How to make a controllable ~55V voltage source - Page 1
Forum section: electronics / projects

author: Fulcrum
author_rank: Regular Contributor
timestamp: July 08, 2016, 11:41:36 pm
content: Hi everyone.I have a bit of a tricky problem (or maybe not, could be a simple solution to it!). I have a circuit with a component that requires a relatively precise voltage of about 55 volts. It needs to be digitally controllable down to +-20mV or thereabouts, with a range of 55V +-3V. The current requirements are low, less than 5mA. I have two options. Either I bring 55V onto the board from a power supply and do the fine control on-board, or I generate the 55V on-board by boosting it with the +6V power rail. However I don't like this idea too much as it introduces noisy booster ICs onto the board, which is in essence a 80MHz bandpass 3000 V/V amplifier. I worry about the emitted noise being picked up.How would you solve this?

author: MosherIV
author_rank: Super Contributor
timestamp: July 09, 2016, 09:45:52 am
content: HiWhat is the component? Or, how clean does the supply have to be to the component?You may find it difficult to find psu that output 55VStart looking at boost converters/regulators.There are many that can easily generate 55V but boost regualtors are noisey (hence my question about how clean does the supply have to be)FYI 55V is considered to be starting to be dangerous (anything above 30V). You will get very large sparks if you accidentally short out that kind of voltage, so be careful.

author: Fulcrum
author_rank: Regular Contributor
timestamp: July 09, 2016, 10:43:41 am
content: Quote from: MosherIV on July 09, 2016, 09:45:52 amThat's the problem. I don't think I can use a booster since they are inherently noisy. I do not have a specific figure on the noise levels I require, but the lower the better since the noise will directly pass into a 3000 V/V amplifier. Let's say no more than 2µVRMS between 30MHz and 100MHz. The noise levels below this is uninteresting as it falls outside the bandpass of the amplifier.

author: TheAmmoniacal
author_rank: Unknown
timestamp: July 09, 2016, 10:49:26 am
content: Can I smell audiophoolery?

author: Fulcrum
author_rank: Regular Contributor
timestamp: July 09, 2016, 11:09:16 am
content: Quote from: TheAmmoniacal on July 09, 2016, 10:49:26 amMore like photon counter foolery! I'm using MPPC silicon photomultipliers to detect scintillation light from muons passing through a plastic scintillator. The MPPCs need very stable bias voltages of around 55 volts, with a 50mV change per degree celsius.

author: TheAmmoniacal
author_rank: Unknown
timestamp: July 09, 2016, 11:17:28 am
content: Quote from: Fulcrum on July 09, 2016, 11:09:16 amThat's more like it.Wouldn't even know where to start building a power supply like that, maybe looks into high voltage linear regulators?How big of a capacitance would you need to do this linearly? (Doubt you'll be able to get variacs/autotransformers with the necessary adjustment resolution though)

author: Fulcrum
author_rank: Regular Contributor
timestamp: July 09, 2016, 11:21:27 am
content: Quote from: TheAmmoniacal on July 09, 2016, 11:17:28 amYeah, I just noticed that LDOs with output voltages in excess of 60V exists, so they'll do the trick. I can also do some heavy filtering on the output to get down to my low noise requirements.http://www.farnell.com/datasheets/2003348.pdf?_ga=1.116720326.1180173967.1467721356

author: Fungus
author_rank: Super Contributor
timestamp: July 09, 2016, 11:33:54 am
content: Quote from: Fulcrum on July 09, 2016, 11:09:16 amThat's a really crazy power requirement.Surely the people who make the sensors can provide a suitable power supply, they're not going to sell many sensors otherwise.

author: TheAmmoniacal
author_rank: Unknown
timestamp: July 09, 2016, 11:41:17 am
content: Quote from: Fungus on July 09, 2016, 11:33:54 amIndeed. Unless the power supply is sold separately at "we're the only ones who sell it" prices.

