There are generally only these reasons why switching power supplies howl!

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There are generally only these reasons why switching power supplies howl! [Copy link]

The switching power supply controls the time ratio of the switch tube in the circuit to open and close, and maintains a stable circuit voltage output. It is a very common power supply design. However, people who have been engaged in the design of switching power supplies know that in the process of testing the switching power supply, some howling sounds are often heard, similar to the leakage sound when the high voltage is poor, or the sound of high voltage arcing. So when these phenomena occur, how should they be solved? Generally speaking, the reasons for the howling of the switching power supply are generally the following. Poor transformer varnishing Including no varnish impregnation. Howling and causing spikes in the waveform, but generally normal load capacity, special note: the greater the output power, the stronger the howling, and the performance of low power may not be obvious. A 72W charger product has had poor load experience, and it was found that there are strict requirements for the material of the magnetic core in this product. In addition, when the design of the transformer is poor, it is also possible to vibrate and produce abnormal noise during operation. PWM IC grounding routing error Usually, the product will show that some parts can work normally, but some parts cannot carry load and may not be able to oscillate. Especially when using some low-power ICs, it is more likely to fail to work properly. For example, the SG6848 test board was laid out hastily based on experience because the performance of the IC was not thoroughly understood at the beginning. As a result, it was not able to perform wide voltage tests during the test. Optocoupler working current point routing error When the position of the optocoupler working current resistor is connected before the secondary filter capacitor, there may be a whistle, especially when the load is higher. Reference voltage regulator IC TL431 grounding error The grounding of the secondary reference voltage regulator IC has similar requirements as the grounding of the primary IC, that is, it cannot be directly connected to the cold ground and hot ground of the transformer. If they are connected together, the load capacity will decrease and the whistle will be 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. In the previous cycle, the duty cycle of the switch tube was too large, the conduction time was too long, and too much energy was transmitted through the high-frequency transformer; the energy storage inductor of the DC rectifier was not fully released in this cycle. After PWM judgment, no drive signal to turn on the switch tube was generated in the next cycle, or the duty cycle was too small. The switch tube is in the cut-off state in the entire subsequent cycle, or the conduction time is too short. After the energy storage inductor has released energy 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, causing the transformer to vibrate at a lower frequency (regular intermittent full cut-off cycles, or frequencies with drastic changes in duty cycle), emitting a lower frequency sound that can be heard by the human ear. At the same time, the output voltage fluctuation will also increase compared to 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 the human ear, and emit a sharp high-frequency "whistle". At this time, the switch transformer is working in a serious overload state, and there is always the possibility of burning out - this is the origin of the "screams" before many power supplies burn out. I believe some users have had similar experiences. When the load is not loaded or very light In this case, the switch tube may also have an intermittent full cutoff cycle. The switch transformer is also working in an overload state, which is also very dangerous. For this problem, it can be solved by presetting a dummy load at the output end, but it still occurs occasionally in some "saving" or high-power power supplies. When there is no load or the load is too light, the back electromotive force generated by the transformer during operation cannot be well absorbed. In this way, the transformer will couple a lot of noise signals to the winding. This noise signal includes many AC components with different 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 a low-frequency self-excitation. The magnetic core of the transformer will not make any sound. We know that the human hearing range is 20--20KHZ. Therefore, when we design the circuit, we generally add a frequency selection circuit to filter out the low-frequency components. It is best to add a bandpass circuit to the feedback loop to prevent low-frequency self-excitation. Or you can make the switching power supply a fixed frequency. This article mainly introduces 6 reasons that cause the switching power supply to howl, and provides corresponding solutions for these 6 reasons. This is a basic article. I hope that through this article, everyone can use the methods in the article to solve the problem by themselves when encountering the switching power supply howling.

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What is the reason for the howling sound when the power goes out?