Author Topic: Low quiescent/ standby boost regulator 3.3-500V  (Read 6481 times)

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Offline AmperTopic starter

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Re: Low quiescent/ standby boost regulator 3.3-500V
« Reply #25 on: July 15, 2019, 07:41:17 pm »
We have gone through this already, at the size i need home brewing is not possible, the inductor we are looking at here is a maximum 5x5mm footprint and there is a height limit as well. Also as said before i have problems with losses at those high frequencies and high winding count, the capacitance at this voltage is not to underestimate at 1MHz and above.
 

Offline AmperTopic starter

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Re: Low quiescent/ standby boost regulator 3.3-500V
« Reply #26 on: July 16, 2019, 07:51:39 pm »
Ans another update:

Sometimes its a nice idea to ave a look at what others have dome and luckily some people have reverse engineered commercial equipment already.



Sadly german but you may still see how it works, a single stage boost converter but controlled by the microcontroller and voltage sensing decoupled by diodes to reduce the discharge while the voltage is still there. This is of cause only possible by a micro "looking after" the voltage every few seconds or minutes by pulsing the boost converter a few times but reduces the total current enough to power the entire device from a single AA sized lithium battery for years (over an entire decade) constantly holding 500V of tube voltage.
« Last Edit: July 16, 2019, 08:47:59 pm by Amper »
 

Offline EmmanuelFaure

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Re: Low quiescent/ standby boost regulator 3.3-500V
« Reply #27 on: July 17, 2019, 12:14:52 pm »
Sometimes its a nice idea to ave a look at what others have dome

You say?

[...] a single stage boost converter but controlled by the microcontroller and voltage sensing decoupled by diodes to reduce the discharge while the voltage is still there. This is of cause only possible by a micro "looking after" the voltage every few seconds or minutes by pulsing the boost converter a few times but reduces the total current enough to power the entire device from a single AA sized lithium battery for years (over an entire decade) constantly holding 500V of tube voltage.

LTC3525 does that without any µC :
https://www.analog.com/media/en/technical-documentation/data-sheets/3525fc.pdf

Iq = 7µA with a 135mAH cell => Autonomy ~ 2 years.
 

Offline AmperTopic starter

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Re: Low quiescent/ standby boost regulator 3.3-500V
« Reply #28 on: July 17, 2019, 05:14:42 pm »
But doesnt this one have an internal synchonous buck and no seperate FB pin? I dont see how i could use it in that topology.
 

Offline SiliconWizard

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Re: Low quiescent/ standby boost regulator 3.3-500V
« Reply #29 on: July 17, 2019, 05:23:29 pm »
Yeah. Looks like it's running in circles a bit. I think we already have been through the idea of using a conventional boost converter to generate a high voltage...

Regarding the commercial equipment you mentioned, I'm not completely sure I got how it's generating the high voltage. Care to share a simplified schematic?
 

Offline AmperTopic starter

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Re: Low quiescent/ standby boost regulator 3.3-500V
« Reply #30 on: July 17, 2019, 06:43:28 pm »
They basically have a box standard boost converter but the output capacitor is isolated by a string of high voltage diodes. In a usual boost converter this wont work without a load but if you do intelligent pulse skipping up so minutes at a time this way the actual boost converter can be at 0V for the majority of the time while the cap stays charged. Since the discharge rate is so slow its enough to do an active measurement every now ant then at predicted intervals depending on count rate and time.

 
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Offline SiliconWizard

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Re: Low quiescent/ standby boost regulator 3.3-500V
« Reply #31 on: July 17, 2019, 08:15:55 pm »
Well, it's just a discrete high voltage boost converter... ;D
Of course you need a transistor that can take the punishment (the BSP125 has a VDSmax of 600V), the inductor should be able to take it as well.

