Six situations that cause switching power supply howling and their solutions

　　The switching power supply controls the time ratio of the switch tube in the circuit to turn on and off, and maintains a stable circuit voltage output. It is a very common power supply design. However, those who have been engaged in the design of switching power supplies know that in the process of testing the switching power supply, some whistling sounds are often heard, similar to the leakage sound when the high voltage is not good, or the sound of high voltage arcing. So when these phenomena occur, how should they be solved?

　　Generally speaking, there are several reasons why switching power supplies howl.

　　Transformer paint impregnation is bad

　　Including no varnish. 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 on the material of the magnetic core in this product. In addition, when the transformer is poorly designed, it is also possible that vibration will 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 oscillate, especially when using some low-power ICs, it is more likely to not work properly. For example, the SG6848 test board was laid out hastily based on experience because the performance of the IC was not fully understood at the beginning. As a result, it was not able to perform wide voltage tests during the experiment.

　　Optocoupler working current point routing error

　　When the working current resistor of the optocoupler is connected before the secondary filter capacitor, there is also the possibility of howling, especially when there is more load.

　　The ground wire of the reference voltage regulator IC TL431 is wrong

　　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 or hot ground of the transformer. If they are connected together, the load capacity will be reduced and the howling sound 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. The duty cycle of the switch tube in the previous cycle 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 did not fully release the energy 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 off state in the entire subsequent cycle, or the conduction time is too short. After the energy storage inductor releases 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 cutoff cycles, or frequencies with drastic changes in duty cycle), emitting lower-frequency sounds 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 cutoff cycles per unit time reaches a considerable proportion of the total number of cycles, it may even reduce the vibration frequency of the transformer that originally worked in the ultrasonic frequency band and enter the frequency range audible to the human ear, emitting a sharp high-frequency "whistle". At this time, the switching transformer is working in a serious overload state and may burn out at any time - this is the origin of the "screams" before many power supplies burn out. I believe some users have had similar experiences.

　　No load or light load

　　In this case, the switch tube may also have intermittent full cut-off cycles, and the switch transformer will also work in an overloaded state, which is also very dangerous. This problem can be solved by presetting a dummy load at the output end, but it still occasionally occurs 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 absorbed well. In this way, the transformer will couple a lot of noise signals to the winding. This noise signal includes many AC components of 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 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.