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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?

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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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    \$\begingroup\$ @BarsMonster, be careful of electrolytic on a "motor". This can easily have voltage reverse and make fireworks out of your cap. \$\endgroup\$
    – Kortuk
    Commented Mar 6, 2011 at 10:03
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    \$\begingroup\$ Inductive loads should always have a reversed >fast< diode added parallel to it. \$\endgroup\$
    – Hans
    Commented Mar 6, 2011 at 10:46
  • \$\begingroup\$ I've just did that on my scheme - PWM with PC FAN. 555 goes into 220 uH inductor, then to 10'000 uF cap. On cap I see beautiful line, with no spikes higher than 20-50mV, and absolutely no audiable noise. So, no danger for electrolyte on usual FAN's. But you indeed need a diode to protect a PWM controller, there are negative spikes up to -12 on that side. \$\endgroup\$
    – BarsMonster
    Commented Mar 6, 2011 at 16:10
  • \$\begingroup\$ Where would the ceramic go? And to what end? \$\endgroup\$
    – Vincent Van Den Berghe
    Commented Mar 8, 2011 at 9:11
  • \$\begingroup\$ ceramic - in parralel to electrolythic cap, just before FAN. That is just to filter high-frequency EM noise and lower heat dissipation of electrolythic cap(but on such low power it's neglible anyway), so it is optional. \$\endgroup\$
    – BarsMonster
    Commented Mar 9, 2011 at 8:48
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Driving a fan by switching the supply is firstly risky. BLDC fans have electronics in them and your switching these on and off at high speed. Not how they were designed. You risk killing the electronics like this over time.

Adding the cap helps because you are removing nasty power spikes into the fan. Adding the inductor is a good idea and if you look at what people are suggesting you will see that the best way to speed control a BLDC fan is with a constant current buck regulator.

This way you PWM the FET that is feeding energy into the inductor (energy store) and place a fly back diode to circulate the power when the FET is off. This will keep a steady flow of power in the fan, minimal noise and not risk killing the fan long term.

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The easy way is to drive it either at an ultrasonic frequency (>20 kHz) or at a lower frequency (<100-200 Hz). That low end there isn't really infrasonic, but a "hum" is usually far less objectionable, if it can be heard at all over the noise of the fan itself.

As for why you see that a lower duty cycle yields more noise, you're essentially increasing the content of the 10 kHz frequency you're sending to the motor until you hit 50%, then it will drop again.

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I keep seeing folks attempting to control a brushless Fan with a PWM signal on the +/- leads of the fan. Clearly this is not in line with the manufacturers intention. There is a brushless controller inside the fan. PWM contol of hte =/- voltage will just mess up the controller. Often the manufacture of the fan make a 4 wire version so you can input a PWM signal. These cost a little more but remove the problem. DC brushless fans have input voltage specs like 5-13.8volts DC not some composite signal.

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    \$\begingroup\$ This is completely incorrect. Most computer fans work fine with a PWM drive. The only reason they produce 4-wire fans is so that you can get a proper tachometer output (otherwise you get the PWM waveform superimposed on the tachometer output). \$\endgroup\$
    – Connor Wolf
    Commented Sep 24, 2012 at 20:56
  • \$\begingroup\$ FYI, basically every computer motherboard I have ever seen has used PWM control for the fans. It's not exactly uncommon. \$\endgroup\$
    – Connor Wolf
    Commented Sep 24, 2012 at 20:56
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    \$\begingroup\$ Common 2 and 3 wire brushless fans are designed to vary speed by either changing the voltage level (linear mode, usually have minimum start voltage of half the rated voltage) or with PWM; however, PWM has quirks. First, the tach output on 3 wire fans becomes invalid; although, smart PWM drivers do "pulse stretching" to restore valid tach output. Second, the PWM frequency must be low (usually < 25kHZ) otherwise it may disrupt the fan's internal brushless control circuitry. Third, the minimum fan speed is usually not as slow as technically possible, but is usually slow enough to make them quiet. \$\endgroup\$
    – Noah Spurrier
    Commented Sep 24, 2014 at 21:38

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