Putting the feedback network BEFORE the diodes (whereas we would normally rather put it after) allows to have it NOT draw current from the output capacitor, but then it requires something a bit more intelligent than just a simple comparator since the feedback voltage is not exactly a fraction of the output voltage... using an MCU to do this solves a lot of problems. :-+
 

Offline AmperTopic starter

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Re: Low quiescent/ standby boost regulator 3.3-500V
« Reply #32 on: July 17, 2019, 10:34:37 pm »
Thats exactly what i was saying ^^ Thats also why it wont work with any off the shelve part that i know of as ist not just pulse skipping. Since every ionization of the tube is a very well defined amount of charge its possible to count them and know the voltage of the cap very closely. Then just have a timer for low count rates and fault conditions, more or less a watchdog and make the converter do a short sounding pulse to the boost.

Though i suspect my tube will reduce battery life a bit more since the si8b is faar more sensitive than the original one used in the gamma scout. I will have a whopping 150 pulses a second with no radioactivity present and up to 5000 per second with sources close to the legal limit.
 

Offline SiliconWizard

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Re: Low quiescent/ standby boost regulator 3.3-500V
« Reply #33 on: July 17, 2019, 10:59:28 pm »
I'm not very comfortable not being able to monitor the output voltage though... I suppose you can indeed estimate it, but I wouldn't be so sure about the "very closely"... but at least here, being able to monitor the transistor's drain voltage allows the circuit to 1/ estimate when the target output voltage has first been reached (approximately) and 2/ avoid burning up the transistor and inductor... unless of course, there is a software bug.



 

Offline AmperTopic starter

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Re: Low quiescent/ standby boost regulator 3.3-500V
« Reply #34 on: July 17, 2019, 11:23:28 pm »
nooooooo. You didnt get how it operates ^^ Its not guessing the voltage, its just predicting the voltage drop so it knows how long it can stay silent taking zero power. When timer or prediction tell that its time for a checkup the boostconverter is powered up, in this moment the diodes will conduct as the boostconverter obviously has a higher output voltage as the cap. This way you can exactly measure the capacitors voltage as long as the converter is running. After shutdown the decay of the voltage is estimated with a safety margin and when it drops a few volts in the calulation the boost converter is started up again for a few cycles and a new measurement can be taken. According to that measurement the boost converter is run until the nominal voltage is exactly reached again, then the next sleep cycle starts and everything is completely currentless besides the microcontroller waiting for interrupts.
 

Offline SiliconWizard

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Re: Low quiescent/ standby boost regulator 3.3-500V
« Reply #35 on: July 17, 2019, 11:50:20 pm »
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!
 

Offline AmperTopic starter

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Re: Low quiescent/ standby boost regulator 3.3-500V
« Reply #36 on: July 18, 2019, 08:04:06 am »
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
 

Offline SiliconWizard

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Re: Low quiescent/ standby boost regulator 3.3-500V
« Reply #37 on: July 18, 2019, 01:23:26 pm »
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.

« Last Edit: July 18, 2019, 02:24:44 pm by SiliconWizard »
 
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Offline AmperTopic starter

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Re: Low quiescent/ standby boost regulator 3.3-500V
« Reply #38 on: July 18, 2019, 03:52:26 pm »
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 ^^
 

Offline SiliconWizard

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Re: Low quiescent/ standby boost regulator 3.3-500V
« Reply #39 on: July 18, 2019, 04:08:06 pm »
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.)
 

Offline AmperTopic starter

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Re: Low quiescent/ standby boost regulator 3.3-500V
« Reply #40 on: July 18, 2019, 05:28:18 pm »
Mhhh well...
Sadly my detector will not be sensitive enough for this particular incident but i can see exactly where your idea is coming from and its definitely a thought in the back of my head i cant get out. My family owns a small hydroelectric power station near lyon (see my older posts about the lithium welder if you are interested) and all the way from the eighties when my dad bought it he wanted to install a permanent radiation logger there for the reason of not trusting governments about this particular topic. Its not like we trust our own one any further but you see where everything is coming from... Maybe some time i will get around to building a better detector, material is almost all in my stock already but the time issue....

