Schottky Leakage Current -- Safe to use to prevent reverse charging? - Page 1
Forum section: electronics / beginners

author: humidbeing
author_rank: Regular Contributor
timestamp: November 27, 2023, 12:45:19 am
content: I have a 4.5V alkaline battery pack for a RTC. It requires a diode to prevent reverse charging because the original device was meant for a rechargeable cell and puts out ~4.7V when powered on to the battery pack header.I want to use a Schottky (1N5817) for it's low forward voltage drop. But in the datasheets they all seem to have around 100uA leakage current (maximum). That's a no go for alkalines.I did some empirical testing with one of Dave's uCurrents, and I only measure ~3uA leakage current when the diode is reverse biased at 5V to a dead short load. Which would be safe for alkalines to endure.Can I trust that these will be ok to use? Or would I, in theory, need to test each one?I can just use a 1N4148, which is only rated for a max of 1uA leakage. But I want to avoid the 0.6V drop.

author: bdunham7
author_rank: Super Contributor
timestamp: November 27, 2023, 01:14:25 am
content: First, your actual reverse voltage will only be a few hundred millivolts unless the batteries are dead. Second, the reverse current will depend more on the temperature than anything else. Is this at room temperature or warmer?

author: humidbeing
author_rank: Regular Contributor
timestamp: November 27, 2023, 01:55:36 am
content: Quote from: bdunham7 on November 27, 2023, 01:14:25 amDevice will be indoors and climate controlled.

author: Peabody
author_rank: Super Contributor
timestamp: November 27, 2023, 04:21:19 am
content: The main risk you have is when the battery has been powering the load for some time, and the diode has become toasty. If you then plug in external power, you could get significant reverse current for a short time until the diode cools down. But you said this is for an RTC. i would think its load current would be tiny, so temperature wouldn't be a problem. I suspect the schottky would work just fine.Just for completeness, the other alternative would be a P-channel mosfet oriented so its body diode blocks reverse current. Its gate would be controlled by the external voltage source. When the external source is connected, the mosfet gate would go high, turning off the mosfet. Its body diode is like a rectifier diode, so essentially no reverse current at all. The advantage is that you have essentially no voltage drop at all when running on the battery.Edit: Should have said: If you find the right chart in the schottky's datasheet, you'll see that the reverse current of a schottky increases exponentially with temperature. For some diodes, it can get up into the milliamp range. That's why this can be an important issue.

author: humidbeing
author_rank: Regular Contributor
timestamp: November 27, 2023, 04:24:53 am
content: Quote from: Peabody on November 27, 2023, 04:21:19 amI tested the RTC with Dave's uCurrent and it pulls 178uA.The output and the battery are connected together. Wouldn't the MOSFET always be turned off? There will always be voltage present to the gate. Either from the battery or from the board.

author: Peabody
author_rank: Super Contributor
timestamp: November 27, 2023, 04:29:46 am
content: I guess I don't understand what your setup is, or what board you have. Under what circumstance does the battery pack take over powering the RTC?

author: humidbeing
author_rank: Regular Contributor
timestamp: November 27, 2023, 04:31:41 am
content: Quote from: Peabody on November 27, 2023, 04:29:46 amIt's pretty simple...when the device is on it outputs a charging voltage (the original pack was rechargeable) hence the need for a diode...When the device is off the charging voltage disappears and the pack provides power to the RTC.

author: Peabody
author_rank: Super Contributor
timestamp: November 27, 2023, 04:44:24 am
content: Is the 4.7V charging voltage actually a 5V supply run through its own schottky diode? If so, then the gate would be connected to the board output *before* that schottky. And you would need a 100K pulldown resistor on the mosfet gate to make sure it didn't float when the board power is off.If you can't determine how the 4.7V is generated, you can always just insert a schottky diode into that line.

author: humidbeing
author_rank: Regular Contributor
timestamp: November 27, 2023, 04:46:36 am
content: Quote from: Peabody on November 27, 2023, 04:44:24 amYes, I understand. Hence my objection. I am not going to modify this device, it is not my property to modify.

author: Smokey
author_rank: Super Contributor
timestamp: November 27, 2023, 04:49:54 am
content: https://www.digikey.com/en/products/detail/texas-instruments/LM66100QDCKRQ1/16341515https://www.ti.com/lit/ds/symlink/lm66100-q1.pdfIdeal Diode for up to 5.5V applications. $0.44 in 1s. 1.5A max current. 150nA quiescent current. Tiny package.Note:Non-Automotive Version in stock at mouser:https://www.mouser.com/ProductDetail/Texas-Instruments/LM66100DCKR?qs=vLWxofP3U2y2yPPrXoBBoA%3D%3D

author: Peabody
author_rank: Super Contributor
timestamp: November 27, 2023, 04:57:57 am
content: Well, just to clarify, what I'm suggesting with the mosfet is shown below.

author: ejeffrey
author_rank: Super Contributor
timestamp: November 27, 2023, 05:49:42 am
content: Quote from: Smokey on November 27, 2023, 04:49:54 amIdeal diodes are great because they completely eliminate the forward drop. However, be careful here, as the MOSFET doesn't turn off until the output voltage exceeds the input by 80 mV. Thus, if the alkaline battery pack is at the very top of it's charge curve it can let significant reverse current through given the external supply of only ~4.7V. From what I understand this is unfortunately where alkaline batteries are most sensitive to recharging. If the supply voltage could be assured to be ~5V I think the ideal diode could be made to work, but I think 4.7 might be a problem.What is the maximum load current?Are you dead set on alkaline batteries? I'm assuming the original batteries you are replacing were NiMH? Have you considered other chemistries? LiFeS2 which are available in AA size would provide enough voltage that you could use a standard silicon diode for reverse protection and still easily have enough voltage for normal operation. As a bonus they have an extremely long shelf life and don't leak.

