Anyone who has done development work has such experience. Test the switching power supply or the sound of leakage sound or high-voltage arcing that is similar to the product in the experiment. The sound is loud or small, or Sometimes it is not; its rhythm is deep or harsh, or it is impermanent.
1. Transformer (dough paint): including non-immersed varnish (Varnish). Howling and causing the waveform to have spikes, but the general loading capacity is normal, especially the description: the greater the output power, the more the whistling, the performance of the small power is not necessarily obvious. I have experienced a bad load in a 72W charger product, and found that the material of the core has strict requirements. (This product customer requirements are more stringent) Add a little, when the design of the transformer is not good, it is possible that the vibration will produce abnormal noise when working.
2, PWM IC grounding line error: usually the product performance will be part of the normal operation, but some products can not be loaded and may not be able to start the vibration, especially when applying some low-power IC, more May not work properly. I have used the SG6848 test board. Since I didn't fully understand the performance of the IC at the beginning, I used the experience to rush to lay out. As a result, I could not do wide voltage test during the test. Sad!
3. Optocoupler operating current point routing error: When the position of the optocoupler's working current resistor is connected before the secondary filter capacitor, there will be a possibility of howling, especially when the load is more.
4. Grounding fault of the reference regulator (Regulator) IC TL431: The grounding of the same secondary reference regulator IC has the same requirements as the grounding of the primary IC, that is, it cannot be directly connected to the cold geothermal ground of the transformer. . If connected together, the load capacity is reduced and the howling is proportional to the output power. When the output load is large and close to the power supply limit, the switching transformer may enter an unstable state: the switching cycle of the previous cycle is too large, the conduction time is too long, and excessive energy is transmitted through the high-frequency transformer. The energy storage inductor of DC rectification is not fully released during this period. It is judged by PWM that there is no driving signal or duty cycle that makes the switch tube conduct in the next cycle; the switch tube is off state in the whole cycle afterwards. , or the on-time is too short; the energy storage inductor is released after more than one full cycle of energy, the output voltage drops, and the duty cycle in the next cycle of the switch tube is large again... so that the transformer generates a lower frequency ( The vibration of a regular intermittent full cut-off cycle or a frequency of drastic changes in the duty cycle emits a lower frequency sound that the human ear can hear. At the same time, the output voltage fluctuations will 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, the vibration frequency of the transformer originally working in the ultrasonic frequency band may be lowered, and the frequency range of the human ear may be heard, and a sharp high-frequency "whistle" is issued. call". At this time, the switching transformer works in a severe overload state, and there is always the possibility of burning - this is the origin of many "screams" before the power supply burns. I believe that some users have had similar experiences.
When the load is very low, or the load is very light, the switch 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 the dummy load at the output, but it still happens occasionally in some "saving" or high-power power supplies. When the load is not loaded or the load is too light, the back EMF generated by the transformer during operation is not well absorbed. This way the transformer will couple a lot of clutter signals to your 1.2 windings. This clutter signal includes the AC components of many different spectra. 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 a low-frequency self-excitation. The core of the transformer does 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 bandpass circuit to the feedback loop to prevent low frequency self-excitation. Or you can make your switching power supply a fixed frequency.
High-power switching power supply short-circuit whistling
I believe that everyone has encountered this situation, the switching power supply suddenly shorts the power supply after full load test, sometimes it will hear the power supply whistling; or when setting the current protection, when the current is debugged to a certain position, there will be Howling, the sound of its howling is swaying, and it is very annoying. The main reasons are as follows:
When the output load is large and close to the power supply limit, the switching transformer may enter an unstable state: the switching cycle of the previous cycle is too large, the conduction time is too long, and excessive energy is transmitted through the high-frequency transformer. The energy storage inductor of DC rectification is not fully released during this period. It is judged by PWM that there is no driving signal or duty cycle which makes the switch tube conduct in the next cycle; the switch tube is cut off in the whole cycle afterwards. The state, or the on-time is too short; the energy storage inductor is released after more than one full cycle of energy, the output voltage drops, and the duty cycle in the next cycle of the switch tube is large again... so that the transformer generates a lower frequency. The vibration of a regular intermittent full cut-off cycle or a frequency at which the duty cycle changes drastically emits a lower frequency sound that the human ear can hear. At the same time, the output voltage fluctuations will 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, the vibration frequency of the transformer originally working in the ultrasonic frequency band may be lowered, and the frequency range of the human ear may be heard, and a sharp high-frequency "whistle" is issued. call". At this time, the switching transformer works in a severe overload state, and there is always the possibility of burning - this is the origin of many "screams" before the power supply burns. I believe that some users have had similar experiences. When the load is very low, or the load is very light, the switch 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 the dummy load at the output, but it still happens occasionally in some "saving" or high-power power supplies. When the load is not loaded or the load is too light, the back EMF generated by the transformer during operation is not well absorbed. This way the transformer will couple a lot of clutter signals to your 1.2 windings. This clutter signal includes the AC components of many different spectra. 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 a low-frequency self-excitation. The core of the transformer does 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 bandpass circuit to the feedback loop to prevent low frequency self-excitation. Or you can make your switching power supply a fixed frequency.
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