High-power switching power supply short-circuit howling analysis - Database & Sql Blog Articles

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I believe many of my friends have 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 is a howling, and the sound of its howling is swaying, which is very annoying. The main reasons are as follows:
When the output load is large and close to the power limit of the switch, the switching transformer may enter an unstable state: the duty cycle of the switch in the previous cycle is too large, the conduction time is too long, and the high-frequency transformer is transmitted too much. The energy of the DC rectified energy storage inductor 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 in the whole cycle afterwards. The cut-off 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 close to the switch tube is large again... so that the transformer is generated again and again Vibrations at lower frequencies (regular intermittent full cut-off periods or frequencies with drastic changes in duty cycle) emit lower frequency sounds 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.
Such problems with high power proximity switches can be solved by presetting the dummy load at the output, but occasionally occur in some "saving" or high power sources. 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. It is best to 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.