increased zener reliability with series / parallel devices? - Page 1
Forum section: electronics / projects

author: max_torque
author_rank: Super Contributor
timestamp: November 09, 2017, 08:35:03 pm
content: I'm going to need to use a zener diode as a low accuracy voltage reference for an emitter follower amplifier. Output voltage will be around 10 vdc (+- 1 v is fine) from a supply that ranges from 18 to 33 vdc.If i use a single zener (of around 11v as i'm using a darlington transistor for the emitter follower) then if that zener fails short then my output drops to zero (load turns off) and if it fails open my output flys up to meet the supply (load goes bang and lets out magic smoke!) Neither is an entirely useful option!So, can i increase the effective robustness of the system to a failure of that single zener?By paralleling two zeners i can mitigate the fail open scenario (second zener steps in to take the shunt current) but having two zeners doubles the chances of a fail to short event, which it cannot mitigate! I guess i could have a series pair meaning a fail to short simply halves the output voltage (or use three in series, which drops output by 1/3 which would probably be ok)But, in reality, does this actually make the system more reliable because i then have many more zeners to fail?Is there some other method/arrangement i have missed that would be better?

author: Siwastaja
author_rank: Super Contributor
timestamp: November 09, 2017, 08:48:32 pm
content: Are the zeners subject to any voltage or current transients, mechanical or themal shock etc. that could trigger either failure mode?If not, adding more components will most likely just hinder reliability, which is practically 100.0000% already, by adding complexity and reducing design time from places where reliability improvements could be made.What I'm trying to say is that zener as-is, when used within specs, should be considered an extremely reliable component.Some crowbar type overvoltage protection (with a fuse) in the output would protect from many more failure modes as well, in fact from the failure caused by any upstream component or design mistake.

author: Whales
author_rank: Super Contributor
timestamp: November 09, 2017, 09:05:25 pm
content: If the zener going open is a problem: attach the zener to the centre of a voltage divider, rather than just a high-side resistor. Some fiddling with values might get you somewhere that's happy both during normal operation and in the event the zener opens.

author: max_torque
author_rank: Super Contributor
timestamp: November 09, 2017, 10:31:08 pm
content: Quote from: Whales on November 09, 2017, 09:05:25 pmAH HA! That's a great idea! The output would move as the input voltage changes, but that would be better than it shooting up to the input voltage?Perhaps that resistive divider approach, in parallel with two series zeners to try to protect from the fail short failure might do it?

author: max_torque
author_rank: Super Contributor
timestamp: November 09, 2017, 10:33:01 pm
content: Quote from: Siwastaja on November 09, 2017, 08:48:32 pmsimple answer: YES!Application is similar to a heavy duty off highway prime mover, so lots of shock, vibration and large swings in environment etc!

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: November 10, 2017, 01:34:34 am
content: From the RAC manual 'Failure Mode Distributions':Diode, Zener, Voltage ReferenceParameter Change 0.69Open 0.18Short 0.13Diode, Zener, Voltage RegulatorOpen 0.45Parameter Change 0.35Short 0.20

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 10, 2017, 02:25:20 am
content: What reliability rate do you need?What are the major failure mechanisms your device will be exposed to?If you're just doing a pass regulator, you're using a very odd example to work reliability upon -- the series resistor can be very reliable in and of itself, and limits current even under surge conditions, even without the surge limiting property of a transformer, rectifier and filter (if applicable)!Zener diodes are extremely reliable. They really only die from environmental, mechanical or thermal problems (corrosion, crushing or cutting, overheating).(Mind I'm not an expert on reliability analysis, but I know enough to know that, unless you're doing amission criticalstudy here, the zener diode is not even on the radar!)Quote from: Neomys Sapiens on November 10, 2017, 01:34:34 amInteresting, do they also give causes/mechanisms, and failure rates? I would think opens to be extremely uncommon, but that's probably because I'm more familiar with transient (overcurrent and overtemp) events as a failure mechanism. It feels kind of funny if it includes corrosion, because that's something that affects all components, and plain old wires too, but if that's the case, it could explain a lot of that. (Of course, if it fails shorted, then draws faulting current and blows open, there's always that, too!)Tim

