I'm looking to make a high voltage supply for some geiger-mueller tubes (the ubiquitous sbm-20 russian surplus tubes that everyone uses). these don't require much current at all, so creating high voltage shouldn't be too tricky.
I'm using a 12 volt source, and I'd really like there to be some way to fine tune the voltage (either by setting a duty cycle, or a current set resistor, or something similar). I'd also like it to be as cheap as possible (meaning probably a preference for discrete components over ICs) while being reasonably sized (say, the size of a clamshell phone).
It seems that a reasonable way to approach this would be to create a two-stage converter: a boost converter to get me up to, say, 48VDC, followed by a series of voltage doublers until I get to 500 - 700 VDC.
This two-phase approach allows me to fine tune the voltage relatively easily, and I should be able to get away with using cheap discrete components.
My questions are:
Is this a reasonable approach? Are there simpler ways to create a similar, charge-pump style voltage supply? I'd really like to have the simplest circuit I can while maintaining some semblance of tunability and stability.
If so, how would I couple the boost converter to a voltage doubler? Do I need a transformer? Can I just use the diode and capacitor as the first diode/capacitor pair in the ladder? Do I need a second FET to switch the boost converter's load capacitor into the voltage ladder?
Any schematics or similar would be extremely helpful.