Reddit mentions of Power Electronics Basics: Operating Principles, Design, Formulas, and Applications

That.... that is not the job of a simple mosfet. It's also beyond my abilities so i can't explain the design process to you either.

Let's consider the hypothetical situation. We have an irlb3034 with a Vgs of 10V, our power supply has infinate current capability with no drop, and everything is connected with lossless conductors.

So when our mosfet is turned on it will have a Rds(on) of 1.7mOhms. I = E / R, so I = 8.4 / 0.0017, I = 4941.18A. But a mosfet is merely a switch, the load is what you want to switch and that dictates the current sunk through a n-channel mosfet. With PWM you're switching it on an off quickly giving you an average voltage, but a lot of factors come into play (mosfet dynamic characteristics) and i don't have the time to explain that all out.

Switching supplies are entirely different animals though, inductors are involved, diodes as well, physics as well. That's all beyond my skill set when it comes to rolling my own SMPS from scratch.

There are some books on it, however they tend to assume you already know a lot or have a formal education in electroncs. That and it'd coost you probably about 400 dollars for two books on the subject if you were to buy them new instead of rent them or check em out from the library.

Sorry i couldn't be of more help. And yes Vgs of 4.5V > is for switching instead of amplifying.

You'd be looking for books such as:
http://www.amazon.com/Power-Electronics-Converters-Applications-Design/dp/0471226939/

http://www.amazon.com/Power-Electronics-Basics-Principles-Applications/dp/1482298791/

Along with a good background in general electronics and design.