**Common Causes of Whistling and Unusual Sounds in Switching Power Supplies**
1. **Poor Transformer Impregnation**: This issue occurs when the transformer is not properly impregnated with varnish, leading to whistling noises and waveform spikes. Although the power supply can still carry normal current, the whistling becomes more noticeable at higher output power levels. At lower power, the sound may be less obvious. I once encountered this problem in a 72W charger, where the core material had to meet very strict requirements due to the customer’s high standards. It's also important to note that if the transformer is poorly designed, mechanical vibrations could cause abnormal noise.
2. **Incorrect Ground Trace for PWM IC**: This error often results in the product functioning normally under certain conditions, but it can prevent the unit from starting or loading properly, especially with low-power ICs. I once used a SG6848 test board without fully understanding its performance, and based my layout on experience alone. As a result, the wide voltage test failed—very frustrating!
3. **Incorrect Routing of Optocoupler Operating Current Resistor**: If the optocoupler’s current-limiting resistor is improperly placed, particularly before the secondary filter capacitor, it can lead to whistling. The effect becomes more pronounced as the load increases.
4. **Ground Fault in Reference Regulator (e.g., ICTL431)**: The ground connection of the secondary reference regulator must not be directly connected to the cold ground of the transformer. Doing so can cause a drop in carrying capacity and increase the whistling proportionally with output power. When the load is near the power limit, the switching transformer may become unstable. The duty cycle of the switching transistor may become too large, causing excessive energy transfer through the high-frequency transformer. If the energy storage inductor isn’t fully discharged, the PWM might reduce the duty cycle, leading to full off periods. This causes the transformer to operate at a lower frequency, which is audible to humans. The output voltage will fluctuate more than usual, and in severe cases, the transformer may overheat and fail. This is often the "screaming" sound heard before a power supply burns out. Even with no load or light load, the same issue can occur, making it dangerous. A dummy load can help, but it’s not always reliable, especially in high-efficiency or high-power designs. When there's no load, the back EMF from the transformer may not be properly absorbed, causing cluttered signals to couple into the windings. These signals contain various AC components, including low frequencies. If these match the natural resonance frequency of the transformer, low-frequency self-excitation can occur. To prevent this, a bandpass filter in the feedback loop is recommended, or you can design the power supply to operate at a fixed frequency.
5. **Whistling Due to Short-Circuit in High-Power Supplies**: This issue often happens when a power supply is under full load and suddenly experiences a short circuit, resulting in a loud whistling sound. It can also occur during current protection debugging when the current reaches a certain level, causing fluctuating whistling. The main cause is similar to what was mentioned earlier: the switching transformer enters an unstable state, leading to irregular duty cycles and audible low-frequency sounds. This can happen even with no load or light load, making the transformer work in an overloaded condition, which is dangerous. Proper grounding, filtering, and stable control loops are essential to avoid such issues.
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