making 24 to 12v step down power supply.

LM2596 is only supply max 3A. and 

even 3A is became too hot 

so instead of LM2596, I tried to make power supply using same principle like LM2596, 

using inductor and capacitor with maxium pwm speed of AT328p 4Mhz. 

faster then LM2596, so if i using right inductor and capacitor , then can make power supply using 30-50A mosfet. I successfuly create 4Mhz pwm using ARDUINO, and then when I using pulse signal 5v to make amplifed 12volt to control MOSFET. but signal became flat... I think, none of transistor or optocoupler, is not fast enough to fallow speed in my hand .I am not sure exist one. so last trried,  I connect to LM2596 output signal to mosfet Gate pin, Mosfet is not fast enough as well..

So far I fail.

but I tried to make LM2596 - 12v fixed one 

But I can not make  3-5 paralle circuit to make power supply 24 to 12v .../.

will not even power load on LM2596 , possibly burn one LM2596 first.

but first have to test and LM2596 need to precies.