other communication equipment, which puts higher requirements on the auxiliary power supply. This article describes the working characteristics and waveforms of the auxiliary power supply in the high-frequency switching power supply, and focuses on the analysis of the problems that should be paid attention to in the design of the high-frequency switching power supply and the selection of parameters based on the experimental data.

smart socket manufacturers