Switching power supply transformer whistling is caused by what?
You said the whistling is the noise generated by the transformer operation, the noise generated mainly for the following three reasons:
1. Mechanical noise generated by the transformer core:
Is due to the iron core in the silicon steel sheet in the alternating magnetic field, will change that magnetostriction, magnetostriction will make the core with the excitation frequency change for periodic vibration noise.
2. The production process is not standardised to produce noise
3. Power transformer thermal noise
Suggest that you can choose to fill the hole (epoxy resin) of the toroidal transformer, the hole filled with epoxy resin of the toroidal transformer can not only reduce the noise and vibration, but also has the effect of thermal conductivity and heat dissipation.
High-frequency whistling may indeed many people can not hear, which is caused by individual differences in the human ear hearing, and not surprising.
The main reason for this is the following:
When the output load is large, close to the power supply power limit, the switching transformer may enter an unstable state: the previous cycle of the switching tube duty cycle is too large, the conduction time is too long, through the high-frequency transformer to transmit too much energy; DC rectifier energy storage inductance this cycle of the energy has not been fully released by the PWM judgement, in the next cycle does not produce a switch to conduct the drive signal or the duty cycle is too small;. Switching tube in the entire cycle after the cut-off state, or conduction time is too short; energy storage inductor after more than the entire cycle of energy release, the output voltage drops, the switching tube in the next cycle of the duty cycle will be large, and so on and so forth, so that the transformer occurs at a lower frequency (regular intermittent cut-off cycle or the duty cycle of the frequency of the drastic changes in the vibration of the lower frequency of the human ear can be heard). sound. At the same time, the output voltage fluctuation will increase compared to normal operation. When the number of intermittent full cut-off cycles per unit of time reaches a considerable proportion of the total number of cycles, it will even make the transformer originally working in the ultrasonic band of vibration frequency reduction, into the frequency range of the human ear can be heard, and emit a sharp high-frequency "whistle". At this time the switching transformer is working in a serious overload state, there is always the possibility of burning - this is the origin of many power supply burned down before the "scream", I believe that some users have had a similar experience. No load, or very light load switching tube may also appear intermittent full cut-off cycle, the switching transformer is also working in the overload state, also very dangerous.
Whistling can be improved by a number of measures:
It can be solved by presetting a dummy load at the output, but it still occurs occasionally in some "frugal" or high power supplies. When there is no load or the load is too light, the counterpotential generated by the transformer during operation is not well absorbed. The transformer will then couple a lot of stray signals into your 1.2 winding. This stray signal consists of AC components of many different spectra. There are also many low frequency waves in it, and when the low frequency waves coincide with the intrinsic oscillation frequency of your transformer, then the circuit will form a low frequency self-excitation. The core of the transformer does not make a sound. We know that the human hearing range is 20 - 20KHZ. So we are designing the circuit, generally coupled with frequency selection circuit. To filter out low frequency components. From your schematic, you'd better add a bandpass circuit on the feedback loop to prevent low-frequency self-excitation. Or just make your switching power supply a fixed frequency.
