Nixie power supply run off single Li+ cell (Nixie imponator) - Page 1
Forum section: electronics / projects

author: magnehh
author_rank: Contributor
timestamp: January 04, 2016, 06:40:29 pm
content: Hi guys!So I'm building this thing called an "imponator". It's basically a device whose sole purpose is to impress other people, in this instance with light.It'll feature anATmega168P(TQFP) microcontroller to control theMMBTA42's (NPN) I'll connect to the single Nixie tube's cathodes (IN-15B). This whole shebang will be powered from a single Lithium cell (~3.7V), which will be charged through aMAX1555EZK. The DC-DCcontrollerI'm planning to use for the 180 volts is theMCP1650S-E/MS. This Nixie tube will draw about2.5mA at 180Vacross the anode resistor and the tube itself. The micro will be connected straight to the battery.In other words, I need to generate180VDC from 3 - 4.2VDC.The most important requirement of this circuit, next to being battery powered, is that it's small (max. 7cm x 7cm) and wearable right around where your heart is pumping away. It should use surface mount components as far as possible. It should really be as small as I can possibly make it.I've never designed a DC/DC converter before, though I have a fairly good idea about how I'll make it (flyback). It seems the conventional boost design just won't cut it due to the high duty cycles required and insane current draws. One online calculator told me I had to pull120Aout of the battery.I searched the forum and foundthis replyin another thread. I think I can get hold of a few small transformers. As far as I'm concerned, at my level of knowledge this seems to be the easiest/best solution. I'd rather not spend my dough if I can make some of these components easily by hand.What do you guys think I should do to make this work? Is it just too ridiculous? I know some people are making Nixie wrist watches, so this power supply of mine should be quite possible to miniaturize.Ok, thanks for reading this far. Happy new year and let's try not to zap ourselves.

author: garre
author_rank: Contributor
timestamp: January 04, 2016, 06:54:57 pm
content: The DC to DC power converter will transfer power in watts. Multiply the 180VDC by the Nixie current draw and divide by the battery voltage to get the battery current. The range is ~ 120 mA not 120 A. There will be some efficiency loss in the converter so the current will be somewhat higher based on the efficiency of your converter design.

author: magnehh
author_rank: Contributor
timestamp: January 04, 2016, 07:06:56 pm
content: Yeah, I should have clarified. The calc said my inductor had to handle a big current. Not sure if I misread the output. Anyway, the current draw from the battery, assuming perfect DC/DC conversion would be closer to 1.2A at 3.7V, wouldn't it?

author: MagicSmoker
author_rank: Super Contributor
timestamp: January 04, 2016, 10:35:11 pm
content: Quote from: magnehh on January 04, 2016, 07:06:56 pmNope, 180V * 2.5mA = 0.45W, so 0.45W / 3.7V = ~122mA (neglecting losses, which should otherwise never be neglected).As for the transformer, the turns ratio for a flyback isn't terribly critical, and you probably don't care if the switching frequency is in the audible range so maybe consider using a small audio transformer, like the kinds used in modems, old "transistor" radios, etc.. The higher the impedance transformation ratio the better (the impedance ratio is the square of the turns ratio).

author: MagicSmoker
author_rank: Super Contributor
timestamp: January 05, 2016, 06:39:58 pm
content: The ATB322515 (and similar) transformers from TDK are an interesting suggestion, but will likely prove difficult to use for the first-time designer of a flyback, mainly because the primary inductance is so low (and to a much lesser extent, because the turns ratio is also non-ideal, but as I said before, the turns ratio isn't that critical in a flyback).Dealing with the easy part first, note that the voltage the switch has to withstand during the flyback period is the sum of the input voltage and the output voltage divided by the sec:pri turns ratio (10:1 for all the transformers in this series), plus a good margin extra to deal with ringing and leakage inductance spikes. At ~4V input and 180V output the switch has to withstand [(180V / 10) + 4V] = 22V, plus the aforementioned margin, so quite a bit more than you might have assumed given the supply voltage. I'd pick a MOSFET rated for at least 40V.The primary inductance of 7uH is much more restrictive, however, because the switching frequency will need be higher than 200kHz to avoid saturation in this application. Personally, I'd look for a similar style transformer with a higher primary inductance at a lower switching frequency would allow the use of a cheaper controller IC with an integrated switch (such as the venerable '34063 and similar).EDIT: design -> designer; 'inductance as' -> 'inductance at'