1. Bad Transformer Impregnation: Including non-impregnated Varnish. Whistling and causing spikes in the waveform, but normal carrying capacity is normal, in particular: the higher the output power is, the more the whistling is, and the lower power is not necessarily obvious. I had experience with bad loading in a 72W charger product and found in this product that there are stringent requirements on the core material. (This product's customer requirements are more rigorous.) It should be added that when the transformer is poorly designed, there is a possibility that vibration may cause abnormal noise.
2, PWMIC ground trace error: usually the product appears to be part of the normal work, but some products can not be loaded and may not be able to start the fault, especially when applying some low-power IC, more likely can not work normally. I have used the SG6848 test board. Because I didn't thoroughly understand the performance of the IC, I was hurriedly laid out based on experience. As a result, the wide voltage test could not be performed during the test. Sad!
3, optocoupler (OptoCoupler) operating current point routing error: When the position of the optocoupler operating current resistance connected to the secondary filter capacitor before there may be howling, especially when the more loaded with more.
4, the reference regulator (Regulator) ICTL431 ground fault: the same secondary reference regulator IC grounding and primary IC ground has the same requirements, that is, can not be directly connected to the transformer cold geothermal ground. If connected together, the carrying capacity drops and the howling 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 switching transistor 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 DC-rectified energy storage inductor is not fully discharged during this cycle. The PWM judges that the driving signal to turn on the switching transistor or the duty ratio is not too small in the next cycle; the switching transistor is in the off state for the entire cycle thereafter. , Or the conduction time is too short; after more than one whole cycle of energy release, the energy storage inductance will drop, the output voltage will drop, and the duty cycle of the switching tube will be larger in the next cycle... so that the transformer will occur at a lower frequency. The frequency of a regular intermittent full cut-off period or a frequency at which the duty cycle changes drastically, gives a lower frequency sound that the human ear can hear. At the same time, output voltage fluctuations will also increase over normal operation. When the number of intermittent full cut-off periods per unit time reaches a considerable proportion of the total number of cycles, even the frequency of the transformer originally operating in the ultrasonic band will be reduced in frequency, entering the audible frequency range of human ears, and emitting a sharp high-frequency “sentinelâ€. call". Switching transformers at this time work in a severely overloaded state, and they may be burned at any moment—this is the reason for the “screaming†before many power supplies are burned. I believe that some users have had similar experiences.
When there is no load, or when the load is very light, the switch may also have an intermittent full off period, and 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 sources. When the load is not loaded or the load is too light, the back electromotive force generated by the transformer during operation cannot be well absorbed. This will cause the transformer to couple many clutter signals to your 1.2 windings. This clutter signal contains many AC components in different spectrums. There are also many low-frequency waves. When the low-frequency waves coincide with the natural oscillation frequency of your transformer, the circuit will form a low-frequency self-excitation. The core of the transformer does not emit sound. We know that people's hearing range is 20--20KHZ. So when we design circuits, we usually add frequency selective loops. 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 fixed frequency.
5, high-power switching power supply short-circuit whistle I believe we have encountered this situation, switching power supply suddenly test the power supply short circuit after full load, and sometimes will hear the power whistle situation; or when setting the current protection, when When the current is debugged to a certain stage, there will be howling, and the whistling sounds will fluctuate. It is annoying. The main reasons for this are:
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 transistor in the previous cycle is too large, the conduction time is too long, and excessive energy is transmitted through the high-frequency transformer. The DC-rectified energy storage inductor is not fully discharged during this cycle. After the PWM judgment, no drive signal to turn on the switch is generated or the duty ratio is too small in the next cycle; the switch is turned off during the entire cycle. The state, or the conduction time is too short; the energy storage inductor releases more than one whole cycle of energy, the output voltage drops, and the duty cycle of the switching tube in the next cycle will be large... This cycle will cause the transformer to generate lower frequencies. (Vibration with a regular intermittent full cut-off period or a frequency at which the duty cycle changes drastically) gives a lower frequency sound that the human ear can hear. At the same time, output voltage fluctuations will also increase over normal operation. When the number of intermittent full cut-off periods per unit time reaches a considerable proportion of the total number of cycles, even the frequency of the transformer originally operating in the ultrasonic band will be reduced in frequency, entering the audible frequency range of human ears, and emitting a sharp high-frequency “sentinelâ€. call". Switching transformers at this time work in a severely overloaded state, and they may be burned at any moment—this is the reason for the “screaming†before many power supplies are burned. I believe that some users have had similar experiences. When there is no load, or when the load is very light, the switch may also have an intermittent full off period, and 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 sources. When the load is not loaded or the load is too light, the back electromotive force generated by the transformer during operation cannot be well absorbed. This will cause the transformer to couple many clutter signals to your 1.2 windings. This clutter signal contains many AC components in different spectrums. There are also many low-frequency waves. When the low-frequency waves coincide with the natural oscillation frequency of your transformer, the circuit will form a low-frequency self-excitation. The core of the transformer does not emit sound. We know that the human hearing range is 20--20 kHz. So we usually add a frequency-selective circuit when designing circuits. 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 fixed frequency.
