Low quiescent/ standby boost regulator 3.3-500V

[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.

[SiliconWizard]

Just an additional quick comment after looking at the LPR6235 transformer's datasheet: with a winding to winding isolation of 300Vrms, I'm not sure this is going to work very well...
To be continued.

[Amper]

This (well, a similar topology without doubler) was something i have seen in a datasheet and forgot which one it was. Nice enough, you actually found it for me without knowing, have a look at page 21 in the LT8494 datasheet you linked. I forgot about the primary side diode but except for that i guess it should work with the 8330 as well. The inductor is 1:100 which is most likely to much but thats the one i have laying around from a LTC3109 converter i played around with. They are 300Vrms rated so i guess it should be fine here and if it turns out the converter works like i invision it i will order the proper one at the right ratio.

Sadly they are not available with middle tap, they are called coupled inductors and i couldnt find one suitable for flyback.

Thanks&cheers!

[Amper]

why do i neeed those things?

@SiliconWizard the 300V is fine using my voltage doubler,going without yes, that could be to close especially since im not sure if moisture may be an issue in the future, i dont want to conformal coat it right from the beginning.

[SiliconWizard]

As I wasn't looking for that in the LT8494 datasheet, I actually missed it! But yes this is it. As I said, it's indeed a flyback converter made out of a boost converter.
Yes there's a missing diode in your design.

Yeah I haven't really taken attention that you added a voltage doubler as the last stage. So you should be OK.

I will still try and simulate that later.

[Amper]

The thing im worried about now is that i dont have enough negative swing on the transformers output due to this diode, that would render the doubler useless. If you want to do a sim that would be cool, im usually to lazy sadly... But i will try putting a test circuit together with a similar controller i already have laying around.

EDIT: And now im not o sure any more about the whole voltage rating thing, i only looked at the primary side but since the doubler is grounded im afraid i will get 500V relative to the primary which is wound below the secondary without additional insulation.... Maybe a cocroft walton is necessary after all, just not so many stages.

[SiliconWizard]

OK, here is what I came up with based on your design so far (I used/modeled the 1:10 version of the transformer with the respective DCRs).

It reaches 500V in about 5ms. Once stabilized, it appears to be drawing an average of ~100mA from the battery for a 10µA load which is not exactly efficient.

[Marco]

Also regulated voltage is a requirement and regulating a flyback sucks or at least has no advantage over using a proper switcher.

Discontinuous mode has the fastest response, that's an advantage.

Siliconwizard, how much worse does it without the doubler?

[Amper]

Thank you very much for your"independant evaluation"! I cana ctually confirm this result by my experiments yesterday. I used a Chinese chip running at 1.2MHz (8330 runs at 2) and had it operate without doubler measuring the output loaded by 2MOhm. It reached only 150V at its best and drew up to 400mA @ 2V. Obviously this is far from the final pcb version using the right parts but it shows that its not the right path. Just running the transformer from a function generator and amplifier gave me the same results, pushing it to 600Vpp the transformer actually heated up to a point near damaging. Though this way i found out that this particular 1:100 transformer works best at 500kHz and looses efficiency fast going into the 1-2MHz range, so no surprise linear is using 250kHz with theirs.

The next experiment was using an lt3757 boost converter as a basis (old prototype laying around) I modified it to fit this topology and it also took over 1W just to hold a voltage of 400V even though the frequency was just 200kHz.

Well, summing this up, transformers seem to suck more at this sort of frequency than i expected however a friend of mine pointed me the right direction yesterday. Its also very close to what you recommended first yesterday using a cocroft walton with a boost stage first:

https://www.maximintegrated.com/en/app-notes/index.mvp/id/3757

This should be roughly at 1-2MHz as well and the no load input current is phenomenal, i will try to put one together today and then have a look how far i can reduce the size of it. Maybe reduce by one or two stages and use sot23 double diodes could get it to acceptable dimensions on the pcb.

[SiliconWizard]

Siliconwizard, how much worse does it without the doubler?

Average current draw drops a little (not much, about -1.5%), which makes sense since there is a bit less loss. But then it takes atrociously longer to reach 500V. And then there is the problem that the transformer selected here wouldn't handle it in real life.

[Amper]

Sooow i just tried the maxim approach without the regulation OP on a breadboard with film caps and 1N4007, it works beautifully. Just had to reduce the frequency to around 500kHz again for reasons of the FET i used having far to much gate charge to be driven by the 74.

Using only 7 capacitors and  diodes i can easily reach 1,1kV without load. The input power using the not suitable components is .8W at 5V. This should drop significantly with the proper parts and regulation.