Oh and there is much more radiologic stuff going on then you may expect, i have a friend who worked in decontamination as a students job and he made me aware of an incident in Munich, some guy in a company took an angle grinder to a 1TBq source and roughly 1% of its contents left the building via the hot cells exhaust, the entire neighborhood was covered in radioactive dust in an amount that is actually a health risk. But except for a tiny article in a local newspaper there was no mention to the public, you could have walked there and absorbed the stuff...
 

Offline AmperTopic starter

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Re: Low quiescent/ standby boost regulator 3.3-500V
« Reply #41 on: October 30, 2019, 07:05:30 pm »
Aaaaand im back.

Had some spare time and continued the prototype a little bit. It now is made almost entirely from surface mount components that are affordable and the footprint will be tiny once on a pcb. Im now controlling it by only an attiny13 and the control scheme mentioned before. Atm im down to 50uA but my method of measuring it is not the best and likely the actual draw will be even smaller. On a single 2Ah lithium battery 40000h or 4.5 years of continuous supply to the geiger tube is now possible. I still aim to get at least a decade just to beat the gammascout and m pretty confident it will work with jus a few tweaks here and there now.

And here is the mess that this ended up being for now but it seems to work beautifully :D
 

Offline jbb

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Re: Low quiescent/ standby boost regulator 3.3-500V
« Reply #42 on: October 30, 2019, 11:34:08 pm »
Hi Amper

Glad to see things are going well. This is obviously a difficult design.
I have a couple of thoughts that might help with things.
  • The internal resistance of your battery will go up a lot as the battery discharges, especially if it’s cold. This can significantly reduce the useful mAh capacity of the cell. The remedy is to try to limit the peak current applied to the cell. You may be able to pursue strategies like trying not to turn on beepers or LEDs or radios at the same time as the boost converter.
  • Large capacitors across the battery can help support peak currents, but their leakage current will drain the battery. There will be a trade off...
  • Have you considered a tapped inductor boost converter? This is a method to get higher voltage gains out of a boost converter without really large duty cycles. Downside is you’ll need a higher voltage diode (due to transformer action in the tapped inductor when the main switch is on).
  • Diodes with good switching characteristics (eg very low capacitance and Qrr) may not have the best leakage current. You could combine a very good switching diode with a very low leakage diode to get the best of both.
  • If you want to sense the main capacitor voltage, you could switch the divider in and out of circuit (eg use a 600V logic level FET). This way you can have reasonable divider resistors without burning too much current
  • Circuit behaviour could be quite different at higher and lower temperatures (diode Qrr and leakage are particularly sensitive to temperature)

And finally, please be careful of the high voltages, especially if you have any large-ish caps about.0
 

Offline schmitt trigger

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Re: Low quiescent/ standby boost regulator 3.3-500V
« Reply #43 on: October 30, 2019, 11:58:13 pm »
on a breadboard with film caps and 1N4007, it works beautifully.


Are you absolutely positive that you are using plain vanilla 1N4007 diodes?
The recovery time for those is orders of magnitude longer than your switching period of 500 Khz.
 

Offline AmperTopic starter

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Re: Low quiescent/ standby boost regulator 3.3-500V
« Reply #44 on: October 31, 2019, 08:44:50 am »
@jbb:

Now lets break this down step by step :D

-Yes, this is a problem, though there is sufficiently low leak super caps and larger lithium batteries will be able to supply quite a bit of current without this problem. The average charge pulse is slightly more than 15mJ (charging 50nF from 0 to 500V and trickling the storage cap). One other reason is that i can actually control the shape of the pulse since i use a micro controller. At the moment i do a few hundred pulses to the boost converter, then wait for 500us, take a voltage reading and then continue charging. This way i can stretch the input current pulse to a certain extent and the 500uS can be swallowed by any ceramic input cap.

