Electronic – How to eliminate PWM noise when driving a fan

PWM frequency is 32 kHz so that's a period of about 32 us and half that time the transistor will be dumping energy into charging up those capacitor. It'll be doing it totally inefficiently too so maybe half that energy again is wasted in transistor heat.

Capacitor acquired energy is \$\dfrac{CV^2}{2} = \dfrac{2.2 \times 10^{-6}\times 144 }{2}\$ = 0.158mJ

BUT because charging the cap is done totally inefficiently, at a guess, the transistor will throw away just as much energy. It does this 32,000 times per second so that's a power of about 5 watts per transistor.

Now I may have over-egged this a bit by assuming that each 2.2uF would become totally discharged in one cycle of PWM but I'd wager that each transistor is doing over a watt just driving these capacitors back to full charge (32,000 times per second). Get rid of the caps and if you still have problems with noise then come back and tell us.

The lower the duty cycle the worse this will be because there will be a longer time for these caps to discharge and hence inefficiently re-acquire more of their 0.158mJ when the transistor switches back on again.

Putting a large electrolytic capacitor across the motor obviously won't work in reverse, but even if that wasn't a problem it's still not a good idea.

When PWM turns the bridge transistors on they will charge the capacitor as quickly as they can, but when turned off it will discharge more slowly through the motor. This has two undesirable side effects:-

1. It will destroy the accuracy of your PWM. With the capacitor 'filling in the gaps' between PWM pulses the motor will spin faster than expected at lower PWM ratios, but less so when heavily loaded.

2. High current spikes will occur as the capacitor charges, putting stress on the bridge transistors and increasing electrical noise in the circuit. There may also be audible effects.

You can eliminate these problems by inserting an inductor in series with the motor/capacitor. However if your PWM frequency is low enough to be audible then the inductance required will be quite large (since you can't use an electrolytic capacitor the amount of capacitance you can apply to the filter is limited, so the inductance must be larger). Also the inductor must pass the relatively high DC motor current without going into saturation, and its resistance must be low to keep losses down.

If space is not a problem then simply use a choke with appropriate inductance and current rating. Then the only problem will be keeping it quiet!

The LC filter's cutoff frequency should be much lower than the PWM frequency, eg. if the PWM frequency is 1kHz then the filter should cut off at 500Hz or lower. If you used a 10uF capacitor then you would need about 10mH of inductance.