Anyone who has done development work have such experience, test switching power supply or in the experiment to hear a similar product to hit the high voltage bad leakage sound or high voltage arc sound uninvited: its sound or large or small, or sometimes not; its rhythm or deep or harsh, or changeable are there.
Transformer (Transformer) poor paint dipping: including the absence of immersion in the Vanlue (Varnish). Whistling and waveforms caused by spikes, but the general carrying capacity is normal, especially: the greater the output power of the whistling, the smaller the power of the performance is not necessarily obvious. I have a 72W charger products in a bad experience with the load, and in this product found in the core material has strict requirements. (This product customer requirements are more stringent) to add, when the transformer design is not good may also work when the vibration generated by the noise. 2.
2. PWM IC ground alignment error: usually the product will be part of the normal work, but some products can not carry the load and may not be able to vibrate the fault, especially when applying some low-power IC, it is more likely not to work properly. I have used SG6848 test board, due to the initial not thoroughly understand the performance of the IC, based on experience will be hastily layoutout, the results of the test can not do wide voltage test. Sad!
3. Optocoupler (Opto Coupler) operating current point alignment error: when the location of the optocoupler operating current resistor is connected to the secondary filter capacitor before the possibility of whistling, especially when the more load is carried, even more.
4. The base voltage regulator (Regulator) IC TL431 grounding line error: the same secondary reference voltage regulator IC grounding and the primary IC grounding has similar requirements, that is, can not be directly connected to the transformer's cold ground hot ground. If the consequences are connected together is a reduction in the carrying capacity and whistling sound and the size of the output power is proportional. The PCB in the previous article has made such a mistake, and then JACKY WANG pointed out to be corrected. When the output load is large, close to the power limit of the power supply, the switching transformer may enter an unstable state: the previous cycle of the switch is too large duty cycle, the on time is too long, through the high-frequency transformer transmitted too much energy; DC rectifier energy storage inductor energy is not fully released in this cycle, the PWM judgment in the next cycle does not generate the switch on the drive signal or duty cycle is too small; switch tube in the whole cycle after the cutoff state, or on time is too short; energy storage inductor after more than a full cycle of energy release, the output voltage drops, the next cycle of the switching tube duty cycle will be large ...... so repeatedly, so that the transformer occurs at a lower frequency (regular intermittent full cutoff cycle or duty cycle of violent frequency) of vibration, emitting a lower frequency of sound 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 of time reaches a significant percentage of the total number of cycles, it may even cause the transformer, which was originally operating in the ultrasonic band, to lower its vibration frequency 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, there is always the possibility of burning - this is the origin of many power supply burned before the "screaming", I believe some users have had a similar experience.
No-load, or very light load switching tubes may also appear intermittent full cut-off cycle, switching transformers also work in the overload state, the same very dangerous. This problem can be solved by pre-setting a dummy load on the output, but it still happens occasionally in some "frugal" or high-power power supplies. When there is no load or the load is too light, the transformer in the work of the counter potential can not be well absorbed. The transformer will then couple a lot of spurious signals to your 1.2 winding. This spurious signal includes many different AC components of the frequency spectrum. It also includes many low frequency waves. When the low frequency waves coincide with the inherent oscillation frequency of your transformer, then 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 - 20KHZ. So we design the circuit, generally coupled with the frequency selection loop. To filter out low frequency components. From your schematic, you'd better add a bandpass circuit on the feedback loop to prevent low-frequency self-excitation. Or your switching power supply into a fixed frequency can be.
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