author: max_torque
author_rank: Super Contributor
timestamp: November 10, 2017, 01:30:26 pm
content: I guess i'm trying to mitigate against a single component failure bringing the whole house down.The application is not mission critical, and so redundant duplicated systems are not necessary, but if i could add a small extra amount of complexity to bring some definite ability to reduce the failure cases, then that would be sensible.Using the parallel resistive divider should bring significant functional failure mitigation to the zener reference failing open, because fixed resistors are highly reliable and very robust (relatively) to operation outside of nominal parameters.The zener failing shut however can only be mitigated by using two (or more) in series.In the original case, i have one zener, but no mitigation of it's failure (single failure = system fails)If i use say 2 series zeners, then the system can survive with a single failure, although that failure is now twice as likely to happen as i have two zeners! (in reality, it won't be that simple, as a failure of one zener, could be caused by a factor that would then result in the second zener failing identically (ie an over voltage surge or similar))

author: Siwastaja
author_rank: Super Contributor
timestamp: November 10, 2017, 05:32:04 pm
content: I guess then that the issue here is strong vibration.The correct way would be to dampen the vibration of the PCB, glue down the components, use conformal coating, etc.And of course, get rid of vibration sensitive components. Zeners shouldn't be on that list. Or if they are, you should do the same analysis for each and every component, including every resistor.I think that the possibility of "going open" failure will be the same for a resistor and a zener: an internal crack. I don't believe a resistor would be inherently much more reliable. They do crack when mishandled physically, I've seen that, but are not susceptible for huge failure rates caused by only "slight abuse", like SMD MLCCs for example.MLCCs require extreme care, as well as any heavy component. But you probably know this already.

author: max_torque
author_rank: Super Contributor
timestamp: November 10, 2017, 06:01:06 pm
content: pcb is pretty well supported on a CNC machined from solid alluminium frame, it's potted too!

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: November 10, 2017, 08:57:05 pm
content: Quote from: T3sl4co1l on November 10, 2017, 02:25:20 amOn the Mil side (RAC), this has always floundered due to organisational problems as well as unclear reporting. And there is the problem that the involved industry would have to disclose where there are undue stress factors in their equipment, which would be like admitting having tricksed during design qualification.I have been personally involved in an attempt to gain more data on the relationship between prevalent stress factors and failure modes of power MOSFETs performed for a major european semiconductor company which was unfortunately cancelled by the usual (budgetary and lack of long-term thinking) reasons when 4/5 done. Ironically, the company in question was sprouted by S*****S, where another data collection on those topics had been lost about 10 years before. I try to grab any information on this I can get hold of, but it is very difficult to without a large-scale collection effort. And ask yourself how many engineers having a sufficiently multidisciplinary approach and diversified knowledge are out there, whp would be able to look into design, manufacturing as well as usage of a technical device in order to find out WHY a component failed. And even if they could do it, they have to be let!Real thorough analysis remains unfortunately limited to a few high-profile cases, which are insufficient to obtain generalized data.

author: danadak
author_rank: Super Contributor
timestamp: November 11, 2017, 12:34:11 am
content: Some ref material.https://www.microsemi.com/document-portal/doc_view/129581-micronote-135-failure-mechanisms-for-transient-voltage-suppressorsRegards, Dana.

author: Zero999
author_rank: Super Contributor
timestamp: November 11, 2017, 12:44:16 am
content: Why are you especially worried about the zener going open circuit, causing the output voltage to go too high? I think you should be more concerned about the pass transistor failing short circuit, which is far more likely to happen!

author: Whales
author_rank: Super Contributor
timestamp: November 11, 2017, 12:52:52 am
content: Quote from: max_torque on November 09, 2017, 10:31:08 pmA zener+resistor reference's output will already move up & down depending on the input voltage. Yes using a divider will make it worse, not sure by how much however (2x? variable?).

author: Simon
author_rank: Unknown
timestamp: November 11, 2017, 01:15:11 pm
content: This is only for pre-regulation so a volt either way does not matter. Should the zener fail the extra resistor will at least guarantee that the output voltage does not hit the input voltage.

author: max_torque
author_rank: Super Contributor
timestamp: November 11, 2017, 05:01:58 pm
content: Quote from: Hero999 on November 11, 2017, 12:44:16 amMainly because the pass element is a big beefy transistor that is being operated nowhere near it's SOA limitation.I have series resistance in, er, series with the Trans as well, and a large clamping diode downstream. I'm in the middle of working out what wins the fight, and the largest value of that series R i can get away with......

