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Low quiescent/ standby boost regulator 3.3-500V

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Amper:
Hi,

Im designing a small geiger counter meant to run of a single 135mAh lithium cell for extended times logging radiation. To extend the possible time between recharges i would like to have a very low power regulator for the roughly 500V supply. Maximum current would be only a few micro amperes. I guess some kind of burst controller would be nice but i dont have a good overview right now.

any recommendations welcome ^^

SiliconWizard:
You'll have a hard time finding a single, integrated boost converter capable of going from ~3.7V input to 500V, and even harder time finding one which has very low quiescent current...
(You said "lithium cell" and said it was rechargeable, so I assumed a Li-Ion/LiPo cell of some sort here...)

But let's start with simple maths first: assuming the output current is 10µA and you can find/design a converter that is 80% efficient (which IMO would be already pretty optimistic here at this output current), that would yield an input current of 10e-6*500/3.7/0.8 ~ 1.7mA. With a 135mAh battery, you'd get ~79h (a little over 3 days) of continuous operation assuming that nothing else is drawing current (which I doubt if your device is supposed to be logging data)... would that be what you'd call "extended times"? Or would you actually have your device log radiation only a fraction of the time, thus having a low duty cycle? This preliminary remark just to make sure your expectations are realistic.

One approach could be to use an efficient, low-power boost converter to go from 3.7V to an higher voltage and then cascading a Cockcroft–Walton multiplier (see: https://en.wikipedia.org/wiki/Cockcroft%E2%80%93Walton_generator , and I think Dave also made a video about it) to go up to 500V. IMO, a Cockcroft–Walton multiplier alone to go from 3.7V to 500V would be bulky and never be efficient enough to get you the level of current draw you're targetting.

One possible such boost converter could be the LT8494: https://www.analog.com/en/products/lt8494.html
(typ. 7µA quiescent current!), set the output at the max. supported: 70V, then cascade a Cockcroft–Walton multiplier for a ~7x multiplication.

Another option would be to use a flyback converter, but this would require a transformer (don't know if it would fit your size requirements) and good luck designing one to draw that little current...

Amper:
Hi,
thanks for the reply, i can see you already found the pieces of information i left out ^^

It doesnt have to be a boost converter, at the moment im tending towards a transformer driven by a boost IC. I have seen such a configuration in some analog datasheet but i just cant find it any more since its not the primary application of the part.

The device will have several modes ranging from continuous logging (for decay experiments and maybe gps logging while hiking in the mountains) to just general monitoring and taking a measurement once a day for 5 minutes. The energy is by far sufficient, i have experience from building a logger for industrial current loop sensor that took much more power. Standby should be around 5uA lasting more than a year without being in use.

Using a multiplier with may stages would be possible but since my space is limited i hope i dont have to go that rout.


Attached is a rough schematic of my plan to this point, dont judge please, its just a sketch and will be refined after im finished coming up with the features needed.

EDIT:
Yes, a flyback would be the obvious simple solution but i stopped liking them for reasons of reliability, also regulated voltage is a requirement and regulating a flyback sucks or at least has no advantage over using a proper switcher.

SiliconWizard:
I just took a quick look. What you're working on kinda looks like a boost DC/DC converter used as a flyback converter. I didn't see such a topology in the suggested applications in its datasheet. Without further analysis, I'm not saying it won't work, but I'd suggest you simulate that first (LTSpice has a model for this converter) and see what happens. Also, have you selected an appropriate transformer yet? (As said above, I had ruled out the flyback topology assuming that the required transformer may be too bulky for your needs - I assumed that based on the choice of a pretty small battery. But maybe you've found a transformer that's small enough.)

Check that it can't destroy the converter under any circumstances. Check the input current. Even if it works, I'd suspect a much lower efficiency than what is typical when used as a basic boost converter.

Then, even if the average input current, once it's in regulation, may fit your requirements (to be checked), also carefuly take a look at the inrush current when it's starting. It may draw currents that your battery would not be happy with.

Regarding size, a 7x Cockcroft–Walton multiplier, at such low currents, would require only very small diodes and caps and would probably not take up too much space compared to a transformer.

Edit: I hadn't seen there was a reference for the transformer on your schematic: https://www.coilcraft.com/lpr6235.cfm
Looks like it's rather small indeed and would be a nice fit. I'll try and set up a simulation in LTSpice.

NiHaoMike:
You could look at using a piezoelectric transformer, no idea where to get them however.

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