Zener accuracy over temperature range - Page 1
Forum section: electronics / projects

author: Simon
author_rank: Unknown
timestamp: June 26, 2014, 02:47:48 pm
content: I'm looking to use a zener as a voltage reference as I need to work down to -46C and most voltage res only go down to less than -40 (farnell's parametric search is crap and does not include temperature ranges) so I'm thinking a zener as it's just a ref for a SMPS, but looking at zener datasheets there does not seem to be any accuracy versus temperature information, does this mean they will stay accurate over the range or is it just misleading ?

author: tautech
author_rank: Super Contributor
timestamp: June 26, 2014, 03:02:23 pm
content: You will need a Temp compensated reference zener.Check this thread for discussion and sample datasheets:https://www.eevblog.com/forum/buysellwanted/wtb-9-1-v-temp-comp-reference-zener/msg459407/#msg459407

author: David Hess
author_rank: Super Contributor
timestamp: June 26, 2014, 05:00:24 pm
content: You are looking at the wrong zener diodes or data sheets. Not all zener diodes are characterized for temperature coefficient.

author: Andreas
author_rank: Super Contributor
timestamp: June 27, 2014, 05:11:31 am
content: Hello,for integrated cirquits you would have to look for hermetically sealed devices like LM136 LM185 (TI) or AD586TQ (AD).If looking for Zeners: the T.C. is voltage dependant. Lowest T.C. (around +/-2mV/K) is around 5.6V for uncompensated Zeners and 6.2V (down to +/-10ppm/K) for compensated Zeners like 1N829A. But you will have to use a constant current source to reach tight zener voltage specifications.Some manufacturers give T.C. specifications (at a certain zener current).http://www.onsemi.com/pub/Collateral/BZX84C2V4LT1-D.PDFWith best regardsAndreas

author: poorchava
author_rank: Super Contributor
timestamp: June 27, 2014, 05:13:05 am
content: Wouldn't a garden variety TL431 be more convenient? It's not expensive at all.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 27, 2014, 05:14:56 am
content: Zeners don't stop zenering, but the voltage varies... you didn't mention how tight your requirement is.Tim

author: Andreas
author_rank: Super Contributor
timestamp: June 27, 2014, 05:31:48 am
content: Quote from: poorchava on June 27, 2014, 05:13:05 amDo you know one that is specced to operate (not stored) below -40 degrees?With best regardsAndreas

author: Simon
author_rank: Unknown
timestamp: June 27, 2014, 06:17:14 am
content: As I can't find much in the way of voltage regs that do under -40 I'm thinking of making my own as i only need a few tens of mA but as the circuit is potted I'm nervous about linear regs as finding something adequate for non potted was hard as I'm dropping from as much as 30V down to 5V. I'm running a MCU so voltage is not too critical but I don't want to drop under 4.5V

author: theatrus
author_rank: Frequent Contributor
timestamp: June 27, 2014, 06:37:07 am
content: I'm curious where you are finding pretty much any parts specified at below -40.

author: Simon
author_rank: Unknown
timestamp: June 27, 2014, 06:46:35 am
content: Well actually most SMD passives and descrete semiconductors will do down to -55 and even logic gates and basic chips like comparators will do -55

author: Andreas
author_rank: Super Contributor
timestamp: June 27, 2014, 06:54:12 am
content: Quote from: Simon on June 27, 2014, 06:17:14 amWhat about good old LM723 in cerdip or TO-100 package

author: Simon
author_rank: Unknown
timestamp: June 27, 2014, 07:09:03 am
content: Might be worth looking at

author: poorchava
author_rank: Super Contributor
timestamp: June 27, 2014, 12:43:50 pm
content: Look at automotive grade parts.

author: Simon
author_rank: Unknown
timestamp: June 27, 2014, 12:44:51 pm
content: Well so long as they are under -46C

author: Dave Turner
author_rank: Frequent Contributor
timestamp: June 27, 2014, 05:32:43 pm
content: If you're not too worried about power consumption (clearly there is some heat loss already dropping from 30V) how about using some power for some temperature control.

author: SeanB
author_rank: Super Contributor
timestamp: June 27, 2014, 05:42:09 pm
content: I used a 723 to make a voltage reference, and the only rail was a nice 60VAC rail, so I used that. A first stage of a 33V zener and then the 723 in a TO100 can with a small push on heatsink and it works still. Was plenty stable enough for the intended use, as a meter offset to give an extended range.

author: Fsck
author_rank: Super Contributor
timestamp: June 27, 2014, 05:43:12 pm
content: Quote from: Simon on June 27, 2014, 12:44:51 pmauto should be -55 to 125C or somewhere thereabout, or industrial/extended operating range/whateverothernamemeaningthesamething.

author: Simon
author_rank: Unknown
timestamp: June 27, 2014, 07:00:41 pm
content: Well Dave with the same surname as methe problem is that this is potted so I'm also concerned about what happens at 50C ambient and it is hard to find a regulator that will cope with the power dissipation without heatsinking even in free air. The product needs to withstand very severe shocks so large items are out and this is why it is potted. I have considered heating for low temperatures as I can't get an MCU that will do -46 but stops at -40 but as it's potted so hard to heat consistently I'd rather heat just the MCU and make sure the rest will work down to -46.

