Searching for an optocoupler with MOSFET output stage - Page 1
Forum section: electronics / projects

author: Lupin III.
author_rank: Contributor
timestamp: January 06, 2020, 04:50:49 pm
content: I'm looking for an optocoupler that has a MOSFET output stage and not the usual bipolar output transistor. I have an analog pin on a chip that I want to pull really close to ground (via an isolated signal), which means that the saturation voltage of a bipolar transistor is already too high. The current will be very low (<1mA), so the internal resistance doesn't matter. Switching speed isn't an issue either. As long as an about 1kHz square wave can get through it's fine. I can't really find such a component that's also easily available and rather cheap (~1€/piece would be nice since I need multiple of them).Has anyone got any suggestions?

author: Wimberleytech
author_rank: Super Contributor
timestamp: January 06, 2020, 04:59:51 pm
content: The physics of a bipolar junction transistor make it very amenable to use as an opto-coupler...MOSFET device physics does not. Could an opto be fabricated in a process that has both MOS and bipolar (e.g., BiCMOS)? Yes, of course, but it would be more expensive and why? I rather doubt one exists.

author: up8051
author_rank: Frequent Contributor
timestamp: January 06, 2020, 05:00:16 pm
content: MOUSER:All Products => Optoelectronics => Optocouplers / Photocouplers => MOSFET Output Optocouplers

author: Wimberleytech
author_rank: Super Contributor
timestamp: January 06, 2020, 05:13:36 pm
content: Quote from: up8051 on January 06, 2020, 05:00:16 pmWell, shut my mouth. I have been schooled...lol!Looking at the datasheet, it appears that they just put an output stage on a more-or-less conventional opto circuit.

author: oPossum
author_rank: Super Contributor
timestamp: January 06, 2020, 05:14:55 pm
content: There are even depletion mode MOSFET optos that are normally closed.

author: David Hess
author_rank: Super Contributor
timestamp: January 06, 2020, 07:42:10 pm
content: Quote from: Lupin III. on January 06, 2020, 04:50:49 pmBipolar Vce saturation is awfully low. You can make it even lower by reversing the collector and emitter.But MOSFET optocouplers are readily available or you can make your own. Internally they use a photodiode stack to generate the voltage to drive the MOSFET's gate. JFET optocouplers also exist but I am not sure how they work.

author: RoGeorge
author_rank: Super Contributor
timestamp: January 06, 2020, 09:38:21 pm
content: Quote from: Lupin III. on January 06, 2020, 04:50:49 pmSearch for Panasonic PhotoMOS-Relais.https://www.panasonic-electric-works.com/eu/photomos-relays.htm

author: wraper
author_rank: Unknown
timestamp: January 06, 2020, 10:07:14 pm
content: Does MOSFET side have any power available, something in 3-15V region (like V+ of the chip you mentioned)? You could use bipolar optocoupler + pull-up/down resistor and cheap MOSFET like 2N7002.

author: thm_w
author_rank: Super Contributor
timestamp: January 06, 2020, 11:04:05 pm
content: We discussed some optoMOS here:https://www.eevblog.com/forum/projects/20-channel-diy-scanner-card-for-keithley-dmms-and-daqs/msg2845486/#msg2845486If its for hobby use they are very cheap on the surplus markets.New they can be pricey ($2-8).

author: DBecker
author_rank: Frequent Contributor
timestamp: January 07, 2020, 07:43:20 pm
content: A second on PhotoMOS relay.They are typically used in test instruments, or for low-volume designs where their ease of design makes up for their high cost and fragility.They are excellent for HV safety isolation when intact, but unlike a physical relay they won't tolerate even a brief excursion outside their SOA (safe operating area). When they fail they typically crack and char the package, which may no longer provide the rated isolation.

author: Lupin III.
author_rank: Contributor
timestamp: January 07, 2020, 09:11:04 pm
content: I found some LCA110 (by IXYS, they call it OptoMOS) on ebay quite cheap (less than 10€ for 10, the guy only had 50, so I guess it was surplus). I'll start experimenting with those.I looked at other vendors for other parts, but most of them had either high shipping costs (mouser wants 20€ to ship to Austria for example), not really that big of a selection, or were very pricey per part (I'll need 10-15 eventually, and since it's for a hobby project, I'm skimping whereever I can).I was thinking of building the bipolar opto/MOSFET switch, but I need multiple of them and I'd rather have to solder a single component than multiple (size is an issue as well; I want to add that onto another circuit into an existing case). I also have an application in mind where I not only don't have any power available, but not even a common ground so I'd need a proper "relay".

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: January 07, 2020, 10:25:09 pm
content: https://www.vishay.com/docs/83469/vom1271t.pdfBeware the ones without turn-off feature. They hold the charge and it can take ages for MOSFET to turn off. Putting a high value resistor across the gate will make sure it actually turns off at all.LeoQuote from: Lupin III. on January 06, 2020, 04:50:49 pm

author: thm_w
author_rank: Super Contributor
timestamp: January 07, 2020, 10:53:31 pm
content: Quote from: Leo Bodnar on January 07, 2020, 10:25:09 pmThe LCA110 OP chose looks reasonable enough, 3ms on, 3ms off max:http://www.ixysic.com/home/pdfs.nsf/www/LCA110.pdf/I'm going to try LH1520, 2ms on/off max:http://www.vishay.com/docs/83818/lh1520ab.pdf