i have designed a dc motor comtroller pwm using 12v 3a current.
now i want to design a 12v dc motor controller pwm using 15a current at output. i designed this circuit using 3 mosfets which is similar to 3a dc motor controller pwm. in this circuit i connect 3 mosfet in parallel. this circuit gives short circuit current of about 2A but i want short circuit currrent of about 15A. what i should be done?

 

Your psu will have to be able to give out at least 15amp, cable sizes to the motor uprated, and put fets on an heatsink .

 

Maybe you can change to some other mosfets and they have lower Rds(on), it will help you to reduce the power dissipation and heat.

You can calculate the Pd(watt) of IRF540 for 5A.
IRF540/Nch/100V/22A/85W/55mΩ

W = V*I = (Id * Rds) *Id
= 5A * 5A * 55mΩ
= 1.375W

And what I bought IRF1010 for 15A.
IRF1010EPbF/Nch/60V/84A/200W/12mΩ

W = V*I = (Id * Rds) *Id
= 15A * 15A * 12mΩ
= 2.7W

if you can buy the STH260N6F6-2/2.4mΩ for 15A
STH260N6F6-2/Nch/60V/180A/300W/2.4mΩ

W = V*I = (Id * Rds) *Id
= 15A * 15A * 2.4mΩ
= 0.54W

FDP5800_Nch_80V60A_4.6mΩ_Vgs10V
IPP042N03L_Nch_30V70A-79W4.2mΩ
IRF1010EPbF_Nch_60V84A200W_12mΩ
IRF3205_Nch_55V110A_8mΩ
STB60NE03L-10_Nch_30V60A10mΩ
STH260N6F6-2_Nch_60V180A300W_2.4mΩ_Vgs10V

 

I'm guessing it all runs off about 12 volts?

Here is a single FET solution with no heat sink required.

http://www.mouser.com/ds/2/302/BUK9507-30B-114318.pdf

The flyback diode will also need to be bigger:

http://www.mouser.com/ds/2/427/sblfb1030-102229.pdf

He will need a small heat sink.

I would also add a decoupling cap from Vcc to ground on the 555.