Generally speaking, the reasons for the howling of switching power supply generally have the following inducements:
The transformer has poor varnishing
includes unimpregnated Van Lishui. Howling causes sharp spikes in the waveform, but generally the load capacity is normal. In particular, the greater the output power, the stronger the howling, and the lower power may not necessarily be obvious. A 72W charger product has the experience of overloading and poor loading, and it is found that there are strict requirements for the material of the magnetic core in this product. To add, when the design of the transformer is not good, it may also cause abnormal noise due to vibration during operation.
PWM IC grounding wiring error
Generally, some products can work normally, but some products cannot be loaded and may not be able to start vibration. Especially when some low-power ICs are used, they are more likely to fail to work normally. For example, the SG6848 test board did not have a thorough understanding of the performance of the IC at the beginning, so it was hurriedly laid out based on experience. As a result, the wide voltage test could not be done during the test.
Optocoupler working current point alignment 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.
The ground wire of the reference voltage regulator IC TL431 is faulty
The grounding of the same secondary reference voltage regulator IC has similar requirements as the grounding of the primary IC, that is, neither can 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.
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 switch tube in the previous cycle is too large, the on-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 energy in this cycle. According to the PWM judgment, in the next cycle There is no driving signal to turn on the switch tube, or the duty cycle is too small. The switch tube is in the cut-off state for the entire period afterwards, or the on-time is too short. After the energy storage inductor is released for more than one entire cycle, the output voltage drops, and the duty cycle of the switch tube in the next cycle will be larger... This cycle repeats, so that the transformer has a lower frequency (regular intermittent full cut-off) Period, or the frequency at which the duty cycle changes drastically), emit 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 sharp high frequencies. Whistle". At this time, the switching transformer is working in a serious overload state, and it may be burnt at any time-this is the origin of many power supplies "screaming" before burning. I believe some users have had similar experiences.
When there is no load or the load is very light
In this case, the switching tube may also have an intermittent full cut-off period. The switching transformer is also working 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 the windings to which a lot of noise signals arrive. 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. Therefore, when we design the circuit, we generally add a frequency selection loop. To filter out low frequency components. It is best to add a band-pass circuit to the feedback loop to prevent low-frequency self-excitation. Or the switching power supply can be made into a fixed frequency.
This article mainly introduces 6 causes of howling in the switching power supply, and provides corresponding solutions to these 6 causes. This is an article that is biased towards basics. I hope that through this article, everyone can use the methods in the article to solve them when they encounter switching power supply whistling.