i have designed a circuit for dc to dc current and my output current is in mili's and i want 3.5 Amp at output.. i don't want to use transformer bcoz of high losses..
Post a schematic diagram of your circuit.
Realize that even if your circuit is 80% efficient, to boost up to 19.5v at 3.5A, over 8.5A at 10v will be required.
Darlington transistor driven into saturation could give you some pretty heavy duty current amplification. You'll have to bias it properly if you require a more exact output.