The most interesting thing is how load sensitive the device is. It takes several seconds to charge the bank of 100nF caps up to 1kV, voltage measurement was done by hp3457a and a 1G resistor giving a rough 1:100 divider, direct 10M loading makes the output instantly crash to 200V.
Running an Si8b Pancake tube from it is very interesting as well as the unregulated supply drops depending on radiation. Just having the thing sit will result in roughly 150 impulses every minute and thee voltage will rise up to 1kV. If 10kBq Strontium90 is used to ionize the gm tube the voltage quickly drops and even reaches 300V, at this point the tube will no longer operate properly and the count rate drops to an equilibrium.

I could imagine a mode of operation first charging the capacitor to a set voltage and then have the tube discharge it over a long period of time until the cap reaches a threshold triggering a comparator interrupt in the microcontroller. Coupled with a real time clock this could provide extremely low power measurement not even requiring any microcntroller to stay awake during measurement. The only disadvantage will be the need or a >1GOhm resistor to do the voltage measurement which will be a moisture problem.

Ill keep you updated as i continue...

[Marco]

That Maxim circuit is quite cool ... I completely forgot that type of boost converter existed (seen it called multilevel boost converter as well).

The way series voltage multipliers droop under load (ripple is not the same for every stage) makes it hard to regulate the output voltage using just the voltage on the lowest stage though. A parallel voltage multiplier might be better.

PS. if you really want it small, VMI makes small multiplier modules like PVM302P08 (example of its use in a neutron detector). No idea what it costs though.

[Amper]

Yes, im curious too how well this regulation will work, on the other hand the average geiger tubes plateu is also around 100V wide so it may be alright to have 10-20% variation. Measuring the output directly would be possible but also requires very very high resistances not to create to much load. As i said before, 10M will completely overload the converter, 1G seems appropriate but even though 1206 parts in this value exist the board it self would be more conductive... Not even speaking of moisture.

I know about these amazing modules though besides the very high price tag they are also very difficult to find anywhere. My current layout is just about double the board area and requires only pretty common components. Changing the fet to a 300V type i could even raise the voltage to 1.5kV since all the other parts are already capable enough.

Something else i just remembered:
There is also the option to use a string of high voltage zeners in series to the sense reistor dropping the first 400V or so. If the 60M divider just sees the last 100V the current should be small enough and the multiplier is loaded evenly without insane resistors.

[SiliconWizard]

I've simulated the "flyback" topology around an LT8494 instead of the LT8330 and I got better results. It would draw ~50mA on average. Still a lot IMO, but it seems hard to do better than this, so if you manage to do better with Maxim's approach, I'll be interested in seeing that. Keep us posted.

The small 1:N coupled inductors from Coilcraft are great, but pretty limited still. They also have miniature flyback transformers that have better specs, except they are only 1:1, so not really usable here. Haven't found much better so far, except with MUCH larger transformers. Or you'd have to have them custom-designed...

I have also tried the approach I had in mind (in simulation only): a conventional boost converter, followed by some oscillator feeding a Cockcroft-Walton multiplier. I was happy to find the LT8415, which has an integrated dual half-bridge, making it ideal for that without the need of any external transistors or additional integrated high-voltage half/full-bridge. Unfortunately, the max switching current of the LT8415 is so low (25mA) that the output of the multiplier can never reach 500V, not even close... Could be doable for a lower output voltage though such as 100V to 200V...

[Marco]

Oh, there's already a complete open source design using the Maxim technique specifically for Geiger tubes.

They also use avalanche diodes for feedback BTW.

[EmmanuelFaure]

There's an easier method : Boost converter with an tapped inductor/autotransformer in place of the inductor. Flyback have an unecessary component count, transformers are not cheap, and you don't need the isolation between input and output.
Schematic for example here, on page 43 :
https://www.analog.com/media/en/technical-documentation/application-notes/an19fc.pdf

[Marco]

An autotransformer has nearly the same complexity as a flyback, the couple of windings saved are hardly relevant, especially given they can be wound with thinner wire.

Isolation has complexity, but you can use the fly back transformer without it.

[Amper]

It could be a nice idea though considering thee isstandard boostconverter ICs up to around 60v so a 1:1 tapped inductor could get rid of the previously needed external fet, that is a legit point he has there.

[Marco]

Boost converters with lots of annoying features which will just make your life hard, compared to a nice and predictable hex inverter ... I'd go for the hex inverter with two transistor from the Theremino GA500 design.

[Amper]

The 7414 does not mean i cant use a transformer, a normal boost topology or what ever afterwards.

[EmmanuelFaure]

It could be a nice idea though considering thee isstandard boostconverter ICs up to around 60v so a 1:1 tapped inductor could get rid of the previously needed external fet, that is a legit point he has there.

And why not a tapped inductor with ratio 1:10, or even more? Buy a ferrite core and do it yourself. Sometimes a bit of handwork is better than browsing catalogs for hours searching the perfect off-the-shelf component.