author: humidbeing
author_rank: Regular Contributor
timestamp: November 27, 2023, 06:41:28 am
content: Quote from: ejeffrey on November 27, 2023, 05:49:42 amThe RTC pulls 178uA from the battery. There is a 100ma fuse on the battery in case something on the board side shorts. May have to investigate lithium cells.

author: humidbeing
author_rank: Regular Contributor
timestamp: November 27, 2023, 08:14:04 pm
content: Quote from: Smokey on November 27, 2023, 04:49:54 amVery cool. Too bad you can't actually buy one. Backorder until 2024 sometime.

author: MrAl
author_rank: Super Contributor
timestamp: November 27, 2023, 08:34:41 pm
content: Quote from: humidbeing on November 27, 2023, 12:45:19 amHi,It's interesting that you brought this up, and that you noticed this.Schottkys are funny. If you measure some of them with an Ohm Meter for reverse resistance, you might measure 20k Ohms at room temperature, which is pretty low. If you measure a regular Si diode, you might measure 500 Megohms. Quite a difference.20k Ohms would be too low for some of my applications.What you could do is look for a low leakage Schottky diode like the ZLLS410 made by what used to be Zetex. That may or may not be good enough. Zetex makes some really good stuff.Another try would be something like the SDM02M30LP3 diode, or something like the BAT20J although you might still run into 100ua at a higher temperature than room ambient temperature.

author: humidbeing
author_rank: Regular Contributor
timestamp: November 27, 2023, 09:11:05 pm
content: Quote from: MrAl on November 27, 2023, 08:34:41 pmThe reverse current specs on the ZLLS410 are quite impressive! This might be a solution. Thanks!

author: shapirus
author_rank: Super Contributor
timestamp: November 27, 2023, 09:32:43 pm
content: Have you calculated the difference between a generic PN diode and a Schottky in the energy lost on the diode over the life span of the battery, knowing the supply current of the RTC, and how much shorter the battery life is going to be with a PN diode?It might turn out to be pointless to go through the hassle of finding a suitable Schottky diode, ordering it etc., if it will be acceptable with a PN diode.(that Zetex diode mentioned above is sure a nice device, though.)p.s. I may be wrong, this is just a guess, but something tells me that the 4.5V battery pack, provided that it's at least 3xAAA, will leak or self-discharge well before the batteries are drained by the RTC supply current regardless of the type of the diode used. Especially if it is expected that the device will be powered by something else besides the battery most of the time (I don't know if it's the case, so again I'm guessing here).

author: Smokey
author_rank: Super Contributor
timestamp: November 27, 2023, 10:08:59 pm
content: Quote from: humidbeing on November 27, 2023, 08:14:04 pmSorry. I linked the Q1 automotive one, which does appear to be out of stock everywhere.The non Q1 version, LM66100DCKR , has 121K in stock at mouser.https://www.mouser.com/ProductDetail/Texas-Instruments/LM66100DCKR?qs=vLWxofP3U2y2yPPrXoBBoA%3D%3D

author: Peabody
author_rank: Super Contributor
timestamp: November 27, 2023, 11:20:19 pm
content: Quote from: shapirus on November 27, 2023, 09:32:43 pmHe says the RTC draws 178uA, which is a lot for an RTC. So on AAs, that would be, what, a year or so? But your other point I think is key. If the RTC is going to be on board power all the time except for rare mains outages, then yeah, stick that 1N4148 in there and be done with it. It will basically be alkaline shelf life regardless of what you do.

author: Manul
author_rank: Super Contributor
timestamp: November 28, 2023, 12:10:35 am
content: The lowest reverse leakage Schottky diodes which I know are from Nexperia like the PMEG10010ELR. Hard to beat in that regard.

author: shapirus
author_rank: Super Contributor
timestamp: November 28, 2023, 01:43:01 am
content: Quote from: Manul on November 28, 2023, 12:10:35 amIts Vf, however, is not much lower than that of the regular PN diodes (only 25% or thereabouts lower at ~200 uA), which I'd say defeats the purpose.

author: spostma
author_rank: Regular Contributor
timestamp: November 28, 2023, 09:02:20 pm
content: there is a rather new WCH CH213B/CH213K single/dual ideal diode chip int SOT23 casewith 2.0-5.5V operating voltage with 500mA current limiting, 80mV type dropout voltage,10uA leakage current, and only 14mV typ. Vin-Vout switchover threshold:https://www.wch-ic.com/downloads/CH213DS1_PDF.htmlThis chip might fit your requirements.

author: Peabody
author_rank: Super Contributor
timestamp: November 29, 2023, 03:47:16 am
content: Does anyone know of an integrated ideal diode with very small hysteresis? The LM66100 appears to require big swings to turn it on or off. The CH213B is more sensitive at 18mV, but is there one with virtually no hysteresis, or maybe one where both of the comparator inputs are brought out to pins so you could adjust hysteresis? Ideally SOT or larger.

author: JustMeHere
author_rank: Frequent Contributor
timestamp: November 29, 2023, 03:52:45 am
content: What RTC are you using? Some you can turn off the battery charger. Others simply only draw from the battery.

author: humidbeing
author_rank: Regular Contributor
timestamp: December 01, 2023, 12:26:31 am
content: Quote from: JustMeHere on November 29, 2023, 03:52:45 amthis is an old school Motorola RTC from the 80s/90s. It doesn't have any fancy battery back circuitry. The mother board just pumps a trickle charge out the battery header by passing the +5V rail through a diode and resistor.