Analysis of the causes of acoustic noise of switching power supply - Epson

Poor transformer impregnation

Contains unimpregnated standing water. Whine and cause the waveform to have spikes, but the general load can be normal, especially clarify: the greater the output power, the stronger the howling, the lower the power is not necessarily significant. A 72W charger product has had a bad load experience, and found that there are strict requirements for the material of the core in this product. To make up for this, when the design of the transformer is not optimal, there is also the possibility that the oscillation will cause abnormal noise during operation.

PWM IC ground wiring error

Generally, some products will be able to operate normally, but some products will not be able to carry load and may not be able to vibrate, especially when some low-power ICs are applied, they may not be able to operate normally. For example, the SG6848 test board did not have a thorough understanding of the IC performance at first, so it was hurriedly laid out based on experience, and the wide voltage test could not be performed during the result test.

Analysis of the causes of acoustic noise of switching power supply - Epson

Analysis of the causes of acoustic noise of switching power supply - Epson

The current point of the optocoupler operation is misrouted

When the orientation of the optocoupler's operating current resistor is connected before the secondary filter capacitor, there is also the possibility of acoustic noise, especially when the load is increasing.

Ground wire error of the reference regulator IC TL431

The ground of the same secondary reference regulator IC has similar requirements as the ground of the primary IC, that is, neither of them can be directly connected to the cold and hot ground of the transformer. If they are connected, the result is that the load can be reduced, and the whistling sound is directly proportional to the size of the output power.

When the output load is large and close to the power limit of the power supply, the switching transformer may enter an unstable condition. The duty cycle of the switch 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 storage inductor of DC rectification is not fully released during this period, and the PWM judges that there is no driving signal to turn on the switch in the next cycle, or the duty cycle is too small. The switch is cut off for the entire subsequent cycle, and the on-time may be too short. The energy storage inductor is released through more than one full cycle, the output voltage drops, and the duty cycle of the switch in the next cycle will be larger...... This cycle causes the transformer to oscillate at a lower frequency (with a regular intermittent full cut-off period, or a frequency with a drastic change in duty cycle) to announce a lower frequency sound that can be heard by the human ear.

In addition, the output voltage fluctuation will be greater than that of 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 oscillation frequency of the transformer that was originally operating in the ultrasonic frequency band, enter the frequency range that can be heard by the human ear, and announce a sharp high-frequency "whistle". At this moment, the switching transformer operation is in a severe overload situation, and there is a possibility of burning out at the time - this is the origin of the "scream" before the power supply burns out, and I believe that some users have had similar experiences.

When the no-load may be very light

When this situation occurs, the switch may also have an intermittent full cut-off period, and the same operation of the switching transformer is in the overload condition, which is very dangerous. This problem can be solved by presetting false loads at the output, but it is still occasionally generated in some "saving" or high-power power supplies.

When there is no load or the load is too light

The back EMF generated by the transformer during operation cannot be well absorbed. In this way, the transformer will couple a lot of clutter signals to the windings. This clutter signal contains many communication components in different spectrums. There are also a lot of low-frequency waves, and when the low-frequency waves are in common with the natural oscillation frequency of your transformer, then the circuit will form low-frequency self-excitation. The core of the transformer does not announce a sound. We know that the human hearing range is 20--20KHZ. Therefore, when we design circuits, we generally add frequency selection loops. to filter out low-frequency ingredients. It is best to add a bandpass circuit to the reaction loop to avoid low-frequency self-excitation. Perhaps the switching power supply can be made to a fixed frequency.

Analysis of the causes of acoustic noise of switching power supply - Epson

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