I'm dealing with a buck converter using an ISP452, an inductor and a capacitor to act as a buck converter driven by a PWM signal coming from a microcontroller. Some part of the circuit is producing a hogh-pitch audible noise when powered, which I would like to get rid of. The circuit is driving some standard 3-pin fans. The carrier frequency of PWM signal is 3.9 Khz. I measured the output of the circuit and verified at the oscilloscope that it is a stable linear voltage. Thus the noise it is not likely coming from the fans but from the circuit itself! My first suspect goes to the inductor which is acting as a loudspeaker. Might this be possible? The question is what to do in order to remove the noise? I cannot really change the capacitor and inductor but I could try to use some "compound" that could absorbe the noise. Would this be a good choice? The ISP452 is limiting the driver PWM signal frequency to max 4 kHz. Any other suggestion?
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