author: Fulcrum
author_rank: Regular Contributor
timestamp: July 09, 2016, 11:44:33 am
content: Quote from: TheAmmoniacal on July 09, 2016, 11:41:17 amYeah, they also sell power supplies. But if I'd have to guess, I'd say they cost no less than 100$ a piece. The detectors themselves were 250$ each.http://www.hamamatsu.com/jp/en/C11204-01.html

author: Gribo
author_rank: Frequent Contributor
timestamp: July 09, 2016, 11:52:41 am
content: If it is only 100$ for such a unit, you better buy those than build your own. You will not be able to reach that level of performance without some major time investment.

author: Fulcrum
author_rank: Regular Contributor
timestamp: July 09, 2016, 11:57:10 am
content: Quote from: Gribo on July 09, 2016, 11:52:41 amI have a budget to keep, and it's already been decimated by the purchase of 18 detectors. Can't afford 18 power supplies in addition. I don't have very stringent voltage level requirements, so I can make do with something "home made".I'm gonna implement it with the high voltage LDO I posted earlier (http://no.farnell.com/texas-instruments/tps7a4001dgnt/ldo-adj-50ma-8msop/dp/1898986), together with a digital potentiometer in the feedback loop. Should give me good enough resolution of at least a few LSB per degree Celsius over the operational range. The noise I can get down with some pi filters on the input and output of the LDO.

author: Someone
author_rank: Super Contributor
timestamp: July 09, 2016, 12:10:29 pm
content: Quote from: Fungus on July 09, 2016, 11:33:54 amOr you stabilise the temperature of the detector...

author: Fulcrum
author_rank: Regular Contributor
timestamp: July 09, 2016, 12:17:24 pm
content: Quote from: Someone on July 09, 2016, 12:10:29 pmYep, that's definitely a possibility. But unfortunately not possible due to the physical layout of the board.

author: chris_leyson
author_rank: Super Contributor
timestamp: July 09, 2016, 12:18:13 pm
content: Used an LM723 for a very low noise power supply a long time ago, tried a few "ultra low noise" regulators from AD and LT but the 723 was the best performer. Can't remember the circuit off hand but it was in "Communications Receivers" 1st edition (I think) by Ulrich Rohde.

author: Ian.M
author_rank: Super Contributor
timestamp: July 09, 2016, 12:24:35 pm
content: *IF* you were going to roll your own, its fairly obvious you need to start from the cleanest possible supply you can get of about 60V. You also need a really clean and stable 50V rail that can sink current as the large divider ratio required for a ground referenced control system for the regulator will make it hard to get the noise and accuracy down where you want them.How much current do the MPPC silicon photomultipliers need and how low does the bias supply source impedance need to be?I'm thinking about a 16 bit DAC with optocoupled control signals, with its ground being the 50V rail driving a power OPAMP operating between the 50V and 60V rails. You'd probably stick a 16 bit ADC up there as well to let you close the loop digitally.To prevent it destroying itself by transients during startup and shutdown, you'd want a really beefy 12V Zener (or a Zener boosted by a common emitter power transistor) between the 50V and 60V rails, + Schottky clamps from the output to each rail.Getting low enough noise, high stability 50V and 60V rails may be interesting. You could go about it the old-skool way with a linear 50V supply and a linear 10V supply stacked on top of it, with an ovenised precision reference for the regulators, but I suspect it will be more convenient to build a fairly low frequency flyback converter with soft recovery rectifiers to minimise the noise in the band of interest then follow that with fixed linear regulators to get the two rails.There's probably a far more elegant way of doing this, but it sounds like the application is either a one-off or an extremely small niche market, and development time costs money . . .However its going to take least a day to build, another day to code the DAC and ADC interface, add temperature compensation and test it, and the parts cost will probably be in the $50 - $150 range. By the time you've figured in your wages and the overheads of employing you, $100 for a pre-built one seems very reasonable. If you needed 50 to 100 of them it might be a different matter, but it would be a high risk decision.************************************************************I've just read your followup. I don't think you'll get there with that regulator and a 10 bit digipot, as keeping the voltage across the digipot within its permitted limits will be difficult. Digipots tend to have either logic level supplies or at most +/-15V OPAMP compatible supplies and all the pot terminals *MUST* remain within the supply rails.The C11204-01 MPPC bias PSU you linked to is only rated for 2mA out, so a DAC => OPAMP solution makes a lot more sense than any sort of LDO regulator.Do you *really* need 18 supplies, or can you get by with one supply with 18 independently controlled outputs? If so, you'd only have to add more DACS, ADCs, OPAMPs and clamping. They could all share the same supply rails, precision reference and control system so the additional cost per output could probably be got down into the $10-$20 range.

