There are plenty of ideas, but the question is: Which one is best for what you want to do? For example the best circuit to boost 3.6V to 300V to charge a Geiger-Muller tube is not the same as the best circuit to boost 3.6V to 300V to power an electric motorcycle engine.
Would you please explain what you want to do with the 300 volts, how much current you need at 300 volts, and why you have put so many batteries in parallel?
I am sorry but according to the User Agreement we are not allowed to discuss rail guns here.
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