DC-DC 5W-15W, 12V to 300V-1.2kV

Current sense transformer - unlikely because the insulation rating on the winding is not intended for anything other than a few tens of volts maximum. However, you might be lucky and get 100V. Frequency of operation is something to consider - if you want a fairly smooth DC output (i.e. small ripple under load) then operating at power AC frequencies is probably not a good idea (although many years ago X-ray machines did use exactly such a technique). These days 50kHz is somewhere about right for switch-mode step up converters but they do tend to use ferrite transformers to achieve this - look at linear technologies website - they have designs that can certainly provide enough voltage for electroluminescent displays (300V ish). Four of these building blocks could have their outputs wired in series. Another thing to remember about these sorts of designs is that the transformer provides much needed isolation and protection both to the primary circuit and the user/operator. Televisions use a transformer to get several kV AC then a diode tripler to get (say) 25kV needed for the tube. I've seen designs (on x-ray machines) that used 7 stages of diode capacitor to boost the voltage but directly from 12V to 1kV would need far too many and I suspect the losses would be too great.

Alternatively I bet you could buy one - what budget do you have? How many units? what ripple on the output? Do you need feedback control to keep the output regulated.

Sorry for the long answer.

I expect to build for 5-15Eur for components and assembly. Without feedback for the same power is possible to get for 3-5Eur. I can't find step-up transformers for photo flash lamps. They might be suitable for me.

Usage 100-1000units per year. Load is not fixed, so feedback is nessecary. Ripples 2% and long therm stability is OK.

I made experiment with current sense transformer. With 3V spike, I get ~300V in the output.

I expect to build for 5-15Eur for components and assembly. Without feedback for the same power is possible to get for 3-5Eur. I can't find step-up transformers for photo flash lamps. They might be suitable for me.

Usage 100-1000units per year. Load is not fixed, so feedback is nessecary. Ripples 2% and long therm stability is OK.

I made experiment with current sense transformer. With 3V spike, I get ~300V in the output.

Try searching for a pdf of an LT3573EMSE#PBFLT3573EMSE#PBF from linear technology - they have a circuit for a 300V isolated output (page 23). Four circuits would work with the outputs wired in series - if you contact LT they may even recommend a transformer that goes straight up to 1.2kV - this would be better because you'd get better control BUT you may not get enough power. They will advise - they are pretty good at helping but don't expect answers as quick as this one!!!

Thanks for fast answer! So far biggest problem to find commercialy available right transformer. I see right transformers in various devices, but googling for part number doesn't work.

Jonas