I'm driving a 12V 0.11A brushless DC fan with PWM using an MSP430Gxxxx --> TC427CPA FET driver --> BS170 N-FET. The fan is on the low side of the FET.
Even with a duty cycle of 90% and a frequency of 10kHz, there's audible buzzing coming from the fan. Lower duty cycle = more noise.
I tried to eliminate the noise by adding a 4.7uF cap in parallel with the fan, and it's a little less noisy, but still very audible.
How do I make the noise go away?
Answer
In fact I am working on exactly same problem at the moment.
1) Freq > 25Khz - first of all
2) BIG cap at the output, 1-4.7uF ceramic + some 100-1000uF electrolytic would do the trick.
3) Add some inductance before the cap + diode in reverse to cut negative spikes.
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