author: eneuro
author_rank: Super Contributor
timestamp: June 27, 2014, 07:50:36 pm
content: Quote from: Andreas on June 27, 2014, 05:31:48 amI used to use in many projects TL431I (Industrial rated -40*C..+85*C operating temperature) but when you look into it's datasheet eg. this onehttp://pdf.datasheetcatalog.com/datasheet/philips/TL431I.pdfthere are many figures with as low as -50*C, so who knows maybe it could survive in such extreme conditions?Maybe it will not keep perfect its parameters, but shouldn't be better than any not trimmed Zener diode without vref temp compensated?

author: Simon
author_rank: Unknown
timestamp: June 27, 2014, 08:10:57 pm
content: Unfortunately i can't rely on random batches being capable of exceeding the manufacturers specs. For the application this is going into I need it to be specified for what I want. the only concession i want to have to make is for the micro controllers because i simply can't get anything below -40 unless it's expensive and complicated and very hard to get hold of so easier to heat +10C to keep them cosy if needed.

author: johansen
author_rank: Super Contributor
timestamp: June 27, 2014, 08:13:51 pm
content: is an REF5050 (or one of the other voltages) too expensive?the datasheet shows a fairly predictable -3ppm/c drop from 0 to -50C

author: lewis
author_rank: Frequent Contributor
timestamp: June 27, 2014, 08:15:11 pm
content: Simon, you always seem to get these weird design jobs with very tight constraints...The best way is definitely to heat the product if you need to go to -46, you'll really struggle to get components rated for that temperature (you'll need -50 to build in margin) in realistic quantity, at realistic prices and with realistic lead-times.(Not only that, but you'll really need to take into account the coefficient of thermal expansion of the potting compound if you need the product to work over a very wide temperature range. I make potted stuff that is spec'd from -40 to 70C, and it ain't easy to get right.)In fact, it's *easier* to heat because it's potted, you can maintain the resin relatively isothermal because the resin is an excellent thermal conductor, rather than relying on heating air in a normal enclosure. The simplest way to do it would be to use some low value resistors dotted around the perimeter of the PCB. When the temperature gets below, say, -10, use a spare pin on your MCU to PWM the resistors (via a mosfet) to begin heating the resin. As the temperature drops, increase the duty cycle to maintain a relatively constant temperature within 10-20 degrees or so. Use any old temperature sensor you like, it doesn't need to be particularly accurate. MCP9700 series springs to mind.If you have a voltage regulator in there dissipating enough power that you need to worry about it at 50C ambient, then you probably have enough heating as it is anyway.

author: Simon
author_rank: Unknown
timestamp: June 27, 2014, 09:13:05 pm
content: Yes i always get asked the impossible, I already suggested heating the whole enclosure but my boss was not happy with the idea.The thing with heating is that I don't want to start the MCU's at -46 as then they may behave strangely and I still don't get heated, so I'd use a thermistor and comparator to kick in various resistors around the board. Does potting compound transfer heat that well ? I'm using scotchcast N8.linear voltage regs are a pain, you will actually struggle to find any that do more than 0.5W even in bigger packages, they seem to end up needing heatsinks, so I'll either use multiple regulators as I can divide the circuit up or a switchmode even if i make it myself

author: Andreas
author_rank: Super Contributor
timestamp: June 27, 2014, 09:50:52 pm
content: Hello,I hope that you don´t need any crystal for the oscillator.They also have sometimes unusual behaviour below -20 deg C.Especially the higher frequencies.With best regardsAndreas

author: tautech
author_rank: Super Contributor
timestamp: June 27, 2014, 09:53:45 pm
content: QuoteThe datasheet will have details of thermal permeability.If it doesn't, use something else.

author: Dave Turner
author_rank: Frequent Contributor
timestamp: June 27, 2014, 11:40:31 pm
content: I'd suggest a peltier but they seem to be very poor with shock resistance. I sit to be corrected though.The issue stands - if you can't obtain economic parts for the working environment then adjust the micro-environment to to available parts.

author: Simon
author_rank: Unknown
timestamp: June 28, 2014, 07:25:53 am
content: No no crystals, they won't take the shocks. I don't know what the internal clock does at -46 but at +50 it speeds up by about 2% which is not too much bother for this application

author: qno
author_rank: Frequent Contributor
timestamp: June 28, 2014, 08:01:27 am
content: Do you know what the problem is with low temperature devices?Usually it is crimping and expansion of the chip enclosure that put stresses on the silicone and bond wire.This can make them break or crack.Military or automotive devices have the widest temperature range.Military devices have been cycled at least once over the specified temperature range.This makes them so expensive.A semiconductor manufacturer will be able to help you.Check out the general specifications of the IC's.It is usually not in the data sheet.You can also go to a test lab. I have done Highly Accelerated Life Test on avionicsthat cycle temperature from -100 to +200 °C in Mannheim Germany.Normal components did survive but it is not cheap.

author: branadic
author_rank: Super Contributor
timestamp: June 28, 2014, 11:19:07 am
content: QuoteUnder a certain temperature the semiconductor becomes an insulator and the device skips its function.

