Analysis of the cause of howling of switching power supply

Anyone who has done development work has such experience. In testing switching power supplies or in experiments, they heard the leakage sound of similar products with poor high voltage or the sound of high voltage arcing uninvited: the sound is loud or small, or Sometimes there is no; its rhythm may be deep or harsh, or changeable.　　 1. Transformer (Transformer) Poor dipping paint: including not impregnated with Varnish. Howling causes sharp spikes in the waveform, but generally the load capacity is normal. Special note: the greater the output power, the worse the whistling, and the lower power may not necessarily be obvious. I have had poor loading experience in a 72W charger product, and found that there are strict requirements on the material of the magnetic core in this product. (Customer requirements for this product are more stringent.) To add, when the design of the transformer is not good, it may vibrate and produce abnormal noise when working. 2. PWM IC grounding wiring error: usually some products can work normally, but some products cannot be loaded and may fail to vibrate, especially when some low-power ICs are used. It may not work properly. I have used the SG6848 test board. Since I did not thoroughly understand the performance of the IC at the beginning, I hurriedly lay out it based on experience. As a result, I couldn't do a wide voltage test during the test. Sad!　　 3. Opto Coupler (Opto Coupler) working current point wiring error: When the position of the optocoupler's working current resistance is connected before the secondary filter capacitor, there is also the possibility of howling, especially when the load is more. 4. The grounding wire of the regulator IC TL431 is faulty: the grounding of the same secondary reference regulator IC has similar requirements as the grounding of the primary IC, that is, it cannot be directly connected to the cold and hot ground of the transformer . If they are connected together, the result is that the load capacity is reduced and the howling sound is proportional to the output power. The PCB in the previous article made such a mistake, and it 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: 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 inductor of the DC rectifier has not fully released the energy in this cycle, and it is judged by PWM that there is no driving signal to turn on the switching tube or the duty cycle is too small; the switching tube is in the off state for the entire period afterwards , Or the conduction time is too short; after more than one cycle of energy release, the output voltage of the energy storage inductor drops, and the duty cycle of the switch tube in the next cycle will be larger again and again, so that the transformer will have a lower frequency (regularly The intermittent full cut-off period or the frequency where the duty cycle changes drastically), 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 send out sharp high-frequency whistles. call'. At this time, the switching transformer is working in a serious overload state, and it may be burnt at all times. This is the origin of many power supplies that 'scream' before burning. I believe some users have had similar experiences.　　 When there is no load, or when the load is very light, the switching tube may also have an intermittent full cut-off period. The switching transformer also works in an overload state, which is also very dangerous. This problem can be solved by presetting a dummy load at the output end, 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 back EMF generated by the transformer during operation cannot be absorbed well. In this way, the transformer will couple a lot of spurious signals to your 1.2 winding. This clutter signal includes many AC components of different frequency spectra. 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 low-frequency self-excitation. The core of the transformer will not make a sound. We know that the human hearing range is 20-20KHZ. So when we design the circuit, we usually add a frequency selection loop. To filter out low frequency components. From your schematic, you'd better add a band-pass circuit to the feedback loop to prevent low-frequency self-excitation. Or you can make your switching power supply a fixed frequency. Disclaimer: The articles published on this site are all from the Internet and do not represent the views of this site. If there is any infringement, please contact to delete WeChat: Disclaimer: Some pictures and content of the articles published on this site are from the Internet. If there is any infringement, please contact delete A: Technical Article—How to Improve the Temperature Measurement Accuracy in the Battery Monitoring System