2, PWMIC ground trace error: usually the product appears to be part of the normal work, but some products can not be loaded and may not be able to start the fault, especially when applying some low-power IC, more likely can not work normally. I have used the SG6848 test board. Because I didn't thoroughly understand the performance of the IC, I was hurriedly laid out based on experience. As a result, the wide voltage test could not be performed during the test. Sad!
3, optocoupler (OptoCoupler) operating current point routing error: When the position of the optocoupler operating current resistance connected to the secondary filter capacitor before there may be howling, especially when the more loaded with more.
4, the reference regulator (Regulator) ICTL431 ground fault: the same secondary reference regulator IC grounding and primary IC ground has the same requirements, that is, can not be directly connected to the transformer cold geothermal ground. If connected together, the carrying capacity drops and the howling 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 switching transistor 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 DC-rectified energy storage inductor is not fully discharged during this cycle. The PWM judges that the driving signal to turn on the switching transistor or the duty ratio is not too small in the next cycle; the switching transistor is in the off state for the entire cycle thereafter. , Or the conduction time is too short; after more than one whole cycle of energy release, the energy storage inductance will drop, the output voltage will drop, and the duty cycle of the switching tube will be larger in the next cycle... so that the transformer will occur at a lower frequency. The frequency of a regular intermittent full cut-off period or a frequency at which the duty cycle changes drastically, gives a lower frequency sound that the human ear can hear. At the same time, output voltage fluctuations will also increase over normal operation. When the number of intermittent full cut-off periods per unit time reaches a considerable proportion of the total number of cycles, even the frequency of the transformer originally operating in the ultrasonic band will be reduced in frequency, entering the audible frequency range of human ears, and emitting a sharp high-frequency “sentinelâ€. call". Switching transformers at this time work in a severely overloaded state, and they may be burned at any moment—this is the reason for the “screaming†before many power supplies are burned. I believe that some users have had similar experiences.
When there is no load, or when the load is very light, the switch may also have an intermittent full off period, and 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 sources. When the load is not loaded or the load is too light, the back electromotive force generated by the transformer during operation cannot be well absorbed. This will cause the transformer to couple many clutter signals to your 1.2 windings. This clutter signal contains many AC components in different spectrums. There are also many low-frequency waves. When the low-frequency waves coincide with the natural oscillation frequency of your transformer, the circuit will form a low-frequency self-excitation. The core of the transformer does not emit sound. We know that people's hearing range is 20--20KHZ. So when we design circuits, we usually add frequency selective loops. 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 fixed frequency.
5, high-power switching power supply short-circuit whistle I believe we have encountered this situation, switching power supply suddenly test the power supply short circuit after full load, and sometimes will hear the power whistle situation; or when setting the current protection, when When the current is debugged to a certain stage, there will be howling, and the whistling sounds will fluctuate. It is annoying. The main reasons for this are:
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 transistor in the previous cycle is too large, the conduction time is too long, and excessive energy is transmitted through the high-frequency transformer. The DC-rectified energy storage inductor is not fully discharged during this cycle. After the PWM judgment, no drive signal to turn on the switch is generated or the duty ratio is too small in the next cycle; the switch is turned off during the entire cycle. The state, or the conduction time is too short; the energy storage inductor releases more than one whole cycle of energy, the output voltage drops, and the duty cycle of the switching tube in the next cycle will be large... This cycle will cause the transformer to generate lower frequencies. (Vibration with a regular intermittent full cut-off period or a frequency at which the duty cycle changes drastically) gives a lower frequency sound that the human ear can hear. At the same time, output voltage fluctuations will also increase over normal operation. When the number of intermittent full cut-off periods per unit time reaches a considerable proportion of the total number of cycles, even the frequency of the transformer originally operating in the ultrasonic band will be reduced in frequency, entering the audible frequency range of human ears, and emitting a sharp high-frequency “sentinelâ€. call". Switching transformers at this time work in a severely overloaded state, and they may be burned at any moment—this is the reason for the “screaming†before many power supplies are burned. I believe that some users have had similar experiences. When there is no load, or when the load is very light, the switch may also have an intermittent full off period, and 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 sources. When the load is not loaded or the load is too light, the back electromotive force generated by the transformer during operation cannot be well absorbed. This will cause the transformer to couple many clutter signals to your 1.2 windings. This clutter signal contains many AC components in different spectrums. There are also many low-frequency waves. When the low-frequency waves coincide with the natural oscillation frequency of your transformer, the circuit will form a low-frequency self-excitation. The core of the transformer does not emit sound. We know that the human hearing range is 20--20 kHz. So we usually add a frequency-selective circuit when designing circuits. 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 fixed frequency.
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