Everyone who has done the development work has such experience, testing switching power supply
or hearing the sound of leakage or high-voltage arc pulling of similar products in the experiment is not invited. The sound is loud or small, or sometimes absent. Its rhythm is deep or harsh, or changeable.
1. Transformer is poorly impregnated, including not containing Varnish whistling and causing spikes in waveforms, but generally has normal carrying capacity. It should be noted that the higher the output power, the more the scream, while the smaller the power, the less obvious the performance. I have had bad experience of overloading in a 72W charger product, and found that there are strict requirements on the material of magnetic cores in this product (the customer requirements of this product are more stringent). Additionally, when the transformer is poorly designed, it is also possible to produce abnormal vibration when working.
2. WMC grounding fault. Usually, some of the products will work normally, but some of them can not be loaded and may not be able to start up. Especially when some low power IC is used, it is more likely that it will not work properly. I have used the SG6848 test board, because I did not have a thorough understanding of IC performance, I rushed to layout based on experience, and the result was that the wide voltage test could not be done.
3. Opto Coupler operating current point wiring error. When the position of the working current resistance of the optocoupler is connected before the secondary filter capacitor, there is also the possibility of screaming, especially when there is more load.
4. Grounding wire fault of Regulator IC TL431. The grounding of the same secondary reference voltage regulator IC has similar requirements as the grounding of the primary IC. That is, they can not be directly connected with the cold, geothermal and geothermal transformer. The consequence of this connection is that the carrying capacity decreases and the whistle is proportional to the output power. The PCB in the last article made such a mistake, which was later corrected by JACKY WANG. When the output load is large and close to the power limit of the power supply, the switching transformer may enter an unstable state. In the previous period, the duty cycle of the switch tube is too large and the turn-on time is too long, so the excessive energy is transmitted through the high-frequency transformer. The energy storage inductance of DC rectifier is not fully released in this cycle, and PWM judges that there is no driving signal or duty cycle which makes the switch on in the next cycle. The switching tube is in the cut-off state or the turn-on time is too short in the following whole cycle. After the energy release of the energy storage inductor for more than one whole cycle, the output voltage drops, and the duty cycle of the switching tube in the next cycle is also large. In this way, the transformer will vibrate at a lower frequency (frequency with regular intermittent full cut-off period or drastic change of duty cycle) and produce a lower frequency sound that can be heard by human ears. At the same time, the fluctuation of output voltage will also increase compared with 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 transformers working in the supersonic band, enter the audible frequency range, and emit sharp high-frequency "whistle". At this time, the switching transformer works in a serious overload state, and it is possible to burn out at all times. This is the origin of "screaming" before many power sources burn down. I believe that some users have had similar experiences.
Intermittent full cut-off cycles may also occur when no load or light load occurs. Switching transformer also works in overload state, which is also very dangerous. This problem can be solved by presetting the false load at the output. But it still happens occasionally in some "saving" or high-power sources. When there is no load or the load is too light, the back EMF generated by the transformer during operation can not be well absorbed. In this way, the transformer will couple many clutter signals to your 1.2 winding. This clutter signal includes many AC components of different spectrum. There are also many low-frequency waves. When the low-frequency wave is consistent with the natural oscillation frequency of your transformer, the circuit will form low-frequency self-excitation. The core of the transformer does not make sound. We know that the range of human hearing is 20-20KHZ. So when we design the circuit, we usually add the frequency selection circuit to filter out the low frequency components. From your schematic diagram, you'd better add a band-pass circuit to the feedback loop to prevent low-frequency self-excitation, or make your switching power supply a fixed frequency.