Electronics > Projects, Designs, and Technical Stuff

Low quiescent/ standby boost regulator 3.3-500V

<< < (8/11) > >>

SiliconWizard:
Well, yes, I got that! ;D

I'm just saying that the system is "blind" until the converter starts again. I have no clue how accurate the output voltage must be here - I guess not very as long as it's high enough, I don't know much about Geiger tubes - so yeah it may not matter at all. But estimating the voltage drop would be pretty inaccurate IMO (of course would depend on the maximum "off" time). You also have to take the capacitor's leakage current into account. Again I suppose it may be negligible in this application, but even ceramic caps have leakage currents that significantly increase with the voltage, so at 500V, it's probably something to consider as well. It could be leaking more current than you think...

As to software bugs: obviously a purely software-controlled high-voltage boost converter is not completely without risks. There are many ways something could go wrong and the transistor (and many things around it) could literally explode. ;D So you better make it robust software-wise...

If the system is not active at all times but only for a small fraction of the time, depending on the average power draw of your tube, it may be simpler or even more efficient to just switch off the converter completely when the system is idle and start it only when needed. Certainly something to be considered by comparing average power in typical scenarios - scenarios which I have no clue about!

Amper:
Allright :D

The voltage is not very critical, more stable is better but +-10V is acceptable. Considering the tube has only 13pF discharging every pulse and the usual pulse rate is 2Hz on average in standby, the storage cap is 200nF and i accept 10V discharge before a measurement has to happen, thats 300 pulses or two minutes. Not a relevant time for capacitor or pcb leakage since everything has to be potted anyways. Since i know the rate will never drop significantly below 100cpm i can just have a watchdog running that will do the checkup every 60s and it will be enough unless i have some sort of hardware failure in which case i will also not get any pulses in those 60s and the device will go into failure mode. In case it wakes up after one minute and notices a voltage drop much larger than expected it will decide something is wrong with the HV side. If a number of pulses more than 200 is detected while asleep it will wake up as well still having a margin to go in voltage and do a recharge cycle. I cant see a way for it to go out of regulation this way, actually if voltage control needs to be tighter for some reason i could still just wake up after every pulse and recharge, still much less energy required than doing proper regulation and holding the voltage divider under voltage throughout wich would draw very very significantly more power than any thing else in the system. For comparison the maxim circuit could do 1kV with no issue in seconds but would completely break down under 10MOhm load, i had to go to 1GOhm to get a usable voltage reading.

Regarding the software damaging the fet you are right considering the gamma scout circuit but i will just put a capacitor and a slight pull down resistor on the gate, using logic level fets thats enough of drive voltage and in case something gets stuck it will just turn off in time. It worked well on half bridge drivers with bootstrap capacitor, they will just run empty in a few milliseconds and thats it, fire prevented. I understand your concern though, if it wasn't for such a wired application i would never ever drive a boost converter from software.

The reason i now want it this way is that i have seen that its possible and i want to go one better on the commercial thing. The gamma scout is a machine hated amongst hobbyists for user unfriendliness and the high price of 400+€ even though it has some very nice features. I have the obligation to build something better after investing so much time i just cant settle before this thing can run continuous mode :D

SiliconWizard:
Yes it looks like a decent approach. The added benefit (don't know if the original device does it that way, and if this point matters) is that you can do all the measurements while the converter is off, which would thus get rid of any noise - don't know if noise on the high voltage could cause measurement issues, but if so, that's a good point.

The risk damaging something is not just with the IO driving the gate getting stuck IMO. It could also be switching away without ever stopping, especially if you use a PWM peripheral in your MCU and some bug prevents your internal control loop from stopping it when the output voltage is reached: in this case the converter could reach dangerous voltages possibly damaging some components. But yes some additional hardware protections can limit the max output voltage whatever happens software-wise.

Amper:
Yes, turning off the converter for low noise is nice, not really necessary with gm tubes though, they are just dump flash lamps completely ionizing and then extinguishing again. However it can  be very useful with neutron counters as they require <2mVpp noise on the supply. Though also at three times the voltage, so a cascade will be needed again. For now ill be happy with just having "ok" regulation.

Regarding a hangup in pwm yes, that would suck and i didn't consider it...
But as you already told, most likely some zeners will be enough to keep it from destroying itself. The Tube can survive 1kV if its only short periode, so most likely the fet will just die and blow the fuse. I will worry about it later, if it will be a problem i will add a zener string for the development board and later i guess it should be sorted out.

After all its not going to be a safety critical device, its just a nice toy and gadget for finding radiation sources in every days life. If it breaks its broken, i wont use it to monitor my dose rate ^^

SiliconWizard:
Maybe this recent event will motivate your quest some more:

https://www.cnews.fr/france/2019-07-18/du-tritium-radioactif-dans-leau-potable-de-64-millions-de-personnes-862178
(Google can translate that for you, didn't find the info yet in foreign media.)

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod