I am trying to control the speed of a 12 volt .5A fan from
a PWM output of a PIC 16c74 microcontroller. I can vary the
duty cycle of this signal from 0-100% in software. But the
frequency of this PWM signal will vary from 5khz - 15khz
depending on the requirement of the second PWM channel of
PIC, used for another application. I have the following problems.

1. I tried a simple mosfet circuit to switch the fan directly
from the PWM output. But the 5khz-15khz frequency causes a
shrill audio tone to resonate in the fan, which of course is
no surprise. Adding an RC filter into the gate of the IRFZ44
N-Channel MOSFET reduces the whine, yet causes erratic and
undefined speed control if any at all.

2. I want to turn this MOSFET into a voltage controlled
resistor which I would think (correct me if I am wrong) will
eliminate the audio problem but cause losses in the MOSFET,
which wouldn't be a problem. Yet I have no idea how to set up
the circuit.

The bottom line is, I want to be able to control the speed of
this fan from 40% to 100% without the audible whine from this
PWM signal. I need a way to filter the PWM output to provide
a stable output voltage proportional to PWM duty cycle. I
would appreciate any advice.

Neil Gandler