-Yes, i considered and even tried this a while ago but i wasnt happy with it. It could work much better than igot it to run but in the end there is a very small selection of smd coupled inductors, its a pain to select one and they are almost never rated 500+V and i dont like to use parts out of spec any more. Due to the winding they usually ave the inductances and maximum currents also dont match this application very nicely. One other downside is, there is geiger tubes and other interesting kit that need up to 1500V, so trying a multiplier topology right fro the beginning gives me an opportunity to just extend and have more voltage. Maybe in the future ill try again.

-The diodes im using right now for all the steps are GSD2004S. They arerated 50ns but at full current which they will never see in my case and a nominal leak of 100nA@ 300V. Since i only do 50kHz and for the "checkvalve" on the storage cap i use two pair of them (150V per diode = turns out to be less than 10nA) there is no real problem.

-The topology i using actually does almost this as i drain the multiplier after each charge. This way im using 66MOhm right now but most likely later it will be only a single 10M precision resistor, mostly for reasons of having less possibility for moisture to change the divider.
Your Idea using a FET is interesting, it may come in handy later... Though in this case it will be a Problem since even using logic level with 2Vth when adding the 1.5-2V divider voltage i wont be able to turn it  on using a 3-3.3V battery. Also while im still using the multiplier approach, i will have to discharge the multiplier anyways since the ceramics have bad leakage characteristics and a dozen film caps are not very surface mount :D
For example the 1.2uF 630V ceramic i first intended turned out to have 80nA leak which corresponds to 6GOhm, the Vishay film cap i turned to now has less than 5nA leak @ 1uF confirmed by the datasheet stating >=100GOhm). Though this also means, any 100GOhm parallel resistance added by parasitics will half my run time between recharges and doing proper continuous measurements of the voltage decay can only be done by such a 100G resistor and an electrometer like my keithley 616.

-Yes, the temperature still worries me. But it will mostly be a Problem of the double pair which can be extended
 without to much additional dropout. The gammascout uses only two BYV26E which are actually really bad, so i hope i will be fine too.

Regarding high voltage, dont worry, i have my fair share of experience with very deadly levels of high voltage up to 20kV 1.5A 3phase, pulsed stuff, tesla, and a whole lot of mains installation.  One small Project i did lately you can see here: https://www.eevblog.com/forum/projects/zvs-driver-lockup/25/ 20uF@500V with a boost converter delivering 100W+, much better chances of killing myself :D

@schmitt trigger
I used 1n4007 only int the first tests because thats what i had stock of. I dont remember the Frequencies i tried but 50-100kHz are no Problem at all. The leakage current is a bigger problem though, these things show their age, 10uA leak compared to just 50-100nA with modern devices. Also the size is not suitable for my application, even the sot23 double diodes take away quite a lot of board space...
 

Offline schmitt trigger

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Re: Low quiescent/ standby boost regulator 3.3-500V
« Reply #45 on: October 31, 2019, 04:25:12 pm »
If you were operating in DCM, it is OK to have slow diodes.
 

Offline AmperTopic starter

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Re: Low quiescent/ standby boost regulator 3.3-500V
« Reply #46 on: October 31, 2019, 05:31:36 pm »
Im actually not even sure im in ccm, never calculated or bothered to measure the timing. Though using the multiplier im not sure if its actually ok since no matter if pushing or pulling on it, there will always be one set of diodes conducting and the other ones isolating. by now everything seems to work nicely, so maybe ill start playing with such detail soon, its just software after all.

One thing im still a bit worried about is that when im between charging cycles and at very high radiation levels i can potentially discharge the capacitor by a continuously conducting tube. This may happen with x-ray as the dose rate is actually large enough so saturate.
 

Offline jbb

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Re: Low quiescent/ standby boost regulator 3.3-500V
« Reply #47 on: November 02, 2019, 01:26:43 am »
It could work much better than igot it to run but in the end there is a very small selection of smd coupled inductors, its a pain to select one and they are almost never rated 500+V and i dont like to use parts out of spec any more. ... Maybe in the future ill try again.

