Reverse leakage current of zener...datasheet contradicts itself - Page 1
Forum section: electronics / beginners

author: opampsmoker
author_rank: Regular Contributor
timestamp: November 20, 2020, 02:54:14 pm
content: Thanks,Actually i notice that the UDZV8.2B (8V2 Zener) is said in its datasheet page 2 to have a Reverse Leakage current of 500nA at VR=5V and 25degC. However, its datasheet on page 4 states that it has a Reverse Leakage current of <10 picoAmps at VR=5V and temp = 25degC.Do you know which one is correct?UDZV8.2https://fscdn.rohm.com/en/products/databook/datasheet/discrete/diode/zener/udzvte-178.2b-e.pdf___---___---___----__----___---___---__---___---___---__Also,Please can you state what is the overall leakage current through these attached back to back zeners?One is a 5V1 zener, and the other an 8V2 zener.UDZV8.2 (8.2V Zener)https://fscdn.rohm.com/en/products/databook/datasheet/discrete/diode/zener/udzvte-178.2b-e.pdfUDZV5.1 (5.1V Zener)https://datasheetspdf.com/pdf-file/1261604/ROHM/UDZV5.1B/1(BTW Page 4 and page 2 of their datasheets contradict each other over leakage current)___----___----____-----____----___-----_____----____Also, do you think the “modified back to back zeners” circuit, (also attached) with a MMBD4148 diode in series with each Zener, will reduce the amount of leakage current whilst having the same “back to back” Zener effect?MMBD4148 datasheethttps://www.diodes.com/assets/Datasheets/ds12003.pdf

author: TimFox
author_rank: Super Contributor
timestamp: November 20, 2020, 03:56:06 pm
content: The larger value of leakage current is a maximum specification. It is a practical value to measure quickly on automated testing equipment in production.The smaller value is a typical or characteristic value, based on careful measurements with laboratory equipment over a range of temperature values, and is not guaranteed from individual part to individual part.This difference between guaranteed performance and typical performance is typical of these datasheets, and not suspicious.

author: bdunham7
author_rank: Super Contributor
timestamp: November 20, 2020, 04:07:17 pm
content: If leakage current is critical for some reason you will have to measure and bin your own under the appropriate test conditions. Keep in mind that temperature will have a huge effect.What is the 'back-to-back zener effect" you are looking for? The circuit you posted makes little sense to me--what is the story?

author: wraper
author_rank: Unknown
timestamp: November 20, 2020, 04:17:08 pm
content: Quote from: opampsmoker on November 20, 2020, 02:54:14 pmWhy would it decrease leakage current? Diode is placed in forward direction of the leakage current. It will only increase clamping voltage by it's forward voltage and will prevent zener from passing current in reverse direction.

author: opampsmoker
author_rank: Regular Contributor
timestamp: November 20, 2020, 04:19:39 pm
content: Hi,Yes thankyou, therefore i modify...would the attched BJT-Zener circuit have the low_leakage_current effect wanted?

author: wraper
author_rank: Unknown
timestamp: November 20, 2020, 04:22:31 pm
content: Quote from: opampsmoker on November 20, 2020, 04:19:39 pmThis circuit is some sort of nonsense and waste of parts. You will be shorting supply via 3 diodes in series basically. D14-> B-C of Q1 -> D7. And the same for the other half (in reverse direction). Nor that something like this could decrease leakage current to begin with, only increase it if anything.

author: srb1954
author_rank: Super Contributor
timestamp: November 20, 2020, 09:38:10 pm
content: Quote from: opampsmoker on November 20, 2020, 04:19:39 pmThis circuit won't reduce the leakage. It will just add the leakage of the transistor on top of the existing leakage of the zeners. Also the clamping voltage will be less well defined.The better way to obtain low leakage is to use low-leakage diodes to clamp the incoming signal to reference supplies biased at slightly more than the maximum signal level desired. There are clamp ICs that incorporate such clamping diodes and and Zener diodes for generating the reference supply. You will need 2 of these if you want bipolar clamping.

author: opampsmoker
author_rank: Regular Contributor
timestamp: November 21, 2020, 12:07:26 am
content: post #6Woops yes...here is the corrected circuit, (attached) with hopefully lower leakage. I think when leakages are in series, the overall leakage seen is the level of the lowest leakage current?

