mrclunk said:
Nice write up on a nixie clock HT supply with the MAX 1771 here. http://desmith.net/NMdS/Electronics/NixiePSU.html
Might be of some use?
I already posted that earlier.
I built my own version of his design, and it's actually quite a challenge because of the high step-up's involved.
EVERYTHING matters in this design: milli-ohms of trace impedance make a big difference, so the layout is critical (especially in how vias tie traces to planes) and component selection is critical as well. You need a fast recovery diode, and the lowest channel resistance FET you can find, and an inductor sized to the worst case output current you need to sustain. Tiny ESR differences in the output cap make a large difference in how much voltage you can squeeze out.
Ian, I would forego the DIP packages: in order to follow the recommendations you really need to stay SMD to keep the loop areas around the feedback path short and low impedance, or MAX1771 chip can oscillate due to parasitic capacitance. Unlike a linear regulator, you can't clobber the feedback path with capacitance to fix things either.
The circuit doesn't work well (if at all) on a breadboard.
The only real chore with getting above 250V is that SMD components rated at those voltages can be hard to find, so if memory serves some of the output caps on my boards were PTH (or you can stack SMD caps in series as well).
Keep in mind that with 60 kHz+ ripple voltages that even small CLC filtering works quite well for even small SMD inductor sizes. I haven't had any ringing or noise issues with vanilla common-cathode gain stages used in microphones or microphone preamps, and you can get a large amount of power out of a very tiny size with very high efficiencies.
Lastly, I powered my board with a commercial wall-wart that supplied 12V at 5A to provide over 20W of power on B+, coupled with a buck SMPS to provide 40W of heater supply (although it's easier to just stack heater coils in series and drive them directly from the raw 12V supply). It's nice to be able to leave the mains voltage as someone else's problem in tube designs.