I am so sick of the constant rehash of this so here is the beginning of the theory end of designing the power supply...something I threw together in response to a NOOB on another board:
using a UC3842 targeting 40% duty cycle for
325V@2.5A from 20V:
"generic" duty cycle is
1-Vin/Vout = 1-20/325 = 94%
100%-94% = 6% on the output but we want 60% so
6/0.6=10, 10-6 = 4, and (94+4)/4 = 24.5
or a turns ratio of 25 to get a 40% duty cycle.
The output current needs to be increased by the duty cycle to compensate for the switch on time that the current is not delivered to the load.
Ityp = Iout/DUTYout = 2.5/0.6 = 4.2A
Iin = Ityp*RATIO = 4.2*25=105A 40% of the time. Your FETs need to handle at least 200A continuous and preferably 400A.
Primary inductance assuming the FET is on 20uS and 30% ripple current:
L = dt*Iin*0.3/V = 20*105*0.3/20 = 30uH
Operating frequency should be F=1/(20uS/0.4)=20KHz
So make a 30uH primary and a secondary with 25 times the primary turns.
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