I have to rotate a dc fan with voltgae 10V to 12V dc. Depending upon voltage the PWM freq will change.
Just for example at 12V, the PWM freq will be 1440HZ with duty cycle = 70%. atteched is the circuit

Now I have connected mosfet so that it will switch between high & low.
DC fan need continuous 12V. But due to PWM it will vary so that average dc voltage is produced.
Now I have to select a capacitor value for this.
What is the formula/how to calculate it.
Also should I connect inductor & capacitor or just capacitor will bve sufficient

 

Is the fan a computer type fan with a brushless motor in it? If not, no filtering is needed for a brushed motor. If so, you need to determine the current drawn by the fan in order to calculate required capacitance to smooth out the ripple, though getting rid of all the ripple will be very difficult, most fans work fine with PWM, try it first.

 

Yes I have to use capacitor.
My fan current is 300mA.
Normally I calculate capacitor by Vr = I/fC.
Vr= ripple, I=current, f= freq, C= cap

But in that I take duty cycle as 50%. I use in case for ac-dc converer

But in my case its 70%. Will formula change.

Also what the formual for avg value. I think its Vavg = Vpk*D. Right????? & Vrms = Vpk*sqr(D)

 

Does the fan work?

What value capacitor are you currently using?

The formula is correct, but if you vary the duty cycle and are driving something picky about duty cycle, calculating for worse case is safest.