This buck converter/step down is based on KA7500c SMPS controller chip,
the following circuit is from the reference design
http://www.ti.com/lit/an/slva001e/slva001e.pdf you can find the details of the circuit and calculations from the document. This is one of the application circuit built around with KA7500C, this buck converter can suply a load current of 5.75Ampere at 5V we could also use TL494 or KA7500c both are same in operation/ratings. The recommended operating voltage range is 7V to 40V DC and the power source i am using for this is a old laptop power supply adapter rated 18V,3.7A. , however the maximum input current the circuit draws is 2A at full load.
circuit diagram:



buck converter can supply load current of 5.75A at 5V and the short circuit current is 7Ampere. the maximum output is 5V×5.75A, the ripple voltage at max power out is 94mV and input power is 18V×2A and the efficiency(n) calculated
n = output power/input power = 5×5.75/18×2
n = 79.86%
Note: C1,C2,C3 are the low ESR <= 50milli ohms electrolytic capacitors add 2 of 100nF,50V rated ceramic capacitors in parallel to C1,C2,C3, to reduce the high frequency noise.
all resistors are 1/4 watt rated except R13 which is 3watt
L1, FS-1RN series toroidal cores made from iron powder (26) for creating inductors, FS-1RN core or similar you can get this one from old desktop PSU’s. here is a useful link of toroids,
http://toroids.info/ Q1 , Q2 are2SB772 2SC4468 they are audio transistors, can be used for switching applications, linear. isolate the tab of transistors and schottky diode with mica/silicone insulator