1. Interference problem of high frequency switching power supply
In the current intelligent switching power supply, there are built-in microprocessors or DSPs, which are used for on-machine monitoring and communication. Micro-processing chips have high requirements for power supply, and the amplitude is required to be quite stable, and it should not have large spikes and burrs, causing 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 the AC input power, the monitoring part must work normally first, and carry out self-test and various conditions detection to determine whether the rectifier can be turned on; in case of extremely high or extremely low AC voltage, although the rectifier has stopped working, But the monitoring part still has to work normally and maintain normal monitoring and communication. During the operation of some power products, there have been phenomena such as unprovoked resets. When designing the auxiliary power supply of high-power switching power supplies, we analyzed them and found that there are many auxiliary power supplies under different AC input voltages and different load conditions. Problems: The AC adaptability is narrow, the load capacity is low, the working waveform is unstable and extremely asymmetrical, biased magnetism occurs, and the electromagnetic interference is extremely serious.
The working principle of the general switching rectifier auxiliary power supply is: the input alternating current is rectified into high-voltage direct current, and then converted into a low-voltage high-frequency square wave through a conversion circuit, and then becomes a stable low-voltage direct current required by the system through a rectifier and filter circuit. The voltage feedback signal of the high-frequency conversion drive pulse control loop is provided by one DC output. The upper series resistance of the main circuit of the power conversion is sampled as the current feedback signal, and the driving pulse of the power conversion tube is controlled by UC3844 and other control chips and its peripheral circuits. produce. (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 be stable. Grab the waveform. For current feedback, the pulse width of the waveform is also wide or narrow, and the duty cycle reaches 47%, while the maximum duty cycle of the 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 load carried by the auxiliary power supply work stably and normally in the full range from no-load to overload, are relatively difficult, which involve several The technical problems in aspects: the withstand voltage and overload capacity of the power device; the design of the high frequency transformer; the selection of the parameters of the driving pulse control loop.
Through certain theoretical analysis and experimental exploration, technicians 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 turns Np on the primary side, reduce the ratio of turns on the primary and secondary sides, and reduce the duty cycle at low voltage, which is much smaller than the upper limit of 45% specified by UC3844; The RC filter network in the feedback link is used to adjust the parameters. After many experiments and exploration, the ideal parameters are finally obtained, and the filter capacitor is increased. Test the same secondary winding of the auxiliary transformer again under the same conditions. From these 4 waveforms, it can be seen that the improved auxiliary power supply is in the case of very high or very 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 more symmetrical and more balanced, and the load capacity is obviously better than that before the improvement. Compared with the low input voltage, the duty cycle after the improvement is 7% lower than the duty cycle before the improvement, which shows that the output voltage of the auxiliary power supply can remain stable under the condition of increasing the load, and the load capacity is obviously strong. Before the improvement, the auxiliary power supply improvement work has achieved obvious results.