author: Marco
author_rank: Super Contributor
timestamp: July 09, 2016, 12:40:01 pm
content: Quote from: Fulcrum on July 09, 2016, 10:43:41 amDoes a slew limited double digit kHz switching power supply with Schottky rectifiers really put out much at all in this bandwidth range?

author: Fulcrum
author_rank: Regular Contributor
timestamp: July 09, 2016, 12:49:38 pm
content: Quote from: Ian.M on July 09, 2016, 12:24:35 pmThanks a lot for the thorough reply! I will require 18 separate supplies, as I have 18 separate detector boards. However, not 18 separate +60 and +50 supplies. To prevent having to generate the 50V and 60V rails on-board with a booster (and therefore noise) I can bring it in off-board from a power supply shared by all the boards. I would prefer to not use this solution though, as it means I will need another power input in addition to the ones I already have: GND, -6, +6, +19, and +60.You say I cannot use a LDO with a digpot because the voltage over the digpot will be too high. I already thought of this. What if I have a regular constant resistor from OUT to FB, and then a digpot from FB to GND? The FB voltage won't be any higher than about 2V anyways so it will be well within the digpot limits.Quote from: Marco on July 09, 2016, 12:40:01 pmIf they don't then that'd be great. It would mean I could generate my higher voltage rails (+19, +60, and possibly +50) on-board, lessening the need for pulling cables everywhere from off-board power supplies.

author: Ian.M
author_rank: Super Contributor
timestamp: July 09, 2016, 01:12:22 pm
content: That TPS7A4001 regulator doesn't use the classic floating HV regulator topology, but has a ground referenced feedback pin. My objections to using a digipot on working voltage range grounds are therefore invalid.However due to the somewhat uncertain wiper resistance of a typical digipot, you'll need to use it as a potentiometer, not a variable resistor, you'll need a 10 bit pot, with series resistors at either track end to set the output range to 55V+/-3V. I'd also advise clamping the top end of the pot to its Vdd supply.The TPS7A4001 only claims +/-2.5% accuracy (1% at fixed nominal temperature, with constant supply voltage and load) so closing the loop digitally with a high resolution ADC is going to be essential.The benefits of onboard PSUs for the extra rails to minimise the wiring are probably marginal, unless it must operate in a high EMI environment, or the supply cable runs are particularly long. Multicore screened cable capable of handling a few hundred mA per core isn't that expensive per meter compared to a custom built switching supply, probably with custom magnetics for the multiple rails required + a screening can and filtering.Edit: realised the TPS7A4001 isn't accurate enough to use open loop.

author: Marco
author_rank: Super Contributor
timestamp: July 09, 2016, 01:19:55 pm
content: Unless you can find someone with experience who knows for sure it can't work I'd breadboard a LT1738/Schottky boost converter on the same board with your 3000V/V amplifier and see what it does on your spectrum analyzer.The ripple is out of the frequency range of interest, it's soft switched, the diode has no snap off to speak off. Can't imagine it doing much.

author: LazyJack
author_rank: Frequent Contributor
timestamp: July 09, 2016, 01:47:32 pm
content: Instead of a switching converter, how about using sine wave? Just like Tek & others did the stabilized HV in their scopes. Low noise, and good stability. Worse efficiency but at the power level you require, I don't think that is a big problem.