author: wraper
author_rank: Unknown
timestamp: November 21, 2020, 12:45:49 am
content: Quote from: opampsmoker on November 21, 2020, 12:07:26 amCorrected, however not useful in any way. Voltage drop across R1/R2 is near zero at zener leakage current. As if zeners were connected directly to the rails. So this circuit has the same leakage current through zener diodes as another two circuits + some additional leakage through transistors. So left side of it is simply useless but the right side of it is broken since collector and emitter of transistor are swapped.

author: cortex_m0
author_rank: Regular Contributor
timestamp: November 21, 2020, 05:38:06 am
content: Quote from: opampsmoker on November 21, 2020, 12:07:26 amIf you stick 100 Megs in series with a zener, you'll cut the leakage significantly, but also prevent it from clamping.Putting a zener in series with a silicon diode which is forward biased is not going to give you lower leakage current, except as it changes the voltage seen by the zener. The forward-biased diode will happily conduct provided it sees 0.6V across its terminals. The zener will happily conduct provided it sees Vz across its terminals.Putting a zener in series with a silicon diode which is reverse-biased is not useful as a clamp.What I can say is that none of the circuits you have provided will have overwhelmingly less leakage than the zener by itself.I created a quick LTSpice model, using the diode models they have, including the 1N4148 silicon diode and the UDZV6_2 and UDZV8_2 zeners (there was no model for the 5.2V zener).With only the 8.2V zener and 5V across it, the leakage is 1.1 nA. Consider that the baseline.Circuit 1, two zeners in series with one forward conducting. SPICE says at 5V, the leakage is 1.0nA.Circuit 2, four diodes in a rather unusual configuration, has 3.7 nA leakage at 5V.Circuit 3, a combination of diodes and BJTs has 6.5 nA leakage at 5V.I certainly wouldn't treat 20 minutes of SPICE as the true observation, simply because the authors of the SPICE Models probably weren't thinking about model accuracy in the nanoamps case. But this does intuitively make sense. Circuits 2 and 3 have components in parallel, so there should be more leakage. Most of the leakage in circuits 2 and 3 comes through the reverse-biased 1N4148 according to SPICE.Circuit 3 is very weird to me. D11 and D13 can never forward-conduct, so they are only providing leakage. No clamping, unless you assume reverse polarity.

author: opampsmoker
author_rank: Regular Contributor
timestamp: November 21, 2020, 12:19:10 pm
content: QuoteThanks, woops, i put the PNP the wrong way round...here it is corrected as attachedBy the way, the shown voltage "7V", is sometimes -4V.The circuit is supposed to protect against high pos or neg voltages......as kindly said by posters here, it doesnt look like it does either

author: bdunham7
author_rank: Super Contributor
timestamp: November 21, 2020, 05:07:49 pm
content: Quote from: opampsmoker on November 21, 2020, 12:19:10 pmThat is useful information. I've been following with no idea of why you were doing all this.Your attempts to reduce the 'leakage' are actually increasing it. In the first circuit, the leakage will be whatever the leakage current of the reverse-biased zener is at the applied voltage less the very small Vf of the forward biased zener. In the second circuit, the leakage in one leg will be the same, except that the Vf of the 1N4148 differs from the zener at the very small leakage current and may cause the voltage across the reverse-biased zener to vary. I think this variation will be very small and I don't know whether it would be more or less. However, the second circuit has another leg and the leakage in that leg will be the reverse leakage current of the 1N4148, which may be small but is still more than the zero.The third circuit will result in even more leakage. I don't want to analyze it, but you've just provided more paths for leakage. The 10K resistor is nothing at these currents, so you have at least the leakage of the original circuit plus whatever leaks through the transistor branch.The fundamental problem here is that to use a zener as a clamp, the zener itself must be exposed somehow to the voltage it is intended to clamp. At that voltage, the zener will leak a certain amount of current and that current has to go somewhere. If you don't allow it to return directly to the other side of the voltage source, there has to be a voltage drop somewhere other than across the zener, whereupon the voltage across the zener is less than the source voltage and the zener no longer works as a clamp. I think you are confusing this leakage issue with circuits that use a zener in regulator that also uses other components to limit the current and power dissipation of the zener.So now that you've told us what voltages your circuit uses and that it switches polarity, can you share what it is that needs the leakage to be so low? And what would be an acceptable leakage amount?