Why does the transformer of the switching power supply sometimes make a sharp noise？

The whistling sound of the transformer is mainly due to the low frequency oscillation contained in the excitation component of the transformer, which causes the magnetic molecules of the magnetic core to move under this low frequency magnetic field to produce mechanical vibration, which causes the vibration of the surrounding air. Since the audible frequency of the human ear is about 20Hz to 20kHz, if the air vibration is within this range, it will eventually reach the human ear and be heard.

There are four main reasons for the whistling of the switching power supply transformer: the process problem of the transformer, the loop problem of the transformer, the core problem of the transformer, and the load problem of the switching power supply, which are analyzed one by one below.

(1) Process problem of transformer

①The impregnation and drying are not in place, which causes the magnetic core to be weak, causing mechanical vibration and noise;

②The length of the air gap is not suitable, causing the transformer's working state to be unstable and making noise;

③If the wire package is not tightly wound, it may cause noise;

④ The magnetic core combination has an air gap, which causes air vibration at high frequency and makes noise (if the transformer is fully immersed in vacuum, it will generally not make a sound).

(2) The loop problem of the transformer

The loop problem of the transformer means that the loop of the transformer oscillates, which causes howling of the transformer.

①Improper wiring of the circuit board, causing interference to cause oscillation, resulting in noise;

②Improper setting of feedback loop parameters causes the loop to become unstable and cause oscillation and noise;

③Quality problems of components in the loop, such as insufficient input filter capacitor capacity, poor quality of output rectifier fast recovery diodes, poor quality of power MOS tubes, poor quality of high-voltage capacitors or diodes in the RCD recoil absorption loop, etc. Problems can cause vibrations and noises.

(3) The core problem of the transformer

When the transformer core is saturated, the current in the coil increases, the transformer heats up and produces self-excited oscillation, and the oscillation of the coil causes the surrounding air to vibrate and make noise.

(4) Load problem of switching power supply

① In the case of no load or light load, the switching power supply will oscillate at certain operating points, which is manifested as the howling of the transformer and the instability of the output.

This phenomenon occurs because the instantaneous turn-on time of the switch is too large when there is no load/light load, which causes the output energy to be too large, and the voltage overshoot is also large, and it takes a long time to recover to the normal voltage, so the switch needs to stop working For a period of time, this switch works in an intermittent working mode, causing the transformer to vibrate at a lower frequency (a regular intermittent full cut-off period or a frequency with a drastic change in duty cycle).

②The transformer is working in a severe overload state, and it may be burnt at all times-this is the origin of the "screaming" before the burning of many power supplies.

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