Picking a MOSFET for low current switching - Page 1
Forum section: electronics / beginners

author: ledtester
author_rank: Super Contributor
timestamp: November 15, 2022, 08:34:03 pm
content: One thing I've noticed is that the general purpose BJT's like the 3904 / 2222, etc. have data about low current operation -- i.e. like just a few milliamps of collector current. For example, consider this graph from the P2N2222A datasheet:However, MOSFET datasheets don't give as precise data about their low I_D and low V_DS operation -- like I_D in the milliamps range and V_DS around 100-200 mV. But this is precisely the situation you have when you are trying to create an inverter out of a MOSFET and a 10K pull-up resistor.This came up in this Stackexchange question where the poster is having trouble using a TN2106 in place of a ZN4424 as a low current switch:https://electronics.stackexchange.com/questions/642138/understanding-the-characteristics-of-a-mosfet-as-a-switchThe TN2106 doesn't seem to be switching as hard as the ZN4424 for the same V_GS (= 3.3V in this case.) Is there something in the datasheets which would point to this?ZN4424 datasheet:https://www.diodes.com/assets/Datasheets/ZVN4424G.pdfTN2106 datasheet:https://ww1.microchip.com/downloads/en/DeviceDoc/TN2106-N-Channel-Enhancement-Mode-Vertical-DMOS-FET-Data-Sheet-20005942A.pdfOne thing I noticed about the two transistors is that their saturation I_D currents are very different for the same V_GS. For instance, at V_GS = 3V the drain current plateaus at 0.9A for the ZVN4424 but only at 0.15A for the TN2106. In general it seems the ZVN4424 I_D curves plateau at higher levels than they do for the TN2106.Could this explain why the TN2106 didn't work for the poster?In general, are the plateaus (which occur at high V_DS) an indication of how the MOSFET will operate at very low V_DS?ZVN4424A I_D vs. V_DS:TN2106 I_D vs. V_DS:

author: PartialDischarge
author_rank: Super Contributor
timestamp: November 15, 2022, 08:46:34 pm
content: I don’t agree in that mosfets don’t have that low current Id data. The vertical axis is 200mA per div in those graphs and you are talking about 1mA.... well clearly you are in the mV range for Vds, but there’s also the rdson to take into account.The question is if you need a low Vgsth device or not depending on the application.

author: Benta
author_rank: Super Contributor
timestamp: November 15, 2022, 08:47:51 pm
content: Well, a big difference is that the ZVN part is specified at a gate foltage of 2.5 V. The TN part is... not.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 15, 2022, 10:13:59 pm
content: Low Vds is trivial... if Vgs is adequate, it's just Rds(on) * Id.If Vgs isn't adequate, you have greater problems, namely that threshold varies a volt or two over manufacturing (i.e. from part to part), so it's not going to be reliable. Solve that, and the above will hold as a consequence.Tim

author: thm_w
author_rank: Super Contributor
timestamp: November 16, 2022, 12:05:33 am
content: If you look at the stackexchange question, both FETs switch to ground just fine. The issue they seem to be having is the voltage does not pull up to a high enough level (4V instead of 5V of the rail), when the FET is OFF.So that should have nothing to do with Vgs at all.The only spec I can think of being relevant is: Zero-Gate Voltage Drain Current (scope shows Vgs=0V)- TN2106 1uA (60V Vds)- ZVN4424G 10uA (240V Vds)But that is nothing.Must be some other part of the circuit, variance, or some detail they forgot to mention causing it.

author: ledtester
author_rank: Super Contributor
timestamp: November 18, 2022, 09:49:12 am
content: Quote from: thm_w on November 16, 2022, 12:05:33 amThanks for pointing this out. The OP of the stackexchange question has resolved the issue -- the MOSFET turned out to be bad and replacing it solved the problem:https://electronics.stackexchange.com/a/642876/95488