I don't think there is an ideal frequency for pwm controlled fans ..
I would start with quite low, say below 50Hz, but if you hear any noise you should try different one ..
I assume that you are after N-channel mosfet, so the popular IRF540 (or similar) will be a good choice ..
Here is an example: **broken link removed**
Regards,
IanP
This mosfet is completely unsuitable for driving a low-voltage fan ..
Look at its parameters:
Rds(on)=5.3Ω
Vds=500V
Vgs .. ( https://www.fairchildsemi.com/ds/FQ/FQD2N50.pdf )
To drive a fan at 24Vdc you should select a mosfet with Rds(on)<0.1Ω
That was the reason I suggested the IRF540 with Vgs>2V, but any low-ohm low-voltage MOSFET will be good for this job ..
Regards,
IanP
I recomend to use single chip switching power supply.
for example you can use LM2576-adj and put potentiometer to feedbak path. In this way you can change output voltage from 1.2 - 24 volts.