sorry for don't provide the full detail.
It's not too late. You can still do so. We've only just had one page of talk about the wrong solution to the problem.What is that power used for? You suggest it's for regulating a fan speed, correct?
Reducing the voltage of a motor with a constant torque load by 50% will reduce the speed about 50%, but a fan is not constant torque, torque varies approximately with the square of speed, suggest testing effect of voltage variation to see what kind of voltage and current needed.
A boost then a buck converter sounds like the wrong solution any day. Not only because there actually are boost/buck type converters which can output the same voltage as on their input, it's also because you would normally control the current through a device directly, not through trying to manipulate its voltage.Many fans can be controlled just fine using PWM instead. Much easier. All you need is a MOSFET and two resistors. Very easy for an Arduino to produce such a signal.Some computer fans actually have a speed control signal and have this speed control built in. Even easier. That speed control signal is usually PWM or similar kind of block wave, which again an Arduino can easily produce.The problem still is that we're 18 posts in and finally at least we know you're trying to control the speed of a fan, but we still don't know exactly what fan (part number).Indeed, sounds like this is yet another example of an XY problem...
Ahhh, so you actually don't want adjustable voltage!? You just want two speed settings for your fans?The easiest solution would still be using 1 MOSFET and using PWM on your Arduino!You will not need any regulators at all, no linear no buck, no boost, nothing, just the MOSFET and your existing 12V supply!You don't have to make it so that you can adjust it, you could just have two settings like 1 button for 100% and another button for 50%!
The MOSFET solution gives you 255 power levels using standard PWM. How you use those levels, and how many of them, is up to you.
or simply just a bare microcontroller, like an ATmega328.