Fair point. In fact, I'm sorry I brought it up again, as you'd already discussed it up-thread.

Quote
-The diodes im using right now for all the steps are GSD2004S. They arerated 50ns but at full current which they will never see in my case and a nominal leak of 100nA@ 300V. Since i only do 50kHz and for the "checkvalve" on the storage cap i use two pair of them (150V per diode = turns out to be less than 10nA) there is no real problem.

The catch with diodes in series is that they may not share off voltage evenly...

Quote
Your Idea using a FET is interesting, it may come in handy later... Though in this case it will be a Problem since even using logic level with 2Vth when adding the 1.5-2V divider voltage i wont be able to turn it  on using a 3-3.3V battery.
Yes, the downside is that you might only be able to count on a 1V full scale output, which might sacrifice some ADC resolution.

Quote
... voltage up to 20kV 1.5A 3phase ...
Yowza.  I was nervous enough about 5kV...

Im actually not even sure im in ccm, never calculated or bothered to measure the timing. Though using the multiplier im not sure if its actually ok since no matter if pushing or pulling on it, there will always be one set of diodes conducting and the other ones isolating. by now everything seems to work nicely, so maybe ill start playing with such detail soon, its just software after all.

DCM / CCM will still be relevant for the main switch and some of the multiplier diodes.  Could you maybe post a schematic?



The nice thing about DCM in a low power converter is that you can load a measured amount of energy into your inductor (proportional to (Ton * Vin)^2), then wait for this energy to flow out into the output stage.

There are two possible control aims on the input side: limiting peak inductor current to avoid saturation, and limiting average input current to avoid crashing the battery supply rail (considering a cold, discharged battery).

To limit peak inductor current in DCM:
Vin = L dI/dt
=> Vin = L Ipk / Ton
=> Ton = L Ipk / Vin
(1)
Where Vin = input voltage, L = inductance, Ipk = peak inductor current

We have now loaded some energy into the inductor, and need to let it out into the assorted capacitor banks:
Vc = L dI/dt
=> Vc = L Ipk / Toff
=> Toff = L Ipk / (Vc-Vin)
=> Toff' = Toff + Tmargin
(2)
Also:
T = Ton + Toff'
Where Vc = bus capacitor voltage scaled by number of charge pump stages, Toff = time for inductor to discharge, Tmargin = extra time to leave some margin, T = total switching period.

We see that Toff' will vary a lot with Vcap.  When Vcap is empty, Toff' will be long.  As Vcap fills up, Toff' gets shorter.  (This is why old school camera flashes make the wheeee sound with increasing pitch.)

We can relate Toff' to Ton:
Ton * Vin / L = Ipk = Toff * (Vc - Vin) / L
=>Ton * Vin = Toff * (Vc-Vin)
=> Toff = Ton * Vin / (Vc-Vin)


Vc and Vin should change fairly slowly, so we can calculate Toff. (Note, initially Vc - Vin = 0 and Toff is allegedly infinite, but resistances and diode drops will take care of this.).  Helpfully, L disappears from this equation (assuming you don't saturate it!).

Now let's look at average input[/t] current Iavg:
Iavg = (1/2 * Ton * Ipk + 1/2 * Toff * Ipk) / T
=> Iavg = 1/2 (Ton + Toff) * Ipk / T


Hmm.  With some rearranging:

Ton + Toff = Ton + Ton * Vin/(Vc-Vin)
=> Ton + Toff = Ton (1 + Vin/(Vc-Vin))
=> Ton + Toff = Ton ((Vc-Vin)/(Vc-Vin) + Vin/(Vc-Vin))
=> Ton + Toff = Ton ((Vc-Vin+Vin)/(Vc-Vin))
=> Ton + Toff = Ton (Vc/(Vc-Vin))

=> Iavg = Ton^2 * Vc/(Vc-Vin) * Vin / (2*L*T)


So, if we just set T = constant (i.e. fixed frequency), we see that Iavg varies a lot (and we can't make it arbitrarily high or the inductor will saturate).  (In this case Tmargin varies a lot.)

