I'm documenting this retro-actively, so I'll be adding more project logs over the next few days/weeks. The first few iterations of the power supply are done. I ended up with a couple of variations on the theme, and I am still tweaking them to get the best performance out of them. I'll add circuit diagrams, parts lists and OSHPark links in due course.
I hope that others will be able to produce their own variations of this and will share them with the public. For me, I need to move on to actually using then in some Nixie clocks!
What seemed to be apparent from the outset was that my design should use a transformer. Nearly all of the power supplies I have seen are boost converters. These are quite simple in principle - at least using physical principles, not engineering ones. They rely on a collapsing magnetic field in an inductor to generate a voltage, the term I was taught for this was 'back-EMF'. To get it to generate more volts out than you put in, you want the field to collapse faster than you build it up. The power stays the same (well, obviously less because of losses in the system). So volts up, current down.
A single inductor can typically give you in the order of a 10x increase in voltage. It seems obvious that one way to go higher is to use a transformer. i.e. let the transformer boost the voltage generated by the back-EMF. Apparently, these are called flyback transformers. You can, quite literally, replace the inductor in a boost converter design with a transformer. At least in principle. Maxim have a great tutorial on this exact idea. 'Great', I thought, 'My search is over!'. Except that the transformers it uses just aren't available. In fact, it turns out, the transformer was going to be the biggest problem. Closely followed by everything else. But seriously, the transformer is the biggest problem. It seems like you can find a suitable inductor for a 12V-180V boost converter in a Chrismtas cracker. Suitable transformers are as rare as hen's teeth. Most people just make their own. That was a step too far for me. I'm too lazy.
Hello Paul, great work ! Most of the voltage converters that one can find on the web are based on the old MC34063 and suffering from low performance and the need of a high input voltage. No chance with a Li accumulator or something else below 9V supply voltage.
And not very often one can find real measured efficiencies. I really appreciate your tables with the measurements. Currently I'm working on a clock project based on cold cathode thyratrons. The thyratrons will visualize the time by their neon light. Need no digital counters or other tubes for displaying. I need 280V and about 20mA of current. Planned to build the HV supply based on MAX668 with about 20V DC in. Maybe I will give your version also a try.
Regarding the transformer issue : yes that is exactly my experience when I tried to follow the Maxim AppNote. I have a lot of transfromers and torroids tested - none of them worked with the performance that was promised by Maxim. But the MAX668 is a nice device for boost converters.
BR, Mark