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?
Best Answer
Try using a P channel MOSFET and switch at an inaudible frequency i.e 50 kHz. Not only will you get the benefit of silent running but your inductor can be much smaller (and cheaper) for the same output ripple voltage: -
If your PWM drive signal is at 5V logic then one further transistor will be needed to perform level shifting like this: -