Causes of Transformer Whistling

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 in the core to move under this low-frequency magnetic field, generating mechanical vibration, thereby causing vibration in the surrounding air. Since the audible frequency of the human ear is 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 the whistling of the switching power supply transformer: transformer process problems, transformer loop problems, transformer core problems and switching power supply load problems, which are analyzed below.

(1) Transformer process issues

① The dip paint is not dried properly, causing the magnetic core to be unstable, causing mechanical vibration and making noise;

②The length of the air gap is inappropriate, causing the transformer to become unstable and make noise;

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

④ There is an air gap in the magnetic core assembly, which causes air vibration at high frequencies and makes noise (if the transformer is fully immersed in vacuum, it will generally not make noise).

(2) Transformer loop problem

The loop problem of the transformer means that the loop of the transformer oscillates, causing the transformer to whistle.

① Improper wiring of the circuit board, causing interference, oscillation, and noise;

② Improper parameter setting of the feedback loop causes the loop to be unstable and cause oscillation and noise;

③ Quality problems of components in the loop, such as insufficient capacity of the input filter capacitor, poor quality of the output rectifier fast recovery diode, poor quality of the power MOS tube, poor quality of the high-voltage capacitor or diode of the RCD recoil absorption circuit, etc., etc. Problems may cause vibration and noise.

(3) Transformer core problem

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

(4) Load problem of switching power supply

① When the switching power supply is under no-load or light-load conditions, oscillation will occur at certain operating points, which is manifested as howling of the transformer and instability of the output.

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

② The transformer is working in a severe overload state and may burn out at any time - this is the reason why many power supplies "scream" before burning out.

HZ2611 Transformer Short Circuit Impedance Tester