author: splin
author_rank: Frequent Contributor
timestamp: July 09, 2016, 03:45:20 pm
content: Quote from: Fulcrum on July 09, 2016, 10:43:41 am2uVrms over 70MHz is 239pV/sqrt (Hz) or the equivalent thermal noise of a 3.5ohm resistor. Sounds like quite a challenge to me. Or did you mean 2uVrms/sqrt (Hz)? On reflection perhaps not too difficult - I see that the impedance of typical MLCCs, below 1uF, are well below 1 ohm up to 100MHz. Parasitic wiring inductance might be a problem though.

author: Fulcrum
author_rank: Regular Contributor
timestamp: July 09, 2016, 05:02:42 pm
content: Quote from: Ian.M on July 09, 2016, 01:12:22 pmAh, good idea about digitally closing the loop, and thanks for the tips. I wouldn't have thought of that. The wires will not have to run very long, about 5 meters. It's just tricky with my prototyping experimental setup to have many wires run to and from the board, since it is enclosed in a light-tight box.Quote from: Marco on July 09, 2016, 01:19:55 pmNo can do, I'm afraid. Running out of time, and I won't be around my workplace for the next few months.Quote from: LazyJack on July 09, 2016, 01:47:32 pmNot sure what you are talking about. Can you elaborate?Quote from: splin on July 09, 2016, 03:45:20 pmHow can the impedance of capacitors lead to noise? And likewise with the parasitic inductance?

author: splin
author_rank: Frequent Contributor
timestamp: July 10, 2016, 01:36:43 am
content: Quote from: Fulcrum on July 09, 2016, 05:02:42 pmI meant that the capacitors you will need for filtering the noise down to 2uVrms will have to have very low parasitic series inductance, in addition to any stray wiring inductance, to be able to filter effectively at 100MHz. It may not turn out to be a big problem - I don't have any direct experience at these frequencies.This Linear Technology application note, AN118FB, may be of interest as it covers high voltage supplies for photodiodes, photomultipliers etc. It doesn't provide a direct solution, offering 200V+ supplies with 100uVrms noise in 100MHz bandwidth but it does cover sinewave as well as switching convertors:http://www.linear.com/docs/26421

author: LazyJack
author_rank: Frequent Contributor
timestamp: July 10, 2016, 06:51:10 am
content: Quote from: Fulcrum on July 09, 2016, 05:02:42 pmTo create the several kV required for the CRT with high stability and low noise generated, most scopes use a sine oscillator. It is then fed into a transformer to generate whatever voltages are required.It is leveled in a closed loop based on the output. The frequency is several tens or hundreds of kHz and since it is sine it is low on harmonics so won't generate noise for other parts of the scope to pick up.

author: Fulcrum
author_rank: Regular Contributor
timestamp: July 10, 2016, 12:22:33 pm
content: Quote from: splin on July 10, 2016, 01:36:43 amThanks! Interesting read.Quote from: LazyJack on July 10, 2016, 06:51:10 amAh, of course. Well, I'm going to go with the 100V LDO I have linked earlier, with a digitally closed loop and digpot. It has only 89uVrms out of the LDO itself at above 10MHz, so it's already pretty clean.Thanks to everyone who helped!

author: bktemp
author_rank: Super Contributor
timestamp: July 10, 2016, 12:36:41 pm
content: You don't need a digipot for adjusting the voltage: Instead of adjusting the voltage divider, you can also use a fixed voltage divider and apply an additional current. Depending on the sign and magnitude this will increase or decrease the output voltage. Because the voltage at the feedback point is constant, generating a current can be as simple as applying a voltage over a resistor into this point.You probably have a voltage reference already in your circuit if you want to regulate the output digitally, so you can use the same voltage for the DAC for generating the fine control voltage. DACs are probably more accurate and stable than a digipot. And you can easily get 10, 12 or 16bit resolution instead of the common 8bit for digipots for much finer steps or a larger adjustment range. You can even use PWM for generating the control voltage.

