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Low quiescent/ standby boost regulator 3.3-500V
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:
--- Quote from: Amper on July 12, 2019, 02:12:06 pm ---Also regulated voltage is a requirement and regulating a flyback sucks or at least has no advantage over using a proper switcher.
--- End quote ---
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:
--- Quote from: Marco on July 13, 2019, 12:09:03 am ---Siliconwizard, how much worse does it without the doubler?
--- End quote ---
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...
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