Analysis of factors causing switching power supply squealing
Time:2023-03-07
Views:1206
Anyone who has done development work has the experience of testing the switching power supply or hearing the leakage sound or high voltage arcing sound of similar products with poor high voltage in the experiment: the sound may be large or small, or sometimes not; Its rhythm is either deep or harsh, or changeable.
1. Poor paint impregnation of transformer:
Including water without immersion (Varnish). The howling will cause sharp waves, but the general load carrying capacity is normal. In particular, the higher the output power, the louder the howling will be, and the lower the power, the less obvious the performance will be. I have had experience of poor load in a 72W charger product, and found that there are strict requirements for the material of magnetic core in this product. (This product is strictly required by the customer) To add, when the design of the transformer is not good, it is also possible that the vibration will produce abnormal noise during operation.
2. PWM IC grounding wiring error:
Usually, some products can work normally, but some products can not carry load and may not start vibration. Especially when some low-power IC is applied, it is more likely to not work normally. I have used the SG6848 test board. Because I didn‘t have a thorough understanding of the performance of the IC at the beginning, I hastily laid it out based on my experience. As a result, I couldn‘t do the wide voltage test during the test. Sad!
3. Opto Coupler working current point wiring error:
When the position of the working current resistance of the optocoupler is connected before the secondary filter capacitor, there is also the possibility of squealing, especially when the load is more.
4. Ground wire error of reference regulator IC TL431:
The grounding of the same secondary reference stabilized voltage IC has the same requirements as the grounding of the primary IC, that is, they cannot be directly connected with the cold ground and hot ground of the transformer. If connected together, the result is that the carrying capacity decreases 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 switch transformer may enter an unstable state: the duty cycle of the switch tube 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 energy storage inductor of DC rectifier is not fully released in this cycle, and PWM determines that there is no driving signal to make the switch on in the next cycle or the duty cycle is too small; The switch tube is in the cut-off state in the following whole cycle, or the conduction time is too short; After more than one full cycle of energy release, the output voltage of the energy storage inductor drops, and the duty cycle of the switch tube in the next cycle will increase again... This cycle will cause the transformer to vibrate at a lower frequency (regular intermittent full cut-off cycle or frequency with drastic change in duty cycle), and produce a lower frequency sound that can be heard by the human ear. At the same time, the output voltage fluctuation will also increase compared with normal operation. When the number of intermittent full cut-off cycles in a unit time reaches a considerable proportion of the total number of cycles, the vibration frequency of the transformer that originally worked in the ultrasonic frequency band will even be reduced, enter the audible frequency range, and emit a sharp high-frequency "whistle". At this time, the switch transformer works in a serious overload state, and it is always possible to burn out - this is the origin of many "screams" before the power supply burns out. I believe that some users have had similar experiences.
The switch tube may also have an intermittent full cut-off cycle when it is no-load or the load is very light. The switch transformer also works in an overload state, which is also very dangerous. To solve this problem, we can preset the false load at the output end, 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 cannot be well absorbed. In this way, the transformer will couple many clutter signals to your 1.2 winding. This clutter signal includes many AC components of different spectrum. 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 low-frequency self-excitation. The magnetic core of the transformer will not make a sound. We know that the range of human hearing is 20-20KHZ. So when we design the circuit, we usually add a frequency selection circuit. To filter out low-frequency components. From your schematic diagram, you‘d better add a band-pass circuit to the feedback circuit to prevent low-frequency self-excitation, or make your switching power supply a fixed frequency.
High-power switching power supply short circuit howling
I believe you have encountered this situation. The switching power supply will suddenly short-circuit the power supply after full load, and sometimes you will hear the power supply howling; Or when the current protection is set, when the current is adjusted to a certain level, there will be a howling sound, which is very annoying. The main reasons are as follows:
When the output load is large and close to the power limit of the power supply, the switch transformer may enter an unstable state: the duty cycle of the switch tube 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 energy storage inductor of DC rectifier is not fully released in this cycle. According to PWM judgment, there is no driving signal to make the switch on in the next cycle or the duty cycle is too small; The switch tube is in the cut-off state in the following whole cycle, or the conduction time is too short; After more than one full cycle of energy release, the output voltage of the energy storage inductor drops, and the duty cycle of the switch tube in the next cycle will increase again... This cycle will cause the transformer to vibrate at a lower frequency (regular intermittent full cut-off cycle or frequency with drastic change in duty cycle), and produce a lower frequency sound that can be heard by the human ear At the same time, the fluctuation of output voltage will also increase compared with normal operation. When the number of intermittent full cut-off cycles in a unit time reaches a considerable proportion of the total number of cycles, it will even reduce the vibration frequency of the transformer that originally worked in the ultrasonic frequency band, enter the audible frequency range, and emit a sharp high-frequency "whistle". At this time, the switch transformer works in a serious overload state, There is always the possibility of burning out - this is the origin of many "screams" before the power supply is burned out. I believe that some users have had similar experiences The switch tube may also have intermittent full cut-off cycle when it is no-load or with light load. The switch transformer also works in overload state, which is also very dangerous
To solve this problem, you can preset a false load at the output end, but it still happens occasionally in some "saved" or high-power power supplies. When the load is not loaded or the load is too light, the back EMF generated by the transformer when it is working cannot be well absorbed. In this way, the transformer will couple a lot of clutter signals to your 1.2 winding. This clutter signal includes many AC components of different spectrum. 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 low-frequency self-excitation. The magnetic core of the transformer will not emit sound. We know that the range of human hearing is 20-20KHZ. So when we design the circuit, we usually add a frequency selection circuit to filter out low-frequency components. From your schematic diagram, you‘d better add a band-pass circuit to the feedback circuit, To prevent low frequency self-excitation, or make your switching power supply a fixed frequency.
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