With the development of power supply technology, high-frequency switching power supply control has gradually evolved from analog circuits to high integration control devices such as microprocessors and DSPs. These devices are small in size and high in precision, but the electromagnetic interference and radiation inside the switching power supply are stronger compared to other communication equipment working environments, which puts higher requirements on auxiliary power supplies. This article elaborates on the working characteristics and waveforms of auxiliary power supplies in high-frequency switching power supplies, and focuses on analyzing the issues that should be paid attention to and the selection of parameters in the design of high-frequency switching power supplies based on experimental data.

1、 Interference issues in high-frequency switching power supplies

In current intelligent switch mode power supplies, internal microprocessors or DSPs are used for monitoring and communication within the machine. Microprocessing chips have high requirements for power supply, with stable amplitude and no large spikes, which can cause electromagnetic interference. Additionally, they require the AC adaptability of auxiliary power supplies to be wider than the normal working range of rectifiers. When the rectifier is connected to AC input power, the monitoring part must first work normally, perform self inspection and various condition checks to determine whether the rectifier can be turned on; In case of extremely high or low AC voltage, although the rectifier has stopped working, the monitoring part still needs to work normally and maintain positive monitoring and communication.

During the operation of certain power supply products, there have been phenomena such as inexplicable reset. When designing auxiliary power supplies for high-power switching power supplies, analysis was conducted and it was found that their auxiliary power supplies have many problems under different AC input voltages and load conditions: narrow AC adaptation range, low load capacity, unstable and extremely asymmetric working waveforms, magnetic bias, and severe electromagnetic interference.

The working principle of a general switch rectifier auxiliary power supply is: the input AC power is rectified to become high-voltage DC power, then transformed into low-voltage high-frequency square waves through a transformation circuit, and then transformed into stable low-voltage DC power required by the system through a rectification and filtering circuit. Generally, it is stabilized by a three-terminal voltage regulator, The voltage feedback signal of the high-frequency conversion drive pulse control loop is provided by a DC output. The current feedback signal is sampled by a series resistor on the main circuit of the power conversion, and the driving pulse of the power conversion transistor is generated by control chips such as UC3844 and their 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 under low AC input voltage and no current feedback conditions, the auxiliary transformer can no longer work properly. The pulse width of its waveform is different, some are wide and some are narrow, and there is jitter. The oscilloscope can no longer stably grasp the waveform. Current feedback, the pulse width of the waveform is also wide and narrow, with a duty cycle of 47%, while the maximum duty cycle of UC3844 is only 50%. If the load is increased, the output voltage will decrease.

How to ensure the stable operation of auxiliary power supply under the upper and lower limit voltage of AC input, and how to ensure the stable and normal operation of the load carried by auxiliary power supply throughout the entire range from no-load to overload, all have significant difficulties. This involves several technical challenges: the withstand voltage and overload capacity of power devices; Design of high-frequency transformers; Selection of driving pulse control circuit parameters.

2、 Solution

Through theoretical analysis and experimental exploration, technicians have made corresponding improvements to the auxiliary transformer and control circuit, and finally solved this problem. The solution is to adjust the turn ratio of the auxiliary transformer, change the number of turns on the original side Np, reduce the ratio of turns on the original and secondary sides, and reduce the duty cycle at low voltage, which is far less than the upper limit of 45% specified in UC3844; By adjusting the parameters of the RC filtering network in the current feedback loop of UC3844 and conducting multiple experiments, the ideal parameters were finally obtained, with an increased filtering capacitance. Test the same secondary winding of the auxiliary transformer again under the same conditions.

From these four waveforms, it can be seen that the improved auxiliary power supply has a more stable working waveform, more symmetrical pulse width, and significantly better load capacity than before, regardless of whether the AC input is extremely high or low (and the starting working voltage is lower than before the improvement), or under no-load or heavy load conditions. Compared to the original duty cycle at low input voltage, the improved duty cycle decreased by 7%, indicating that the AC input of the auxiliary power supply can still maintain stable output voltage even with increased load, and the load capacity is significantly stronger than before. The improvement of the auxiliary power supply has achieved significant results.

3、 Experience summary

In the process of improving the auxiliary power supply, technicians have started from multiple aspects, including changing the PI adjustment parameters of the voltage feedback loop, changing the pulse frequency, increasing the filtering capacitor after secondary rectification, etc. However, the root cause of the problem has not been found. Under the conditions of high and low AC input voltage, light load, and overload, the waveform still shakes severely, and the DC output voltage is unstable. When adjusting the RC filtering network parameters of the current feedback link of UC3844, Multiple experiments were also conducted to find suitable matching parameters, indicating that engineers still need to verify and improve the results through continuous experimentation after conducting theoretical analysis.

The above conclusion is also useful for other low-power switching power supplies that use the same circuit. By changing the RC filtering network parameters of the current feedback link of the control chip using this method, significant results have been achieved. The specific parameters vary depending on the difference in each circuit, but the direction of improvement is the same.