Low quiescent current 3.7V to 170V boost converter - Page 1
Forum section: electronics / projects

author: RawCode
author_rank: Regular Contributor
timestamp: July 26, 2019, 08:55:48 pm
content: Hello there.I would like to make a circuit to power a Nixie tube out of a small lipo battery.Looking around for some schematics I couldn't find anything with a small footprint that doesn't involve a bulky ccfl transformer.I found out that there are some people that are using a MAX771 for this purpose, but since I can't simulate any circuit in Ltspice with this particular IC (due to the lack of a model of it), I can't verify of this IC is suitable for this purpose.Do you have any suggestion?Inviato dal mio LG-H930 utilizzando Tapatalk

author: David Hess
author_rank: Super Contributor
timestamp: July 27, 2019, 03:28:21 pm
content: Any single inductor or flyback transformer boost converter is suitable however an external transistor switch may be necessary.What is your current requirement?

author: RawCode
author_rank: Regular Contributor
timestamp: July 27, 2019, 06:22:25 pm
content: What do you suggest? Which kind of design?There are 4 nixie tubes, but i think that i'll multiplex them, so i would need something between 1-2mA

author: SiliconWizard
author_rank: Super Contributor
timestamp: July 27, 2019, 06:40:43 pm
content: You can find an equivalent of the MAX771 (which is a boost controller requiring an external transistor) with Linear parts (if you're going to want to simulate this with LTSpice).Look up "External Power Switch Boost".Something like the LT1619 should be usable?Beware that the transistor, coil, diode and capacitors at the output will all have to be able to handle the high voltage.

author: David Hess
author_rank: Super Contributor
timestamp: July 28, 2019, 12:15:41 am
content: Quote from: RawCode on July 27, 2019, 06:22:25 pmFigure 17 on page 7 of Linear Technology application note 118 shows an example of the boost converter which would be suitable.An alternative which does not require the external high voltage transistor is to place a charge pump voltage multiplier on the output as shown here:https://www.analog.com/en/analog-dialogue/articles/high-voltage-boost-and-inverting-converters-for-communications.htmlBoth could be combined if you wanted a really high output voltage.

author: RawCode
author_rank: Regular Contributor
timestamp: July 29, 2019, 12:44:11 pm
content: I've done some simulations.I started first with the LT1619 as SiliconWizard kindly suggested me, and it worked until i placed the mosfet i planned to use in the circuit: it didn't get to 170V, don't know why. maybe this transistor is not suitable for such a low voltage in the gate.Then i tried to follow the suggestion given kindly by David Hess, getting rid off the high voltage transistor. For this purpose i used the LT8364 suggested in the link provided by David. It works, it can output 170V in a simplified load of 170kOhm(170V/1mA). It get to 170V in about 20ms.What do you think about it? Is it a reasonable time?

author: voltsandjolts
author_rank: Unknown
timestamp: July 29, 2019, 01:39:03 pm
content: The link says LT8365 but I can't find any info on it.Must mean LT8364?

author: RawCode
author_rank: Regular Contributor
timestamp: July 29, 2019, 01:41:52 pm
content: Quote from: voltsandjolts on July 29, 2019, 01:39:03 pmYes, i think so. It's probably a typo

author: SiliconWizard
author_rank: Super Contributor
timestamp: July 29, 2019, 01:41:59 pm
content: Quote from: RawCode on July 29, 2019, 12:44:11 pmNot sure. You could post the LTSpice sim so I can take a look. But yes the VGS threshold is possibly the culprit. Many of the high-voltage MOSFETs that have a model in LTSpice have relatively high thresholds so I wouldn't be surprised. (And many MOSFETs in general...)Also do not connect the DRV pin (which powers the MOSFET driver) to the output voltage: it would destroy the LT1619 (and most likely the transistor as well, whichever dies first). Connect it to Vin instead. But then yes you need an appropriate MOSFET with a relatively low threshold.There was a similar thread but for powering Geiger tubes. The BSP125 came up:https://www.infineon.com/cms/en/product/power/mosfet/12v-800v-small-signal-mosfet/bsp125/It looks like a nice fit for this kind of high voltage but low output current applications. It has a relatively high RDSon so expect the starting time to be longer (well OTOH without a charge pump, it will be faster, so...), but it can be driven @3.3V without a problem. There is a Spice model, integrating it in LTSpice shouldn't be difficult.Quote from: RawCode on July 29, 2019, 12:44:11 pmWell, given there is an additional charge pump, it definitely looks reasonable. I don't think you can get much better than this with this topology.Note the average power consumption of this circuit with a 1mA load once it has reached 170V is still pretty high. Maybe higher than you expected. (I seem to be getting something ~85mA from simulation. Not too bad: it's approx. 60% efficiency, which is decent with this step-up + charge pump topology.)Conversely, during start-up, if the input voltage source can't deliver enough current (I don't know if it's going to be from a battery or whatever), the starting time will be much longer than 20ms.Edit: I re-read your initial post and it seems you're going to use a "small" lipo battery, so what I said above will probably apply. Also consider the power consumption. What is going to be your battery's capacity? Just consider that to estimate how long the battery's going to last.

