Reasons for Transformer Whistling

The whistling sound of a 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, resulting in mechanical vibration and causing vibration in the surrounding air. Due to the audible frequency of the human ear being approximately 20Hz to 20kHz, if the vibration of the air is within this range, it will eventually reach the human ear and be heard.

There are four main reasons for transformer whistling in switch mode power supplies: transformer process issues, transformer loop issues, transformer iron core issues, and switch mode power supply load issues. The following will analyze them one by one.

(1) Process issues of transformers

① Insufficient immersion and drying of the paint resulted in unstable magnetic cores, causing mechanical vibrations and producing noise;

② The length of the air gap is not suitable, resulting in unstable working conditions of the transformer and making noise;

③ The loose winding of the wire bundle may also cause noise;

④ There is an air gap in the magnetic core combination, which causes air vibration and produces noise at high frequencies (transformers that are fully immersed in vacuum generally do not produce sound).

(2) Loop problem of transformer

The loop problem of a transformer refers to the oscillation of the transformer's loop, which causes the transformer to howl.

① Improper wiring of the circuit board can cause interference and oscillation, resulting in noise;

② Improper setting of feedback loop parameters leads to instability of the loop, resulting in oscillation and noise;

③ The quality issues of components in the loop, such as insufficient input filtering capacitor capacity, poor quality of output rectification fast recovery diode, poor quality of power MOS transistor, poor quality of high voltage capacitor or diode in RCD recoil absorption circuit, etc., can all lead to oscillation and noise.

(3) Iron core problem of transformer

When the iron core of a transformer saturates, the current in the coil increases, causing the transformer to heat up and produce self-excited oscillations. The oscillation of the coil causes vibrations in the surrounding air, resulting in a sound.

(4) Load problem of switch mode power supply

① Switching power supplies may experience oscillation at certain operating points under no-load or light load conditions, manifested as transformer whistling and unstable output.

This phenomenon occurs due to the momentary opening time of the switch being too long during no-load/light load, resulting in excessive output energy and significant voltage overshoot. It takes a long time to restore normal voltage, so the switch needs to stop working for a period of time. As a result, the switch operates in an intermittent mode, causing the transformer to vibrate at lower frequencies (regular intermittent full cutoff periods or frequencies with drastic changes in duty cycle).

② Transformers work in a severely overloaded state, and there is always a possibility of burning out - this is the reason why many power sources scream before burning out.

HZ-600A Sweep Frequency Response Analysis Transformer