Step Up DC-DC 12V to 400v converter

[NikoTico]

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Hi
I was needing a variable power supply between about 50V and 400V, with a current of only a couple of milliamps, to reform electrolytic capacitors in valve equipment. To do this, I came up with the idea of designing a boost converter using a 555, with a small feedback towards pin 5. However, the problem is that I don't have mosfets that handle those voltages (I only have a couple of IRF630s), but it does have transistors like the 13007, 2SC5353 and RC3866, as well as other small bjts. As diodes, I have FR307. I also have unwound transformers from ATX sources, in a flyback type power supply I find myself more lost.

I would appreciate it if someone could advise me on calculations and design to get the best configuration. [Google translated from Spanish]

 

[BradtheRad]

Many recommend using a variac to gradually turn on old equipment from house voltage. I've used one rated for 1.75A, 120VAC.

Did you rule out building a simple voltage doubler? Even a half-wave supply (one diode) should give 330VDC.

I built a bridge-style doubler to step up 170V peak, to a high enough voltage that I could run a xenon flash tube as an automobile ignition tester.
However if you really need a supply referenced to ground, then a Villard doubler is suitable.

 

[prairiedog]

Also search for "nixie tube power supply" there are many circuits out there using boost converters with a 555 or MC34063 etc.
I have seen old CCFL transformers or even a backwards connected mains transformer like 6V-220V used to make HVDC.
You can use a DC-DC converter salvaged from a disposable camera (xenon flash), or buy the DC-DC converters from Aliexpress.
It depends on what you want. Easy, low cost or to learn and have fun.