author: Ian.M
author_rank: Super Contributor
timestamp: November 11, 2017, 05:19:06 pm
content: Consider using three TL431 shunt regulators, each with a resistor in series with its cathode so it cant pass more than 50% of the no load current at the nominal regulated voltage, and with its own potential divider for Adj. Normally, one will be full on and draw 50% of the current, another will be regulating and the last will be off. Which is which depends on their precise reference voltage and the potential divider tolerances. If one fails open or shorted or if there is any single component or connection failure in the potential dividers, they will just redistribute the current amongst themselves and maintain regulation so long as the current through them does not drop below slightly over 50% of the no-load current.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 12, 2017, 01:30:13 am
content: Heh... if the thresholds are wider than the offset spread, that could work. But nothing prevents two random devices from being unusually well matched, and then what? Oscillation seems possible.As with all voltage regulators and amplifiers, you can at least always make regulation worse by adding a resistor (in series with the anode), and add individual damping by using some series resistance on the K and REF pin, and an R+C from K to REF (for each device).Tim

author: Ian.M
author_rank: Super Contributor
timestamp: November 12, 2017, 01:54:39 am
content: If the tolerances fall out so two of them have virtually the same regulated voltage, they will just share current, with both in their linear region, provided there is enough capacitance to ground at the regulated node. A single TL431 needs at least 6uF on the regulated node to guarantee unconditional stability. I suspect 18uF would be the minimum for three effectively in parallel. The capacitance would swamp any fast interaction with their peers. Extra capacitance at the Adj node introduces a phase shift and is likely to cause instability. K to Adj R-C compensation networks could also get a bit hairy, as there is no design info for such usage.Obviously the regulated node capacitor and the dropper resistor to it from the supply should be series parallel combos so they can tolerate a single component failure. 4x 47uF should do for the capacitor. The resistor should be split into more parallel strings so an individual short or open doesn't take the trio of TL431 regulators out of their failsafe current range.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 12, 2017, 02:24:31 am
content: Quote from: Ian.M on November 12, 2017, 01:54:39 amAh, but how do you know?And, will it be like C-loading an LM339-come-op-amp, where it's still fidgeting internally, but the output ripple is squashed enough that you can't tell -- until you put it on a low-noise system and find out the hard truth? Or will it behave properly? How can you guarantee that?Reminder, the datasheet only gives stability for a lone part. Who knows how that curve changes with more? Or over temperature, or load condition (ESR, ESL).What if the design requires fully bounded specs -- you can only use min and max parameters from the datasheet. What C load must you use?QuoteAh, a cookbook engineer...Extra capacitanceADJ to GNDintroduces a phase shift, but that's not what I described. TL431 is just an open-collector op-amp with a whopping (albeit precision) input offset voltage. When you think of it this way, it becomes intuitively obvious how to place compensation around it -- and obvious that the above will strictly improve the situation (given the choice of optimal values of R and C, of course -- because we're talking compensation here, and the situation can also be made strictly worse by using maliciously chosen values, too!).But there is design info, too!From On Semi, at least:https://www.onsemi.com/pub/Collateral/TL431-D.PDFEveryone has their version of Fig. 12, by the way, and assuming minimum phase, you can use Kramers-Kronig to reconstruct the actual phase shift. (But probably not, because look at which way Fig. 14 shows the bounce. Granted it's a cartoon of a waveform, but it might've been intended that way, too.)Anyway, scroll down to Fig. 31 for the small signal equivalent model.Even this is insufficient, because the real device is not all-pole, and the poles shift around with load voltage and current. The SPICE models are notoriously inaccurate; I forget whose is the best, but there's also a discrete implementation out there, based on the internal schematic (http://www.audio-perfection.com), which performs accurately including nonlinearities and dependencies.Tim

author: Ian.M
author_rank: Super Contributor
timestamp: November 12, 2017, 06:31:11 am
content: Thanks for pointing me at the On-Semi version datasheet. Saved. Its *MUCH* more comprehensive than the T.I. one I was referring to. I now agree there *IS* enough data to successfully compensate it with a R-C network from K to Adj as you describe.However the Elephant in the room is: How does the reliability of a TL431 compare to a typical Zener?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 12, 2017, 10:47:54 am
content: Simple, again, treat it like an op-amp. How reliable is an op-amp, anyway?Should be some transient capacity there (ESD tolerance 2kV HBM), so even for surges outside the operating range (100mA), with current limiting, I would think it would be pretty good.It doesn't have thermal protection, unlike say a 7805, so there's that to worry about.Tim

author: Zero999
author_rank: Super Contributor
timestamp: November 12, 2017, 02:26:51 pm
content: Quote from: max_torque on November 11, 2017, 05:01:58 pmAnd if the zener is operated well below its maximum power rating, it should be just as reliable as the pass transistor.