The switching frequency of currently available SMPS is generally over the 100-kHz range, so the switching frequency in normal PWM operation does not enter the audible frequency band. However, when the frequency (1/tP_Load) of the load current (IOUT) change enters the audible frequency band (generally 20 Hz to 20 kHz) as described below, audible noise may occur.
Figure1. Timing chart of the switching operation with the fixed PWM mode (DD3V of S6BP502A)
In the Auto PWM / PFM mode, the switching cycle (tP) changes depending on the load current as described below because switching operation pauses at light load (For details of the Auto PWM / PFM mode, see KBA224577).
Therefore, audible noise may occur when the frequency of intermittent operation (1/tP) enters the audible frequency band.
Figure 2. Timing chart of the switching operation with the auto PWM/PFM mode (DD3V of S6BP502A)
The human auditory system has the characteristics as illustrated in the following chart. Frequency band of 2 kHz - 5 kHz, to which human ear is most sensitive, should be avoided.
Figure3. Equal-Loudness contour
*Cited from Wikipedia (https://en.wikipedia.org/wiki/Equal-loudness_contour)
Use the following methods to reduce the audible noise from the inductor:
- Change the load current frequency by changing the operation cycle of the load device to avoid audible frequency band
- Select fixed PWM mode under light load current conditions. (Operation mode is controlled by SYNC pin in S6BP501A / S6BP502A)
- Coat or fill the inductor with varnish
- Change the PCB mechanical resonance frequency by changing the inductor placement or by using different type of inductor (e.g. Changing from a ferrite wiring inductor to a metal composite inductor).
Contact the inductor vendor for more detailed advice on how to handle audible noise.
Author: yukinorim_01 Version: **
Translation - Japanese: SMPSインダクタからの可聴ノイズを低減させる – KBA226199 - Community Translated (JA)