Thanks Christian, that's very helpful and hopefully useful to others here too.
Yes, of course MMBTA42 is a NPN transistor and not a FET, what was I thinking??? I have a few mosfets around (N-channel and P-channel) I'll check their datasheet, so great to have yours as a reference point. I've also ordered a high side switch such as TPS27081A, it seems to control inrush currents and doesn't take too much real estate on the pcb. I'll report back as soon as I have received the parts and tested them. Thanks everyone. Il giorno lunedì 7 giugno 2021 alle 06:20:02 UTC+1 Christian Riise Wagner ha scritto: > Sorry, it's early morning here and I'm apparently not fully awakened yet. > Meant to say that it was the lowest Rds(on) P-channel FET I could find in a > SOT-23 package from LCSC, which is where I shop most of my components. > You'd be able to get away with a slightly higher Rds(on) FET. If it's too > high though, it will start to affect the efficiency, leading to a higher > current consumption when on, due to the boost converter compensating for > the voltage drop. > > mandag den 7. juni 2021 kl. 07.10.53 UTC+2 skrev Christian Riise Wagner: > >> The MMBTA42 is a bipolar transistor and not a FET. Secondly it's NPN, >> making it unsuitable as a high side switch, because the base needs to be at >> least at 0.7V higher than the voltage at the emitter for the transistor to >> turn on. I used a Winsok WST2339 because that was the lowest Rds(on) >> P-channel FET I could find in a SOT-23 package. However you will no doubt >> be able to find something similar from Mouser, Digikey, etc. It will be >> active low, so the gate should be connected to the input voltage rail >> through a pull-up resistor. Your microcontroller will then have to pull the >> gate low to enable the power supply. If the supply voltage of the power >> supply is within the voltage rating of your microcontroller's GPIO pins, >> you can connect it directly. If not, you should use a N-channel MOSFET or >> an NPN transistor to pull the gate low. >> søndag den 6. juni 2021 kl. 22.24.19 UTC+2 skrev [email protected]: >> >>> That's great to hear as a confirmation. As soon as I get a chance to >>> spend some time on the bench, I'll give it a try. It should be quick >>> enough. I'm going to use a standard MMBTA42, please let me know what you >>> have used, if you can share that piece of information. >>> >>> Thanks Christian. >>> >>> Il giorno domenica 6 giugno 2021 alle 21:17:00 UTC+1 Christian Riise >>> Wagner ha scritto: >>> >>>> I've successfully used a P-channel MOSFET as a high side load switch >>>> for a LM3478 based tapped inductor booster. IIRC the off current was >>>> immeasurable, so sub 100nA. >>>> søndag den 6. juni 2021 kl. 19.33.50 UTC+2 skrev [email protected]: >>>> >>>>> Thanks David, indeed you are absolutely right. The power supply with >>>>> LT1308B is very reliable and not as sensitive to layout design as other >>>>> ones that I have tried. >>>>> >>>>> LT1308B has in Iq of 0.01uA in stand-by, as you say. I've built your >>>>> nixie watch (picture attached) and in my design it seems to use 2.3uA. If >>>>> I >>>>> build the HVPS on a breadboard, I can measure 0.1uA or less, which is >>>>> great >>>>> (I guess the resolution of my multimeter Keysight U1241B cannot measure >>>>> less than 0.1uA accurately but that's great at 0.1uA or less). Of course >>>>> I >>>>> have tried to troubleshoot my LT1308B HVPS with no success. In my quest >>>>> for >>>>> a more efficient HVPS, I have designed a power supply using the MAX668 >>>>> chip (as per datasheet and similar to many commercial HVPS), this was >>>>> tricky to design with no noise, it works well now (after 6 different >>>>> designs) however the stand-by power is 2.4uA, as per datasheet. So I can >>>>> have higher efficiency than with LT1308B but also higher stand-by current >>>>> (I believe because MAX668 has an internal LDO). >>>>> >>>>> Which is why I was thinking whether I can use a high load switch to >>>>> truly bring shutdown current to zero using a power supply built around >>>>> MAX668. Of course, this is a bit of a (pointless) extreme challenge to >>>>> reduce stand-by current as much as possible to break even with solar >>>>> panels, but that's where the fun is for me. And of course I could keep on >>>>> troubleshooting the LT1308B design, it actually works perfectly other >>>>> than >>>>> it gives me higher stand-by current than on the datasheet, not sure if >>>>> that >>>>> depends on the design layout. But then again, I'm only a hobbyist and not >>>>> an engineer, so I may be missing something. >>>>> >>>>> So any suggestions on why my LT1308B draws more than 1uA with SHTN=0V >>>>> welcome (I have tried 4 different chips, same thing on the PCB, fine on >>>>> the >>>>> breadboard). Or any other suggestions to use any other power supply with >>>>> perhaps a high load switch to bring standby current to less than 1uA, >>>>> which >>>>> I worry is unnecessary additional components and introduces a potential >>>>> start-up delay. >>>>> >>>>> >>>>> Il giorno domenica 6 giugno 2021 alle 18:12:46 UTC+1 nixiebunny ha >>>>> scritto: >>>>> >>>>>> Max, >>>>>> The LT1308B has a .01 uA typical, 1 uA maximum standby current when >>>>>> shut down, according to the data sheet, shown on page 2 as Iq (the >>>>>> third line with Vshdn = 0V). >>>>>> >>>>>> I don't think you have a problem here. >>>>>> >>>>>> >>>>>> On 6/6/21, Max DN <[email protected]> wrote: >>>>>> > Hello, >>>>>> > >>>>>> > I'm working on a solar powered nixie watch (similar to the Kopriso >>>>>> Nixie >>>>>> > watch). >>>>>> > >>>>>> > It's crucial that I dramatically reduce stand-by power as much as >>>>>> possible >>>>>> > and within 2.5uA, which is where the solar panels will balance off >>>>>> the >>>>>> > stand-by current. >>>>>> > >>>>>> > Regardless of which power supply I use, even using the shutdown >>>>>> pin, the >>>>>> > power supply still takes 2.5uA. This is perfectly in line with the >>>>>> > electrical specifications of the datasheets. >>>>>> > >>>>>> > So, my question is, can I 'brute force' a shutdown, effectively >>>>>> removing >>>>>> > the power from the VIN pin via a PNP high speed mosfet such as >>>>>> MMBTA42 or >>>>>> > it will introduce too much of a delay in starting the power supply? >>>>>> > >>>>>> > I'm going to try it on the fly but if anyone has any suggestions on >>>>>> how to >>>>>> > bring to 0 or to 0.01uA the stand-by current that would be great. >>>>>> > >>>>>> > As an example, I'm using the LT1308B DC/DC converter as on page 17 >>>>>> of the >>>>>> > datasheet >>>>>> > >>>>>> https://www.analog.com/media/en/technical-documentation/data-sheets/1308abfb.pdf >>>>>> >>>>>> > >>>>>> > Any suggestions will be much appreciated. >>>>>> > >>>>>> > Thank you, >>>>>> > Max >>>>>> > >>>>>> > -- >>>>>> > You received this message because you are subscribed to the Google >>>>>> Groups >>>>>> > "neonixie-l" group. >>>>>> > To unsubscribe from this group and stop receiving emails from it, >>>>>> send an >>>>>> > email to [email protected]. >>>>>> > To view this discussion on the web, visit >>>>>> > >>>>>> https://groups.google.com/d/msgid/neonixie-l/d45edcd8-0176-4205-95f1-b04eb34a32f1n%40googlegroups.com. >>>>>> >>>>>> >>>>>> > >>>>>> >>>>> -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. 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