High Current fan controlled with Arduino pwm

There is no relationship between frequency and duty.
You can run 50% duty PWM at any frequency.

Frequency selection is a fairly important point in the use of PWM.

It's not a matter of calculating it; you need to determine it either by referring to the datasheet of the fans (if available; probably not...) or by experimentally determining what frequency they perform well at. It really helps you're only going to be using a 50% duty cycle as most frequency-related problems manifest themselves at low duty cycles (<25%).

Just refer to the datasheet of e.g. the IR4427 you were linked to; right on the first page there's a very clear and simple schematic...

oh i thought they were directly linked....... what am i missing here, how do i get frequency then.

You decide it! :grin:

If you use IR4427, it looks like this.

MOSFET not selected.
The D,G,S pin numbering has not been decided.

Will the ir4427 be able the power the MOSFETs carrying 30amps, the data sheet confused me a bit lol

And what are the capacitors for? And why specifically those size capacitors??

Most of the case it will do that easily.
     In essence, depending on the MOSFET of your choice.
     But as long as you make a common sense choice, there is no problem at all.

This is a bypass capacitor. (a.k.a. decoupling capacitor)
Which is must necessary for the stable operation of the IC.
For the capacitance, the value used in the IC evaluation conditions of the datasheet was used.

Please find out about bypass capacitors.

Some recommendations:

FET specs I find I need to check are:-
Vds, Vgs, Id and Vgs(th)
The Vgs (often +/-10 or 20V) limits the maximum gate voltage
The Vgs(th) sets whether or not you can drive it from an Arduino pin (5V or 3v3 chips)
Then I look at the plot of Id versus V drain to source, to see if a 3v3 (or 5v) drive will give me a low voltage drop across the FET
This voltage drop time the current is approx the power the FET will dissipate.