author: voltsandjolts
author_rank: Unknown
timestamp: July 10, 2016, 01:28:13 pm
content: Interesting topic. In science arena the Keithley 2400 seems popular for supplying bias voltage but of course they cost a bit more than the Ham C11204!Some related info:http://fusion.ncbj.gov.pl/wp-content/uploads/2015/12/poster_MPPC_SYMPOZJUM_2015_NCBJ_last.pdfArduino and RaspPi being used in MPPC space electronics dev, hackers rule:http://astroteilchenschule.nat.fau.de/schule2015/teilnehmer_vortraege/Huber.pdf

author: Fulcrum
author_rank: Regular Contributor
timestamp: July 10, 2016, 03:32:47 pm
content: Quote from: bktemp on July 10, 2016, 12:36:41 pmOh, that's much easier... Throws a several hours of work out the window on my part though. How would I go about calculating the additional voltage and resistor required for a certain voltage change from the ldo output?EDIT:Should have googled before asking, duh. Found this:http://www.microchip.com/forums/m688260.aspx

author: David Hess
author_rank: Super Contributor
timestamp: July 10, 2016, 10:10:42 pm
content: A 6 volt range divided by 20 millivolts is 300 counts so DAC resolution will need to be 9 bits or higher. There is a complication though since if you want absolute accuracy without trimming, the reference and feedback need to be good to 3000 counts or 12 bit or better but that is not a difficult problem.There are high voltage operational amplifiers which could be used to produce the output or directly drive an emitter/source follower but I would use a normal operational amplifier and level shift its output to +55 volts using a zener diode to drive a high voltage emitter/source follower; the zener diode is bypassed to preserve high frequency and low noise performance. Include another transistor to rob base/gate drive from the high voltage pass transistor implementing a fast current limit.A good public example of this is how the Tektronix 7854/7904A/7934/7104 oscilloscopes regulate their +/- 50 volt supplies. They even provide the output transistor bias with the same transistor which implements foldback current limiting I doubt you need that with a 5 milliamp maximum output.A more modern alternative is to replace the zener level shift and discrete output follower with an integrated floating regulator which has built in fault protection. The operational amplifier then drives the bottom of the floating regulator's resistor divider *and* takes feedback from its output so the floating regulator's accuracy is irrelevant. This could even be done using a 317 regulator but its limited input to output voltage rating would present a problem during startup and in the event of an output short. An LT3010 would be ideal except for the 8 pin MSOP package and possibly being too slow.Slightly more complicated would be to use the integrated floating regulator as described above with a simple discrete input cascode to handle the high voltage during startup and faults but this allows using any integrated low voltage floating regulator.

author: Fulcrum
author_rank: Regular Contributor
timestamp: July 10, 2016, 10:29:48 pm
content: Quote from: David Hess on July 10, 2016, 10:10:42 pmYou're a bit late unfortunately... Already put a lot of work into the high voltage LDO + digitally closing the loop. I'm interested in learning more about this subject though. Could you show me some circuits of what you are talking about? Especially the whole level-shifting by zener diode thing.

author: Fulcrum
author_rank: Regular Contributor
timestamp: July 10, 2016, 11:47:07 pm
content: Actually, in order to measure the voltage out I would need to muck about with ADC ICs... It would be much cleaner if I could just use the built-in 10bit ADCs on my microcontroller. Any way I could somehow level shift the 55V output and so only measure a certain range around this level? No need to waste resolution below 45V or above 65V. Perhaps use a precise voltage divider to get the 55V down to a manageable level, and then use a reference close to the divided voltage plus an in-amp to amplify the difference... Any good ideas?

author: David Hess
author_rank: Super Contributor
timestamp: July 11, 2016, 05:24:38 am
content: Quote from: Fulcrum on July 10, 2016, 10:29:48 pmThe service manuals for the Tektronix 7854, 7904A, 7934, and 7104 oscilloscopes are available online for free and include one of the better examples of how it is done in their +/- 50 volt regulators.