But what if we set Tmargin to be small, i.e. Tmargin = 0 (note, in practical designs Tmargin must be > 0 so we don't enter CCM!)?

=>T ~= Ton + Toff
=> Iavg ~= (1/2 * Ton * Ipk + 1/2 * Toff * Ipk) / (Ton + Toff)
=> Iavg ~= 1/2 * (Ton + Toff) * Ipk / (Ton + Toff)
=> Iavg ~= 1/2 * Ipk
=> Iavg ~= Vin * Ton / (2*L)


Solving for Ton, we get:
Ton = 2 * L * Iavg / Vin
And:
Ipk ~= 2 * Iavg
Toff = Ton * Vin / (Vc-Vin)
Tmargin = constant

As a practical matter, you could round Vin into a few bins and use a small lookup table (maybe 8 - 16 entries) to get Ton (scaled directly to clock cycles).  A similar approach would probably work for Toff.

This method gets you reasonable control of the input current (good for battery) without saturating the inductor or requiring a current sensor.  I think it could reasonably be implemented using an 8 bit MCU.  Also note that because no current sensor is required, you probably don't need to update every switching cycle.

If your MCU PWM unit includes a one-shot mode, this would be perfect.  Your software can request the one-shot (duration Ton) from the PWM unit, then configure a timer interrupt to go off at (Ton+Toff) at which point you calculate the next step.  If the software hangs on something, the PWM unit will push out one pulse and then stop, so nothing blows up :-)


I did a lot of unnecessary algebra below...

Substituting for T:
T = Ton + Toff + Tmargin
=> T = Ton (Vc/(Vc-Vin)) + Tmargin
=> Iavg = [Ton^2 * Vc/(Vc-Vin) * Vin] / [2 * L * (Ton * Vc/(Vc-Vin)) + Tmargin)]


If we want to solve for Ton, this is an unmanageable mess.  Let's make some assumptions:
Tmargin is small.  This means variable switching frequency!!
Vc >> Vin
=> Vc / (Vc-Vin) ~= 1


Now we can simplify a bit:
Iavg ~= [Ton^2 * 1 * Vin] / [2 * L * Ton]
=> Iavg ~= Ton * Vin / (2*L)


Solving for Ton:
Ton ~= 2 * L * Iavg / Vin (3)

For a control algorithm, we can use the minimum of equations 1 and 3.  This provides limited peak inductor current when Vc is small (i.e. startup) and approximately limited input power when Vc is large.
Ton ~= min(L * Ipk / Vin, 2 * L * Iavg / Vin)
=> Ton ~= min(Ipk, 2*Iavg) * L / Vin


Dammit, I could have save a lot of work!

 

Offline Miti

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Re: Low quiescent/ standby boost regulator 3.3-500V
« Reply #48 on: November 04, 2019, 12:17:24 am »
I know you're looking for a schematic that works from 3.3V but I have some inside pictures of a Radaler 50. It works from a 9V battery and the supply current is only around 2mA. And it has a transformer and multiplier.
Fear does not stop death, it stops life.
 

Offline profdc9

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Re: Low quiescent/ standby boost regulator 3.3-500V
« Reply #49 on: November 04, 2019, 04:54:04 am »
Perhaps a charging unit from a photostrobe would work.  I've taken them out of disposable cameras and they can get 300 or 400 volts and they run off of a 3 volt disposable battery.  Maybe take a look at these:

https://www.goldmine-elec-products.com/products.asp?dept=1480

https://www.goldmine-elec-products.com/prodinfo.asp?number=G22744
https://www.goldmine-elec-products.com/prodinfo.asp?number=G22986
https://www.goldmine-elec-products.com/prodinfo.asp?number=G23390


 


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