I believe that you have encountered such a situation, the switching power supply suddenly short-circuit the power supply test after full load, sometimes you will hear the power supply has a loud situation; Or when the current protection is set, when the current is adjusted to a certain section, there will be a noise, and the noise will be very annoying with its cadence and cadence. The reasons are mainly as follows

When the output load is large and close to the power limit, the switching transformer may enter an unstable state: the duty ratio of the switching 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 of the dc rectified energy storage inductor is not fully released in this period. According to THE PWM judgment, no drive signal or low duty ratio to make the switch tube conduction is produced in the next period. The switching tube is in the cut-off state in the whole cycle after, or the conduction time is too short; After more than one full cycle of energy release, the output voltage drops, and the duty cycle of the switch tube in the next cycle will be large...... This cycle causes the transformer to vibrate at a lower frequency (the frequency of regular intermittent full cut-off cycles or drastic changes in duty ratio), producing a lower frequency sound that can be heard by the human ear.

At the same time, the output voltage fluctuation will increase of their normal work. When the intermittent all by the number of cycles per unit time reaches a considerable proportion in the total number of cycles, even made the work in the ultrasonic frequency transformer vibration frequency is reduced, into the ear of the audible frequency range, a sharp high frequency "noise". At this time of switching transformer work on the serious overload state, moment could have burned - this is the origin of many power supply before burning "scream", believe that some users have had similar experiences. Light, or when the load is very light switch tube is also likely to be intermittent cycle of whole globe, Switching transformers also work in the overload state, also very dangerous.