Why most people can't hear the whistling sound of the power transformer
The main reasons for this are as follows:
When the output load is large and approaching the power limit of the power supply, the switching transformer may enter an unstable state: the duty cycle of the switching transistor in the previous cycle was too high, the conduction time was too long, and too much energy was transmitted through the high-frequency transformer; The energy storage inductor of DC rectification has not fully released its energy in this cycle. According to PWM judgment, there is no driving signal that causes the switch tube to conduct or the duty cycle is too small in the next cycle; The switch tube remains in a cut-off state throughout the entire cycle, or the conduction time is too short; After more than the entire cycle of energy release, the output voltage of the energy storage inductor decreases, and the duty cycle of the switch tube in the next cycle will be larger. This cycle repeats itself, causing the transformer to vibrate at a lower frequency (regular intermittent full cutoff period or frequency with drastic changes in duty cycle), emitting a lower frequency sound that can be heard by the human ear. At the same time, the output voltage fluctuation will also increase compared to normal operation. When the number of intermittent full cutoff cycles per unit time reaches a considerable proportion of the total number of cycles, it may even cause the vibration frequency of transformers that originally worked in the ultrasonic frequency band to decrease, entering the audible frequency range of the human ear and emitting sharp high-frequency "whistling" sounds. At this time, the switching transformer is working in a severely overloaded state, and there is always a possibility of burning out - this is the origin of many power sources' screams before burning out, and I believe some users have had similar experiences before. When there is no load or when the load is very light, the switching tube may also have intermittent full cutoff cycles, and the switching transformer also works in an overloaded state, which is also very dangerous.
Screaming can be improved by taking some measures:
It can be solved by pre setting false loads at the output end, but it still occasionally occurs in some "energy-saving" or high-power power supplies. When there is no load or the load is too light, the back electromotive force generated by the transformer during operation cannot be well absorbed. This way, the transformer will couple a lot of clutter signals to your 1.2 winding. This clutter signal includes many AC components of different spectra. There are also many low-frequency waves among them. When the low-frequency waves are consistent with the natural oscillation frequency of your transformer, the circuit will form low-frequency self-excited. The magnetic core of the transformer does not make any sound. We know that the human auditory range is 20-20KHz So when we design circuits, we usually add a frequency selection circuit. To filter out low-frequency components. From your schematic, it would be best to add a bandpass circuit to the feedback loop to prevent low-frequency self excitation. Alternatively, you can make your switching power supply a fixed frequency.
