Designing a 5v to 12v converter


so my latest idea is to make a converter that runs off the 5v and converts to 12v. there i would add a 6pin pci-e connector so i can power the gfx card off of a cheap old psu that is heavy on 5v amperage...
so it needs to be at least 12v, 6a, (72w) which is close enough the the 75w that is required of the pci-e connector usually.
That is going to be pretty difficult to do, if you were going from 12V to 5V you could just use a voltage divider. How much power does that old PSU have available on its 5V rail? For a cheaper voltage boosting system i would only expect about 60-80% efficency, and you dont want to have it only capable of providing 75W otherwise it will be running at full power all the time and will burn out, i would build it for ~90W to stay on the safer side and if your little circuit is only about 60% efficiency that means you could be pulling up to 150W from the 5V rail at peak.

There are a few smaller prebuilt ones like this, but it only has a max output current of 1A so you would need to put 7 of them in parrallel to get the 6.25A the PCI-E connector needs.
Be simpler and cheaper to just buy a larger PSU. Any DC-DC converter that you buy will cost more than a larger PSU. And building one of that power level will require some pretty specialized (another word for "expensive") parts.

I wouldn't want to tackle a project like that, and I have decades of electronics experience.