What is the cause of the whistling of the power transformer
The howling you mentioned is the noise generated when the transformer is running. The noise is mainly caused by the following three reasons:
1. Mechanical noise generated by transformer core:
It is because the silicon steel sheet in the iron core will change under the action of the alternating magnetic field, that is, magnetostriction, and the magnetostriction will make the iron core vibrate periodically with the change of the excitation frequency to generate noise.
2. Noise generated by non-standard manufacturing process
3. Power transformer thermal noise
It is recommended that you choose a toroidal transformer with a middle hole filled with glue (epoxy resin). The toroidal transformer with a middle hole filled with epoxy resin can not only reduce noise and vibration, but also have the effect of heat conduction and heat dissipation
Why can't most people hear the howling sound of the power transformer
High-frequency howling may indeed be inaudible to many people. It is not surprising that this is caused by individual differences in human hearing.
The main reasons are as follows:
When the output load is large and close to the power limit of the power supply, the switching transformer may enter an unstable state: the duty cycle of the switching tube in the previous cycle is too large, the conduction time is too long, and too much energy is transmitted through the high-frequency transformer ;The energy storage inductance of the DC rectifier has not fully released the energy in this cycle. According to the PWM judgment, there is no driving signal to turn on the switch tube in the next cycle or the duty cycle is too small; the switch tube is cut off in the entire cycle after that. state, or the conduction time is too short; the energy storage inductance releases more energy than the entire cycle, the output voltage drops, and the duty cycle of the switch tube in the next cycle will increase again, so that the transformer will generate a lower frequency (some Regular intermittent full cut-off period or frequency of drastic change in duty cycle) vibration, which emits a lower frequency sound that can be heard by the human ear. At the same time, the output voltage fluctuation will be larger than normal operation. When the number of intermittent full cut-off cycles per unit time reaches a considerable proportion of the total number of cycles, it will even reduce the vibration frequency of the transformer originally working in the ultrasonic frequency band, enter the frequency range audible to human ears, and emit a sharp high-frequency "whistle" Call". At this time, the switching transformer is working in a serious overload state, and it may burn out at any time-this is the origin of many power supplies "screaming" before burning out. I believe that some users have had similar experiences. No-load, or when the load is very light, the switching tube may also have intermittent full cut-off periods, and the switching transformer is also working in an overloaded state, which is also very dangerous.
Howling can be improved by taking some measures:
It can be solved by presetting a false load at the output, but it still happens occasionally in some "saving" or high-power power supplies. When there is no load or the load is too light, the counter electromotive force generated by the transformer during operation cannot be well absorbed. This transformer will couple a lot of clutter to your 1.2 winding. This clutter signal includes many AC components of different frequency spectrums. There are also many low-frequency waves. When the low-frequency waves are consistent with the natural oscillation frequency of your transformer, the circuit will form a low-frequency self-excitation. The magnetic core of the transformer will not make sound. We know that the human hearing range is 20--20KHZ. So when we design the circuit, we generally add a frequency-selective circuit. to filter out low frequency components. From your schematic it looks like you'd be better off adding a bandpass circuit to the feedback loop to prevent low frequency self-excitation. Or make your switching power supply a fixed frequency.
