Common causes of howling in switching power supplies (2)

News     |      2021-10-11 10:36

Abstract: This article continues from the previous issue to explain the common causes of switching power supply whistling.

table of Contents

4. The ground wire of the reference voltage regulator IC TL431 is faulty

5. When no load or very light load

6. When there is no load or the load is too light

 

4. The ground wire of the reference voltage regulator IC TL431 is faulty

When the output load is large, approaching the power limit of the Switching power supply, the switching transformer may enter an unstable state. The duty cycle of the switch tube in the previous cycle is too large, the on-time is too long, and too much energy is transmitted through the high-frequency transformer; the energy storage inductor of the DC rectifier is not fully released in this cycle. According to the PWM judgment, in the next cycle, There is no driving signal to turn on the switch tube, or the duty cycle is too small. The switch tube is in the cut-off state for the entire period afterward, or the on-time is too short. The energy storage inductor is released after more than one entire cycle of energy, the output voltage drops, and the duty cycle of the switch tube in the next cycle will be larger…This cycle repeats so that the transformer has a lower frequency (regular intermittent full cut-off) Period, or the frequency at which the duty cycle changes drastically), emits a lower frequency sound that can be heard by the human ear.

PCB board

At the same time, the output voltage fluctuation will be larger than normal operation. When the number of intermittent full cut-off cycles per unit time reaches a considerable proportion of the total number of cycles, it will even reduce the vibration frequency of the transformer originally working in the ultrasonic frequency band, enter the frequency range audible to human ears, and emit sharp, high frequencies. Whistle”. At this time, the switching transformer is working in a severe overload state, and it may be burnt at any time. This is the origin of many power supplies “screaming” before burning. I believe some users have had similar experiences.

 

5. When no load or very light load

In this case, the switching tube may also have an intermittent full cut-off period, and the switching transformer is also working in an overload state, which is also very dangerous. This problem can be solved by presetting a dummy load at the output end, but it still happens occasionally in some “saving” or high-power power supplies.

 

 PCB

6. When there is no load or the load is too light

The back electromotive force generated by PCB board the components transformer cannot be absorbed well. In this way, the transformer will couple the windings to which a lot of clutter signals arrive. This clutter signal includes many AC components of different frequency spectra. There are also many low-frequency waves. When the low-frequency waves are consistent with the natural oscillation frequency of your transformer, the circuit will form low-frequency self-excitation. The magnetic core of the PCB components transformer will not make a sound. We know that the human hearing range is 20-20KHZ. Therefore, when we design the circuit, we generally add a frequency selection loop. To filter out low-frequency components. It is best to add a band-pass circuit to the feedback loop to prevent low-frequency self-excitation. Or the Switching power supply can be made into a fixed frequency.

 Switching power supply