author: Simon
author_rank: Unknown
timestamp: June 29, 2014, 06:27:02 am
content: Interesting and one of the reasons I'm reluctant to just say "I'll be ok"

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 29, 2014, 07:00:38 am
content: But that varies. Majority carrier devices (FETs, schottky diodes) work at most any temperature. FETs (GaAs or GaN I guess?) are routinely used in LHe for physics front ends. BJTs and junction diodes "freeze out" because they depend on thermally generated minority carriers. And for that matter, I'm not sure of schottky diodes; they are majority carrier devices, but obey the same rules as regular diodes.Zener or avalanche should work down at absolute zero, give or take what voltage it ends up going at (due to tempco). These are due to breakdown effects which should depend on the material being *made of* electrons, not the relative availability thereof.But by all means, don't take my word for it. I don't know this stuff. Find the literature and go from there. If nothing else, phone a few manufacturer reps...Tim

author: SeanB
author_rank: Super Contributor
timestamp: June 29, 2014, 07:42:17 am
content: Just remember as well your solder might not perform at low temperature if it is lead free.....http://en.wikipedia.org/wiki/Tin_pest

author: Simon
author_rank: Unknown
timestamp: June 29, 2014, 08:23:45 am
content: I'm allowed to use leaded solder

author: lewis
author_rank: Frequent Contributor
timestamp: June 29, 2014, 11:36:21 am
content: Quote from: Simon on June 27, 2014, 09:13:05 pmHe might not be happy with the idea, but it's really tough tit. Unless you want to use some really exotic and expensive components.QuoteComponents rated for -40 operation often have a storage temperature rating down to -55. As you say, if the device will be unpowered while at -46, the heater control might not start up. So use some sort of passive heater control to get the enclosure up to -30 or something before operation begins. MOSFET + Thermistor + heater resistors, or even put a load of series connected PTCs directly across the power input (use them as heaters) for some rough temperature self-regulation. Make it as simple as possible.QuoteVery well - orders of magnitude better than air. Not as good as copper or ally, but good enough to ensure your board will be relatively isothermal, and to use as heatsinking for the voltage regulator.QuoteWhat are the input/output requirements of the reg? D2PAK on a 1 square inch pad with thermal vias through to another 1 square inch pad underneath, and potted, will easily sink 0.5W You'd get away with 2-3W

author: David Hess
author_rank: Super Contributor
timestamp: June 29, 2014, 02:57:45 pm
content: Quote from: T3sl4co1l on June 29, 2014, 07:00:38 amI have used bipolar devices including operational amplifiers and bandgap references down to LN2 temperatures without problems but had to qualify the parts myself.

author: Edwin G. Pettis
author_rank: Frequent Contributor
timestamp: June 29, 2014, 05:34:20 pm
content: Motorola published the Zener Diode Handbook, 1st edition in 1967, it had a lot of information on zeners and using them. Some years later, Motorola republished the handbook but I do not consider it to be as good as the original (I have both of them). These books have become difficult to find and while they are probably available in PDF form somewhere (ON Semi), they were not meant to be precision voltage references with the exception of certain types:The old 1N827/A voltage reference zener which dates back to the 1950s from Motorola, this special zener could exhibit a TCV of 5 PPM/°C but this wasn't the only Motorola zener that could do that; the 1N4569/A, 1N4574/A, 1N4579/A, 1N4584/A, 1N4779/A, 1N4784/A, 1N939/A/B, 1N4769/A, 1N4774/A, 1N2624/A/B, 1N2169/A, 1N2170/A, 1N2171A and the 1N945/A/B. These zeners all had 5 PPM/°C TCV and voltages between 6.2V and 11.7 V nominally. The zener currents varied between 0.5mA and 10mA for best performance and the temperature extremes could be as wide as -55°C and +150°C, pretty impressive I'd say.Some of these diodes may still be available.

author: Simon
author_rank: Unknown
timestamp: June 29, 2014, 06:14:40 pm
content: Getting a comparator for -55 is fairly easy I can use it to drive a mosfet and drive some resistors to heat the board, We don't usually expect -46 but being safety critical as it is supposed to be speced for -46 that is what I'll do. I need to drop 30 down to 5V with about 50mA of current so 1.25W SOIC8 regulators only do 0.5W and you can supposedly get more with a 2oz copper ground plain but it needs to be significant and I won't have the room for that, what I can do is use a basic SMPS made with discrete parts to pre-regulate to say 7-8V so tempco on a zener won't be a problem and then the linear regs, that way I don't have problems with trying to adapt the circuit to work at 20-30V as what i normally do is use a series resistor on the regulator to help take some of the load but with 20-30V input that still makes life hard.

author: lewis
author_rank: Frequent Contributor
timestamp: June 29, 2014, 06:31:48 pm
content: LT3470 is an excellent switched-mode regulator I've used many times, and you can buy them from Farnell. Works up to 40V too, so your high input voltage requirement is taken care of. The internal reference is very stable, and it's effing tiny.http://cds.linear.com/docs/en/datasheet/3470fd.pdf