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What you need to investigate are switching boost convertors. Visit websites of Switching IC manufacturers like National Instruments, Maxim, ST, Philips etc and look at the data sheets. You will find application circuits that will meet your needs.
Ya use a convert boost DC-DC!
You should use a powerMosfer with vds>440V like a IRF443 or something
And my suggestion is TL594 and a UCC27324P to control it! If you need schematics contact me! i made some of this