author: RawCode
author_rank: Regular Contributor
timestamp: July 31, 2019, 10:28:38 am
content: Quote from: SiliconWizard on July 29, 2019, 01:41:59 pmThanks you for the help, very appreciatedYes I plugged the DRV pin to Vin, since i noticed that it can handle something like 20V at maximum.I used a BSP89(which is already inside LTSPICE), which is similar to that one you suggested, and i can get only 100V out of this circuit. Maybe 3.7V as Vin is too small :/If i try to use a STN3N45K3 by ST and the situation is even worse.I'll attach the simulation file.Quote from: SiliconWizard on July 29, 2019, 01:41:59 pmDo you think that with a standard boost converter i can get a more efficient "conversion"?How did you get the input current, though? I can only get a poorly readable graph for the current.Anyway, I'm using a 380mAh lipo battery. I know, it's not very huge, but i plan to power on the Nixies only when needed.How can i calculate the battery life out of this information?

author: SiliconWizard
author_rank: Super Contributor
timestamp: July 31, 2019, 03:06:24 pm
content: Quote from: RawCode on July 31, 2019, 10:28:38 am(The STN3N45K3 has a much higher threshold so yes it won't work properly here @3.3V. All typical figures are given @VGS = 10V.)Well, to begin with, the values of the feedback resistors you selected would not yield an output of 170V but ~249V. [ 1.24*(1+2e6/10e3) ]I made the following changes to your schematic:* More typical values for the compensation network,* Appropriate values for the feedback divider (~168V),* More realistic value for the series resistor of the input power supply, picked an inductor from a real reference, picked the output cap from a real reference too and changed to diode to a Schottky one (although this last point shouldn't make much of a difference here).It does settle to ~168V in ~25ms.I was unimpressed with the average current draw once settled though (~204mA), so an efficiency of ~25%. Tried to figure out where the excessive losses would come from. The suspect was the transistor with its relatively high RDSon. Adding the following trace helped showing that:(V(vd)-V(vs))*Id(Q1)=> an average dissipation of ~404mW in the transistor (for a total power draw of ~673mW!)You can then further try playing with the current limit (Rsense), try other transistors, consider generating an elevated voltage for the MOSFET driver...Quote from: RawCode on July 31, 2019, 10:28:38 amTheoretically yes, as this simple charge pump topology is not very efficient, and you're cumulating (multiplying) the efficiencies of the two cascaded stages (boost converter + charge pump).For significantly higher output currents, you would probably not even have a choice.But in practice, at these high voltage, low output current levels and high conversion ratio, it's hard to find something appropriate that's reasonably efficient.The typical compromise is between the switching losses and the gate charge losses. Higher gate voltage (RDSon will be lower) and faster rise and fall times (the transistor will spend less time in the linear region) will limit power dissipation (drain-source), but increase the losses to drive the gate... At this low output current, this compromise gets critical.So the solution with a LT8364 + a charge pump looks like a simple an not too bad solution here.Some people could suggest a flyback topology, but I think it will be pretty hard to do better than 60%.Quote from: RawCode on July 31, 2019, 10:28:38 amLTSpice can compute average and RMS values for any trace in the waveform viewer.For average current draw, I take the average current from a current trace.To do that, hold the CTRL key and left click on the trace's title (not the trace itself). A message box pops up wih the average and RMS values (or integral depending on the dimension/unit of the trace) of the portion of trace that is currently displayed (so zoom in/out to have it computed on the portion you want).Quote from: RawCode on July 31, 2019, 10:28:38 amWell, you simply have to compute the average current draw using a weighted sum based on the duty cycle(s) of your system and the average current draw of each scenario (display on, display off, MCU running or in sleep mode, etc)Then divide the battery's capacity by the average current draw and there you get your approximate battery life.

author: RawCode
author_rank: Regular Contributor
timestamp: July 31, 2019, 08:25:15 pm
content: Quote from: SiliconWizard on July 31, 2019, 03:06:24 pmYes, i was trying to get an higher voltage without any results. I probably didn't save the file with the right feedback resistors :/Quote from: SiliconWizard on July 31, 2019, 03:06:24 pmThank you very much, i'll dig a bit more on this circuit.Quote from: SiliconWizard on July 31, 2019, 03:06:24 pmThank you for all the effort and the time you spent for me! I really really appreciate it!