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

author: Infraviolet
author_rank: Super Contributor
timestamp: December 12, 2022, 09:20:35 pm
content: Can anyone suggest MOSFET choices for use in analogue switching applications. I haven't had much experience with discrete electronics, mainly just using BJTs for digital switches, so I'm not sure what the key properties I'd be looking for are. My use case here is using MOSFETs as analogue switches, as smaller-board-space cheaper-price options instead of analogue switching ICs. I've read over the MOSFETs section in Horowitz and Hill, but I suspect all its discussion of options for MOSFETs is probably quite out of date when searching for arts to buy. I do know I'm never going to need >150mA current, going to be working with 5V supplies, I want to have the most minimal voltage drop across them, want to avoid super-sensitive parts with ESD vulnerabilities so severe you can't handle them easily, and want parts which are available as through-hole for breadboard prototyping then SOT-23 for PCB design.Thanks

author: Zero999
author_rank: Super Contributor
timestamp: December 12, 2022, 10:47:08 pm
content: I don't see how going discrete is going to save you any board space or cost.Four discrete MOSFETs, plus a logic inverter are required to build a discrete analogue switch. This is because two are needed to be connected back-to-back, because of the body diodes. An IC analogue switch only requires two MOSFETs, plus a logic inverter.

author: BrianHG
author_rank: Super Contributor
timestamp: December 13, 2022, 12:06:09 am
content: It's not as cheap, but you can get opto-mosfet AC/DC switches.You can find 150ma to 350ma support in DIP8 package.Complete isolation from the driver circuits.If you wan to pay even more, you can fin 1amp through 10amp support.

author: BrianHG
author_rank: Super Contributor
timestamp: December 13, 2022, 01:32:14 am
content: Something like this:https://www.digikey.com/en/products/detail/toshiba-semiconductor-and-storage/TLP592A-F/1823624or this:https://www.digikey.com/en/products/detail/ixys-integrated-circuits-division/CPC1002NTR/1023460(You wont beat this 0.55ohm on resistance, 700ma switch in such a small package unless you want to pay $$.)

author: thm_w
author_rank: Super Contributor
timestamp: December 13, 2022, 01:53:52 am
content: Quote from: Infraviolet on December 12, 2022, 09:20:35 pmlol, don't see that happening.BTW SMD to through hole adapter PCBs are readily available, there is no need for parts to be supplied in through hole packages for prototyping. You're cutting out a large percentage of good parts if you stick with that requirement.You might also consider if/why 150mA is needed, if there is a way to reduce that, you can use 4051's which are 10c each.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 13, 2022, 02:08:17 am
content: Oh that's easy -- since your hard requirement is "least drop", you'll just have to deal with the body diode, capacitance, possible bootstrap drive, etc., and use this:https://www.digikey.com/en/products/detail/vishay-siliconix/SIR178DP-T1-RE3/13175746https://www.vishay.com/docs/77598/sir178dp.pdfA couple hundred microohms is as low as DK has right now; I'm afraid you'll have to go looking elsewhere, or use some in parallel, if you need even less.A few microvolts drop should be feasible, or potentially down to zero if you can consider cryogenically cooled superconductors. What's your budget, anyway?So, not to be too condescending here, just emphasizing the value of clear specifications. This seems really overkill for 150mA; if you can be more specific about what you're doing, a more reasonable answer may be available. Most signal switches are in the 1-100 ohms range, so the request for "minimal" suggests an oversight somewhere. If you can explain what you're doing, or how you're trying to do it, there may be another method that avoids the voltage drop entirely. (In other words, what's known as an X-Y problem: your underlying problem is X, with proposed [but potentially unworkable] solution Y, and you're only asking about Y, when you should be asking about X.)Tim

author: BrianHG
author_rank: Super Contributor
timestamp: December 13, 2022, 03:59:42 am
content: This guy is 0.3ohm, 700ma and costs a bit less.https://www.digikey.com/en/products/detail/toshiba-semiconductor-and-storage/TLP170AM-V4TPL-E/12390883Also it is low led current.